would be reviewed and updated as appropriate in the context of each
periodic review of the NAAQS.

	The Policy Assessment developed specific F factors for each ecoregion
based on the approach discussed above, using alternative percentiles and
alternative national target ANC levels.  The results of this analysis
for ecoregions characterized as acid sensitive are presented in Table
7-1a-d in the Policy Assessment.

C.	Averaging Time

	As discussed in section 7.3 of the Policy Assessment, aquatic
acidification can occur over both long- and short-term timescales.
Long-term cumulative deposition of nitrogen and sulfur is reflected in
the chronic acid-base balance of surface waters as indicated by measured
annual ANC levels.  Similarly, the use of steady state critical load
modeling, which generates critical loads in terms of annual cumulative
deposition of nitrogen and sulfur, means that the focus of ecological
effects studies based on critical loads is on the long-term equilibrium
status of water quality in aquatic ecosystems.  Much of the evidence of
adverse ecological effects associated with aquatic acidification, as
discussed above in section II.A, is associated with chronically low ANC
levels.  Protection against a chronic ANC level that is too low is
provided by reducing overall annual average deposition levels for
nitrogen and sulfur.

	Reflecting this focus on long-term acidifying deposition, the Policy
Assessment developed the AAI that links ambient air indicators to
deposition-related ecological effects, in terms of several factors, F1
through F4.  As discussed above, these factors are all calculated as
annual average values, whether based on water quality and hydrology data
or on CMAQ model simulations.  In the context of a standard defined in
terms of the AAI, the Policy Assessment concludes that it is appropriate
to consider the same annual averaging time for the ambient air
indicators as is used for the factors in the AAI equation.

	We also recognize that short-term (i.e., hours or days) episodic
changes in water chemistry, often due to changes in the hydrologic flow
paths, can have important biological effects in aquatic ecosystems.
Such short-term changes in water chemistry are termed “episodic
acidification.”  Some streams may have chronic or base flow chemistry
that is generally healthy for aquatic biota, but may be subject to
occasional acidic episodes with potentially lethal consequences.  Thus,
short-term episodic ecological effects can occur even in the absence of
long-term chronic acidification effects.

	Episodic declines in pH and ANC are nearly ubiquitous in drainage
waters throughout the eastern U.S.  Episodic acidification can result
from several mechanisms related to changes in hydrologic flow paths.
For example, snow can store nitrogen deposited throughout the winter and
snowmelt can then release this stored nitrogen, together with nitrogen
derived from nitrification in the soil itself, in a pulse that leads to
episodic acidification in the absence of increased deposition during the
actual episodic acidification event.  The Policy Assessment notes that
inputs of nitrogen and sulfur from snowpack and atmospheric deposition
largely cycle through soil.  As a result, short-term direct deposition
inputs are not necessarily important in episodic acidification.  Thus,
as noted in chapter 3 of the ISA, protection against episodic acidity
events can be achieved by establishing a higher chronic ANC level.

	Taken together, the above considerations support the conclusion that it
is appropriate to consider the use of a long-term average for the
ambient air indicators NOy and SOx for an aquatic acidification standard
defined in terms of the AAI.  The use of an annual averaging time for
NOy and SOx concentrations would be appropriate to provide protection
against low chronic ANC levels, which in turn would protect against both
long-term acidification and acute acidic episodes.

	The Policy Assessment has also considered interannual variability in
both ambient air quality and in precipitation, which is directly related
to the deposition of oxides of nitrogen and sulfur from the ambient air.
 While ambient air concentrations show year-to-year variability, often
the year-to-year variability in precipitation is considerably greater,
given the highly stochastic nature of precipitation.  The use of
multiple years over which annual averages are determined would dampen
the effects of interannual variability in both air quality and
precipitation.  For the ambient air indicators, the use of multiple-year
averages would also add stability to calculations used to judge whether
an area meets a standard defined in terms of the AAI.  Consequently, the
Policy Assessment concludes that an annual averaging time based on the
average of each year over a consecutive 3 to 5 year period is
appropriate to consider for the ambient air indicators NOy and SOx.  In
reaching this conclusion, the Policy Assessment notes that in its
comments on the second draft Policy Assessment, CASAC  agreed that a 3
to 5 year averaging time was appropriate to consider (Russell and Samet,
2010b).

D.	Level

	As discussed above, the Policy Assessment concludes that ANC is the
ecological indicator best suited to reflect the sensitivity of aquatic
ecosystems to acidifying deposition from oxides of nitrogen and sulfur
in the ambient air.  ANC is an indicator of the aquatic acidification
expected to occur given the natural buffering capacity of an ecosystem
and the loadings of nitrogen and sulfur resulting from atmospheric
deposition.  Thus, the Policy Assessment developed a new standard for
aquatic acidification that is based on the use of chronic ANC as the
ecological indicator as a component in the AAI.

	The level of the standard would be defined in terms of a single,
national value of the AAI.  The standard would be met at a monitoring
site when the multi-year average of the calculated annual values of the
AAI was equal to or above the specified level of the standard.   The
annual values of the AAI would be calculated based on the AAI equation
using the assigned ecoregion-specific values for factors F1 through F4
and monitored annual average NOy and SOx concentrations.  Since the AAI
equation is based on chronic ANC as the ecological indicator, the level
chosen for the standard would reflect a target chronic ANC value.  As
noted above, the assigned F factors for each ecoregion would be
determined by EPA in the rulemaking to set the NAAQS, based on water
quality and hydrology data, CMAQ modeling, the selected percentile that
is used to identify a representative critical load within the ecoregion,
and the selected level of the standard.  The combination of the form of
the standard, discussed above in section III.B, defined by the AAI
equation and the assigned values of the F factors in the equation, other
elements of the standard including the ambient air indicators (section
III.A) and their averaging time (section III.C), and the level of the
standard determines the allowable levels of NOy and SOx in the ambient
air within each ecoregion.  All of the elements of the standard together
determine the degree of protection from adverse aquatic acidification
effects associated with oxides of nitrogen and sulfur in the ambient
air.  The level of the standard plays a central role in determining the
degree of protection provided and is discussed below.

	The Policy Assessment focuses primarily on information that relates
degrees of biological impairment associated with adverse ecological
effects to aquatic ecosystems to alternative levels of ANC in reaching
conclusions regarding the range of target ANC levels that is appropriate
to consider for the level of the standard.  The Policy Assessment
develops the rationale for identifying a range of target ANC levels that
is appropriate to consider by addressing questions related to the
following areas:  (1) associations between ANC and pH levels to provide
an initial bounding for the range of ANC values to be considered;  (2)
evidence that allows for the delineation of specific ANC ranges
associated with varying degrees of severity of biological impairment
ecological effects;  (3) the role of ANC in affording protection against
episodic acidity; (4) implications of the time lag response of ANC to
changes in deposition; (5) past and current examples of  target ANC
values applied in environmental management practices; and (6) data
linking public welfare benefits and ANC.

1.	Association Between pH Levels and Target ANC Levels

	As discussed above in section II.A and more fully in chapter 3 of the
Policy Assessment, specific levels of ANC are associated with differing
levels of risk of biological impairment in aquatic ecosystems, with
higher levels of ANC resulting in lower risk of ecosystem impacts, and
lower ANC levels resulting in risk of both higher intensity of impacts
and a broader set of impacts.  While ANC is not the causal agent
determining biological effects in aquatic ecosystem,  it is a useful
metric for determining the level at which a water body is protected
against risks of acidification.  There is a direct correlation between
ANC and pH levels which, along with dissolved aluminum, are more closely
linked to the biological causes of ecosystem response to acidification.

y causal indicator of effects related to aquatic acidification, this
suggests that ANC values below approximately -50 μeq/L (the apparent
point in the relationship between pH and ANC where pH reaches a minimum)
are not likely to result in further damage.  In addition, ANC values
around and above approximately 100 μeq/L (the apparent region in the
relationship where pH reaches a maximum) are not likely to confer
additional protection.  As a result, the initial focus in the Policy
Assessment was on target ANC values in the range of -50 to 100 μeq/L.

2.	ANC Levels Related to Effects on Aquatic Ecosystems

	As discussed above in section II.A, the number of fish species present
in a water body has been shown to be positively correlated with the ANC
level in the water, with higher values supporting a greater richness and
diversity of fish species.  The diversity and distribution of
phyto-zooplankton communities also are positively correlated with ANC.

ly -50 to 100 μeq/L, linear and sigmoidal relationships are observed
between ANC and ecosystem effects.  On average, fish species richness is
lower by one fish species for every 21 μeq/L decrease in ANC in
Shenandoah National Park streams (ISA, section <IP_ADDRESS>).  As shown in
Table II-1, ANC levels have been grouped into five categories related to
expected ecological effects, including categories of acute concern ( <0
μeq/L), severe concern (0-20 μeq/L), elevated concern (20-50 μeq/L),
moderate concern (50-100 μeq/L), and low concern (>100 μeq/L).  This
categorization is supported by a large body of research completed
throughout the eastern U.S. (Sullivan et al., 2006).

Policy Assessment has focused on target ANC levels no lower than 0
μeq/L.

	As discussed in the Policy Assessment, biota generally are not harmed
when ANC values are >100 μeq/L, due to the low probability that pH
levels will be below 7.  In the Adirondacks, the number of fish species
also peaks at ANC values >100 μeq/L.  This suggests that at ANC levels
greater than 100 μeq/L, little risk from acidification exists in many
aquatic ecosystems.  At ANC levels below 100 μeq/L, overall health of
aquatic communities can be maintained, although fish fitness and
community diversity begin to decline.  At ANC levels ranging from 100
down to 50 μeq/L, there is increasing likelihood that the fitness of
sensitive species (e.g., brook trout, zooplankton) will begin to
decline.  When ANC concentrations are below 50 μeq/L, the probability
of acidification increases substantially, and negative effects on
aquatic biota are observed, including large reductions in diversity of
fish species and changes in the health of fish populations, affecting
reproductive ability and fitness, especially in water bodies that are
affected by episodic acidification.  While there is evidence that ANC
levels above 50 can confer additional protection from adverse ecological
effects associated with aquatic acidification in some sensitive
ecosystems, the expectation that such incremental protection from
adverse effects will continue up to an ANC level of 100 is substantially
reduced.  The Policy Assessment concludes that the above considerations
support a focus on target ANC levels up to a level greater than 50
μeq/L but below 100 μeq/L, such as up to a level of 75 μeq/L.

	In considering the available scientific evidence, as summarized here
and discussed in more detail in the ISA and REA, in its review of the
second draft Policy Assessment, CASAC expressed the following views
about the range of biological responses that corresponds to this range
of ANC levels (i.e., 0-100 μeq/L):

ects at ANC levels below 20 μeq/L, and reasonable confidence that there
are adverse effects below 50 μeq/L.  Levels of 50 μeq/L and higher
would provide additional protection, but the Panel has less confidence
in the significance of the incremental benefits as the level increases
above 50 μeq/L. (Russell and Samet, 2010b)

	The Policy Assessment concludes that the above considerations,
including the views of CASAC, provide support for focusing on target ANC
levels in the range of 20 to 75 μeq/L.

3.	Consideration of Episodic Acidity

	As discussed in the Policy Assessment, across the broad range of ANC
values from 0 to 100 μeq/L, ANC affords protection against the
likelihood of decreased pH (and associated increases in Al) during long
or short periods.  In general, the higher the ANC within this range, the
lower the probability of reaching low pH levels where direct effects
such as increased fish mortality occur, as shown in Table 3-1 of the
Policy Assessment.  Accordingly, greater protection would be achieved by
target chronic ANC values set high enough to avoid pH depression to
levels associated with elevated risk.

	The specific relationship between ANC and the probability of reaching
pH levels of elevated risk varies by water body and fish species.  ANC
levels below 20 μeq/L are generally associated with high probability of
low pH, leading to death or loss of fitness of biota that are sensitive
to acidification (US EPA, 2008, section <IP_ADDRESS>; US EPA, 2009, section
<IP_ADDRESS>).  At these levels, during episodes of high acidifying
deposition, brook trout populations may experience lethal effects.  In
addition, the diversity and distribution of zooplankton communities
decline sharply at ANC levels below 20 μeq/L.  Overall, there is little
uncertainty that significant effects on aquatic biota are occurring at
ANC levels below 20 μeq/L.

	It is clear that at ANC levels approaching 0 μeq/L (Table II-1), there
is significant impairment of sensitive aquatic ecosystems with almost
complete loss of fish species.  Avoiding ANC levels approaching 0 μeq/L
is particularly relevant to episodic spikes in acidity that occur during
periods of rapid snow melt and during and after major precipitation
events.  Since the ANC range considered in the Policy Assessment
reflects average, long-term base flow values, it is appropriate to
consider protecting against episodic drops in ANC values to a level as
low as 0 μeq/L.  Staddard et al. (2003) noted on average a 30 μeq/L
depression of ANC between spring and summer time values, indicating the
need to maintain higher base flow ANC levels to protect against ANC
levels below 0 μeq/L.  The above considerations do not provide support
for a target chronic ANC level as low as 0 μeq/L for a standard that
would protect against significant harm to aquatic ecosystems, including
harm from episodic acidification.  The Policy Assessment concludes that
these considerations also support a lower end of the range for
consideration no lower than 20 μeq/L.

	The CASAC agreed with this conclusion in its comments on the second
draft Policy Assessment (Russell and Samet, 2010b).  CASAC noted that
“there are clear and marked biological effects at ANC values near 0
μeq/L, so this is probably not an appropriate target value” for the
AAI.  With regard to the likelihood of impairment of aquatic ecosystems
due to episodic acidification, in terms of specific target levels for
chronic ANC, CASAC expressed the following view:

] approximately 50 μeq/L.  Thus, based on these studies, a long term
ANC target level of 75 μeq/L would generally guard against effects from
episodic acidification down to a level of about 25 μeq/L. (Russell and
Samet, 2010b)

4.	Consideration of Ecosystem Response Time

	The Policy Assessment notes that when considering a standard level to
protect against aquatic acidification, it is appropriate to take into
account both the time period to recovery as well as the potential for
recovery in acid-sensitive ecoregions.  Ecosystems become adversely
impacted by acidifying deposition over long periods of time and have
variable time frames and abilities to recover from such perturbations.
Modeling presented in the REA (US EPA, 2009, section 4.2.4) shows the
estimated ANC values for Adirondack lakes and Shenandoah streams under
pre-acidification conditions and indicates that for a small percentage
of lakes and streams, natural ANC levels would have been below 50
μeq/L.  Therefore, for these water bodies, reductions in acidifying
deposition are not likely to achieve an ANC of 50 μeq/L or greater.
Conversely, for some lakes and streams the level of perturbation from
long periods of acidifying deposition has resulted in very low ANC
values compared to estimated natural conditions. For such water bodies,
the time to recovery would be largely dependent on future inputs of
acidifying deposition.

el of the ecological indicator by a given time.  For example, to achieve
an ANC level of 20 μeq/L by 2030, it might be necessary to specify a
higher target ANC level of, for example, 50 μeq/L, such that the
depositional loading would be reduced more quickly than would occur if
the depositional loading was based on achieving a target ANC level of 20
μeq/L as a long-term equilibrium level.  In this example, the target
ANC of 50 μeq/L would ultimately be realized many years later.

	The above considerations have implications for selecting an appropriate
standard level, in that the standard level affects not only the ultimate
degree of protection that would be afforded by the standard, but also
the time frame in which such protection would be realized.  However, the
Policy Assessment recognizes that there is a great deal of heterogeneity
in response times among water bodies and that there is only very limited
information from dynamic modeling that would help to quantify recovery
time frames in areas across the country.  As a consequence,
quantification of a general relationship between critical loads
associated with a specific long-term target ANC level and target loads
associated with achieving the target ANC level within a specific time
frame is not currently possible.  Thus, while the time frame for
recovery is an important consideration in selecting an appropriate range
of levels to consider, the Policy Assessment concludes that it can only
be considered in a qualitative sense at this time.

5.	Prior Examples of Target ANC Levels

	A number of regional organizations, states, and international
organizations have developed critical load frameworks to protect against
acidification of sensitive aquatic ecosystems.  In considering the
appropriate range of target ANC levels for consideration in this review,
it is informative to evaluate the target ANC levels selected by these
different organizations, as well as the rationale provided in support of
the selected levels.  Chapter 4 of the Policy Assessment provides a
detailed discussion of how critical loads have been developed and used
in other contexts.  Specific target values and their rationales are
summarized below.

	The UNECE has developed critical loads in support of international
emissions reduction agreements.  As noted in chapter 4 of the Policy
Assessment, critical loads were established to protect 95 percent of
surface waters in Europe from an ANC less than 20 µeq/L based on
protection of brown trout.  Individual countries have set alternative
ANC targets; for example, Norway targets an ANC of 30 µeq/L based on
protection of Atlantic salmon.

Several states have established target ANC or pH values related to
protection of lakes and streams from acidification.  While recognizing
that some lakes in the Adirondacks will have a naturally low pH, the
state of New York has established a target pH value of 6.5 for lakes
that are not naturally below 6.5.  As noted above, this level is
associated with an ANC value that is likely to be between 20 and 50
µeq/L or possibly higher.  New Hampshire and Vermont have set ANC
targets of 60 µeq/L and 50 µeq/L, respectively.  Tennessee has
established site-specific target ANC values based on assessments of
natural acidity, with a default value of 50 µeq/L when specific data
are not available.

hat target ANC values between 20 and 60 μeq/L have been selected by
states and other nations to provide protection of lakes and streams in
some of the more sensitive aquatic ecosystems.

6.	Consideration of Public Welfare Benefits

	The point at which effects on public welfare become adverse is not
defined in the CAA.  Characterizing a known or anticipated adverse
effect to public welfare is an important component of developing any
secondary NAAQS.  According to the CAA, welfare effects include:

…effects on soils, water, crops, vegetation, manmade materials,
animals, wildlife, weather, visibility, and climate, damage to and
deterioration of property, and hazards to transportation, as well as
effect on economic values and on personal comfort and well-being,
whether caused by transformation, conversion, or combination with other
air pollutants. (CAA, section 302(h)).

While the text above lists a number of welfare effects, the NAAQS is
aimed at protection from adverse effects to public welfare.
Consideration of adversity to public welfare in the context of the
secondary NAAQS for oxides of nitrogen and sulfur can be informed by
information about losses in ecosystem services associated with
acidifying deposition and the potential economic value of those losses,
as summarized above in section II.C and discussed more fully in chapter
4 of the Policy Assessment.

, the study implies benefits to the New York population roughly on the
order of $600 million per year (in constant 2007$).  The survey
administered in this study recognized that participants were thinking
about the full range of services provided by the lakes in question –
not just the recreational fishing services.  Therefore the estimates of
willingness to pay include resident’s benefits for potential hunting
and birdwatching activities and other ancillary services. These results
are just for New York populations.  The Policy Assessment concludes that
if similar benefits exist for improvements in other acid sensitive
lakes, the economic value to U.S. populations could be very substantial,
suggesting that, at least by one measure of impact on public welfare,
impacts associated with ANC less than 50 μeq/L may be adverse to public
welfare.

7.	Summary of Alternative Levels

	Based on all the above considerations, the Policy Assessment concludes
that consideration should be given to a range of standard levels from 20
to 75 μeq/L.  The available evidence indicates that target ANC levels
below 20 μeq/L would be inadequate to protect against substantial
ecological effects and potential catastrophic loss of ecosystem function
in some sensitive aquatic ecosystems.  While ecological effects occur at
ANC levels below 50 μeq/L in some sensitive ecosystems, the degree and
nature of those effects are less significant than at levels below 20
μeq/L.  Levels at and above 50 μeq/L would be expected to provide
additional protection, although uncertainties regarding the potential
for additional protection from  adverse ecological effects are much
larger for target ANC levels above about 75 μeq/L, as effects are
generally appreciably less sensitive to changes in ANC at such higher
levels.

	In reaching this conclusion in the Policy Assessment, consideration was
given to the extent to which a target ANC level within this range would
protect against episodic as well as long-term ecological effects.
Levels in the mid- to upper part of this range would be expected to
provide greater protection against short-term, episodic peaks in aquatic
acidification, while lower levels within this range would give more
weight to protection from long-term rather than episodic acidification.
Similarly, levels in the mid- to upper part of this range would be
expected to result in shorter time periods for recovery given the lag in
ecosystem response in some sensitive ecosystems relative to levels in
the lower part of this range.  The Policy Assessment also note that this
range encompasses target ANC values that have been established by
various States and regional and international organizations to protect
against acidification of aquatic ecosystems.

	The Policy Assessment recognizes that the level of the standard
together with the other elements of the standard, including the ambient
air indicators, averaging time, and form, determine the overall
protectiveness of the standard.  Thus, consideration of a standard level
should reflect the strengths and limitations of the evidence and
assessments as well as the inherent uncertainties in the development of
each of the elements of the standard.  The implications of considering
alternative standards, defined in terms of alternative combinations of
levels and percentile values that are a critical component of factor F1
in the form of the standard, are discussed below in section III.E.  Key
uncertainties in the various components of the standard are summarized
and considered below in section III.F.

E.	Combined Alternative Levels and Forms

	To provide some perspective on the implications of various alternative
multi-pollutant, AAI-based standards, the Policy Assessment presented
the number of acid-sensitive ecoregions that would likely not meet
various sets of alternative standards.  The alternative standards
considered were based on combinations of alternative target ANC levels,
within the range of 20 to 75 µeq/L, and alternative forms,
characterized by alternative representative percentiles within the range
of the 70th to 90th percentile.  These alternative standards are also
defined in terms of the other elements of the standard:  ambient air
indicators NOy and SOx, discussed above in section III.A; other elements
of the form of the standard, including ecoregion-specific values for
factors F1 through F4 in the AAI equation, discussed above in section
III.B.5; and an annual averaging time for NOy and SOx, discussed above
in section III.C.  With regard to the averaging time, the assessment did
not consider multi-year averaging of the calculated annual AAI values
due to data limitations, including, for example, the lack of CMAQ
modeling for multiple consecutive years.  In this assessment, we
characterize an ecoregion as likely not meeting a given alternative
standard if the calculated AAI value is less than the target ANC level
of the standard, recognizing that higher AAI values are more protective
than lower values.

 In all cases, these relatively non-acid sensitive ecoregions were
estimated to meet all of the alternative standards considered in this
assessment.

	As described above, the AAI values presented in Table 7-1a-d of the
Policy Assessment are based in part on data from 2005 CMAQ model
simulations, which was used to generate values for F2 through F4 in the
AAI equation, as well as to estimate annual average ambient air
concentrations of NOy and SOx that reflect recent air quality in the
absence of currently available monitored concentrations in sensitive
ecoregions across the country.  Water quality and hydrology data from
water bodies within each ecoregion were also used in calculating the AAI
values.  Such data were initially used to calculate critical loads for
each water body with sufficient data within an ecoregion so as to
identify the nth percentile critical load representative of the
ecoregion used in calculating the F1 factor for the ecoregion.  As
expected, the number of ecoregions that likely would not meet
alternative standards increases with increasing percentile values and
target ANC levels (US EPA, 2011, Table 7-2).  Out of 22 acid-sensitive
ecoregions, the number of ecoregions that would likely not meet the
alternative standards  ranges from 22 for the most protective
alternative standard considered (75 µeq/L, 90th percentile) to 4 for
the least protective alternative standard (20 µeq/L, 70th percentile).
It is apparent that both the percentile and the level chosen have a
strong influence, over the ranges considered, in determining the number
of areas that would likely not meet this set of alternative standards.

	The Policy Assessment observes that there is one grouping of these
acid-sensitive ecoregions that would likely not meet almost all
combinations of level and form under consideration (US EPA, 2011, Table
7-2 and Appendix D).  This group is made up of southern Appalachian
mountain areas, including North Central Appalachians, 5.3.3; Ridge and
Valley, 8.4.1; Central Appalachians, 8.4.2; Blue Ridge, 8.4.4; and
Southwestern Appalachians, 8.4.9.   In addition, these ecoregions
exhibit the highest amounts of exceedance relative to alternative
standards.

	The Northern Appalachian and Atlantic Maritime Highlands (5.3.1), which
includes the Adirondacks, and the Northern Lakes and Forests (5.2.1) of
the upper midwest exhibit similar patterns with respect to in the role
of level and percentile in identifying regions not likely to meet
alternative standards, although there are considerably fewer cases
compared to the regions in the Appalachians.

	In the mountainous west, the Sierra Nevada (6.2.12), Idaho Batholith
(6.2.15) and the Cascades (6.2.7) ecoregions likely would not meet
alternative standards in fewer cases relative to eastern regions, with
the Sierra Nevada ecoregion exhibiting relatively greater sensitivity
compared to all western regions.  Only in the upper part of the ranges
of level and percentile do regions in the northern and central Rockies
likely not meet alternative standards.

	In considering these findings, the Policy Assessment observes that the
standard as defined by the AAI behaves in an intuitively logical manner.
 That is, an increase in ecoregions likely not to meet the standard is
associated with higher alternative levels and percentiles, both of which
contribute to a lower regionally representative critical load.
Moreover, the areas of known adverse aquatic acidification effects are
identified, mostly in high elevation regions or in the northern
latitudes -- the Adirondacks, Shenandoahs, northern midwest lakes and
the mountainous west.  These results reflect the first application of a
nationwide model that integrates water quality and atmospheric processes
at a national scale and provides findings that are consistent with our
basic understanding of the extent of aquatic acidification across the
U.S.  What is particularly noteworthy is that this model is not
initialized with a starting ANC based on water quality data, which
likely would result in a reproduction of water quality observations.
Rather, this standard reflects the potential of the changes in
atmospheric concentrations of NOy and SOx to induce long-term sustained
changes in surface water systems.  The Policy Assessment notes that the
fact that the patterns of adversity based on applying this standard are
commensurate with what is observed in surface water systems provides
confidence in the basic underlying formulation of the standard.

	The Policy Assessment notes that the Appalachian mountain regions merit
further inspection as they stand out as areas with the largest relative
exceedances from a national perspective.  Water quality data from these
regions as well as an emissions sensitivity CMAQ simulation were
considered to better understand the simulated behavior of these regions.
 The maps and tables in appendix D of the Policy Assessment include
paired comparisons of the CMAQ 2005 and emissions sensitivity
simulations.  The emissions sensitivity simulation reflects domain-wide
reductions in NOy and SOx emissions of 48% and 42%, respectively,
relative to 2005 base year emissions.  The Policy Assessment assumes
that this emissions sensitivity simulation is indicative of future
conditions.

	The emissions sensitivity results project that many of the regions that
likely would not meet the alternative standards based on recent air
quality, especially at alternative levels of 20 and 35 µeq/L, would
likely meet such standards in the future year scenario for the
Appalachian mountain regions.  It is apparent that the AAI calculations
are especially sensitive to changes in SOx emissions as the Appalachian
regions have the highest SOx concentrations and deposition rates (US
EPA, 2011,section 2), and the AAI equation  responds as expected to
modeled reductions in SOx.  The emissions sensitivity scenario is a
prospective application of the standard, in the sense that rules derived
from the air quality management process result in reductions of NOy and
SOx emissions.   Expected emission changes over the next two decades
should be far greater than the 42 and 48% SOx and NOy reductions used in
this analysis, with a consequent further reduction in areas that would
likely not meet alternative standards.

	The Appalachian mountain regions generally have low DOC levels, average
runoff rates, moderately low base cation supply and highly elevated
sulfate concentrations.  Collectively, those attributes do not suggest
naturally acidic conditions as the availability of anthropogenic
contributions of mineral acids is likely responsible for observed low
ANC values in those regions.

	The Policy Assessment notes the Sierra Nevada region as an interesting
case study, as it has some of the lowest critical load values nationally
(US EPA, 2011, Table D-3).  Water quality data indicate extremely low
sulfate, as expected given the relatively low SO2 emissions in the
western U.S.   Extremely low base cation supply and low Neco, which
mitigate the effect of nitrogen deposition, explain the low critical
load values.  Low Neco values appear to associate well with high
elevation western U.S. regions, perhaps reflecting the more arid and
reduced vegetation density relative to eastern U.S. regions. The
proximity to high level nitrogen emissions combined with very low base
cation supply explains the cases where the Sierra region likely does not
meet alternative standards.  Because Neco values are low in the Sierras,
the system responds effectively to reductions of NOx emissions, as
illustrated in the maps and tables of Appendix D of the Policy
Assessment.  Although Neco affords protection from the acidifying
effects of nitrogen deposition, the availability of excessive nitrogen
neutralization capacity also means that reductions in nitrogen are not
as effective as reductions in SOx in reducing the calculated AAI.

	In reviewing these results, the Policy Assessment observes that the
analysis of the alternative combinations of level and form presented
provide context for considering the impact of different standards.
Since the AAI equation has been newly developed in the Policy
Assessment, these examples of estimated exceedances help to address the
question of whether the AAI equation responds in a reasonable manner
with regard to identifying areas of concern and to prospective changes
in atmospheric conditions likely to result from future emissions
reduction strategies.  The Policy Assessment concludes that the behavior
of the AAI calculations is both reasonable and explainable, which the
Policy Assessment concludes serves to increase confidence in considering
a standard defined in terms of the AAI.

F.	Characterization of Uncertainties

	This section summarizes discussions of the results of analyses and
assessments, presented more fully in the Policy Assessment (US EPA,
2011, section 7.6 and Appendices F and G), intended to address the
relative confidence associated with the linked atmospheric-ecological
effects system described above.  An overview of uncertainties is
presented in the context of the major structural components underlying
the standard, as well as with regard to areas of relatively high
uncertainty.  The section closes with a discussion of data gaps and
uncertainties associated with the use of ecological and atmospheric
modeling to specify the factors in the AAI equation, which can be used
to guide future field programs and longer-term research efforts.

1.	Overview of Uncertainty

	 As discussed in the Policy Assessment (US EPA, 2011,Table 7-3), there
is relatively low uncertainty with regard to the conceptual formulation
of the overall structure of the AAI-based standard that incorporates the
major associations linking biological effects to air concentrations.
Based on the strength of the evidence that links species richness and
mortality to water quality, the associations are strongly causal and
without any obvious confounding influence.  The strong association
between the ecosystem indicator (ANC) and the causative water chemistry
species (dissolved aluminum and hydrogen ion) reinforces the confidence
in the linkage between deposition of nitrogen and sulfur and effects.
This strong association between ANC and effects is supported by a sound
mechanistic foundation between deposition and ANC.  The same mechanistic
strength holds true for the relationship between ambient air levels of
nitrogen and sulfur and deposition, which completes the linkage from
ambient air indicators through deposition to ecological effects.

	There are relatively higher uncertainties, however, in considering
specific elements within the structure of an AAI-based standard,
including the deposition of SOx, NOy, and NHx as well as the critical
load-related component, each of which can vary within and across
ecoregions.  Overall system uncertainty relates not just to the
uncertainty in each such element, but also to the combined uncertainties
that result from linking these elements together within the AAI-based
structure.  Some of these elements – including, for example, dry
deposition, pre-industrial base cation production, and reduced nitrogen
deposition – are estimated with less confidence than other elements
(US EPA, 2011, Table 7.3).  The uncertainties associated with all of
these elements, and the combination of these elements through the AAI
equation, are discussed below and in the following sections related to
measured data gaps and modeled processes for both air quality and water
quality.

	The lack of observed dry deposition data is constrained by resources
and the lack of efficient measurement technologies.  Progress in
reducing uncertainties in dry deposition will depend on improved
atmospheric concentration data and direct deposition flux measurements
of the relevant suite of NOy and SOx species.

Pre-industrial base cation productivity by definition is not observable.
 Contemporary observations and inter-model comparisons are useful tools
that would help reduce the uncertainty in estimates of preindustrial
base cation productivity used in the AAI equation.  In characterizing
contemporary base cation flux using basic water quality measurements
(i.e., major anioni and cation species as defined in equation 2.11 in
the Policy Assessment), it is reasonable to assume that a major
component of contemporary base cation flux is associated with
pre-industrial weathering rates.  To the extent that multiple models
converge on similar solutions, greater confidence in estimating
pre-industrial base cation production would be achieved.

	Characterization of reduced nitrogen (NHx) deposition has been evolving
over the last decade.  The relatively high uncertainty in characterizing
NHx deposition is due to both the lack of field measurements and the
inherent complexity of characterizing NHx with respect to source
emissions and dry deposition.  Because ammonia emissions are generated
through a combination of man-made and biological activities, and ammonia
is semi-volatile, the ability to characterize spatial and temporal
distributions of NHx concentrations and deposition patterns is
challenging.  While direct measurement of NHx deposition is resource
intensive because of the diffuse nature of sources (i.e., area-wide and
not point sources), there have been more frequent deposition flux
studies, relative to other nitrogen species, that enable both the
estimation of both emissions and dry deposition.  Also, while ammonia
has a relatively high deposition velocity and traditionally was thought
to deposit close to the emissions release areas, the semi-volatile
nature of ammonia results in re-entrainment back into the lower boundary
layer resulting in a more dispersed concentration pattern exhibiting
transport type characteristics similar to longer lived atmospheric
species.  These inherent complexities in source characterization and
ambient concentration patterns raise the uncertainty level of NHx in
general.  However, the Policy Assessment notes that progress is being
made in measuring ammonia with cost efficient samplers and anticipates
the gradual evolution of a spatially robust ammonia sampling network
that would help support analyses to reduce underlying uncertainties in
NHx deposition.  Also, from an aquatic acidification perspective, NHx is
not as important a driver relative to as NOy and SOx in the mountainous
areas in the eastern U.S.  However, the relative importance of NHx is
likely to increase over time, in light of air quality rules in place
designed to reduce emissions of NOy and SOx.

2.	Uncertainties Associated with Data Gaps

	In summarizing uncertainties with respect to available measurement data
and the use of ecological and atmospheric models, the Policy Assessment
indentified data gaps and model uncertainties in relative terms by
comparing, for example, the relative richness of data between geographic
areas or environmental media.  With regard to relevant air quality
measurements, the Policy Assessment notes that such measurements are
relatively sparse in the western U.S.  While the spatial extent of
CASTNET coverage has gradually incorporated western U.S. locations with
support from the National Parks Service (NPS), the relative density of
monitoring sites is much less than that in the eastern U.S.  This
relative disparity in spatial density of monitors is exacerbated as air
quality patterns in the mountainous west generally exhibit greater
spatial heterogeneity due to dramatic elevation gradients that impact
meteorology and air mass flow patterns.  Similarly, water quality data
coverage is far more comprehensive in the eastern U.S. relative to the
west

	Measurements of NOy notably are lacking in both eastern and western
acid-sensitive ecoregions.  This adds uncertainty to the use of the AAI
equation as the lack of NOy data limits efforts to evaluate air quality
modeling of NOy that is the basis for quantifying factor F3 in the AAI
equation.  The lack of NOy measurements also limits efforts to
characterize the variability and representativeness of modeled NOy
concentrations within and across ecoregions.  Currently, the Agency’s
ability to define the protection likely to be afforded by alternative
standards (in terms of alternative levels and percentiles) is
compromised by the lack of a full set of ambient air quality indicator
measurements, notably including NOy, throughout sensitive ecoregions
across the U.S.

	Further, obtaining measurement of the dominant species that comprise
NOy (HNO3, true NO2, NO, p–NO3, and PAN) would be useful to evaluate
performance of NOy samplers.  Beyond the more well known dominant
components of NOy, research efforts would be needed to characterize
total reactive nitrogen that may include significant amounts of
organically-bound nitrogen (beyond PAN) which is poorly understood with
regard to emission sources and concentration levels.

Field measurements of NHx have been extremely limited, but have begun to
be enhanced through the National Atmospheric Deposition Program’s
(NADP) passive ammonia network (AMoN).  AMoN measures ammonia at over 50
sites, with more than 35 at CASTNET locations.  Enhanced spatial
coverage of reduced nitrogen measurements, particularly to understand
within and across ecoregion variability, and the inclusion of some
continuous observations would provide a better understanding of the
uncertainty in the F2 factor in the AAI equation and of the s
representativeness of modeled NHx deposition within and across
ecoregions.

	With regard to water quality data, the Policy Assessment notes that
such data are typically limited relative to air quality data sets, and
are also relatively sparse in the western U.S.  The TIME/LTM water
quality sampling program  in the eastern U.S. (as described in chapter 2
of the Policy Assessment) is an appropriate complement to national air
monitoring programs as it affords consistency across water bodies in
terms of sampling frequency and analysis protocols.  Consideration
should be given to extending the TIME/LTM design to all acid sensitive
ecoregions, with priority for areas in the western mountains that are
data limited and showing initial signs of adversity particularly with
respect to aquatic acidification.  The lack of a regulatory requirement
for TIME/LTM often jeopardizes funding support of this resource that is
especially valuable and cost effective.  While there are several state
and local agency water quality data bases, it is unclear the extent to
which differences in sampling, chemical analysis and reporting protocols
would impact the use of such data for the purpose of better
understanding the degree of protectiveness that would be afforded by an
AAI-based standard within sensitive ecoregions across the country.  In
addition, our understanding of water quality in Alaska and Hawaii and
the acid sensitivity of their ecoregions is particularly limited.

	Water quality data and modeling support the standard setting process.
As more water bodies are sampled, the critical load data bases would
expand, enabling clearer delineation of ecoregion representative
critical loads in terms of the nth percentile.  This would provide more
refined characterization of the degree of protection afforded by a given
standard.  Longer term, the availability of water quality trend data
(annual to monthly sampled) would support accountability assessments
that examine if an ecoregion’s response to air management efforts is
as predicted by earlier model forecasting.  The most obvious example is
the long-term response of water quality ANC change to changes in
calculated AAI, deposition, ambient NOy and SOx concentrations, and
emissions.  In addition, water quality trends data provide a basis for
evaluating and improving the parameterizations of processes in critical
load models applied at the ecoregion scale related to nitrogen retention
and base cation supply.  A better understanding of soil processes,
especially in the southern Appalachians, would enhance efforts to
examine the variability within ecoregions of the soil-based adsorption
and exchange processes which moderate the supply of major cations and
anions to surface waters and strongly influence the response of surface
water ANC to changes in deposition of nitrogen and sulfur.

3.	Uncertainties in Modeled Processes

	As discussed in the Policy Assessment, from an uncertainty perspective,
gaps in field measurement data are related to uncertainties in modeled
processes and in the specific application of such models.  As noted
above, processes that are embodied in an AAI-based standard are modeled
using the CMAQ atmospheric model and steady state ecological models.
These models are characterized in the Integrated Science Assessment as
being well established and they have undergone extensive peer review.
Nonetheless, the application of these models for purposes of specifying
the factors in the AAI equation, on an ecoregion scale, is a new
application that introduces uncertainties, as noted below, especially in
areas with limited observational data that can be used to evaluate this
specific application.  Understanding uncertainties in relevant modeled
process thus involves consideration of the uncertainties associated with
applying each model as well as the combination of these uncertainties as
the models are applied in combination within the AAI framework.

With regard to the application of CMAQ for purposes of use in an
AAI-based standard, the modeling of dry deposition has been identified
as having a relatively high degree of uncertainty.  Due to a combination
of system complexity and resource constraints, there is no routine
observational basis for directly comparing modeled dry deposition and
measurements.  Periodic dry deposition flux experiments covering a
variety of vegetation, surfaces and meteorology across seasons would
enable a more robust evaluation of modeled deposition of nitrogen and
sulfur.  Given the difficulty in acquiring dry deposition observations,
it becomes especially important to evaluate the model’s ability to
capture temporal and spatial ambient air patterns of individual nitrogen
and sulfur species which are used to drive dry deposition calculations
in models.  For example, reducing a generally acknowledged positive bias
in model-predicted SO2 relative to observations is especially relevant
to the AAI-based standard, as SO2 deposition is a dominant contributor
to total acidifying deposition in the eastern U.S.  With respect to
oxidized nitrogen, observations of individual NOy species are important
as air quality models calculate the individual deposition of each
species.  The modeled transference ratios, TNOy and TSOx used in factors
F3 and F4 rely on CMAQ’s ability to characterize both deposition and
concentration.   Consequently, a better understanding of the variability
of these factors within and across ecoregions could be achieved by
improved availability of measured ambient concentrations and deposition
observations.

	Steady state biogeochemical ecosystem modeling is used to develop
critical load estimates that are incorporated in the AAI equation
through factor F1.  Consequently, the Policy Assessment notes that an
estimate of the temporal response of surface water ANC to deposition and
air concentration changes is not directly available.  Lacking a
predicted temporal response impairs the ability to conduct
accountability assessments down to the effects level.  Accountability
assessments would examine the response of each step in the emissions
source through air concentration – deposition -- surface water quality
– biota continuum.  The steady state assumption at the ecosystem level
does not impair accountability assessments through the air
concentration/deposition range of that continuum. However, in using
steady state ecosystem modeling, several assumptions are made relative
to the long-term importance of processes related to soil adsorption of
major ions and ecosystem nitrogen dynamics.  Because these models often
were developed and applied in glaciated areas with relatively thin and
organically rich soils, their applicability is relatively more uncertain
in areas such as those in the non-glaciated clay-based soil regions of
the central Appalachians.  Consequently, it is desirable to develop the
information bases to drive simple dynamic ecosystem models that
incorporate more detailed treatment of subsurface processes, such as
adsorption and exchange processes and sulfate absorption.

4.	Applying Knowledge of Uncertainties

	An understanding of the relative uncertainties in a system assists in
setting priorities for data collection efforts and research, with the
expectation that such efforts would reduce uncertainties over time and
afford greater confidence in applications of an AAI-based standard.
Because of the uniquely wide breadth of pollutants and environmental
media addressed by an AAI-based multi-pollutant standard, there are a
wide range of uncertainties that are important to consider relative to
single pollutant standards that typically address only direct effects of
ambient air exposures.  For an AAI-b ased standard, an improved
understanding reduction of the uncertainties across the various modeled
processes at the ecoregion scale would lead to greater confidence in the
degree of protection afforded by the standard.

	The Policy Assessment notes that there is generally low uncertainty
with regard to the conceptual development and related major components
of this standard.  In recognizing the scientific soundness of the basic
structure of this standard, the Policy Assessment notes that future
efforts would be appropriately directed at expanding the availability of
relevant data for ecoregion-specific evaluation and application of the
relevant modeling of ecological and atmospheric processes, as identified
above.  Such efforts would further support consideration of an AAI-based
standard and would guide field studies and analyses designed to improve
the longer-term confidence in such a standard.

G.	CASAC Advice

	The CASAC has advised EPA concerning the Integrated Science Assessment,
the Risk and Exposure Assessment, and the Policy Assessment.  CASAC has
endorsed EPA’s interpretation of the science embodied in the
Integrated Science Assessment and the assessment approaches and
conclusions incorporated in the Risk and Exposure Assessment.

	Most recently, CASAC has considered the information in the final Policy
Assessment in providing its recommendations on the review of the new
multi-pollutant standard developed in that document and discussed above
(Russell and Samet, 2011a).  In so doing, CASAC has expressed general
support for the conceptual framework of the standard based on the
underlying scientific information, as well as for the conclusions in the
Policy Assessment with regard to indicators, form, averaging time, and
level of the standard that are appropriate for consideration by the
Agency in reaching decisions on the review of the secondary NAAQS for
oxides of nitrogen and sulfur:

The final Policy Assessment clearly sets out the basis for the
recommended ranges for each of the four elements (indicator, averaging
time, level and form) of a potential NAAQS that uses ambient air
indicators to address the combined effects of oxides of nitrogen and
oxides of sulfur on aquatic ecosystems, primarily streams and lakes. As
requested in our previous letters, the Policy Assessment also describes
the implications of choosing specific combinations of elements and
provides numerous maps and tabular estimates of the spatial extent and
degree of severity of NAAQS exceedances expected to result from possible
combinations of the elements of the standard.

We believe this final PA is appropriate for use in determining a
secondary standard to help protect aquatic ecosystems from acidifying
deposition of oxides of sulfur and nitrogen. EPA staff has done a
commendable job developing the innovative Aquatic Acidification Index
(AAI), which provides a framework for a national standard based on
ambient concentrations that also takes into account regional differences
in sensitivities of ecosystems across the country to effects of
acidifying deposition. (Russell and Samet, 2011a)

	CASAC also recommended that as EPA moves forward in the regulatory
process “some attention should be given to our residual concern that
the available data may reflect the more sensitive water bodies and thus,
the selection of percentiles of waterbodies to be protected could be
conservatively biased” (Russell and Samet, 2011a).  In additional,
CASAC found some improvements could be made to the uncertainty analysis,
as noted below.

With respect to indicators, CASAC supports the use of SOx and NOy as
ambient air indicators (discussed above in section III.A) and ANC as the
ecological indicator (discussed above in section III.B.1):

	The use of total reactive oxidized nitrogen (NOy) and sulfur oxides
(SOx) as atmospheric  indicators of oxidized nitrogen (N) and sulfur (S)
atmospheric concentrations is well justified.  The use in the Aquatic
Acidification Index (AAI) of NOy and SOx as atmospheric indicators of N
and S concentrations is useful and corresponds with other efforts by
EPA. As we have stated previously, CASAC also agrees that acid
neutralizing capacity (ANC) is the most appropriate ecological indicator
of aquatic ecosystem response and resiliency to acidification. (Russell
and Samet, 2011a)

	With respect to the form of the standard (discussed above in section
III.B), CASAC stated the following:

EPA has developed the AAI, an innovative “form” of the NAAQS itself
that incorporates the multi-pollutant, multi-media, environmentally
modified, geographically variable nature of SOx/NOy deposition-related
aquatic acidification effects. With the caveats noted below, CASAC
believes that this form of the NAAQS as described in the final Policy
Assessment is consistent with and directly reflective of current
scientific understanding of effects of acidifying deposition on aquatic
ecosystems. (Russell and Samet, 2011a)

CASAC agrees that the spatial components of the form in the Policy
Assessment are reasonable and that use of Omernick’s ecoregions (Level
III) is appropriate for a secondary NAAQs intended to protect the
aquatic environment from acidification . . . (Russell and Samet, 2011a)

	The “caveats” noted by CASAC include a recognition of the
importance of continuing to evaluate the performance of the CMAQ and
ecological models to account for model uncertainties and to make the
model-dependent factors in the AAI more transparent.  In addition, CASAC
noted that the role of DOC and its effects on ANC would benefit from
further refinement and clarification (Russell and Samet, 2011a).  While
CASAC expressed the view that the “division of ecoregions into
‘sensitive’ and ‘non-sensitive’ subsets, with a more protective
percentile applied to the sensitive areas, also seems reasonable”
(Russell and Samet, 2011a), CASAC also noted that there was the need for
greater clarity in specifying how appropriate screening criteria would
be applied in assigning ecoregions to these categories. Further, CASAC
identified potential biases in critical load calculations and in the
regional representativeness of available water chemistry data, leading
to the observation that a given percentile of the distribution of
estimated critical loads may be protective of a higher percentage of
surface waters in some regions (Russell and Samet, 2011a).

	With respect to averaging time (discussed above in section III.C),
CASAC stated the following:

A longer averaging time would mask possible trends of AAI, while a
shorter averaging time would make the AAI being more influenced by the
conditions of the particular years selected. (Russell and Samet, 2011a)

	With respect to level as well as the combination of level and form as
they are presented as alternative standards (discussed above in sections
III.D-E), CASAC stated the following:

CASAC agrees with EPA staff’s recommendation that the “level” of
the alternative AAI standards should be within the range of 20 and 75
μeq/L. We also recognize that both the “level” and the form of any
AAI standard are so closely linked in their effectiveness that these two
elements should be considered together. (Russell and Samet, 2011a)

amount of deposition for a given ecoregion. However, when these
percentile ranges are combined with alternative levels within the
staff-recommended ANC range of 20 to 75 microequivalents per liter
(μeq/L), the results using the AAI point to the ecoregions across the
country that would be expected to require additional protection from
acidifying deposition.  Reasonable choices were made in developing the
form.  The number of acid sensitive regions not likely to meet the
standard will be affected both by choice of ANC level and the percentile
of the distribution of critical loads for lakes to meet alternative ANC
levels in each region.  These combined recommendations provide the
Administrator with a broad but reasonable range of minimally to
substantially protective options for the standard. (Russell and Samet,
2011a)

	CASAC also commented on EPA’s uncertainty analysis, and provided
advice on areas requiring further clarification in the proposed rule and
future research.  CASAC found it “difficult to judge the adequacy of
the uncertainty analysis performed by EPA because of lack of details on
data inputs and the methodology used, and lack of clarity in
presentation” (Russell and Samet, 2011a).  In particular, CASAC
identified the need for more thorough model evaluations of critical load
and atmospheric modeling, recognizing the important role of models as
they are incorporated in the form of the standard.  In light of the
innovative nature of the standard developed in the Policy Assessment,
CASAC identified “a number of areas that should be the focus of
further research” (Russell and Samet, 2011a).  While CASAC recognized
that EPA staff was able to address some of the issues in the Policy
Assessment, they also noted areas “that would benefit from further
study or consideration in potential revisions or modifications to the
form of the standard.”  Such research areas include “sulfur
retention and mobilization in the soils, aluminum availability, soil
versus water acidification and ecosystem recovery times.”  Further,
CASAC encouraged future efforts to monitor individual ambient nitrogen
species, which would help inform further CMAQ evaluations and the
specification of model-derived elements in the AAI equation (Russell and
Samet, 2011a).

H.	Administrator’s Proposed Conclusions

As discussed above, the Administrator notes that the Integrated Science
Assessment concludes that the available scientific evidence is
sufficient to infer a causal relationship between acidifying deposition
of nitrogen and sulfur in aquatic ecosystems, and that the deposition of
oxides of nitrogen and sulfur both cause such acidification under
current conditions in the U.S.   Further, the Integrated Science
Assessment concludes that there are well-established water quality and
biological indicators of aquatic acidification as well as
well-established models that address deposition, water quality, and
effects on ecosystem biota, and that ecosystem sensitivity to
acidification varies across the country according to present and
historic nitrogen and sulfur deposition as well as geologic, soil,
vegetative, and hydrologic factors.  Based on these considerations, the
Administrator agrees with the conclusion in the Policy Assessment, and
supported by CASAC, that there is a strong scientific basis for
development of a standard with the general structure presented in the
Policy Assessment.

	The Administrator also recognizes that the conceptual framework for an
ecologically relevant, multi-pollutant standard, which was initially
explored in the Risk and Exposure Assessment and further developed in
the Policy Assessment, builds on the information in the Integrated
Science Assessment.  She notes that the structure of the standard
addresses the combined effects of deposition from oxides of nitrogen and
sulfur by characterizing the linkages between ambient concentrations,
deposition, and aquatic acidification, and that the structure of the
standard takes into account relevant variations in these linkages across
the country.  She recognizes that while the standard is innovative and
unique, the structure of the standard is well grounded in the science
underlying the relationships between ambient concentrations of oxides of
nitrogen and sulfur and the aquatic acidification related to deposition
of nitrogen and sulfur associated with such ambient concentrations.

As discussed above, these uncertainties and complexities generally
relate not to the structure of the standard, but to the quantification
of the various elements of the standard, such as the F factors discussed
earlier in this section and their representativeness at an ecoregion
scale.  These uncertainties and complexities currently limit efforts to
characterize the degree of protectiveness that would be afforded by
determine an appropriate form an level for such a standard, within the
ranges of levels and forms identified in the Policy Assessment, and the
representativeness of F factors in the AAI equation described above and
in the Policy Assessment.  These important uncertainties have been
generally categorized as limitations in available field data as well as
uncertainties that are related to reliance on the application of
ecological and atmospheric modeling at the ecoregion scale to specify
the various elements of the AAI.

	With regard to data limitations, the Administrator observes that there
are several important limitations in the available data upon which
elements of the AAI are based.  For example, while ambient measurements
of NOy are made as part of a national monitoring network, the monitors
are not located in locations that are representative of sensitive
aquatic ecosystems.  While air and water quality data are generally
available in areas in the eastern U.S., there is relatively sparse
coverage in mountainous western areas where a number of sensitive
aquatic ecosystems are located.  Further, even in areas where relevant
data are available, small sample sizes impede efforts to characterize
the representativeness of the available data, which was noted by CASAC
as being of particular concern.  Also, measurements of reduced forms of
nitrogen are available from only a small number of monitoring sites, and
emission inventories for reduced forms of nitrogen used in atmospheric
modeling are subject to considerable uncertainty.

	With regard to uncertainties related to the use of ecological and
atmospheric modeling, the Administrator notes in particular that model
results are difficult to evaluate due to a lack of relevant
observational data.  For example, relatively large uncertainties are
introduced by a lack of data with regard to pre-industrial environmental
conditions and other parameters that are necessary inputs to critical
load models that are the basis for factor F1 in the AAI equation.  Also,
observational data are not generally available to evaluate the modeled
relationships between nitrogen and sulfur in the ambient air and
associated deposition, which are the basis for the other factors (i.e.,
F2, F3, and F4) in the AAI equation.

	In combination, these limitations and uncertainties result in a
considerable degree of uncertainty as to how well the quantified
elements of the AAI standard would predict the actual relationship
between varying ambient concentrations of oxides of nitrogen and sulfur
and steady state ANC levels across the distribution of water bodies
within the various ecoregions in the U.S.  Because of this, there is
considerable uncertainty as to the actual degree of protectiveness that
such a standard would provide, especially for acid-sensitive ecoregions.
 The Administrator recognizes that the AAI equation, with factors
quantified in the ranges discussed above and described more fully in the
Policy Assessment, generally performs well in identifying areas of the
country that are sensitive to such acidifying deposition and indicates,
as expected, that lower ambient levels of oxides of nitrogen and sulfur
would lead to higher calculated AAI values.  However, the uncertainties
discussed here are critical for determining the actual degree of
protection that would be afforded such areas by any specific target ANC
level and percentile of water bodies that would be chosen in setting a
new AAI-based standard, and thus for determining an appropriate
AAI-based standard that meets the requirements of the CAA.

	In considering these uncertainties, the Administrator notes that CASAC
acknowledged that important uncertainties remain that would benefit from
further study and data collection efforts, which might lead to potential
revisions or modifications to the form of the standard developed in the
Policy Assessment.  She also notes that CASAC encouraged the Agency to
engage in future monitoring and model evaluation efforts to help inform
the specification of model-derived elements in the AAI equation.

	Based on the above considerations, the Administrator has determined
that it is not appropriate tp to set a new multi-pollutant standard to
address deposition-related effects of oxides of nitrogen and sulfur on
aquatic acidification at this time.  Setting a NAAQS generally involves
consideration of the degree of uncertainties in the science and other
information, such as gaps in the relevant data and, in this case,
limitations in the evaluation of the application of relevant ecological
and atmospheric models at an ecoregion scale.  The issue here is
identifying a specific form and level for the standard that would
provide an appropriate level of public welfare policy judgment to be
made protection, consistent with the requirements of the CAA, in light
of the specific uncertainties and limitations that are discussed in
detail throughout this preamble.  As noted above, it is not a question
of uncertainties about the scientific soundness of the structure of the
AAI, but instead uncertainties in the quantification and
representativeness of the elements of the AAI as they vary in ecoregions
across the country.  At present, these uncertainties prevent an
understanding of the degree of protectiveness that would be afforded to
various ecoregions across the country by a new standard defined in terms
of a specific nationwide target ANC level and a specific percentile of
water bodies for acid-sensitive ecoregions, and thus prevent
identification of an appropriate form and level for the standard.  The
Administrator has considered whether these uncertainties could be
appropriately accounted for by choosing either a more or less protective
target ANC level and percentile of water bodies than would otherwise be
chosen if the uncertainties did not substantially limit the confidence
that can appropriately be ascribed to the quantification of the AAI
elements.  However, in the Administrator’s judgment, the uncertainties
are of such nature and magnitude that there is no reasoned way to choose
such a specific nationwide target ANC level or percentile of water
bodies that would appropriately account for the uncertainties, since
neither the direction nor the magnitude of change from the target level
and percentile that would otherwise be chosen can reasonably be
ascertained at this time.

	Based on the above considerations, the Administrator judges that the
current limitations in relevant data and the uncertainties associated
with specifying the elements of the AAI based on modeled factors are of
such nature and degree as to prevent her from reaching a reasoned
decision such that she is adequately confident as to what level and form
(in terms of a selected percentile) of such a standard would provide the
any particular intended degree of protection of public welfare that the
Administrator determined satisfied the requirements of the CAA.  	While
acknowledging that CASAC supported moving forward to establish the
standard developed in the Policy Assessment, the Administrator also
observes that CASAC supported conducting further field studies that
would better inform the continued development or modification of such as
standard.  Given the large uncertainties and complexities inherent in
quantifying the elements of such a standard, largely deriving from the
unprecedented nature of the standard under consideration in this review,
and having fully considered CASAC’s advice, the Administrator
provisionally concludes that it is premature to set a new,
multi-pollutant secondary standard for oxides of nitrogen and sulfur at
this time, and as such she is proposing not to set such a new secondary
standard.

	While it is premature to set such a multi-pollutant standard at this
time, the Administrator determines that the Agency should undertake a
field pilot program to gather additional data, and that it is
appropriate that such a program be undertaken before, rather than after,
reaching a decision to set such a standard.  As described below in
section IV, the purpose of the program is to collect and analyze data so
as to enhance our understanding of the degree of protectiveness that
would likely be afforded by a standard based on the AAI as developed in
the Policy Assessment.  This will provide a better basis for the
Administrator to identify a form and level of the standard that is
requisite to protect the public welfare consistent with the CAA,
specifically with respect to the acidifying effects of deposition of
oxides of nitrogen and sulfur.  Data generated by this field program
will also support development of an appropriate monitoring network that
would work in concert with such a standard to result in the intended
degree of protection.  The data and analyses generated as a result of
this program will serve to inform the next review of the NAAQS for
oxides of nitrogen and sulfur.  The information generated during the
field program can also be used to help state agencies and EPA better
understand how an AAI-based standard would work in terms of the
implementation of such a standard.

add secondary standards of the ecologically relevant form to address
acid depositions, it will directionally provide some degree of
additional protection.  This is consistent with the view that the
current secondary standards are neither sufficiently protective nor
appropriate in form, but that it is not appropriate to propose to set a
new, ecologically relevant multi-pollutant secondary standard at this
time, for all of the reasons discussed above.

	While not a basis for this decision, the Administrator also recognizes
that a new, innovative AAI-based standard would raise significant
implementation issues that would need to be addressed consistent with
the CAA requirements for implementation-related actions following the
setting of a new NAAQS.  It will take time to address these issues,
during which the Agency will be conducting a field pilot program to
gather relevant necessary data and the environment will benefit from
reductions in oxides of nitrogen and sulfur resulting from the new NO2
and SO2 primary standards, as noted above, as well as reductions
expected to be achieved from EPA’s proposed Transport Rule and Mercury
and Air Toxics standards.  These implementation-related issues are
discussed in more detail below in section IV.A.5.

	The Administrator solicits comment on all aspects of this proposed
decision, including the framework and elements of a multi-pollutant
standard for oxides of nitrogen and sulfur to address deposition-related
effects on sensitive ecosystems, with a focus on aquatic acidification,
and the uncertainties and complexities associated with the development
of such a standard at this time.  The Administrator also solicits
comment on the field pilot program and related monitoring methods as
discussed below in section IV.

IV.	Field Pilot Program and Ambient Monitoring

This section describes EPA’s plans for a field pilot program and the
evaluation of monitoring methods for ambient air indicators of oxides of
nitrogen (NOy) and oxides of sulfur (SOx) to implement the
Administrator’s decision to undertake such a field monitoring program
in conjunction with her decision to propose not to set a new
multi-pollutant secondary standard in this review, as discussed above in
section III.H.  As noted above and discussed below in section IV.A, the
field pilot program is intended to collect and analyze data so as to
enhance our understanding of the degree of protectiveness that would
likely be afforded by a standard based on the AAI as developed in the
Policy Assessment.  Data generated by this field program would also
support development of an appropriate monitoring network that would work
in concert with such a standard to result in the intended degree of
protection.  As discussed below in section IV.B, the evaluation of
monitoring methods focuses on the development of Federal Reference
Methods/Federal Equivalent Methods (FRM/FEM) for NOy and SOx.  EPA notes
that the monitoring program described here is intended to supplement
existing monitoring programs and will require additional resources.

A.	Field Pilot Program

	This section presents the objectives of a field pilot program (section
IV.A.1) that would gather relevant field data over a five-year period in
a sample of 3 to 5 sensitive ecoregions across the country.  An overview
of the scope and structure of the field program, with a focus on
measurements of ambient air indicators of oxides of nitrogen and sulfur,
is presented in section IV.A.2.  Section IV.A.3 explains the role of
additional complementary measurements beyond the ambient air indicators
that would be included in the program, and section IV.A.4 discusses a
parallel longer-term research agenda, both of which are guided by the
uncertainties discussed above in section III.  Section IV.A.5 identifies
implementation challenges presented by an AAI-based standard that could
be addressed in parallel with a field pilot program.  Section IV.A.6
discusses engagement with stakeholder groups as part of the planned
pilot program.

1.	Objectives

Consideration of a new multi-pollutant standard to address
deposition-related effects on sensitive aquatic ecoregions raises unique
challenges relative to those typically raised in reviews of existing
NAAQS for which an established network of FRM/FEM monitors, designed to
measure the indicator pollutant, is generally available.  The primary
goal of this field pilot program, and the related monitoring program
discussed in section IV.B, is to enhance our understanding of the degree
of protectiveness that would likely be afforded by a standard based on
the AAI, as described above in section III, and to support the
identification of a specific form and level for such a standard that
would be requisite to protect public welfare consistent with the
requirements of the CAA, through the following objectives:

(1)  evaluate measurement methods for the ambient air indicators of NOy
and SOx and consider designation of such methods as FRMs;

(2)  examine the variability and improve characterization of
concentration and deposition patterns of oxides of nitrogen and sulfur,
as well as reduced forms of nitrogen, within and across a number of
sensitive ecoregions across the country;

(3)  develop updated ecoregion-specific factors (i.e., F1 through F4)
for the AAI equation based in part on new observed air quality data
within the sample ecoregions as well as on updated nationwide air
quality model results and expanded critical load data bases, and explore
alternative approaches for developing such representative factors;

(4)  calculate ecoregion-specific AAI values using observed NOy and SOx
data and updated ecoregion-specific factors to examine the extent to
which the sample ecoregions would meet a set of alternative AAI-based
standards;

(5)  develop air monitoring network design criteria for an AAI-based
standard;

(6)  assess the use of total nitrate measurements as a potential
alternative indicator for oxides of nitrogen; and

(7)  support related longer-term research efforts, including
enhancements to and evaluation of modeled dry deposition algorithms.

2.	Overview of Field Pilot Program

	The CASTNET program (Figure IV-1) affords an available infrastructure
relevant to an AAI-based standard, given the location of sites in some
acid-sensitive ecoregions and various measurements of sulfur and
nitrogen species.  The EPA plans to use CASTNET sites in selected
acid-sensitive ecoregions to serve as the platform for this pilot
program, potentially starting in late 2012 and extending through 2018.
CASTNET sites in three to five ecoregions in acid-sensitive areas would
collect NOy and SOx (i.e., SO2 and p-SO4) measurements over a five-year
period.  The initial step in developing a data base of observed ambient
air indicators for oxides of nitrogen and sulfur requires the addition
of NOy samplers at the pilot study sites so that a full complement of
indicator measurements are available to calculate AAI values.  These
CASTNET sites would also be used to make supplemental observations
useful for evaluation of CMAQ’s characterization of factors F2 –F4
in the AAI equation.

The selected ecoregions would account for geographic variability by
including regions from across the U.S., including the east, upper
midwest and west.  Each selected region would have at least two existing
CASTNET sites.  Each of the pilot CASTNET sites would be used to verify
and enhance the performance of the established methods, data retrieval
and reporting procedures.  This would in turn inform the future
selection of specific parameters for, and application of the
measurements into the AAI equation.

Figure IV-1.  Location of CASTNET sites in relation to acid sensitive
ecoregions.

Over the course of this five-year pilot program, the most current
national air quality modeling, based on the most current national
emissions inventory, would be used to develop an updated set of F2 –
F4 factors.  A parallel multi-agency national critical load data base
development effort would be used as the basis for calculating updated F1
factors.  As discussed above in section III.B, these factors would be
based on average parameter values across an ecoregion.  Using this new
set of F factors, observations of NOy and SOx derived from the pilot
program, averaged across each ecoregion, would be used to calculate AAI
values in the sample ecoregions.   The data from the pilot program would
also be used to examine alternative approaches to generating
representative air quality values, such as examining the appropriateness
of spatial averaging in areas of high spatial variability.

3 . 	Complementary Measurements

Complementary measurements may be performed at some sites in the pilot
network to reduce uncertainties in the recommended methods and better
characterize model performance and application to the AAI.  The CASAC
Air Monitoring and Methods Subcommittee (AMMS) advised EPA that such
supplemental measurements were of critical importance in a field
measurement program related to an AAI-based standard (Russell and Samet,
2011b).

Candidate complementary measurements to address sulfur, in addition to
those provided by the CASTNET filter pack (CFP), include trace gas
continuous SO2 and speciated PM2.5 measurements. The co-located
deployment of a continuous SO2 analyzer with the CFP for SO2 will
provide test data for determining suitability of continuous SO2
measurements as an FEM, as well as producing valuable time series data
for model evaluation purposes.  The weekly averaging time provided by
the CFP adequately addresses the annual-average basis of an AAI-based
secondary standard, but would not be applicable to short-term (i.e.,
1-hour) averages associated with the primary SO2 standard.  Conversely,
because of the low concentrations associated with many acid-sensitive
ecoregions, existing SO2 FRMs designated for use in determining
compliance with the primary standard would not necessarily be
appropriate for use in conjunction with an AAI-based secondary standard.

Co-locating the PM2.5 sampler used in the EPA Chemical Speciation
Network and the IMPROVE network at pilot network sites would allow for
characterizing the relationship between the CFP-derived p-SO4 and the
speciation samplers used throughout the state and local air quality
networks.  Note that CASTNET already has several co-located IMPROVE
chemical speciation samplers.  Because the AAI equation is based on
concentration of p-SO4, the original motivation for capturing all
particle size fractions is not as important relative to simply capturing
the concentration of total p-SO4.

Candidate measurements to complement oxidized nitrogen measurements, in
addition to the CFP, include a mix of continuous and periodic sampling
for the dominant NOy species, namely NO, true NO2, PAN, HNO3, and p-NO3.
 While there are several approaches to acquiring these measurements,
perhaps the most efficient strategy would take advantage of the
available CFP for total nitrate, and add a three-channel
chemiluminescence instrument that will cycle between NOy, true NO2 and
NO by adding photolytic detection for true NO2.  Other options for
measuring true NO2 would include adding either a stand-alone photolytic
or cavity ring-down spectroscopy instrument.  Measurements of PAN may be
acquired either on a periodic basis through canister sampling and
subsequent laboratory analysis or through emerging in-situ sampling and
analysis methods.  Although the CFP yields a reliable measurement of
total nitrate, the t-NO3 (i.e., the sum of HNO3 and p-NH4) value, strong
consideration may be given to direct measurement of HNO3, which has the
highest deposition velocity of all the dominant NOy species.  Similar to
the use of continuous SO2 data, these speciated NOy data serve two
purposes: by evaluating total NOy instrument behavior, and air quality
model evaluation.  The measurement of individual NOy species can be used
to generate site-specific NOy values for comparison to modeled NOy, and
will likely provide insight into and improvement of modeled dry
deposition.

The CASAC AMMS (Russell and Samet, 2011b) recommended that EPA consider
the use of t-NO3 obtained from CASTNET sampling as an indicator for NOy,
reasoning that t-NO3 is typically a significant fraction of deposited
oxidized nitrogen in rural environments and CASTNET measurements are
widely available.  Collection of this data would support further
consideration of using the CFP for t-NO3 as the indicator of oxides of
nitrogen for use in an AAI-based secondary standard.

The CASAC AMMS also recommended that total NHx (NH3 and p-NH4) be
considered as a proxy for reduced nitrogen species, reasoning that the
subsequent partitioning to NH3 and p-NH4 may be estimated using
equilibrium chemistry calculations. Reduced nitrogen measurements are
used to evaluate air quality modeling which is used in generating factor
F2. Additional studies are needed to determine the applicability of NHx
measurements and calculated values of NH3 and NH4 to the AAI.

The additional supplemental measurements of speciated NOy, continuous
SO2 and NHx will be used in future air quality modeling evaluation
efforts.  Because there often is significant lag in the availability of
contemporary emissions data to drive air quality modeling, the complete
use of these data sets will extend beyond the five-year collection
period of the pilot program.  Consequently, the immediate application of
those data will address instrument performance comparisons that explore
the feasibility of using continuous SO2 instruments in rural
environments, and using the speciated NOy data to assess NOy instrument
performance.   Although contemporary air quality modeling will lag
behind measurement data availability, the observations can be used in
deposition models to compare observed transference ratios with the
previously calculated transference ratios to test temporal stability of
the ratios.

An extended water quality sampling effort should parallel the air
quality measurement program to address some of the uncertainties related
to factor F1 and the representativeness of the nth percentile critical
load as discussed in section III.B.5.b.i.  The objective of the water
quality sampling would be to develop a larger data base of critical
loads in each of the pilot ecoregions such that the nth percentile can
adequately be characterized in terms of representing all water bodies.
Opportunities to leverage and perhaps enhance existing ecosystem
modeling efforts enabling more advanced critical load modeling and
improved methods to estimate base cation production would be pursued.
For example, areas with ongoing research studies producing data for
dynamic critical load modeling would be considered when selecting the
pilot ecoregions.

4.	Complementary areas of research

The EPA recognizes that a source of uncertainty in an AAI-based
secondary standard that would not be directly addressed in the pilot
program stems from the uncertainty in the model used to link atmospheric
concentrations to dry deposition fluxes.  Currently, there are no
ongoing direct dry deposition measurement studies at CASTNET sites that
can be used to evaluate modeled results.  It was strongly recommended by
CASAC AMMS that a comprehensive sampling-intensive study be conducted in
at least one, preferably two sites in different ecoregions to assess
characterization of dry deposition of sulfur and nitrogen.  These sites
would be the same as those for the complementary measurements described
above, but they would afford an opportunity to also complement dry
deposition process research that benefits from the ambient air
measurements collected in the pilot program.  The concerns regarding
uncertainties underlying an AAI-based secondary standard suggests that
research that includes dry deposition measurements and evaluation of dry
deposition models should be a high priority.

Similar leveraging should be pursued with respect to ecosystem research
activities.  For example, studies that capture a suite of soil,
vegetation, hydrological, and water quality properties that can help
evaluate more advanced critical load models would complement the
atmospheric-based pilot program.  In concept, such studies could provide
the infrastructure for true multi-pollutant, multi-media “super”
sites assuming the planning, coordination, and resource facets can be
aligned.  While this discussion emphasizes the opportunity of leveraging
ongoing research efforts, consideration could be given to explicitly
including related research components directly in the pilot program.

5.	Implementation challenges

	The CAA requires that once a NAAQS is established, designation and
implementation must move forward.  With a standard as innovative as the
AAI-based standard considered in this review, the Administrator believes
that its success will be greatly improved if, while additional data are
being collected to reduce the uncertainties discussed above, the
implementing agencies and other stakeholders have an opportunity to
discuss and thoroughly understand how such a standard would work.   And
since, as noted above, emissions reductions that are directionally
correct to reduce aquatic acidification will be occurring as a result of
other CAA programs, the Administrator believes that this period of
further discussion will not delay progress but will ensure that once
implementation is triggered, agencies will be prepared to implement it
successfully.

Consideration of an AAI-based secondary standard for oxides of nitrogen
and sulfur would present significant implementation challenges because
it involves multiple, regionally-dispersed pollutants and relatively
complex compliance determinations.  The anticipated implementation
challenges fall into three main categories:  monitoring and compliance
determinations for area designations, pre-construction permit
application analyses of individual source impacts, and State
Implementation Plan (SIP) development.  Several overarching
implementation questions that we anticipate will be addressed in
parallel with the field pilot program’s five-year data collection
period include:

(1)  What are the appropriate monitoring network density and siting
requirements to support a compliance system based on ecoregions?

(2)  Given the unique spatial nature of the secondary standard (e.g.,
ecoregions), what are the appropriate parameters for establishing
nonattainment areas?

(3)  How can new or modified major sources of oxides of nitrogen and
oxides of 	sulfur emissions assess their ambient impacts on the standard
and demonstrate that they are not causing or contributing to a violation
of the NAAQS for preconstruction permitting?

(4)  What additional tools, information, and planning structures are
needed to assist states with SIP development, including the assessment
of interstate pollutant transport and deposition?

(5)  Would transportation conformity apply in nonattainment and
maintenance areas for this secondary standard, and, if it does, would
satisfying requirements that apply for related primary standards (e.g.,
ozone, PM2.5, and NO2) be demonstrated to satisfy requirements for this
secondary standard?

6.	Final monitoring plan development and stakeholder participation

          The CASAC AMMS generally endorsed the technical approaches
used in CASTNET, but concerns were raised by individual representatives
of State agencies concerning the perception of EPA-controlled management
aspects of CASTNET and data ownership. Potential approaches to resolve
these issues will be developed and evaluated in existing NACAA/EPA
ambient air monitoring workgroups.  The EPA Office of Air and Radiation
(which includes the Office of Air Quality Planning Standards, OAQPS; and
the Office of Atmospheric Program’s Clean Air Markets Division,
OAP-CAMD), and their partners on the NACAA monitor steering committee
will develop a prioritized specific plan that identifies the 3-5
ecoregions and the instrumentation to be deployed.  EPA anticipates that
a cost estimate of the plan with priorities and options will be
developed by January, 2012.   Although this pilot program is focused on
data collection, the plan will include details of the data analysis
approaches as well as a vehicle that incorporates engagement from those
within EPA and SLTs to foster progress on the implementation questions
noted above in section IV.A.5.

If an AAI-based secondary standard were to be set in the future,
deployment of a full national network would follow the pilot monitoring
program.  The number of sites deployed in the network will lead to
increased confidence in capturing spatial patterns of air quality.
Recognizing that this section presents the general elements of the field
pilot programs, EPA intends to develop a more detailed field pilot
program plan through a process that will engage the air quality
management and research (atmospheric and ecosystem) communities, as well
as other federal agencies, state and local agencies, and non-government
based centers of expertise.  The EPA is seeking comment and input on all
aspects of this field pilot program.

.B.	Evaluation of Monitoring Methods

The EPA generally relies on monitoring methods that have been designated
as FRMs or FEMs for the purpose of determining the attainment status of
areas with regard to existing NAAQS.  Such FRMs or FEMs are generally
required to measure the air quality indicators that are compared to the
level of a standard to assess compliance with a NAAQS.  Prior to their
designation by EPA as FRM/FEMs through a rulemaking process, these
methods must be determined to be applicable for routine field use and
need to have been experimentally validated by meeting or exceeding
specific accuracy, reproducibility, and reliability criteria established
by EPA for this purpose.  As discussed above in section III.A, the
ambient air indicators being considered for use in an AAI-based standard
include SO2, particulate sulfate (p-SO4), and total reactive oxides of
nitrogen (NOy).

The CASTNET provides a well established infrastructure that would meet
the basic location and measurement requirements of an AAI-based
secondary standard given the rural placement of sites in acid sensitive
areas.  In addition, CFPs currently provide very economical weekly,
integrated average concentration measurements of SO2, p-SO4, ammonium
ion (NH4) and t-NO3, the sum of HNO3 and p-NO3.

While routinely operated instruments that measure SO2, p-SO4, NOy and/or
t-NO3 exist, instruments that measure p-SO4, NOy, t-NO3, or the CFP for
SO2 have not been designated by EPA as FRMs or FEMs   The EPA’s ORD
has initiated work that will support future FRM designations by EPA for
SO2 and p-SO4 measurements based on the CFP.  Such a designation by EPA
could be done for the purpose of facilitating consistent research
related to an AAI-based standard and/or in conjunction with setting and
supporting an AAI-based secondary standard.

Based on extensive review of literature and available data, the EPA has
identified potential methods that appear suitable for measuring each of
the three components of the indicators.  These three methods are being
considered as new FRMs to be used for measuring the ambient
concentrations of the three components that would be needed to determine
compliance with an AAI-based secondary standard.

For the SO2 and p-SO4 measurements, EPA is considering the CFP method,
which provides weekly average concentration measurements for SO2 and
p-SO4.  This method has been used in the EPA’s CASTNET monitoring
network for 15 years, and strongly indicates that it will meet the
requirements for use as an FRM for the SO2 and p-SO4 concentrations for
an AAI-based secondary standard.

Although the CFP method would provide measurements of both the SO2 and
p-SO4 components in a unified sampling and analysis procedure,
individual FRMs will be considered for each.  The EPA recognizes that an
existing FRM to measure SO2 concentrations using ultra-violet
fluorescence (UVF) exists (40 CFR Part 50, Appendix A-1) for the purpose
of monitoring compliance for the primary SO2 NAAQS.  However, several
factors suggest that the CFP method would be superior to that UVF FRM
for monitoring compliance with an AAI-based secondary standard and will
be discussed in more detail below.

	For monitoring the NOy component, a continuous analyzer for measuring
NOy is commercially available and is considered to be suitable for use
as an FRM.  This method is similar in design to the existing NO2 FRM
(described in 40 CFR Part 50, Appendix F), which is based on the ozone
chemiluminescence measurement technique.  The method is adapted to and
further optimized to measure all oxides of nitrogen (NOy).  However,
this NOy method requires further evaluation before it can be fully
confirmed as a suitable FRM.  The EPA is currently completing a full
scientific assessment of the NOy method to determine whether it would be
appropriate to consider for designation by EPA as an FRM.  Specific
details on these three methods are given below.

             On February 16, 2011, EPA presented this set of potential
FRMs to the CASAC AMMS for their consideration and comment.  In
response, the CASAC AMMS stated that, overall, it believes that EPA’s
planned evaluation of methods for measuring NOy, SO2 and p-SO4 as
ambient air indicators are suitable approaches in concept.   On
supporting the CFP method as a potential FRM for SO2, CASAC stated that
they felt that the CFP is adequate for measuring long-term average SO2
gas concentrations in rural areas with low levels (less than 5 ppbv) and
is therefore suitable for consideration as an FRM.  For p-SO4, CASAC
generally supports the use of the CFP as a potential FRM for measuring
p-SO4 for an AAI-based secondary standard.  The method has been
relatively well-characterized and evaluated, and it has a documented,
long-term track record of successful use in a field network designed to
assess spatial patterns and long-term trends.

On supporting the photometric NOy method as a potential FRM, CASAC
concluded that the existing NOy method is generally an appropriate
approach for the indicator.  However, CASAC agrees that additional
characterization and research is needed to fully understand the method
in order to designate it as a FRM.  The EPA is now soliciting public
comment on these methods as to their adequacy, suitability, and relative
merits as FRMs for purposes of monitoring to determine compliance with
an AAI-based secondary standard.

1.	Potential FRMs for SO2 and p-SO4

The CFP is a combined, integrated sampling and analysis method based on
the well-established measurement technology that has been used
extensively in EPA’s CASTNET monitoring network (see  HYPERLINK
"http://www.epa.gov/castnet" http://www.epa.gov/castnet ).  This method
is in current use at over 80 monitoring sites and has been in use at not
less than 40 sites for over 15 years.  This method employs a relatively
simple and inexpensive sampler and uses four 47-mm filters placed in an
open-faced filter pack to simultaneously collect integrated filter
samples for the SO2 and p-SO4 components.  In addition, the CFP is also
capable of the collection of t-NO3, the sum of HNO3 and p-NO3.

The first stage of the filter pack assembly contains a Teflon® filter
that collects p-SO42- and p-NO3, the second stage contains a nylon
filter that collects SO2 (as SO42-) and HNO3, and the third stage
contains two cellulose fiber filters impregnated with potassium
carbonate (K2CO3) that collect any remaining SO2 (as SO42-).  The
sampler collects one-week integrated samples at a very low, controlled
flow rate (1.5 or 3 L/min) in an attempt simulate actual deposition.
Weekly averaged SO2 and p-SO4 concentrations could then be averaged over
a 1-year period to calculate annual average values.

Upon sample completion, the species-specific filters are extracted, with
subsequent analysis by the well-established and documented ion
chromatographic (IC) analytical technique. During the IC analysis, an
aliquot of a filter extract is injected into a stream of eluent (ion
chromatography mobile phase, generally a millimolar-strength solution of
carbonate-bicarbonate) and passed through a series of ion exchangers.
The anions of interest are separated on the basis of their relative
affinities for a low capacity and the strongly basic anion exchanger
(guard and separator column).  The separated anions are directed onto a
cation exchanger (suppressor column) where they are converted to their
highly conductive acid form, and the eluent is converted to a weakly
conductive form.  The now-separated anions, each in their acid form, are
measured by conductivity.  They are identified on the basis of retention
time compared to that of standards and quantified by measurement of peak
area compared to the peak areas of calibration standards.

Calibration and quality assurance for the method are applied to the
sample filters, the analytical processes, and the flow rate measurement
and control aspects of the sampler.  Overall method performance is
typically assessed with collocated samplers.  These quality assurance
techniques are routinely used and have proved adequate for other types
of FRMs and equivalent methods in air monitoring network service.

The measurement and analytical procedures and past performance data
associated with the CFP method are well documented and available through
Quality Assurance Performance Plans (QAPPs), Standard Operating
Procedures (SOPs) and annual reports (US EPA, 2010a and 2010b).  The
accumulated database on the CFP method is substantial and indicates that
the method is sound, stable and has good reliability in routine, field
operation.  Data quality assessment results show the method to have good
reproducibility, with collocated and analytical precision values in the
range of 2% – 10% (excluding very low concentration measurements near
the method detection limits; US EPA 2010b).

Data quality objectives (DQOs) for a new FRM would be based upon current
DQOs being used for this method by EPA’s OAP/CAMD and the National
Park Service (NPS), the federal managers of CASTNET (US EPA, 2010a).  In
its current state, the CFP method is expected to meet or exceed (as past
CASTNET data have indicated; US EPA, 2010b) the expected FRM DQOs, even
when deployed in new monitoring networks outside of CASTNET.  In
addition, CASTNET samples have agreed favorably with other measures of
SO2 and p-SO4 in comparison studies.  For example, in direct comparison
with an annular denuder sampler (ADS) method, CASTNET/ADS ratios for SO2
and p-SO4 were generally on the order of 0.9-1.1 (Lavery et al, 2009;
Sickles et al, 1999; Sickles et al, 2008), thus illustrating the
accuracy of the CFP method in the determination of long-term average SO2
and p-SO4 concentrations.  The EPA believes that the CFP method would be
fully adequate as an FRM in determining yearly average SO2 and p-SO4
concentrations for compliance determination purposes.

The EPA recognizes that an existing FRM for SO2 has proven adequate for
the purposes of monitoring compliance for the primary SO2 NAAQS,
specifically the newly-promulgated 1-hour standard.  However, this FRM
is better suited to the shorter-term, higher concentration primary and
secondary SO2 NAAQS, and there is substantial uncertainty as to the
adequacy of this SO2 FRM for monitoring the lower concentrations
relevant to determining compliance with an AAI-based secondary standard.
 The performance specifications for SO2 FRM analyzers (40 CFR Part 53,
Table B-1) require a lower detectable limit (LDL) of 0.002 ppm for the
standard measurement range and 0.001 ppm for the lower measurement
range.  These requirements correspond to mass per unit volume
concentrations of 5.24 and 2.62 µg/m3, respectively.   Analysis of 2009
CASTNET data shows that of the 84 CASTNET sampling sites, 63 measured
annual average SO2 concentrations below even the lower of these LDL
requirements of 2.62 µg/m3 for the lower range SO2 FRM (US EPA, 2010a).
 In addition, 11 of the 84 sites measured annual (2009) average SO2
concentrations very near or below the manufacturers’ reported
detection limits for trace level UVF SO2 monitors.  Further, it is
likely that the number of sites with annual average SO2 concentration
below both the SO2 FRM LDL and the manufacturers reported detection
limits will increase due to expected declines in mean SO2 concentrations
(US EPA, 2010b). For these reasons, EPA is considering the CFP method
for use as the FRM for monitoring the SO2 component of an ambient air
indicator for oxides of sulfur, with a recommendation for additional
study and data collection to further evaluate the possible applicability
of the continuous UVF SO2 FRM for this purpose.

2.	Potential FRM for NOy

 Atmospheric concentrations of NOy are measured continuously by an
analyzer that photometrically measures the light intensity, at
wavelengths greater than 600 nanometers (nm), resulting from the
chemiluminescent reaction of ozone (O3) with NO in sampled air.  This
method is very similar to the chemiluminescence NO/NO2 analyzers widely
used to collect NO2 monitoring data for determining compliance with the
NO2 NAAQS.  The various oxides of nitrogen species, excluding NO, are
first quantitatively reduced to NO by means of a catalytic converter.
These species include NO2, HNO2, PANs, HNO3 and p-NO3.  The NO, which
commonly exists in ambient air, passes through the converter unchanged,
and, when combined with the NO resulting from the catalytic conversion
of the other oxides of nitrogen, a measurement of the total NOy
concentration results.  To maximize the conversion of the more
chemically active oxides of nitrogen species, the converter is located
externally, at or near the air sample inlet probe.  This location
minimizes losses of these active species that could otherwise occur from
chemical reactions and wall losses in the sample inlet line.

The NOy analyzer is a suitable, commercially produced continuous
chemiluminescence analyzer that includes an ozone generator, a reaction
cell, a photometric detector, wavelength filters as necessary to reduce
sensitivity to wavelengths below 600 nm, a pump and flow control system
to draw atmospheric air through the converter and into the reaction
cell, a suitable converter, a system to control the operation of the
analyzer, and appropriate electronics to process and quantitatively
scale the photometric signals.  The converter contains a catalyst such
as molybdenum and is heated to an optimum temperature designed to
optimize the conversion of the various oxides of nitrogen to NO.  It is
connected to the analyzer via suitable lengths of Teflon® tubing.
Hourly NOy measurements obtained by the analyzer would be averaged over
the same 7-day period used by the CFP method to measure the SO2 and
p-SO4 components, with further averaging over a 1-year period.

Commercial NOy analyzers are currently available, and the analyzers have
been used for a variety of monitoring applications.  During the 2006
TexAQS Radical and Aerosol Measurement Project (TRAMP), Luke et. al.,
2010, compared measured NOy concentrations obtained with an NOy
instrument based upon the above mentioned methodology with the sum of
measured individual NOy species (i.e., NOyi =
NO+NO2+HNO3+PANs+HNO2+p-NO3).  This comparison yielded excellent overall
agreement during both day ([NOy](ppb) = [NOyi](ppb) × 1.03 – 0.42; r2
= 0.9933) and night time ([NOy](ppb) = [NOyi](ppb) × 1.01 – 0.18; r2
= 0.9975) periods (Luke et al, 2010).  The results of this study show
that this NOy method is capable of the accurate determination of all the
atmospherically relevant NOy components, resulting in an accurate
determination of total NOy concentrations.  NOy instruments have been
routinely operated in networks such as SEARCH, dating back several
years.  In addition, state monitoring agencies across the U.S. have
begun, starting in 2009, the routine operation of commercially available
NOy instrumentation in anticipation of EPA’s NCore network
transitioning to full operation in 2011.

These initial assessments described above are promising and indicate
that the photometric NOy method appears to be accurate, reliable, and
capable of routine network operation.  As a result, the method is most
likely capable for use as an FRM for determining atmospheric NOy
concentrations as a component in determining compliance with an
AAI-based secondary standard.  However, as described below, this
continuous method for NOy requires additional time for further
evaluation before it can be fully confirmed for adoption as a FRM.  The
EPA has identified measurement uncertainties and some remaining science
questions associated with this method.  Among these are (a) the ability
of the method to capture all components of NOy relevant to nitrogen
deposition, (b) the efficiency of the molybdenum converter in converting
all oxides of nitrogen to NO for detection (excluding NO2, as this
conversion is already well documented), (c) appropriate inlet height
specifications to minimize any bias associated with vertical
concentration gradients of key NOy components, (d) identification and
quantification of potential measurement interferences in the NOy
determination, and (e) development and demonstration of effective
calibration/challenge procedures to best represent the various mixtures
of NOy components that are expected to be present in the different air
sheds across the U.S.

To address these NOy method uncertainties and to fully assess this
method for use as the NOy FRM, EPA has developed a detailed research
plan (Russell and Samet, 2011b) which was presented to the CASAC AMMS on
February 16, 2011.  In response, CASAC recognized the need for, and
supported the general outline of EPA’s research plan to evaluate the
NOy method for potential designation as an FRM (US EPA, 2011).  In
addition, the CASAC AMMS suggested additional areas of research
associated with the photometric NOy method that warrant further
assessment prior to final designation of the method as the NOy FRM.
These include operation of the method during extremely low temperature
conditions to investigate possible condensation in sample lines, method
detection limits relative to low levels expected in remote areas, and
ambient-based method evaluations in various air sheds across the U.S.
In response to these CASAC AMMS suggestions, EPA is carrying out
studies, in addition to the tasks outlined in the research plan, for the
NOy method.  The results of these studies will likely take a year or
more to become available.  As noted previously, EPA anticipates that
these results will be favorable and will confirm the adequacy of the NOy
method as a suitable FRM for determining compliance with an AAI-based
secondary standard.

V.	Statutory and Executive Order Reviews

A.	Executive Order 12866: Regulatory Planning and Review and Review and
Executive Order 13563:  Improving Regulation and Regulatory Review

	 Under Executive Order 12866 (58 FR 51735, October 4, 1993), this
action is a “significant regulatory action.” because it was deemed
to “raise novel legal or policy issues.”  Accordingly, EPA submitted
this action to the Office of Management and Budget (OMB) for review
under Executive Orders 12866 and 13563 (76 FR 3821, January 21, 2011),
and any changes made in response to OMB recommendations have been
documented in the docket for this action.

B.	Paperwork Reduction Act

	This action does not impose an information collection burden under the
provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.
Burden is defined at 5 CFR 1320.3(b). There are no information
collection requirements directly associated with the establishment of a
NAAQS under section 109 of the CAA.

C.	Regulatory Flexibility Act

	For purposes of assessing the impacts of today’s rule on small
entities, small entity is defined as:  (1) a small business that is a
small industrial entity as defined by the Small Business
Administration’s (SBA) regulations at 13 CFR 121.201;  (2) a small
governmental jurisdiction that is a government of a city, county, town,
school district or special district with a population of less than
50,000; and (3) a small organization that is any not-for-profit
enterprise which is independently owned and operated and is not dominant
in its field.

	After considering the economic impacts of today's proposed rule on
small entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities.  This
proposed rule will not impose any requirements on small entities.
Rather, this rule establishes national standards for allowable
concentrations of oxides of nitrogen and sulfur in ambient air as
required by section 109 of the CAA.  See also American Trucking
Associations v. EPA. 175 F. 3d at 1044-45 (NAAQS do not have significant
impacts upon small entities because NAAQS themselves impose no
regulations upon small entities).  We continue to be interested in the
potential impacts of the proposed rule on small entities and welcome
comments on issues related to such impacts

D.	Unfunded Mandates Reform Act

 	Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public
Law 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and Tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with “Federal mandates” that
may result in expenditures to State, local, and Tribal governments, in
the aggregate, or to the private sector, of $100 million or more in any
1 year.  Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and to adopt
the least costly, most cost-effective or least burdensome alternative
that achieves the objectives of the rule. The provisions of section 205
do not apply when they are inconsistent with applicable law.  Moreover,
section 205 allows EPA to adopt an alternative other than the least
costly, most cost-effective or least burdensome alternative if the
Administrator publishes with the final rule an explanation why that
alternative was not adopted. Before EPA establishes any regulatory
requirements that may significantly or uniquely affect small
governments, including Tribal governments, it must have developed under
section 203 of the UMRA a small government agency plan. The plan must
provide for notifying potentially affected small governments, enabling
officials of affected small governments to have meaningful and timely
input in the development of EPA regulatory proposals with significant
Federal intergovernmental mandates, and informing, educating, and
advising small governments on compliance with the regulatory
requirements.

	This action contains no Federal mandates under the provisions of Title
II of the Unfundated Mandates Reform Act of 1995 (UMRA), 2 U.S.C.
1531-1538 for State, local, or tribal governments or the private sector.
 Therefore, this action is not subject to the requirements of sections
202 or 205.  Furthermore, as indicated previously, in setting a NAAQS
EPA cannot consider the economic or technological feasibility of
attaining ambient air quality standards; although such factors may be
considered to a degree in the development of State plans to implement
the standards.  See also American Trucking Associations v. EPA, 175 F.
3d at 1043 (noting that because EPA is precluded from considering costs
of implementation in establishing NAAQS, preparation of a Regulatory
Impact Analysis pursuant to the Unfunded Mandates Reform Act would not
furnish any information which the court could consider in reviewing the
NAAQS).  Accordingly, EPA has determined that the provisions of sections
202, 203, and 205 of the UMRA do not apply to this proposed decision.
The EPA acknowledges, however, that any corresponding revisions to
associated SIP requirements and air quality surveillance requirements,
40 CFR part 51 and 40 CFR part 58, respectively, might result in such
effects.  Accordingly, EPA will address, as appropriate, unfunded
mandates if and when it proposes any revisions to 40 CFR parts 51 or 58.

E.	Executive Order 13132: Federalism

	This proposed rule does not have federalism implications.  It will not
have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government, as
specified in Executive Order 13132 because it does not contain legally
binding requirements. Thus, the requirements of Executive Order 13132 do
not apply to this rule.

	EPA believes, however, that this proposed rule may be of significant
interest to State governments.  However, aAs also noted in section E
(above) on UMRA, EPA recognizes that States will have a substantial
interest in this rule and any corresponding revisions to associated SIP
requirements and air quality surveillance requirements, 40 CFR part 51
and 40 CFR part 58, respectively.  Therefore, in the spirit of Executive
Order 13132 and consistent with EPA policy to promote communications
between EPA and State and local governments, EPA specifically solicits
comment on this proposed rule from State and local officials.

F.	Executive Order 13175: Consultation and Coordination with Indian
Tribal Governments

	Executive Order 13175, entitled “Consultation and Coordination with
Indian Tribal Governments” (65 FR 67249, November 9, 2000), requires
EPA to develop an accountable process to ensure “meaningful and timely
input by tribal officials in the development of regulatory policies that
have tribal implications.”  This rule concerns the establishment of
national standards to address the public welfare effects of oxides of
nitrogen and sulfur.

	This action does not have Tribal implications, as specified in
Executive Order 13175 (65 FR 67249, November 9, 2000).  It does not have
a substantial direct effect on one or more Indian tribes, since tribes
are not obligated to adopt or implement any NAAQS.  Thus, Executive
Order 13175 does not apply to this rule.

G.	Executive Order 13045: Protection of Children from Environmental
Health & Safety Risks

 action is not subject to EO 13045 because it is not an economically
significant rule as defined in EO 12866.  addresses the effects on
public welfare of oxides of nitrogen and sulfur and does not establish
an environmental standard intended to mitigate health or safety risks.

H.	Executive Order 13211: Actions that Significantly Affect Energy
Supply, Distribution or Use

	This action is not a “significant energy action” as defined in
Executive Order 13211 (66 FR 28355, May 22, 2001), because it is not
likely to have a significant adverse effect on the supply, distribution,
or use of energy.  This action concerns the establishment of national
standards to address the public welfare effects of oxides of nitrogen
and sulfur.   This action does not prescribe specific pollution control
strategies by which these ambient standards will be met.  Such
strategies will be developed by States on a case-by-case basis, and EPA
cannot predict whether the control options selected by States will
include regulations on energy suppliers, distributors, or users.

I.	National Technology Transfer and Advancement Act

	Section 12(d) of the National Technology Transfer and Advancement Act
of 1995 (NTTAA), Public Law No. 104-113, 12(d) (15 U.S.C. 272 note)
directs EPA to use voluntary consensus standards in its regulatory
activities unless to do so would be inconsistent with applicable law or
otherwise impractical.  Voluntary consensus standards are technical
standards (e.g., materials specifications, test methods, sampling
procedures, and business practices) that are developed or adopted by
voluntary consensus standards bodies.  The NTTAA directs EPA to provide
Congress, through OMB, explanations when the Agency decides not to use
available and applicable voluntary consensus standards.

  EPA is not aware of any voluntary consensus standards that are
relevant to the provisions of this proposed rule.  EPA welcomes any
feedback on such standards that may be applicable.

J.	Executive Order 12898: Federal Actions to Address Environmental
Justice in Minority Populations and Low-Income Populations

	Executive Order 12898 (59 FR 7629 (Feb. 16, 1994)) establishes federal
executive policy on environmental justice.  Its main provision directs
federal agencies, to the greatest extent practicable and permitted by
law, to make environmental justice part of their mission by identifying
and addressing, as appropriate, disproportionately high and adverse
human health or environmental effects of their programs, policies, and
activities on minority populations and low-income populations in the
United States.

	EPA has determined that this proposed rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it retains the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health or
environmental effects on any population, including any minority or
low-income population.

References

Baker JP; Bernard DP; Christensen SW; Sale MJ. 1990. Biological effects
of changes in surface water acid-base chemistry. (State of science /
technology report #13).Washington DC: National Acid Precipitation
Assessment Program (NAPAP).

Bulger AJ; Cosby BJ; Webb JR. 2000. Current, reconstructed past, and
projected future status of brook trout (Salvelinus fontinalis) streams
in Virginia. Can J Fish Aquat Sci, 57, 1515-1523.

Banzhaf, S., D. Burtraw, D. Evans, and A. Krupnick. 2006. “Valuation
of Natural Resource Improvements in the Adirondacks.” Land Economics
82:445-464.

Lavery, T.F. C.M. Rogers, R. Baumgardner, and K.P. Mishoe. 2009.
Intercomparison of Clean Air Status and Trends Network Nitrate and
Nitric Acid Measurements with Data from Other Monitoring Programs. J.
Air & Waste Manage. Assoc., 59, 214-226.

Lien L; Raddum GG; Fjellheim A. 1992. Critical loads for surface waters:
invertebrates and fish. (Acid rain research report no 21). Oslo, Norway:
Norwegian Institute for Water Research.

Luke, W.T., P.K. Barry, L. Lefer, J. Flynn, B. Rappenglück, M.
Leuchner, J.E. Dibb, L.D. Ziemba, C.H. Anderson, and M. Buhr. 2010.
Measurements of primary trace gases and NOY composition in Houston,
Texas.  Atmospheric Environment, 44, 4068-4080.

MacAvoy SW; Bulger AJ. 1995. Survival of brook trout (Salvelinus
fontinalis) embryos and fry in streams of different acid sensitivity in
Shenandoah National Park, USA. Water Air Soil Pollut, 85, 445-450.

McNulty SG; Cohen EC; Myers JAM; Sullivan TJ; Li H. 2007. Estimates of
critical acid loads and exceedances for forest soils across the
conterminous United States. Environ Pollut, 149, 281-292.

NAPAP. 1990. Acid Deposition: State of Science and Technology. National
Acid Precipitation Assessment Program. Office of the Director,
Washington, DC.

NAPAP. (2005). National acid precipitation assessment program report to
Congress: An integrated assessment.
http://www.esrl.noaa.gov/csd/aqrs/reports/napapreport05.pdf. Silver
Spring, MD: National Acid Precipitation Assessment Program (NAPAP);
Committee on Environment and Natural Resources (CENR) of the National
Science and Technology Council (NSTC).

NRC (National Research Council). 2004. Air quality management in the
United States. Washington, DC: National Research Council (NRC); The
National Academies Press.

Russell, A and J. M. Samet, 2010a. Review of the Policy Assessment for
the Review of the Secondary National Ambient Air Quality Standard for
NOx and SOx: First Draft. EPA-CASAC-10-014.

Russell, A and J. M. Samet, 2010b. Review of the Policy Assessment for
the Review of the Secondary National Ambient Air Quality Standard for
NOx and SOx: Second Draft. EPA-CASAC-11-003.

Russell, A and J. M. Samet, 2011. Review of the Policy Assessment for
the Review of the Secondary National Ambient Air Quality Standard for
NOx and SOx: FINAL. EPA-CASAC-11-005.

Russell and Samet, 2011b Review of EPA Draft Documents on Monitoring and
Methods for Oxides of Nitrogen (NOx) and Sulfur (SOx)  HYPERLINK
"http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommittees/CASAC"
http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommittees/CASAC

Sickles II, J.E., L. L. Hodson, and L. M. Vorburger. 1999. Evaluation of
the filter pack for long-duration sampling of ambient air. Atmospheric
Environment, 33, 2187-2202.

Sickles II, J.E. and D.S. Shadwick. 2008. Comparison of particulate
sulfate and nitrate at collocated CASTNET and IMPROVE sites in the
eastern US.  Atmospheric Environment, 42, 2062-2073.

Smyth SC, W. Jiang, and H. Roth. 2008. A comparative performance
evaluation of the AURAMS and CMAQ air quality modeling systems. Atmos
Envir 43:1059-1070.

Stoddard J; Kahl JS; Deviney FA; DeWalle DR; Driscoll CT; Herlihy AT;
Kellogg JH; Murdoch PS; Webb JR; Webster KE. (2003). Response of surface
water chemistry to the Clean Air Act Amendments of 1990 (No. EPA
620/R-03/001). Research Triangle Park, NC; National Health and
Environmental Effects Research Laboratory; Office of Research and
Development; U.S. Environmental Protection Agency.

Sullivan TJ; Driscoll CT; Cosby BJ; Fernandez IJ; Herlihy AT; Zhai J;
Stemberger R; Snyder KU; Sutherland JW; Nierzwicki-Bauer SA; Boylen CW;