SO2 concentrations and respiratory symptoms, ED visits, and
hospitalizations were conducted in areas where ambient SO2
concentrations were well below the level of the current annual NAAQS
(REA, section 10.4.2).  The REA noted that this evidence suggested that
the current annual standard was not providing adequate protection
against health effects associated with shorter-term SO2 concentrations
found in epidemiologic studies (REA, section 10.4.2).

b.	Air quality, exposure, and risk-based considerations

Results of the risk characterization based on the air quality assessment
provided additional insight into whether there is a need to revise the
current annual standard, focusing again on the extent to which the
annual standard may be providing protection against effects associated
with short-term exposures.   In general, analyses presented in the REA
described the extent to which the current annual standard provided
protection against 5-minute peaks of SO2 in excess of potential health
effect benchmark levels (REA, Chapter 7).  The REA found that many of
the monitors where frequent 5-minute exceedances were reported had
annual average SO2 concentrations well below the level of the current
annual standard.  Moreover, the REA found that there was little to no
correlation between the annual average SO2 concentration and the number
of 5-minute daily maximum concentrations above potential health effect
benchmark levels at these monitors (REA section 7.3.1).  Thus, the REA
concluded that the annual standard adds little in the way of protection
against 5-minute peaks of SO2 (REA, section 10.4.3).

c.	Summary of considerations from the REA regarding the annual standard

.e. ≤ 24-hours) SO2 exposures.  Thus, the policy chapter of the REA
accordingly concluded that consideration should be given to either
revoking the annual standard or retaining it without revision, in
conjunction with setting an appropriate short-term standard(s).

3.	CASAC views regarding the adequacy of the current 24-hour and annual
standards

With regard to the adequacy of the current standards, CASAC conclusions
were consistent with the views expressed in the policy assessment
chapter of the REA.  CASAC agreed that the primary concern in this
review is to protect against health effects that have been associated
with short-term SO2 exposures, particularly those of 5 - 10 minutes
(Samet 2009).  CASAC also agreed that the current 24-hour and annual
standards are not sufficient to protect public health against the types
of exposures that could lead to these health effects.  Given these
considerations, and as noted in their letter to the EPA Administrator,
CASAC agreed “that the current 24-hour and annual standards are not
adequate to protect public health, especially in relation to short term
exposures to SO2 (5 - 10 minutes) by exercising asthmatics” (Samet,
2009, p. 15).    CASAC also noted: “assuming that EPA adopts a one
hour standard in the range suggested, and if there is evidence showing
that the short-term standard provides equivalent protection of public
health in the long-term as the annual standard, the panel is supportive
of the REA discussion of discontinuing the annual standard” (Samet
2009, p. 15).  With regard to the current 24-hour standard, CASAC was
generally supportive of using the air quality analyses in the REA as a
means of determining whether the current 24-hour standard was needed in
addition to a new 1-hour standard to protect public health.  CASAC
stated: “the evidence presented [in REA Table 10-3] was convincing
that some of the alternative one-hour standards could also adequately
protect against exceedences of the current 24-hour standard” (Samet
2009, p. 15)    Discussion regarding CASAC’s views on how the standard
should be revised is provided below within the context of discussions on
the elements (i.e., indicator, averaging time, form, level) of a new
short-term standard.

4.	The Administrator’s conclusions regarding adequacy of the current
24-hour and annual standards

Based on the epidemiologic evidence, the risk and exposure data set out
in this section, as well as CASAC’s advice and recommendations, the
Administrator concludes (subject to consideration of public comment)
that the current standards are not adequate to protect public health
with an adequate margin of safety.  The basis for this conclusion is as
follows.  First, the Administrator accepts and agrees with the ISA’s
conclusion that the results of controlled human exposure and
epidemiologic studies form a plausible and coherent data set that
supports a causal relationship between short-term (5-minutes to
24-hours) SO2 exposures and adverse respiratory effects.  The
Administrator further agrees that the epidemiologic evidence (buttressed
by the clinical evidence) indicates that the effects seen in the
epidemiologic studies are attributable to exposure to SO2.  She also
accepts and agrees with the conclusion of the ISA that “[i]n the
epidemiologic studies, respiratory effects were observed in areas where
the maximum ambient 24-h avg SO2 concentration was below the current
24-h avg NAAQS level …..” (ISA, section 5.2, p. 5-2.) and so would
occur at ambient SO2 concentrations that are present in locations
meeting the current 24-hour NAAQS.  The Administrator also notes that
these effects occurred in areas with annual air quality levels
considerably lower than those allowed by the current annual standard,
indicating that the annual standard also is not providing protection
against such effects.  Existence of epidemiologic studies showing
adverse effects occurring at levels allowed by the current standards is
an accepted justification for finding that it is appropriate to revise
the existing standards.  See, e.g. American Trucking Ass’n v. EPA, 283
F. 3d 355, 370 (D.C. Cir. 2002).

With regard to the exposure and risk results, the Administrator notes
and agrees with the analyses in the REA supporting that 5-minute
exposures, reasonably judged important from a public health perspective,
were associated with air quality adjusted upward to simulate just
meeting the current standards.  The Administrator especially notes the
results of the St. Louis exposure analysis which, as summarized above,
indicates that substantial percentages of asthmatic children at moderate
or greater exertion would be exposed, at least once annually, to air
quality exceeding the 400 and 200 ppb benchmarks.   Moreover, in
addition to the health evidence and risk-based information, the
Administrator agrees with CASAC’s conclusion that the current SO2
standards do not adequately protect the public’s health.

In considering approaches to revising the current standards, the
Administrator is proposing that it is appropriate to consider setting a
new short-term standard. The Administrator initially notes that a 1-hour
standard could provide increased public health protection, especially
for members of at-risk groups, from health effects described in both
controlled human exposure and epidemiologic studies, and hence, health
effects associated with 5-minute to 24-hour exposures to SO2.  As
discussed in section II.F.5 below, depending on the degree of protection
afforded by such a standard, it may be appropriate to replace, and not
retain, the current 24-hour and annual standards in conjunction with
setting a new short-term standard.

F.	Conclusions on the elements of a proposed new short-term standard

In considering alternative SO2 primary NAAQS, the Administrator notes
the need to protect at-risk populations from: 1) 1-hour daily maximum
and 24-hour average exposures to SO2 that could cause the types of
respiratory morbidity effects reported in epidemiologic studies; and 2)
5 - 10 minute SO2 exposure concentrations reported in controlled human
exposure studies to result in moderate or greater lung function
responses and/or respiratory symptoms.  Considerations with regard to
potential alternative standards and the specific options being proposed
are discussed in the following sections in terms of indicator, averaging
time, form, and level (sections II.F.1 to II.F.4).

1.	Indicator

In the last review, EPA focused on SO2 as the most appropriate indicator
for ambient SOx.  In making a decision in the current review on the most
appropriate indicator, the Administrator has considered the conclusions
of the ISA and REA as well as the views expressed by CASAC.  The REA
noted that, although the presence of gaseous SOx species other than SO2
has been recognized, no alternative to SO2 has been advanced as being a
more appropriate surrogate for ambient gaseous SOx.  Controlled human
exposure studies and animal toxicology studies provide specific evidence
for health effects following exposure to SO2.  Epidemiologic studies
also typically report levels of SO2, as opposed to other gaseous SOx.
Because emissions that lead to the formation of SO2 generally also lead
to the formation of other SOx oxidation products, measures leading to
reductions in population exposures to SO2 can generally be expected to
lead to reductions in population exposures to other gaseous SOx.
Therefore, meeting an SO2 standard that protects the public health can
also be expected to provide some degree of protection against potential
health effects that may be independently associated with other gaseous
SOx even though such effects are not discernable from currently
available studies indexed by SO2 alone.  Given these key points, the REA
concluded that the available evidence supports the retention of SO2 as
the indicator in the current review (REA, section 10.5.1).  Consistent
with this conclusion, CASAC stated in a letter to the EPA Administrator
that “for indicator, SO2 is clearly the preferred choice (Samet 2009,
p. 14).”   The Administrator agrees with this consensus, and therefore
proposes to retain SO2 as the indicator for oxides of sulfur in the
current review.

2.	Averaging time

In considering whether it is appropriate to revise the averaging times
of the current standards, the first consideration is what health effects
the standard is addressing, and specifically whether those effects are
associated with short-term (i.e., 5-minutes to 24-hours), and/or
long-term (i.e. weeks to years) exposure to SO2.  There are distinct
differences in the causality judgments in the ISA as to short-term
versus long-term health effects of SO2.  The ISA found evidence relating
long-term (weeks to years) SO2 exposures to adverse health effects to be
“inadequate to infer the presence or absence of a causal
relationship” (ISA, Table 5-3).  In contrast, the ISA judged evidence
relating short-term (5-minutes to 24-hours) SO2 exposure to respiratory
morbidity to be “sufficient to infer a causal relationship” (the
strongest possible conclusion as to causality) and short-term exposure
to SO2 and mortality to be “suggestive of a causal relationship”
(ISA, Table 5-3).  Taken together, the REA concluded that these
judgments most directly supported standard averaging time(s) that focus
protection on SO2 exposures from 5-minutes to 24-hours (REA, section,
10.5.2).

a.	Evidence and air quality, exposure, and risk-based considerations

In considering the level of support available for specific short-term
averaging times, the REA noted the strength of evidence from human
exposure and epidemiologic studies evaluated in the ISA.  As previously
mentioned, controlled human exposure studies exposed exercising
asthmatics to 5 - 10 minute peak concentrations of SO2 and consistently
found decrements in lung function and/or respiratory symptoms.
Importantly, the ISA described the controlled human exposure studies as
being the “definitive evidence” for its conclusion that there exists
a causal association between short-term (5-minutes to 24-hours) SO2
exposure and respiratory morbidity (ISA, section 5.2).  In addition to
the controlled human exposure evidence, there is a relatively small body
of epidemiologic studies describing positive associations between 1-hour
daily maximum SO2 levels and respiratory symptoms as well as hospital
admissions and ED visits for all respiratory causes and asthma (ISA
Tables 5.4 and 5.5).  In addition to the evidence from these 1-hour
daily maximum epidemiologic studies, there is a considerably larger body
of epidemiologic studies reporting positive associations between 24-hour
average SO2 levels and respiratory symptoms, as well as hospitalizations
and ED visits for all respiratory causes and asthma.  However, as in
previous reviews, there was uncertainty as to whether these positive
associations are due to 24-hour average SO2 exposures, or exposure (or
multiple exposures) to short-term peaks of SO2 within a 24-hour period.
More specifically, when describing epidemiologic studies observing
positive associations between ambient SO2 and respiratory symptoms, the
ISA stated “that it is possible that these associations are determined
in large part by peak exposures within a 24-hour period” (ISA, section
5.2 at p. 5-5).  The ISA also stated that the respiratory effects
following 5 - 10 minute SO2 exposures in controlled human exposure
studies provide a basis for a progression of respiratory morbidity that
could result in increased ED visits and hospital admissions (ISA,
section 5.2).

The policy chapter of the REA found that the controlled human exposure
evidence described above provided support for an averaging time that
protects against 5-10 minute peak SO2 exposures (REA, section 10.5.2).
In addition, the REA found that results from epidemiologic studies
provided support for both 1-hour and 24-hour averaging times (REA,
section 10.5.2).  However, it is worth noting again that the ISA
suggested effects observed in epidemiologic studies up to 24-hours, may
be due, at least in part, to shorter-term peaks of SO2.  Taken together,
the REA concluded that a primary concern with regard to averaging time
is the level of protection provided against 5 - 10 minute peak SO2
exposures (REA, section 10.5.2).  The REA also found that the evidence
described above suggested it would also be appropriate to consider the
degree of protection potential averaging times under consideration
provide against both 1-hour daily maximum and 24-hour average SO2
concentrations (REA, section 10.5.2).

In considering the level of support available for specific short-term
averaging times, the policy assessment chapter of the REA also took into
account air quality considerations.  More specifically, since the
shortest averaging time for the current primary SO2 standard is
24-hours, the REA evaluated the potential for a standard based on
24-hour average SO2 concentrations to limit 5-minute peak SO2 exposures
(REA, section <IP_ADDRESS>).  The REA evaluated ratios between 99th
percentile 5-minute daily maximum and 99th percentile 24-hour average
SO2 concentrations for 42 monitors reporting measured 5-minute data for
any year between 2004-2006 (REA, Table 10-1).  Across this set of
monitors, ratios of 99th percentile 5-minute daily maximum to 99th
percentile 24-hour average SO2 concentrations spanned a range of 2.0 to
14.1 (REA, Table 10-1).    These results suggested a standard based on
24-hour average SO2 concentrations would not likely be an effective or
efficient approach for addressing 5-minute peak SO2 concentrations.
That is, the REA concluded using a 24-hour average standard to address
5-minute peaks would likely result in over- controlling in some areas,
while under-controlling in others (REA, section 10.5.2).  This analysis
also suggested that a 5-minute standard would not likely be an effective
or efficient means for controlling 24-hour average SO2 concentrations
(REA, section 10.5.2).

The REA also reported ratios between 99th percentile 5-minute daily
maximum and 99th percentile 1-hour daily maximum SO2 levels from this
set of monitors.  Compared to the ratios discussed above (5-minute daily
maximum to 24-hour average), there was far less variability between
5-minute daily maximum and 1-hour daily maximum ratios.  More
specifically, 39 of the 42 monitors had 99th percentile 5-minute daily
maximum to 99th percentile 1-hour daily maximum ratios in the range of
1.2 to 2.5 (REA, Table 10-1).  The remaining three monitors had ratios
of 3.6, 4.2 and 4.6 respectively.  Overall, the REA found that this
relatively narrow range of ratios (compared to the range of ratios
presented above with respect to 5-minute daily maximum to 24-hour
average) suggested that a standard with a 1-hour averaging time would be
more efficient and effective at limiting 5-minute peaks of SO2 than a
standard with a 24-hour averaging time (REA, section <IP_ADDRESS>).  This
analysis also suggested that a 5-minute standard could be a relatively
effective means of controlling 1-hour daily maximum SO2 concentrations.

ns ≥ 36 ppb (Table 1).  In addition, given air quality adjusted to
just meet a 100 ppb 99th percentile 1-hour daily maximum standard, only
6 of the 39 counties (Linn, Union, Bronx, Fairfax, Hudson, and Wayne)
included in this 2004 analysis were estimated to have 99th percentile
24-hour average SO2 concentrations ≥ 36 ppb (Table 1).  The REA
repeated this analysis for the years 2005 and 2006 and found similar
results (REA, Appendix Tables D1 and D2).

Table 1.  99th percentile 24-hour average SO2 concentrations for 2004
given just meeting the alternative 1-hour daily maximum 99th and 98th
percentile potential standards analyzed in the air quality assessment
(SOURCE: REA Table 10-2).

ntration ≥ 36 ppb).  In addition, based on the air quality and
exposure analyses presented in chapters 7 and 8 of the REA, there is
also a strong likelihood that a 1-hour standard will limit 5 - 10 minute
peaks of SO2 shown in human exposure studies to result in decrements in
lung function and/or respiratory symptoms in exercising asthmatics (see
especially: REA Tables 7-11 to 7-14 and Figure 8-19).  Such analyses are
also summarized in section II.F.4 of this notice.  Taken together, these
results support that a 1-hour daily maximum standard, with an
appropriate form and level, can provide adequate protection against the
range of health outcomes associated with averaging times from 5-minutes
to 24-hours (REA, section <IP_ADDRESS>).

The REA also considered the possibility of a 5-minute averaging time
based solely on the controlled human exposure evidence.  However, the
REA did not favor such an approach (REA <IP_ADDRESS>).  As in past NAAQS
reviews, the stability of the design of pollution control programs in
considering the elements of a NAAQS was considered, since more stable
programs are more effective, and hence result in enhanced public safety.
 American Trucking Associations v. EPA, 283 F. 3d 355, 374-75 (D.C. Cir.
2002).   In this review, there were concerns about the stability of a
standard using a 5-minute averaging time.  Specifically, there was
concern that compared to longer averaging times (e.g., 1-hour, 24-hour),
year-to-year variation in 5-minute SO2 concentrations were likely to be
substantially more temporally and spatially diverse.  Consequently, the
REA concluded that a 5-minute averaging time would not provide a stable
regulatory target and therefore would not be the preferred approach to
provide adequate public health protection.  However, as noted above,
analyses in the REA support that a 1-hour averaging time, given an
appropriate form and level (discussed below in sections II.F.3 and
II.F.4, respectively) can adequately limit 5-minute SO2 exposures and
provide a more stable regulatory target than setting a 5-minute
standard.

b.	CASAC views

CASAC agreed with the conclusions of the policy assessment chapter of
the REA that a primary consideration of the SO2 NAAQS should be the
protection provided against health effects associated with short-term
exposures.  In their letter to the EPA Administrator, CASAC stated that
they were “in agreement with having a short-term standard and finds
that the REA supports a one-hour standard as protective of public
health” (Samet 2009, p. 1).   Furthermore, CASAC agreed with the REA
that a “one-hour standard is the preferred averaging time” (Samet
2009, p.15).”

c.	Administrator’s conclusions on averaging time

In considering the most appropriate averaging time(s) for the SO2
primary NAAQS, the Administrator notes the conclusions and judgments
made in the ISA about the available scientific evidence, conclusions
from the REA, and CASAC recommendations discussed above.  Based on these
considerations, the Administrator proposes to set a new standard based
on 1-hour daily maximum SO2 concentrations to provide increased
protection against effects associated with short-term (5-minutes to
24-hour) exposures.  First, the Administrator agrees with the REA’s
conclusion that the standard should focus protection on short-term SO2
exposures from 5-minutes to 24-hours.  As noted above, CASAC’s strong
recommendation supports this approach as well.  Second, the
Administrator agrees that the standard must provide requisite protection
from 5-10 minute exposure events (the critical issue in the previous
review), but believes (subject to consideration of public comment) that
this can be done without having a standard with a 5-minute averaging
time.  The Administrator agrees with the REA conclusion that it is
likely a 1-hour standard -- with the appropriate form and level -- can
substantially reduce 5 - 10 minute peaks of SO2 shown in controlled
human exposure studies to result in respiratory symptoms and/or
decrements in lung function in exercising asthmatics.  The Administrator
further believes that a 5-minute averaging time would result in
significant and unnecessary instability and is undesirable for that
reason.   The Administrator also notes the statements from CASAC
addressing whether a one-hour averaging time can adequately control 5-10
minute peak exposures and whether there should be a 5-minute averaging
time.  CASAC stated that the REA had presented a “convincing
rationale” for a one-hour standard, and that “a 1-hour standard is
the preferred averaging time” (Samet 2009, p. 16).

Third, the Administrator agrees that a one-hour averaging time (again,
with the appropriate form and level) would provide protection against
the range of health outcomes associated with averaging times of one hour
to 24 hours.  Specifically, the Administrator finds that a 1-hour
standard can substantially reduce: 1) by definition, 99th percentile
1-hour daily maximum air quality concentrations in cities observing
positive effect estimates in epidemiologic studies of hospital
admissions and ED visits for all respiratory causes and asthma; and 2)
99th percentile 24-hour average air quality concentrations found in U.S.
cities where ED visit and hospitalization studies (for all respiratory
causes and asthma) observed statistically significant associations in
multi-pollutant models with PM.   Finally, the Administrator notes that
the proposal to establish a new 1-hour averaging time is in agreement
with CASAC recommendations.  That is, as noted above, CASAC stated that
they were “in agreement with having a short-term standard and finds
that the REA supports a one-hour standard as protective of public health
(Samet 2009, p. 1).”

3.	Form

When evaluating alternative forms in conjunction with specific levels,
the REA considered the adequacy of the public health protection provided
by the combination of level and form to be the foremost consideration.
In addition, the REA recognized that it is important that the standard
have a form that is reasonably stable.  As just explained in the context
of a five-minute averaging time, a standard set with a high degree of
instability could have the effect of reducing public health protection
because shifting in and out of attainment could disrupt an area’s
ongoing implementation plans and associated control programs.

a.	Evidence, air quality, and risk- based considerations

As previously mentioned, the policy chapter of the REA recognized that
the adequacy of the public health protection provided by a 1-hour daily
maximum potential alternative standard will be dependent on the
combination of form and level.  It is therefore important that the
particular form selected for a 1-hour daily maximum potential
alternative standard reflect the nature of the health risks posed by
increasing SO2 concentrations.  That is, the REA noted that the form of
the standard should reflect results from controlled human exposure
studies demonstrating that the percentage of asthmatics affected, and
the severity of the respiratory response (i.e. decrements in lung
function, respiratory symptoms) increases as SO2 concentrations
increase.  Taking this into consideration, the REA concluded that a
concentration-based form, averaged over three years, is more appropriate
than an exceedance-based form (REA, section 10.5.3).  This is because a
concentration-based form averaged over three years would give
proportionally greater weight to years when 1-hour daily maximum SO2
concentrations are well above the level of the standard, than to years
when 1-hour daily maximum SO2 concentrations are just above the level of
the standard.  In contrast, an expected exceedance form would give the
same weight to years when 1-hour daily maximum SO2 concentrations are
just above the level of the standard, as to years when 1-hour daily
maximum SO2 concentrations are well above the level of the standard.
Therefore, the REA concluded that a concentration-based form, averaged
over three years (which also increases the stability of the standard)
better reflects the continuum of health risks posed by increasing SO2
concentrations (i.e. the percentage of asthmatics affected and the
severity of the response increases with increasing SO2 concentrations;
REA section 10.5.3).

In considering specific concentration-based forms, the REA recognized
the importance of: 1) minimizing the number of days per year that an
area could exceed the level of the standard and still attain the
standard; 2) limiting the prevalence of 5-minute peaks of SO2; and 3)
providing a stable regulatory target to prevent areas from frequently
shifting in and out of attainment.  Given this, the REA focused on 98th
and 99th percentile forms averaged over 3 years.  The REA first noted
that in most locations analyzed, the 99th percentile form of a 1-hour
daily maximum standard would correspond to the 4th highest daily maximum
concentration in a year, while a 98th percentile form would correspond
approximately to the 7th to 8th highest daily maximum concentration in a
year (REA, Table 10-5 and Thompson, 2009).  In addition, results from
the REA air quality analysis suggested that at a given SO2 standard
level, a 99th percentile form is appreciably more effective at limiting
5-minute peak SO2 concentrations than a 98th percentile form (REA,
section 10.5.3 and Figures 7-27 and 7-28).  For example, the REA
reported that compared to the same standard with a 99th percentile form,
a 98th percentile 1-hour daily maximum standard set at a level of 100
ppb allows for on average, an estimated 90 and 74% more days per year
when SO2 concentrations would likely exceed the 200 and 400 ppb
benchmark values respectively (REA, section 10.5.3 and Figure 7-28).
Moreover, in the counties selected for analysis in the REA air quality
assessment, the estimated number of benchmark exceedances using a 98th
percentile 1-hour daily maximum standard set at a level of 200 ppb was
similar to the corresponding 99th percentile standard set at a level of
250 ppb (REA section 10.5.3 and Tables 7-11 through 7-14).  Similarly,
the estimated number of benchmark exceedances considering a 98th
percentile standard set at a level of 100 ppb fell within the range of
benchmark exceedances estimated for 99th percentile standards set at
levels of 100 and 150 ppb (id).

As an additional matter, the REA compared trends in 98th and 99th
percentile design values, as well as design values based on the 4th
highest daily maximum from 54 sites located in the 40 counties selected
for the detailed air quality analysis (REA section 10.5.3 and Thompson,
2009).  These results suggested that at the vast majority of sites,
there would have been similar changes in 98th and 99th percentile design
values over the last ten years (i.e. based evaluating overlapping three
year intervals over the last ten years; see REA Figure 10-1 and
Thompson, 2009).  These results also demonstrated that design values
based on the 4th highest daily maximum are virtually indistinguishable
from design values based on the 99th percentile (REA Figure 10-1 and
Thompson, 2009).  As part of this analysis, all of the design values
over this ten year period for all 54 sites were aggregated and the
standard deviation calculated (REA Figure 10-2 and Thompson, 2009).
Results demonstrated similar standard deviations – i.e. similar
stability -- based on aggregated 98th or aggregated 99th percentile
design values over the ten year period (see REA Figure 10-2 and Thompson
2009).

Considering the evidence and air quality analyses presented above, the
REA concluded that a concentration-based form provides the best
protection against the health risks posed by increasing SO2
concentrations (REA, section 10.5.3).  Moreover, the REA found that at a
given standard level, a 99th percentile or 4th highest daily maximum
form provides appreciably more public health protection against 5-minute
peaks than a 98th percentile or 7th - 8th highest daily maximum form
(REA, section 10.5.3).  In addition, over the last 10 years and for the
vast majority of the sites examined, there appears to be little
difference in 98th and 99th percentile design value stability (REA,
section 10.5.3).  Thus, the REA ultimately concluded that consideration
should be given primarily to a 1-hour daily maximum standard with a 99th
percentile or 4th highest daily maximum form (REA, section <IP_ADDRESS>).

b.	CASAC views

CASAC agreed with the importance of considering the public health
protection provided by the combination of form and level.  Moreover,
CASAC was in general agreement with the forms being considered.  In a
letter to the Administrator, CASAC stated: “there is adequate
information to justify the use of a concentration-based form averaged
over 3 years” (Samet 2009, p. 16).  Moreover, when considering 98th
vs. 99th percentile forms, CASAC encouraged EPA to consider analyses in
the REA (and perhaps additional analyses) with respect to the number of
days per year 98th vs. 99th percentile forms would allow SO2
concentrations to exceed the selected level.  CASAC also encouraged EPA
to consider analyses such as those presented above with respect to the
number exceedences of 5-minute benchmarks given 98th vs. 99th percentile
forms at a given standard level (Samet 2009).

c.	Administrator’s conclusions on form

When considering alternative forms, the Administrator notes and agrees
with the views expressed in the REA and the recommendations from CASAC,
as described above.  In particular, she agrees that the standard should
use a concentration-based form averaged over three years in order to
give due weight to years when 1-hour SO2 concentrations are well above
the level of the standard, than to years when 1-hour SO2 concentrations
are just above the level of the standard.  The Administrator agrees
further, for the reasons given above, that a 99th percentile (or 4th
highest) form could be appreciably more protective than a 98th (or 7th
or 8th highest) form, and thus, should be utilized.  Given these
considerations, and in light of the specific range proposed for level
below, the Administrator proposes to adopt either a 99th percentile or a
4th highest form, averaged over 3 years.

4.	Level

In assessing the level of a one-hour standard with either a 99th
percentile or 4th highest average form (averaged over three years in
either case) to propose, the Administrator has considered the broad
range of scientific evidence assessed in the ISA, including the
epidemiologic studies and controlled human exposure studies, as well as
the results of air quality, exposure, and risk analyses presented in the
REA.   In light of this body of evidence and analyses, the Administrator
reiterates that it is necessary to provide increased public health
protection for at-risk populations against an array of adverse
respiratory health effects related to short-term (i.e., 5 minutes to 24
hours) exposures to ambient SO2.  In considering the most appropriate
way to provide this protection, the Administrator is mindful of the
extent to which the available evidence and analyses can inform a
decision on the level of a standard.  Specifically, the range of
proposed standard levels discussed below is informed by epidemiologic
and controlled human exposure studies.

a.	Evidence-based considerations

Evidence-based considerations take into account the full body of
scientific evidence assessed in the ISA.  When considering the extent to
which this scientific evidence can inform a decision on the level of a
1-hour standard, it is important to note that SO2 concentrations
represent different measures of exposure when drawn from experimental
versus epidemiologic studies.  Concentrations of SO2 tested in
experimental studies, such as controlled human exposure studies,
represent exposure concentrations in the breathing zone of the
individual test subjects.  In cases where controlled human exposure
studies report effects, those effects are caused directly by exposure to
a specified concentration of SO2.  In contrast, concentrations of SO2
drawn from epidemiologic studies are often based on ambient monitoring
data.  SO2 concentrations recorded at these ambient monitors are used as
surrogates for the distribution of SO2 exposures across the study area
and over the time period of the study.

Since the last review, there have been more than 50 peer reviewed
epidemiologic studies published worldwide dealing with SO2 exposure and
effects.  Overall, the ISA concluded that these studies provide evidence
of an association between ambient SO2 concentrations and respiratory
symptoms, as well as ED visits and hospitalizations for all respiratory
causes and asthma (ISA, section 3.1.4).  Moreover, the ISA indicates
that many of these epidemiologic studies have reported that children and
older adults may be at increased risk for SO2- associated adverse
respiratory effects (ISA, section 5.2).  In assessing the extent to
which these studies and their associated air quality information can
inform the level of a new 99th percentile 1-hour daily maximum standard
for the U.S., the REA considered U.S. and Canadian air quality
information to be most relevant.  EPA sent a request to the authors of
U.S. and Canadian epidemiologic studies (studies were identified from
Tables 5-4 and 5-5 of the ISA) for 99th (and 98th) percentile 1-hour
daily maximum SO2 concentrations from the monitor recording the highest
SO2 level in the location and time period corresponding to their studies
(see Thompson and Stewart (2009)).  Air quality information was received
from authors of both U.S. and Canadian studies; however, as noted in the
REA (REA, section 5.5), SO2 concentrations reported for Canadian studies
are not directly comparable to those reported for studies in the U.S.
because SO2 levels reported for Canadian analyses represent the average
1-hour daily maximum level across multiple monitors in a given city (see
REA Figure 5-5), rather than the concentration from the single monitor
that recorded the highest SO2 concentration (see Thompson and Stewart,
2009).   Thus, the REA noted that SO2 concentrations associated with
Canadian studies would be relatively lower (potentially significantly
lower) than those levels presented for U.S. epidemiologic studies, and
therefore the REA focused on 99th percentile air quality information
from U.S. studies for informing potential 1-hour standard levels.

 Figures 1 to 4 present 99th (and 98th) percentile 1-hour daily maximum
SO2 concentrations from ten U.S. epidemiologic studies (some of which
were conducted in multiple cities) of ED visits and hospital admissions
(Figures 5-1 to 5-4 in the REA).  The REA noted that this information
provides the most direct evidence for effects in cities with particular
99th percentile 1-hour SO2 levels, and hence, was of particular
relevance for identifying standard levels that could protect against the
SO2 concentrations observed in these studies.  The air quality
information presented in these figures generally shows that positive
associations between ambient SO2 concentrations and ED visit and
hospitalizations have been reported in cities where 99th percentile
1-hour daily maximum SO2 concentrations ranged from approximately 50 –
460 ppb.  More specifically, seven of these studies were in cities where
99th percentile 1-hour daily maximum SO2 concentrations ranged from
approximately 75 -150 ppb.  Among these epidemiologic studies in the
range of 75 – 150 ppb, there is a cluster of three studies
demonstrating statistically significant results in multi-pollutant
models with PM.  Specifically, in epidemiologic studies conducted in the
Bronx, NY (NYDOH 2006,) and in NYC, NY (Ito et al., 2007), the SO2
effect estimate remained positive and statistically significant in
multi-pollutant models with PM2.5 when 99th percentile 1-hour daily
maximum SO2 levels were 78 and 82 ppb respectively. (ISA, Table 5-5).
Moreover, in an epidemiologic study conducted in New Haven, CT (Schwartz
et al., 1995), the SO2 effect estimate remained positive and
statistically significant in a multi-pollutant model with PM10 when the
99th percentile 1-hour daily maximum SO2 concentration was 150 ppb.  The
REA noted that although statistical significance in co-pollutant models
is an important consideration, it is not necessary for appropriate
consideration of and reliance on such epidemiologic evidence.  However,
as noted earlier, there is special sensitivity in this review in
disentangling PM-related effects (especially sulfate PM) from
SO2-related effects in interpreting the epidemiologic studies; thus,
these studies are of particular relevance here, lending strong support
both to the conclusion that SO2 effects are generally independent of PM
(ISA, section 5.2) and that these independent adverse effects of SO2
have occurred in cities with 1-hour daily maximum, 99th percentile
concentrations in the range of 78 - 150 ppb.

In addition to the study locations where SO2 concentrations ranged from
75 - 150 ppb, the REA noted that two epidemiologic studies included
cities reporting positive associations between ambient SO2 levels and ED
visits when 99th percentile 1-hour daily maximum SO2 concentrations were
approximately 50 ppb (Wilson et al., (2005) in Portland, ME and Jaffe et
al., (2003) in Columbus, OH).  These studies reported generally positive
and sometimes statistically significant results using single pollutant
models (Figures 1 and 2), and did not evaluate potential confounding
through the use of multi-pollutant models.  Nonetheless, these studies
provide limited evidence of an association between ED visits and 99th
percentile 1-hour daily maximum SO2 concentrations in locations where
SO2 levels were approximately 50 ppb.  Finally, the REA noted that
studies conducted in Cleveland and Cincinnati, OH (Schwartz et., al.
1996 and Jaffe et., al. 2003) reported positive associations between ED
visits and hospital admissions when 99th percentile 1-hour daily maximum
SO2 concentrations ranged from 170 – 457 ppb (REA, section 5.5).  The
REA found the SO2 level in Cincinnati (Jaffe et., al. 2003; REA section
5.5) to be of particular concern.  The 99th percentile 1-hour daily
maximum SO2 level in Cincinnati was > 400 ppb (Figure 2), which in 5-10
minute controlled human exposure studies, was an SO2 concentration range
consistently shown to result in clearly adverse health effects in
exercising asthmatics (i.e., decrements in lung function accompanied by
respiratory symptoms).

ncentrations ≥ 78 ppb).  Moreover, standard levels at or below 75 ppb
recognize the limited evidence from two epidemiologic studies reporting
mostly positive and sometimes statistically significant associations in
single pollutant models when 99th percentile 1-hour daily maximum SO2
concentrations were approximately 50 ppb (Wilson et al., (2005) in
Portland, ME and Jaffe et al., (2003) in Columbus, OH; see Figures 1 and
2).

Figure 1.  Effect estimates for U.S. all respiratory ED visit studies
and associated 98th and 99th percentile 1-hour daily maximum SO2 levels.

Figure 2.  24-hour effect estimates for U.S. asthma ED visit studies and
associated 98th and 99th percentile 1-hour daily maximum SO2 levels.

Figure 3.  1-hour effect estimates for U.S. asthma ED visit studies and
associated 98th and 99th percentile 1-hour daily maximum SO2 levels.

Figure 4.  24-hour effect estimates for U.S. hospitalization studies and
associated 98th and 99th percentile 1-hour daily maximum SO2 levels.

ons ≥ 400 ppb, decrements in lung function are often statistically
significant at the group mean level, and are frequently accompanied by
respiratory symptoms (ISA, Table 5-1).

Controlled human exposure studies have also demonstrated decrements in
lung function in exercising asthmatics following 5 - 10 minute SO2
exposures starting as low as 200 - 300 ppb in free-breathing chamber
studies.  At concentrations ranging from 200 - 300 ppb, the lowest
levels tested in free breathing chamber studies, approximately 5 - 30%
percent of exercising asthmatics are likely to experience moderate or
greater decrements in lung function.  Moreover, although these
individuals experienced lung function decrements, they were not
frequently accompanied by respiratory symptoms and at these SO2
concentrations, group mean changes in lung function have not been shown
to be statistically significant.  However, the ISA and REA noted that
for evident ethical reasons, the subjects participating in the
controlled human exposure studies described above do not necessarily
represent the most SO2 sensitive individuals (e.g., the most severe
asthmatics). Thus, the REA found it is reasonable to anticipate that
individuals who are more sensitive to SO2 would have a greater response
at 200 - 300 ppb SO2, and/or would respond to SO2 concentrations even
lower than 200 ppb.  Similarly, the REA noted that there is no evidence
to suggest that 200 ppb represents a threshold below which no adverse
respiratory effects occur.  In fact, limited evidence from two
mouthpiece exposure studies suggests that exposure to 100 ppb SO2 can
result in small decrements in lung function.

 concentrations ≥ 400 ppb, and appreciable protection against 5-minute
SO2 concentrations ≥ 200 ppb (REA, section 10.5.4).

b.	Air quality, exposure and risk-based considerations

In evaluating the extent to which 99th percentile 1-hour daily maximum
alternative standard levels limit 5-minute SO2 concentrations ≥ 400
and ≥ 200 ppb, the REA first considered key results of the air quality
analysis.  As previously noted, the results generated from the air
quality analysis were from 40 counties and considered a broad
characterization of national air quality and human exposures that might
be associated with these 5-minute SO2 concentrations (see section II.C).
 However, there is uncertainty associated with the assumption that SO2
air quality measured at fixed site monitors can serve as an adequate
surrogate for total exposure to ambient SO2.  Actual exposures might be
influenced by factors not considered in this analysis including small
scale spatial variability in ambient SO2 concentrations (which might not
be captured by the network of fixed-site ambient monitors) and
spatial/temporal variability in human activity patterns.

  Table 2 reports the maximum mean number of days per year 5-minute
daily maximum SO2 levels would be expected to exceed a given 5-minute
potential health effect benchmark level in any of the 40 counties
included in the air quality analysis, given air quality simulated to
just meet the current, and potential alternative 99th percentile 1-hour
daily maximum standards analyzed in the REA.  In addition, although not
directly analyzed in the REA, these tables include air quality results
given a 99th percentile 1-hour daily maximum standard at 75 ppb; this
concentration was included in these tables because as mentioned above,
the epidemiologic evidence suggested consideration of a standard level
at or below 75 ppb.  Table 2 shows that at standard levels ranging from
50 – 100 ppb, there would be at most two days per year when
statistically estimated 5-minute SO2 concentrations in these counties
exceed the 400 ppb benchmark, while at standard levels of 150 ppb and
above there is a marked increase in the maximum number of days per year
the 400 ppb benchmark is exceeded.  Similar trends are seen with respect
to the 300 ppb benchmark level.  With respect to the 200 and 100 ppb
benchmarks, the 50 ppb standard is clearly the most effective at
limiting these 5-minute SO2 concentrations.  However, compared to
standards at 150 ppb and above, standards in the range of 75 - 100 ppb
would allow considerably less exceedence of the 200 and 100 ppb
benchmarks.  Additional and more detailed results from the air quality
analysis can be found in Chapter 7 of the REA.

Table 2.   Maximum mean number of days per year in any of the counties
included in the air quality analysis when 5-minute daily maximum SO2
concentrations exceed the 100, 200, 300, and 400 ppb potential health
effect benchmark values given air quality adjusted to just meet the
current standards, or alternative 99th percentile 1-hour daily maximum
standards.

Exposure Benchmarks

(5-minute exposures)	Air Quality Scenarios

	Just Meeting Current Standards	99th percentile 1-hour Daily Maximum
Standards

50 ppb

	75 ppb

	100 ppb

	150 ppb

	200 ppb

	250 ppb

400 ppb	102	0	(0 - 2)	2	7	13	18

300 ppb	130	0	(0 - 5)	5	13	20	27

200 ppb	171	2	(2 - 13)	13	24	42	69

100 ppb	234	13	(13- 43)	43	93	133	180

While the air quality analysis results presented in Table 2 used
estimated 5-minute SO2 concentrations as a surrogate for exposure, the
results from the exposure analysis considered the likelihood that an
asthmatic at elevated ventilation rate would come into contact with a
5-minute SO2 concentration at or above a given benchmark level one or
more times per year.  As previously noted, this resource intensive
analysis was performed for St. Louis and Greene County, MO, but results
from the St. Louis analysis were found to be more informative with
respect to informing standard levels given that the St. Louis results:
1) suggested that the current standards were not adequate to protect
public health; and 2) likely provide useful insights into exposures and
risk for other urban areas in the U.S. with similar population and SO2
emissions density (i.e., areas where SO2 exposures are more likely).

 ≥ 400 ppb per year.   Similarly, a standard at 150 ppb is estimated
to protect ~ 99% of asthmatic children at moderate or greater exertion
from experiencing at least one SO2 exposure ≥ 400 ppb.  Compared to
standards ranging from 50 -150 ppb, standards at 200 and 250 ppb are
estimated to allow appreciably more exposures ≥ 400 ppb (Table 3).
With respect to the 300 ppb benchmark, standards at 50, 75, and 100 ppb
provide similar protection, while there is a marked increase in
exposures of asthmatic children at moderate or greater exertion at
standard levels ≥ 150 ppb (Table 3).  Considering the 200 ppb
benchmark level, it is estimated that 1-hour standard levels ranging
from 50 – 100 ppb limit 5-minute SO2 exposures ≥ 200 ppb
considerably more than 1-hour standard levels ≥ 150 ppb.  More
specifically, standards in the range of 50 – 100 ppb are estimated to
protect approximately 97 to > 99% of asthmatic children at moderate or
greater exertion from experiencing at least one 5-minute exposure ≥
200 ppb per year, while standards ranging from 150 – 250 ppb are
estimated to protect approximately 60 to 88% of these children from
experiencing at least one 5-minute SO2 exposure ≥ 200 ppb per year.
Finally, similar to the air quality analysis, a standard at 50 ppb is
clearly most effective at limiting 5-minute SO2 exposures ≥ 100 ppb.
Additional and more detailed results from the exposure assessment can be
found in Chapter 8 of the REA.

Table 3.   Estimated percent of asthmatic children in St. Louis at
moderate or greater exertion expected to experience at least one
5-minute exposure above the 100, 200, 300, and 400 ppb potential health
effect benchmark levels given air quality adjusted to just meet the
current standards, or alternative 99th percentile 1-hour daily maximum
standards.

Exposure Benchmarks

(5-minute exposures)	Air Quality Scenarios

	Just Meeting Current Standards	99th percentile 1-hour Daily Maximum
Standards

50 ppb

	75 ppb

	100 ppb

	150 ppb

	200 ppb

	250 ppb

400 ppb	24%	< 1%	< 1%	< 1%	~1%	2.7%	6.3%

300 ppb	43.8%	< 1%	< 1%	< 1%	2.7%	8%	16%

200 ppb	73.1%	< 1%	(~1 to 2.7%)	2.7%	11.6%	24.5%	40%

100 ppb	96.7%	2.7%	(2.7 to 24.5%)	24.5%	54.5%	73.6%	84.8%

≥ 100% increase in sRaw per year, while standards around and below 75
ppb would be estimated to provide exposed asthmatic children with
protection approaching 99% or greater.  Additional and more detailed
risk analyses can be found in Chapter 9 of the REA.

Table 4. Estimated percent of asthmatic children in St. Louis at
moderate or greater exertion expected to experience a ≥ 100% increase
in sRaw given air quality adjusted to just meet either the current
standards, or alternative 99th percentile 1-hour daily maximum
standards.

Air Quality Scenarios

Just Meeting Current Standards	99th percentile 1-hour Daily Maximum
Standards

	50 ppb

	75 ppb

	100 ppb

	150 ppb

	200 ppb

	250 ppb

19.1 -19.2%	0.4 -  0.9%	(0.4 - 2.9%)	2.1 -2.9%	4.6 - 5.4%	7.4 –

  8.1%	10.4 -10.9%

c.	Observations based on evidence and risk-based considerations

The policy assessment chapter of the REA considered the scientific
evidence and the air quality, exposure, and risk information as they
relate to considering alternative 1-hour SO2 standards that could be
judged to be requisite to protect public health with an adequate margin
of safety.  This evidence and information supports the following
conclusions:

Given the U.S. epidemiologic evidence and their associated air quality
levels (see Figures 1-4), 99th percentile 1-hour standard levels at and
below 75 ppb should be considered to provide protection against the
effects reported in these studies

Based on the air quality and exposure results, 1-hour standard levels in
the range of 50 - 100 ppb should be considered to substantially limit
5-minute SO2 concentrations ≥ 400 ppb and appreciably limit 5-minute
SO2 concentrations ≥ 200 ppb.

Based on the air quality and exposure results, compared to a 1-hour
standard in the range of 50 - 100 ppb, a 1-hour standard level at 150
ppb would be expected similarly limit 5-minute SO2 concentrations ≥
400 ppb, but would limit 5-minute SO2 concentrations ≥ 200 ppb
considerably less.

If significant weight is placed on the uncertainties in the
epidemiologic and controlled human exposure evidence, levels up to 150
ppb could be considered, recognizing the questions as to the adequacy of
protection that would be raised by levels at the higher end of this
range.

If significant weight is placed on the uncertainty that participants in
controlled human exposure studies do not represent the most SO2
sensitive individuals, standard levels down to 50 ppb could be
considered.

d.	CASAC views

CASAC expressed their views on potential levels for a standard in a
letter to the EPA Administrator (Samet, 2009) within the context of
their review of the 2nd draft REA, which also contained the draft policy
assessment chapter.  In drawing conclusions regarding the level of a
short-term standard, CASAC considered the scientific evidence evaluated
in the ISA, the air quality, exposure, and risk results presented in the
2nd draft REA, and the evidence- and risk-based considerations presented
in the policy assessment chapter of the 2nd draft REA.  CASAC concurred
with the conclusion from the policy assessment chapter for a range of
standard levels beginning at 50 ppb: “ [that chapter 10] clearly
provides sufficient rationale for the range of levels beginning at a
lower limit of 50 ppb” (Samet 2009, p. 16).  For instance, CASAC has
previously indicated that EPA should consider in its analyses the
uncertainty that asthmatics participating in controlled human exposure
studies do not represent the most SO2 sensitive asthmatics (Henderson
2008 p. 6).  With respect to the upper end of the range, CASAC stated,
“an upper limit of 150 ppb posited in Chapter 10 could be justified
under some interpretations of weight of evidence, uncertainties, and
policy choices regarding margin of safety,” (Samet 2009, p. 16)
although the letter did not provide any indication of what
interpretations, uncertainties, or policy choices might support
selection of a level as high as 150 ppb.  Further, CASAC stated that
“the draft REA appropriately implies that levels greater than 150 ppb
are not adequately supported” (id).  Moreover, CASAC stated that:
“the panel agrees that the posited range of 50 to 150 ppb and the
exposition of factors to consider when comparing values within the range
are appropriately conveyed (Samet 2009, p. 16).”

e.	Administrator’s conclusions on level for a 1-hour standard

In considering the appropriate level for an SO2 standard based on the
3-year average of the 99th percentile (or 4th highest) 1-hour daily
maximum SO2 concentration, the Administrator has considered the broad
body of scientific evidence and air quality, exposure, and risk
information.  As explained in the decision to propose an averaging time
of one-hour, she draws from that evidence and information the need to
protect at-risk populations from the array of health effects that have
been linked to short-term (5-minutes to 24-hour) SO2 exposures.

Specifically, the Administrator has considered the extent to which a
variety of standard levels would be expected to protect at-risk
populations against decrements in lung function, respiratory symptoms,
and respiratory-related emergency department visits and hospital
admissions.  The Administrator notes that these health endpoints are
logically linked together in that the evidence for decrements in lung
function and respiratory symptoms in asthmatics is part of the body of
clinical evidence that the ISA recognized as supporting the plausibility
of associations between ambient SO2 and the respiratory morbidity
endpoints (i.e., respiratory symptoms, emergency department visits, and
hospital admissions) reported in epidemiologic studies.

In making judgments regarding the weight to place on the scientific
evidence and air quality, exposure, and risk information, the
Administrator notes the following:

Based on epidemiologic and controlled human exposure evidence, the ISA
concluded that there is a causal relationship between short-term
(5-minutes to 24-hour) SO2 exposure and respiratory morbidity (ISA,
section 5.2).

Epidemiologic studies provide evidence of the most serious effects
following exposure to SO2 (i.e., ED visits and hospitalizations for all
respiratory causes and asthma), and the ISA reported generally
consistent associations were observed between these serious health
outcomes and ambient SO2 concentrations particularly among children and
older adults.

Limited available evidence indicates that the effect of SO2 on
respiratory health outcomes appears to be generally robust and
independent of the effects of gaseous copollutants, including NO2 and
O3, as well as particulate copollutants, particularly PM2.5 (ISA,
section 5.2; p. 5-9). Given this conclusion, along with conclusions from
the ISA regarding the consistency and the coherence of results across
the relatively large number of SO2 epidemiologic studies and the
evidence from controlled human exposure studies (ISA, section 5.2), the
Administrator has judged it appropriate to place weight on the
epidemiologic results.

Controlled human exposure studies provide direct evidence of a
relationship between 5-10 minute exposure to SO2 and respiratory
morbidity.  These studies report that 5-10 minute exposures to SO2 can
result in decrements in lung function and/or respiratory symptoms in
exercising asthmatics.  Given the potential public health importance of
this effect, due to the large size of the asthmatic population in the
U.S., the Administrator judges that it is also appropriate to place
weight on this evidence when identifying an appropriate range of levels
to propose.

The results of the risk and exposure analyses presented in the REA
provide information on the potential public health implications of
setting the standard at different levels.  The Administrator
acknowledges the uncertainties associated with these analyses which, as
discussed in the REA, could result in either over- or underestimates of
SO2-associated health risks.  However, she also notes that those
uncertainties should be similar across different air quality simulations
within the air quality, exposure, and risk analyses.  Therefore, the
Administrator judges that these analyses are useful for considering the
relative levels of public health protection that could be provided by
specific standard levels.

After considering the scientific evidence and the air quality, exposure,
and risk information (see sections II.B, II.C, and II.F.4.a, b), the
Administrator concludes that the strongest support is for a standard
level in the range of 50 - 100 ppb.  The Administrator’s rationale in
reaching this conclusion is provided below.

In considering the epidemiologic evidence, the Administrator notes that
there have been more than 50 peer reviewed epidemiologic studies
evaluating SO2 published worldwide and overall, the ISA concluded that
these studies provide evidence of an association between ambient SO2
concentrations and respiratory symptoms, as well as ED visits and
hospitalizations for all respiratory causes and asthma (ISA, section
<IP_ADDRESS>).  The Administrator also finds that in assessing the extent to
which these studies and their associated air quality information can
inform the level of a new a 99th percentile 1-hour daily maximum
standard, U.S. and Canadian air quality information is most relevant.
However as described in section II.F.4.a, SO2 concentrations reported
for Canadian studies are not directly comparable to those reported for
U.S. studies.  That is, concentrations reported for Canadian analyses
represent the average 99th percentile 1-hour daily maximum level across
multiple monitors in a given city (see REA Figure 5-5), rather than the
concentration from the single monitor that recorded the highest SO2
level (see Thompson and Stewart, 2009).   Thus, the Administrator
focused on 99th percentile air quality information from U.S. studies for
informing potential 1-hour standard levels.

 The Administrator notes that Figures 1 to 4 include 99th percentile
1-hour daily maximum SO2 concentrations from ten U.S. epidemiologic
studies of ED visits and hospital admissions (Figures 5-1 to 5-4 in the
REA).  The Administrator agrees with the REA finding that this
information provides the most direct evidence for effects in cities with
particular 99th percentile 1-hour SO2 levels, and hence, is of
particular relevance for identifying standard levels that could protect
against the SO2 concentrations observed in these studies.  These figures
report mostly positive associations between ambient SO2 concentrations
and ED visit and hospital admissions in locations where 99th percentile
1-hour daily maximum SO2 levels ranged from 50 - 460 ppb.  Moreover,
within this broader range of SO2 concentrations, seven of these studies
were in locations where 1-hour SO2 concentrations were in the range of
75 -150 ppb.  The Administrator particularly notes the cluster of three
epidemiologic studies between 78 - 150 ppb where the SO2 effect estimate
remained positive and statistically significant in multi-pollutant
models with PM (NYDOH (2006), Ito et al., (2007), and Schwartz et. al,
(1995).  The Administrator also notes the limited evidence from two
epidemiologic studies employing single pollutant models that found
mostly positive, and sometimes statistically significant associations
between ambient SO2 and ED visits in locations where 1-hour SO2
concentrations were approximately 50 ppb (Figures 1 and 2).  Taken
together, the Administrator concludes that the epidemiologic studies
described above provide support for consideration of a 99th percentile
1-hour daily maximum standard at or below 75 ppb.

ions ≥ 400 ppb results in health effects that are clearly adverse.
That is, exposure to ≥ 400 ppb SO2 for 5-10 minutes results in
moderate or greater decrements in lung function that are frequently
accompanied by respiratory symptoms.  The Administrator also focused on
exceedences of the 200 ppb benchmark because at 200 – 300 ppb (the
lowest SO2 concentrations tested in free-breathing chamber studies),
moderate or greater decrements in lung function occur in approximately
5-30% of exercising asthmatics.  The Administrator further notes that
while concentrations as low as 200 ppb have not been frequently
accompanied by respiratory symptoms, these effects are considered
adverse when considering CASAC advice and ATS guidelines.

Considering the results of the air quality, exposure, and risk analyses
as they relate to limiting 5-minute SO2 concentrations ≥ 400 ppb and
≥ 200 ppb, and being mindful that more severe effects occur following
5-minute SO2 exposures ≥ 400 ppb, the Administrator finds the most
support for 1-hour standards up to 100 ppb to protect against 5-minute
SO2 exposures.  She notes that the 40 county air quality analysis
estimates that a 100 ppb standard would allow at most 2 days per year on
average when statistically estimated 5-minute daily maximum SO2
concentrations exceed the 400 ppb benchmark, and at most 13 days per
year on average when 5-minute SO2 concentrations exceed the 200 ppb
benchmark (Table 2).  Furthermore, given a 100 ppb standard level, most
counties in the air quality analysis would experience 0 and ≤ 3 days
per year on average when 5-minute SO2 concentrations exceed the 400 and
200 ppb benchmarks respectively (see REA, Tables 7-14 and 7-12).  In
addition, the St. Louis exposure analysis estimates that a 1-hour
standard at 100 ppb would protect > 99% of asthmatic children at
moderate or greater exertion from experiencing at least one 5-minute
exposure ≥ 400 ppb per year, and approximately 97% of asthmatic
children at moderate or greater exertion from experiencing at least one
exposure ≥ 200 ppb per year.  Finally the Administrator notes that the
St. Louis risk assessment estimates that a 1-hour standard at 100 ppb
would protect about 97- 98% of exposed asthmatic children from
experiencing at least one moderate or greater lung function response
(defined as a ≥ 100% increase in sRaw).

 concentrations ≥ 400 and ≥ 200 ppb.

In considering the lower or upper ends of the proposed range further,
the Administrator notes that if it is emphasized that the most severe
effects associated with SO2 exposure are those reported in epidemiologic
studies (i.e. respiratory-related ED visits and hospitalizations), then
consideration could be given to a standard level as low as 50 ppb.  The
Administrator finds that a 99th percentile 1-hour daily maximum standard
at 50 ppb would provide a margin of safety against the air quality
levels observed in the cluster of three epidemiologic studies observing
statistically significant positive associations between SO2 and
respiratory-related ED visits and hospitalizations in multi-pollutant
models with PM (i.e. 99th percentile 1-hour daily maximum SO2
concentrations ≥ 78 ppb).  It also recognizes limited evidence from
two epidemiologic studies that reported positive associations between
ambient SO2 and ED visits in cities when SO2 concentrations were
approximately 50 ppb, while also recognizing that these studies did not
look for potential confounding by co-pollutants through multi-pollutant
models.

≥ 100 ppb per year (Table 3; REA, Figure 8-19).

ions ≥ 200 ppb.  However, it should be noted that although a 1-hour
standard at 100 ppb would be estimated to protect about 97% of asthmatic
children at moderate or greater exertion in St. Louis from experiencing
at least one 5-minute SO2 exposure ≥ 200 ppb, a 1-hour standard at 50
ppb is estimated to be more effective, protecting > 99% of these
children from 5-minute SO2 exposures ≥ 200 ppb.

As stated above, the Administrator proposes to set the level of a new
1-hour standard between 50 ppb and 100 ppb.  In so doing, the
Administrator proposes to place emphasis on reported findings from both
epidemiologic and controlled human exposure studies.   The Administrator
solicits comment on the appropriateness of this proposed range of
standard levels as well as on the approach she has used to identify the
range.  Specifically, the Administrator solicits comment on the
following:

The weight she has placed on the epidemiologic evidence, the controlled
human exposure evidence, and the air quality, exposure, and risk
information, and the uncertainties associated with each of these.

The most appropriate part of the proposed range in which to set the
standard level given the available scientific evidence, and air quality,
exposure, and risk information, and the uncertainties associated with
each.

With regard to the proposed range of standard levels, the Administrator
notes that the proposed range is consistent with CASAC advice that there
is clearly sufficient evidence for consideration of standard levels
starting at 50 ppb (Samet 2009).  Moreover, the proposed range is in
agreement with the broader CASAC indication that standards up to 150 ppb
“could be justified under some interpretations of weight of evidence,
uncertainties, and policy choices regarding margin of safety” (Samet
2009, p. 16).

 exposures ≥ 200 ppb.

5.	Implications for retaining or revoking current standards

The REA recognized that the particular level selected for a new 1-hour
daily maximum standard would have implications for reaching decisions on
whether to retain or revoke the current 24-hour and annual standards.
That is, with respect to SO2-induced respiratory morbidity, the lower
the level selected for a 99th percentile 1-hour daily maximum standard,
the less additional public health protection the current standards would
be expected to provide.  As previously mentioned (see section II.E.3),
CASAC expressed a similar view following their review of the 2nd draft
REA: “assuming that EPA adopts a one hour standard in the range
suggested, and if there is evidence showing that the short-term standard
provides equivalent protection of public health in the long-term as the
annual standard, the panel is supportive of the REA discussion of
discontinuing the annual standard” (Samet 2009, p. 15).  With regard
to the current 24-hour standard, CASAC was generally supportive of using
the air quality analyses in the REA as a means of determining whether
the current 24-hour standard was needed in addition to a new 1-hour
standard to protect public health.  CASAC stated: “the evidence
presented [in REA Table 10-3] was convincing that some of the
alternative one-hour standards could also adequately protect against
exceedances of the current 24-hour standard” (Samet 2009, p. 15).

In accordance with the REA findings and CASAC recommendations mentioned
above, the Administrator notes that the 1-hour standards being proposed
(i.e., 99th percentile 1-hour daily maximum SO2 standards at 50 – 100
ppb) would have the effect of maintaining 24-hour and annual SO2
concentrations generally well below the levels of the current 24-hour
and annual NAAQS (see REA Tables 10-3 and 10-4 and REA Appendix Tables
D-3 to D-6).  Thus, if a new 99th percentile 1-hour daily maximum
standard is set in the proposed range of 50 – 100 ppb, than the
Administrator proposes to revoke the current 24-hour and annual
standards.  However, if a standard is set at a level >100 ppb and up to
150 ppb, then the Administrator proposes to retain the existing 24-hour
standard, recognizing that a 99th percentile 1-hour daily maximum
standard at 150 ppb would not have the effect of maintaining 24-hour
average SO2 concentrations below the level of the current 24-hour
standard in all locations analyzed (see REA Appendix Table D-4).
However, the Administrator would revoke the current annual standard
recognizing: 1) 99th percentile 1-hour daily maximum standards in the
range of 50 -150 ppb would maintain annual average SO2 concentrations
below the level of the current annual standard (see REA Table 10-4 and
REA Appendix tables D-5 and D-6); and 2) the lack of sufficient evidence
linking long-term SO2 exposure to adverse health effects.

G.	Summary of proposed decisions on the primary standard

For the reasons discussed above, and taking into account information and
assessments presented in the ISA and REA as well as the advice and
recommendations of CASAC, the Administrator proposes that the current
24-hour and annual standards are not requisite to protect public health
with an adequate margin of safety.  The Administrator proposes to
establish a new 1-hour standard that will afford increased protection
for asthmatics and other at-risk populations against an array of adverse
respiratory health effects related to short-term (5-minutes to 24-hours)
SO2 exposure.  These effects include increased decrements in lung
function (defined in terms of sRaw and FEV1), increases in respiratory
symptoms, and related serious indicators of respiratory morbidity
including emergency department visits and hospital admissions for
respiratory causes.

Specifically, the Administrator proposes to set a new short-term primary
SO2 standard with a 1-hour (daily maximum) averaging time and a form
defined as the 3-year average of the 99th percentile or the 4th highest
daily maximum concentration.  The level for the new standard is proposed
to be within the range of 50 - 100 ppb.  The Administrator also solicits
comment on levels as high as 150 ppb.  In addition to setting a new
1-hour standard in the proposed rage of 50 – 100 ppb, the
Administrator proposes to revoke the current 24-hour and annual
standards recognizing that a 1-hour standard set in the proposed range
of 50 - 100 ppb will have the effect of generally maintaining 24-hour
and annual SO2 concentrations well below the levels of the current
24-hour and annual standards.  Moreover, the Administrator notes that
there is little health evidence to support an annual standard for the
purpose of protecting against health effects associated with long-term
SO2 exposures.

III.	Proposed Amendments to Ambient Monitoring and Reporting
Requirements

EPA is proposing changes to the ambient air monitoring, reporting, and
network design requirements for the SO2 NAAQS. This section discusses
the changes we are proposing that are intended to support the proposed
1-hour NAAQS, and the possible retention of the existing 24-hour NAAQS
depending on the selected level of the 1-hour NAAQS, as described in
Section II above.  Ambient SO2 monitoring data are used to determine
whether an area is in violation of the SO2 NAAQS. Ambient SO2 monitoring
data are collected by state, local, and tribal monitoring agencies
(“monitoring agencies”) in accordance with the monitoring
requirements contained in 40 CFR parts 50, 53, and 58.

A.	Monitoring Methods

To be used in a determination of compliance with the SO2 NAAQS, SO2 data
must be collected using either a Federal Reference Method (FRM) or a
Federal Equivalent Method (FEM) as defined in 40 CFR Parts 50 and 53.
The current monitoring methods in use by most State and local monitoring
agencies are FEM analyzers based on the ultraviolet fluorescence (UVF)
measurement principle.  These continuous analyzers were implemented into
the SO2 monitoring networks in the early 1980s, and the current manual
FRM for SO2 is no longer used for field monitoring. The current list of
all approved FRMs and FEMs capable of providing ambient SO2 data for use
in attainment designations may be found on the EPA website
http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent
-methods-list.pdf.

For reasons explained subsequently, EPA proposes to establish a new FRM
for measuring SO2 in the ambient air.  This proposed new FRM for SO2
would be an automated method based on UVF (the same type of analyzers
now in widespread use), and it would be specified in the form of a
reference measurement principle and a calibration procedure.  It would
be in a new Appendix A-1 to 40 CFR Part 50.  Analyzers approved as FRMs
for SO2 after the effective date of the final rule would be subject to
performance specifications and other requirements set forth in 40 CFR
Part 53, under associated amendments proposed for Part 53. The existing
FRM for SO2 (a wet-chemical, manual method) would be retained for some
period of time, thereby permitting continued use of currently designated
FEMs to avoid any disruption to existing SO2 monitoring networks.

1. 	Background

FRMs, as set forth in several appendices to 40 CFR Part 50, serve either
or both of two primary purposes.  The first is to provide a specified,
definitive methodology for routinely measuring concentrations of various
ambient air pollutants for comparison to the NAAQS in Part 50 and for
other air monitoring objectives.  The second is to provide a standard of
comparison for determining equivalence to the specified reference method
of alternative and perhaps more practical pollutant measurement methods
(FEMs) that can be used in lieu of the FRM for routine monitoring.

	Some of the FRMs contained in appendices to Part 50 (such as the
current SO2 FRM) are manual methods that are completely specified within
their respective appendices.  Others (such as the ozone FRM) are in the
form of a measurement principle and associated calibration procedure
that must be implemented in a commercial FRM analyzer model.  Such FRM
analyzers must be tested and shown to meet explicit performance and
other requirements that are set forth in 40 CFR Part 53 (Ambient Air
Monitoring Reference and Equivalent Methods).  Each of these analyzer
models is considered to be an FRM only upon specific designation as such
by EPA under the provisions of Part 53.

	From time to time, as pollutant measurement technology advances, the
reference methods in these Part 50 appendices need to be assessed to
determine if improved or more suitable measurement technology has become
available to better meet current FRM needs as well as potential future
FRM requirements.  Such new technology can either be presented to EPA
for evaluation by an FEM applicant under §53.16, or (as in this case)
EPA can originate the process itself as provided in §53.7.  If, after
reviewing a new methodology, the Administrator determines that the new
methodology is substantially superior, §53.16 of Part 53 provides for
supersession of FRMs under these circumstances.

	The FRM for measuring SO2 in the ambient air was promulgated on April
30, 1971 (36 FR 8186), in conjunction with EPA’s establishment
(originally as 42 CFR Part 410) of the first national ambient air
quality standards (NAAQS) for six pollutants (including sulfur dioxide)
as now set forth in 40 CFR Part 50.  This SO2 FRM is specified in
Appendix A of Part 50 and identified as the pararosaniline method.  It
is a manual, wet-chemical method requiring sample air to be bubbled
through an absorbing reagent (tetrachloromecurate), which is then
returned to a laboratory for chemical analysis.  At the time of its
promulgation, the method was considered the best available method and
was in considerable use for monitoring SO2 in the air.  However, newly
developed automated continuous analyzers approved as FEMs rapidly
supplanted use of this manual method for air monitoring in the U.S.  By
the 1990’s, the FRM was no longer used at all in domestic air
monitoring (EPA, 2009b), and since then the method has been used mainly
as a comparison reference method for the testing and designation of
candidate FEMs for SO2 in accordance with 40 CFR Part 53.

The pararosaniline manual FRM has served its role for many years, but
now a better method is needed that more fully meets the needs of
contemporary SO2 monitoring.  The existing FRM is primarily a 24-hour
integrated method, whereas a 1-hour SO2 FRM measurement capability would
be needed to implement the proposed 1-hour SO2 NAAQS.   Existing FEM
analyzers can and do provide 1-hour measurement capability, but EPA
wishes to facilitate the approval of new monitoring technologies as
well.  While the existing manual reference method can produce 1-hour
averages, it is clearly impractical for routine use in making 1-hour SO2
measurements.  Also, the 1-hour mode of the manual method is not a good
standard for approving new FEMs with 1-hour measurement capability,
because scores of 1-hour measurements would be needed during equivalency
testing.  Further, the existing FRM is cumbersome to use and requires a
mercury-containing reagent that is potentially hazardous to operators or
to the environment if it is mishandled.

These operational shortcomings suggest that the existing FRM should be
replaced with a more suitable methodology.  Fortunately, the existing
SO2 instrumental measurement technique based on the UVF measurement
principle offers superior performance and substantial operational
advantages, as reported in an FRM evaluation for EPA produced by
Research Triangle Institute (Rickman, 1987).  Analyzers using this
technique can well provide the needed detection limits, precision, and
accuracy and fulfill other purposes of an FRM, including use as an
appropriate standard of reference for testing and designation of new FEM
analyzers.  After reviewing these factors, EPA has determined that a
new, automated FRM for SO2 based on the UVF measurement principle should
be adopted.  EPA is proposing to add the new FRM in a new Appendix A-1
to Part 50.

	In association with the proposed new FRM, EPA is also proposing to
update the performance-based requirements for FEM SO2 analyzers
currently in 40 CFR Part 53.  These requirements were established in the
1970’s, based primarily on the wet-chemical measurement technology
available at that time.  Those initial requirements have become
significantly outdated and should be modified to match current
technology, particularly because they would apply to new FRM analyzers
under the proposed new FRM.  The better instrumental performance
available with the proposed new UVF reference method technique allows
the performance requirements for SO2 in 40 CFR Part 53 to be made more
stringent for both FRM and FEM analyzers (EPA, 2009c).

2. 	Proposed New FRM Measurement Technique

Since the 1970’s, a variety of measurement principles have been
successfully used to produce continuous analyzers for SO2, some of which
have qualified for EPA designation as equivalent methods (found at
http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent
-methods-list.pdf).  These include methods based on ultraviolet
fluorescence, flame photometry, differential optical absorption
spectroscopy, coulometric and conductometric techniques, and second
derivative ultraviolet absorption spectrometry.  Although some of these
techniques saw considerable utilization in the 1970’s, their use
dwindled after the introduction of UVF analyzers because of various
shortcomings such as non-specificity for SO2, susceptibility to
interferences, marginal performance, or operational disadvantages (e.g.
requiring hydrogen gas or wet-chemical reagents).  Consequently, the UVF
technique has emerged as the clearly dominant measurement technique for
SO2, providing a majority of the domestic air monitoring data obtained
over the last three decades, and virtually 100% of the current
monitoring data (EPA, 2009b).  As the proposed new reference method, the
UVF technique would be specified in performance-based form, with a
generic reference measurement principle and associated calibration
procedure in a new Appendix A-1 to 40 CFR Part 50.  Associated
performance requirements applicable to candidate UVF FRM analyzers would
be specified in 40 CFR Part 53.  This form of the FRM is consistent with
that specified for FRMs for CO, O3, and NO2 in Appendices C, D, and F
(respectively) to 40 CFR Part 50.

	Reasonable commercial availability of high quality analyzers utilizing
the reference measurement principle that can be offered by multiple
manufacturers, ideally over many years, is an important aspect of any
new reference measurement principle.  EPA has designated more than a
dozen UVF analyzers as equivalent to the current reference method over
the last 30 years.  Although most of the early model UVF analyzers are
no longer in production, many have been replaced by redesigned and
improved models, and entirely new models continue to become designated
as FEMs.  Currently, more than a half-dozen designated FEM models
offered by multiple manufacturers are commercially available.  The
widespread use of the method has three important technical advantages
for an FRM:  1) a variety of analyzer models are available and will
likely continue to be available from multiple manufacturers for many
years to come, 2) analyzer manufacturers have had (and continue to have)
a strong marketing incentive to improve, refine, perfect, and continue
to market such analyzers, and 3) the number of accumulated UVF field
monitoring datasets (including related QC data) provide an extensive,
available performance track record that can be evaluated to assess the
performance of the analyzers in actual monitoring use.

	The only other equivalent method measurement technique that has even a
small representation among currently available FEM analyzers is the
differential optical absorption spectrometric method.  The open-path
nature of this method (measurement of pollutants in the open air without
a closed measurement cell) is not suitable for many of the purposes of a
reference method.  Further, this method is only available as two product
models from two manufacturers, and very few State and local monitoring
agencies are using such analyzers.

	The UVF technique is not without some imperfections as a reference
method.  Analyzers utilizing the technique are, to a limited degree,
susceptible to interference from aromatic hydrocarbon species and
potentially other compounds at existing levels or levels that may occur
at many monitoring sites.  However, analyzer manufacturers have
developed very effective ways to reduce these potential limitations,
including careful selection of wavelengths, optimum optical design, and
sample air scrubbers, such that typical interferences are minimal.

	All UVF analyzers that have been designated as SO2 FEMs have been
tested and shown to meet the existing performance requirements of 40 CFR
Part 53.  These include required testing for both positive and negative
potential interferents, minimum level of measurement, zero and span
drift, and precision.  The results of these tests have been submitted to
EPA and are in the archived FEM applications for these analyzers.  Many
newer models substantially exceed those requirements, with sensitivities
down to less than 1 ppb, and typically commensurate levels of signal
noise, precision, and zero drift (EPA, 2009c).  In addition, UVF
analyzers can accommodate a wide range of concentration measurement
ranges.  They are quite well suited to measure high, short-term SO2
concentrations near sources, and they can also be used to measure
trace-level concentrations in clean areas.

For these reasons, EPA has decided to propose a new automated SO2 FRM
based on the UVF measurement technology.  EPA is confident that
commercially available UVF instrument models would provide capability to
serve not only current monitoring and FRM applications but anticipated
monitoring and FRM needs well into future years.  EPA solicits comment
on the proposal to promulgate an FRM for SO2 that would be an automated
method based on ultraviolet fluorescence, which would be specified in
the form of a reference measurement principle and calibration procedure,
as stated here, and contained in a new Appendix A-1 to 40 CFR Part 50.

3. 	Technical Description of the Proposed UVF FRM

The proposed new reference method is based on automated measurement of
the intensity of the characteristic fluorescence released by SO2 in an
ambient air sample when irradiated by ultraviolet light.   The SO2
fluorescence produced is also in the ultraviolet range, but is measured
at a longer wavelength.  An analyzer implementing this measurement
principle would include a measurement cell, an ultraviolet light source
of appropriate wavelength, an ultraviolet detector system with
appropriate wavelength sensitivity, and a pump and flow control system
for sampling the ambient air.  Generally, the analyzer also requires a
means to reduce concentrations of aromatic hydrocarbons and possibly
other compounds (depending on target wavelengths and other parameters
used) in the air sample to control for potential measurement
interferences.  The analyzer is calibrated by referencing the
instrumental fluorescence measurements to SO2 standard concentrations
traceable to a NIST (National Institute of Standards and Technology)
primary standard for SO2.  This generic description of the FRM would be
contained in Appendix A-1 to 40 CFR Part 50 and would be coupled with
explicit analyzer performance requirements specified in Subpart B of 40
CFR Part 53.  To qualify as an FRM, an analyzer model based on this
principle would have to be tested in accordance with test procedures in
Subpart B Part 53 and shown to meet the performance requirements
specified in that Subpart.  EPA could then designate the analyzer model
as an FRM analyzer, as provided in Part 53.

4. 	Implications to Air Monitoring Networks

Under §53.16, EPA must consider the benefits of a proposed supersession
of an existing reference method, the potential economic consequences of
such action for State and local monitoring agencies, and any disruption
of State and local air quality monitoring programs that might result
from such action.  Supersession of an existing reference method, as
described in §53.16, presumes that the existing FRM would be deleted
from Part 50 and replaced with a new FRM, and that all equivalent
methods based on the old FRM would be cancelled.  In the case of SO2,
essentially all current domestic air monitoring activity is carried out
using FEM UVF analyzers.  Cancellation of the FEM designations of all
these analyzers now would be potentially very disruptive to State,
local, and other monitoring networks, even though §53.16 alludes to a
possible transition period to allow monitoring agencies some period of
time to replace cancelled FEM analyzers.

EPA recognizes that these existing SO2 FEMs are providing monitoring
data that are adequate for the current and the proposed SO2 NAAQS and
for many other purposes, and there appears to be no need or purpose
served by their withdrawal.  Therefore, in this case, EPA proposes
instead to retain the existing manual FRM for SO2 and to promulgate an
entirely new automated FRM for SO2.  The new FRM description would be
contained in a new Appendix A-1 to 40 CFR Part 50, and the existing FRM
would be re-codified as Appendix A-2 to 40 CFR Part 50, with both
reference methods coexisting.  Following adoption of the new Appendix
A-1, new language proposed for §53.2(a) and (b) would provide that new
FRM and FEM analyzers for SO2 be designated only with reference to the
proposed new Appendix A-1.  At the same time, retention of the existing
SO2 reference method will preclude the need to cancel the designations
of all existing FEMs for SO2.

Under this proposal, no monitoring agencies would be required to change
their SO2 monitoring procedures as a result of the proposed changes, so
it would have no economic costs for implementation and no disruptive
effects on state, local, or tribal air quality monitoring programs.
Further, since UVF FEM analyzers have been in dominant use for many
years, no bias or discontinuity in any aspect of the monitoring data
obtained subsequently would result from the proposed change in the SO2
reference methodology.

In conjunction with the proposed new FRM, EPA is also proposing to adopt
updated performance requirements in 40 CFR Part 53, applicable to both
FRM and FEM analyzers, consistent with the automated methods and in
anticipation of future NAAQS needs.  This would ensure that, going
forward, all new SO2 monitors would have improved performance.   EPA
believes that the proposal to retain the existing FRM while adding the
new FRM would provide for a smooth, evolutionary transition from the
older, manual FRM to the new, modern, automated FRM and FEM technology
and the associated better performance requirements, with no immediate
impact to current monitoring activities.  For purposes of comparing SO2
monitoring data to the SO2 NAAQS, the EPA believes that the UVF FEMs are
appropriate for continued use under the current standards and under the
option being considered for a new 1-hour averaged primary SO2 NAAQS.
After several years, at a time when either a new SO2 NAAQS would require
higher monitoring data quality or there would be no further potential
for disruption to monitoring agencies, EPA would plan to withdraw the
older reference method and it’s associated FEMs.

5. 	Proposed Revisions to 40 CFR Part 53

Several amendments associated with the proposed new SO2 reference
measurement principle are proposed to 40 CFR Part 53.  The most
significant of these would update the performance requirements for both
new FRM and new FEM analyzers for SO2, as set forth in proposed revised
Table B-1.  Based on typical performance capabilities available for UVF
analyzers, EPA is proposing to reduce the allowable noise from 5 ppb to
1 ppb, the lower detectable limit from 10 ppb to 2 ppb, and the
allowable interference equivalent limits from ±20 ppb to ±5 ppb for
each interferent and from 60 ppb to 20 ppb for the total of all
interferents.  Also, EPA proposes to change the allowable zero drift
limits from ±20 ppb  to ±4 ppb, and to delete the specified limits for
span drift at 20% of the upper range limit (URL) for SO2 analyzers.
Review of FEM analyzer performance test results has shown that the 20%
URL span limit requirements are unnecessary because drift performance
requirements are adequately covered by the zero drift and 80% URL span
drift limits.  EPA proposes to change the lag time allowed from 20 to 2
minutes and change the rise and fall time limits from 15 to 2 minutes.
For precision, EPA proposes to change the form of the precision limit
specifications from ppm to percent (of the URL) for SO2 analyzers and to
set the limit at 2 percent for both 20% and 80% of the URL.  Two percent
is equivalent to 10 ppb for the standard (500 ppb) range, which is
equivalent to the existing limit value for precision at 20% of the URL,
but would be a reduction from 15 ppb to 10 ppb for the limit value at
80% of the URL.  This change in units from ppm (or ppb as given here) to
percent makes the requirement responsive to higher and lower measurement
ranges. Also, a new footnote is proposed to be added to Table B-1 to
clarify how noise tests are to be carried out for candidate analyzers
having an adjustable or automatic time constant capability.

EPA recognizes that SO2 monitoring needs can vary widely, from
monitoring background levels in pristine areas to measuring short-term
(1-hour) or even very short-term (less than 1-hour) high-level averages
in the vicinity of substantial sources of SO2.  To address the need for
more sensitive, lower measurement ranges for SO2 analyzers, EPA is
proposing a separate set of performance requirements that would apply
specifically to narrower measurement ranges, i.e. ranges extending from
zero to concentrations less than 0.5 ppm.  These additional requirements
are listed in the proposed revised Table B-1.  A candidate analyzer that
meets the Table B-1 requirements for the standard measurement range (0
to 0.5 ppm) could optionally have one or more narrower ranges included
in its FRM or FEM designation by further testing to show that it meets
these supplemental, narrower-range requirements.

	At the other (high) end of the concentration measurement spectrum,
another related change proposed for §53.20 would allow optional
designation of measurement ranges for SO2 up to 2 ppm rather than 1 ppm
as is now permitted, and designation of these higher ranges would be
applicable to both FRM and FEM analyzers.  Such higher ranges are often
needed for measurement of short-interval SO2 averages.  Finally, EPA is
proposing to clarify in §53.20 that optional testing for auxiliary
higher or lower measurement ranges (for all gaseous pollutants) may
include tests for only some of the performance parameters, since the
test results for the other performance parameters carried out for the
standard measurement range would be technically applicable and adequate
for the higher and/or lower ranges as well.

	EPA believes that these changes in performance requirements are
appropriate, based on analyzer performance data available from analyzer
manuals and recent FEM applications. EPA solicits comments especially
from UVF instrument users and manufacturers on these proposed changes,
particularly in regard to whether they are reasonable, appropriate, of
significant benefit, and achievable without undue cost.  Comments are
also requested on such issues as the trade off between a high
measurement range and the need for adequate resolution at concentrations
near the annual NAAQS, a similar trade off between noise level and
response time (some analyzers allow these parameters to be adjusted by
the operator or may adjust them automatically based on the rate of
change of the concentration level), and whether such performance
parameters should be addressed in more detail in 40 CFR Part 53.  In
particular, should SO2 analyzer requirements address the potential need
for faster measurement response time to permit more accurate monitoring
of short-term intervals such as 5-minute or 10-minute averages, and are
the special performance requirements EPA is proposing for measuring very
low levels (trace levels) of SO2 appropriate and effective?

	Another significant change proposed to 40 CFR Part 53 would add some
low and medium level 1-hour comparability tests to the Subpart C
comparability test requirements, as specified in Table C-1.  These would
help to ensure that the 1-hour measurement performance of candidate FEMs
are adequate, relative to the FRM.  Also, EPA proposes to amend Table
A-1 in Subpart A to reflect the new FRM description in proposed new
Appendix A-1 of 40 CFR Part 50.  This table would also be amended to
correct some printing errors in the current table as well as to add new
entries related to the new FRM for lead in PM10 that was recently
promulgated.  Other minor changes would be necessary in the wording of a
few sections of Subparts A and B due to the proposed change in the
nature of the SO2 FRM from a manual to an automated method or to update
the language.  These changes are reflected in the proposed regulatory
text section of this notice.

	EPA proposes additional minor revisions to Tables B-2 and B-3 of
Subpart B.  The changes proposed to Table B-2 would update some of the
analytical methods for generation or verification of SO2 and interferent
test concentrations and their associated references.  Similarly, Table
B-3 would be updated to add a specific listing for ultraviolet
fluorescent methods and to add a few additional interferent test species
for some other measurement techniques that have been found from
experience to be needed.

B.	Network Design

1. 	Background

The basic objectives of an ambient monitoring network, as noted in 40
CFR Part 58 Appendix D, include 1) providing air pollution data to the
general public in a timely manner, 2) supporting compliance with ambient
air quality standards and emissions strategy development, and 3)
providing support for air pollution research.  The SO2 network was
originally deployed to support implementation of the SO2 NAAQS
established in 1971.  Although the SO2 standard was established in 1971,
EPA did not establish uniform minimum monitoring requirements for SO2
monitoring until May 1979.  From the time of the implementation of the
1979 monitoring rule, through 2008, the SO2 network has steadily
decreased in size from approximately 1496 sites in 1980 to the
approximately 488 sites operating in 2008 (Watkins and Thompson, 2009).
The reduction in network size is due in part to the change in the source
sector contributions to the overall SO2 inventory and the general
decline of ambient SO2 levels over time. In the early decades of the SO2
network, particularly the 1970’s, there were a wider variety of more
ubiquitous SO2 sources in urban areas, including residential coal and
oil furnaces, when compared to the stationary source, Electric
Generating Unit (EGU) dominated inventories of today (see below).  The
situation in the 1970s led to a network design keyed on population, an
appropriate approach at the time considering the close proximity of
sources and people, particularly in urban, residential settings (Watkins
and Thompson, 2009).

An analysis of the approximately 488 monitoring sites comprising the
current (2008) SO2 monitoring network (Watkins and Thompson, 2009)
indicates that just under half (~46%) of the sites in the current SO2
network are reported to be for the assessment of concentrations for
general population exposure.  As for the present day inventory, the 2005
NEI (  HYPERLINK "http://www.epa.gov/ttn/chief/net/2005inventory.html"
http://www.epa.gov/ttn/chief/net/2005inventory.html ) indicates that SO2
emissions from EGUs contribute approximately 70% of the anthropogenic
non-fire SO2 emissions in the U.S.  However, only approximately one
third (~35%) of the network is reported to be addressing locations of
maximum (highest) concentrations, likely linked to a specific source or
group of sources such as EGUs.

The current network is reporting hourly data to EPA’s Air Quality
System (AQS) database, as required in 40 CFR §58.12, since the network
utilizes the continuous UVF FEM, which can provide aggregated data on
the order of minutes, hours, or longer.  The routine submittal of hourly
data by state, local, and tribal air monitoring agencies to AQS is
suitable for use in comparison to both of the current primary 24-hour
and annual NAAQS.  There are a few monitoring agencies who also report
5-minute data voluntarily to AQS. The current network is sited at a
variety of spatial scales, however a majority of the network, just over
sixty percent, is sited at the neighborhood spatial scale (Watkins and
Thompson, 2009).  Although there are 488 SO2 monitors operating in the
network (2008 estimate), there are currently no minimum monitoring
requirements for SO2 in 40 CFR part 58 Appendix D, other than the
following three: 1) SO2 must be monitored at National Core (NCore)
monitoring sites (discussed below), 2) the EPA Regional Administrator
must approve the removal of  any existing monitors, and 3) that any
ongoing SO2 monitoring must have at least one monitor sited to measure
the maximum concentration of SO2 in that area.  These current CFR
requirements regarding the existing monitors are intended to ensure that
any ongoing measurements are for maximum concentrations relative to both
the 24-hour and the annual standards.

The SO2 monitors that are required as part of the National Core
monitoring network (NCore) were not required solely for providing direct
support of the SO2 NAAQS.  The monitoring rule promulgated in 2006 (71
FR 61236) and codified at 40 CFR Part 58 and its Appendices created the
NCore multi-pollutant network to support integrated air quality
management data needs.  Further, NCore is intended to establish
long-term sites providing data for air quality trends analysis, model
evaluation, and, for urban sites, tracking metropolitan area statistics.
 To do this, NCore sites are required to measure various pollutants,
including SO2, but are not sited to monitor maximum concentrations of
SO2.  NCore sites provide data representing concentrations at the
broader neighborhood and urban spatial scales and the data from the
NCore sites will be compared to the NAAQS.  The NCore network, which
will be fully implemented by January 1, 2012, will result in
approximately 83 sites, each with SO2 monitors, with approximately 60
sites being located in urban areas.

As set out in detail in section II.B of the preamble, there is a causal
relationship between short-term SO2 exposure and respiratory morbidity,
with ‘short-term’ meaning exposures from 5-10 minutes up to and
including 24-hours.  This finding was based primarily on results from
controlled human exposure studies of 5-10 minutes as well as
epidemiologic studies using mostly 1-hour daily maximum and 24-hour
average SO2 concentrations.   Importantly, the ISA described the
controlled human exposure studies of 5-10 minutes as being the
“definitive evidence” for this conclusion (ISA, section 5.2).  In
addition, when describing epidemiologic studies observing positive
associations between ambient SO2 and respiratory symptoms, the ISA
stated “that it is possible that these associations are determined in
large part by peak exposures within a 24-hour period” (ISA, section
5.2 at p. 5-5).  The ISA also stated that the respiratory effects
following 5 - 10 minute SO2 exposures in controlled human exposure
studies provide a basis for a progression of respiratory morbidity that
could result in increased ED visits and hospital admissions (ISA,
section 5.2).  Thus, the monitoring network to be proposed to support
the proposed NAAQS should be focused on identifying the expected maximum
short-term concentrations in any particular area.

The ISA (Section 2.1) indicates that point (i.e. stationary) sources
account for approximately 95% of the total anthropogenic SO2 emissions
in the U.S.  According to the 2005 National Emissions Inventory (
HYPERLINK "http://www.epa.gov/ttn/chief/net/2005inventory.html"
http://www.epa.gov/ttn/chief/net/2005inventory.html ), electrical
generating units (EGUs) emit approximately 70% of the anthropogenic
non-fire SO2 emissions in the U.S.  The 2005 NEI indicates that the
total anthropogenic non-fire emission inventory of SO2 is approximately
14,742 thousand tons per year.  Of those 14,742 thousand tons per year
of emitted SO2, approximately 85% were emitted by stationary sources
that emit 100 or more tons per year (comprising approximately 1,928 of
the 32,988 facilities listed in the 2005 NEI).  This information
indicates that a relatively small number (6 %) of all SO2 emitting
stationary sources are responsible for a large majority of the total
anthropogenic emissions inventory (85%) in the U.S.  Therefore, monitors
sited to reflect locations of expected maximum concentrations should be
primarily oriented towards locations influenced by one or a cluster of
high SO2 emitting sources.

As noted in the key observations of the exposure analysis of the REA
(REA, Section 8.12), there are a variety of factors that influence
exposures to ground-level concentrations in a given area, including
population density and proximity to sources, emissions density in an
area, and source specific emission parameters such as stack height,
among other factors.  In general, however, it is expected that any
short-term peaks that may occur in an area are more likely to occur
nearer to a source or sources, or in an area where multiple sources are
significantly contributing to increased ground level concentrations (an
area with high emissions density).  Given that maximum ground-level
concentrations of SO2 are usually directly traceable to specific
sources, or a cluster of sources, the proposed network design would
support implementation of the proposed 1-hour SO2 NAAQS by targeting
maximum ground-level concentrations in areas of both higher population
and higher emissions.

2. 	Proposed Changes

In conjunction with the proposed 1-hour primary NAAQS and (if EPA should
adopt a standard at the upper end of the range of levels for which the
Agency is soliciting comment) the potential retention of the current
24-hour NAAQS, we are proposing a number of changes to the SO2
monitoring network.  As just noted, there are currently no minimum
monitoring requirements for SO2, except for NCore sites.  The proposal
for a new 1-hour NAAQS necessitates the re-introduction of minimum
monitoring requirements.  An analysis of the approximately 488
monitoring sites comprising the current (2008) SO2 monitoring network
(Watkins and Thompson, 2009) indicates that just under half (~46%) of
the sites in the current SO2 network are reported to be for the
assessment of concentrations for general population exposure.  The
current network was not originally deployed to address short-term, peak
concentrations, such as those locations nearer to stationary sources or
in areas of higher emissions densities, where maximum hourly and 5-10
minute concentrations are likely to occur. The Agency has data
indicating that only about one third of the existing SO2 network may be
source-oriented monitors and/or sited in locations of expected maximum
concentrations (Watkins and Thompson, 2009).

 To fully support the proposed SO2 NAAQS, the monitoring network needs
to identify where short-term, peak ground- level concentrations – i.e.
concentrations from 5 minutes to one hour (or potentially up to 24
hours) -- may occur. Due to the multiple variables that affect ground
level SO2 concentrations caused by one or more stationary sources, it is
difficult to specify a source specific threshold, algorithm, or metric
by which to require monitoring in a rule such as this.  To achieve this
goal, EPA is proposing a two-pronged network design to ensure that
States perform a sufficient amount of monitoring of ambient
concentrations of SO2 to determine attainment of the proposed SO2 NAAQS
that intends to prevent exposure to peak concentrations.  EPA
anticipates this two-pronged network would require approximately 345
monitors nationwide, providing data for comparison with both the
proposed 1-hour and the 24-hour standard if retained.  The network would
be wholly comprised of monitors sited at locations of expected maximum
hourly concentrations.  EPA is proposing that the two prongs of this SO2
network design would be distributed based on: 1) a Population Weighted
Emissions Index (PWEI) and 2) the state-level contribution to the
national, non-fire SO2 emissions inventory.  EPA notes that although we
propose that the network include a minimum number of required monitors,
State, local, and tribal agencies always have the right to conduct
additional monitoring above the minimum requirements.  If those monitors
satisfy all applicable requirements in 40 CFR Part 58, the data from
those monitors would be comparable to the NAAQS.  EPA estimates that
one-half to two-thirds of the monitors in the existing network
(excluding any currently operating NCore sites) may have to be moved to
become source- oriented and sited at locations of expected maximum
short-term concentrations of SO2.

We solicit comment on whether the estimated 345 monitors required by
this proposal, distributed based on the two network design components
presented below, are too few, too many, or suitable to establish a
minimum network sufficient to meet the monitoring objectives noted
above, including supporting compliance with the proposed  1-hour SO2
NAAQS.

We propose that State and, where appropriate, local air monitoring
agencies submit a plan for deploying SO2 monitors in accordance with the
proposed requirements presented below by July 1, 2011. We also propose
that the SO2 network being proposed be physically established no later
than January 1, 2013. Considering the proposed timeline and criteria
presented in the network design, we solicit comment on whether
alternative dates would be more appropriate as deadlines for state and
local monitoring agencies to submit a monitoring plan.  We also solicit
comments on whether alternative dates would be more appropriate as
deadlines for state and local monitoring agencies to physically deploy
monitors.

a. 	Population Weighted Exposure Index (PWEI) Triggered Monitoring

The EPA proposes that the first prong of the ambient SO2 monitoring
network account for SO2 exposure by requiring monitors in locations
where population and emissions may lead to higher potential for exposure
to peak hourly SO2 concentrations.  In order to do this, EPA has
developed a Population Weighted Exposure Index (PWEI) that uses
population and emissions inventory data at the CBSA level to assign
required monitoring for a given CBSA (population and exposure being
obvious relevant factors in prioritizing numbers of required monitors).
The PWEI for a particular CBSA is calculated by multiplying the
population (using the latest Census Bureau estimates) of a CBSA by the
total amount of SO2 emissions in that CBSA. The CBSA emission value is
in tons per year, and is calculated by aggregating the county level
emissions for each county in a CBSA.  We then normalize by dividing the
resulting product of CBSA population and CBSA SO2 emissions by 1,000,000
to provide a PWEI value in millions of people-tons per year.  This
calculation has been performed for each CBSA and has been posted in the
docket as “CBSA PWEI Calculation, 2009”.  EPA believes that using
this PWEI metric to inform where monitoring is required is more
appropriate for the SO2 network design than utilizing a population-only
type of approach, so that we may focus monitoring resources in areas of
the country where people and emission sources are in greater proximity.
In addition, EPA’s initial view is that this PWEI concept is
appropriate for SO2 but is not necessarily transferrable to the other
criteria pollutants.  From a very broad vantage point, SO2 is
exclusively a primarily emitted pollutant (i.e. unlike PM2.5 and ozone
there is no secondary formation of SO2), is almost exclusively emitted
by stationary sources (unlike NO2, CO, PM2.5, thoracic coarse PM, and
ozone), and is a gaseous pollutant which is somewhat more subject to
transport (unlike Pb in the Total Suspended Particulate (TSP) and PM10
size fractions).

We propose that the first prong of the SO2 network design require
monitors in CBSAs, according to the following criteria. Those CBSAs with
a PWEI value of 1,000,000 or more would be required to operate three
monitors within that CBSA.  Those CBSAs with a PWEI value between
1,000,000 and 10,000 would be required to operate two monitors within
that CBSA. Those CBSAs with a PWEI value between 10,000 and 5,000 would
be required to operate one monitor within that CBSA.  EPA believes that
the monitors required within these breakpoints provide a reasonable
minimum number of monitors in a CBSA that consider the combination of
population and emissions that exist in a given CBSA.  This proposed
requirement is based on factors that will ensure highly populated areas
will receive monitoring even if the emissions in that area are moderate,
due to the fact that the greater population creates increased potential
for exposure to those moderate sources.  Alternatively, this proposed
requirement ensures that those areas with higher emissions or emission
densities, with moderate or modest populations will receive monitoring
since those increased emissions are likely to have a significant impact
on whatever population may exist nearby.

EPA estimates that these criteria will result in 231 required sites in
132 CBSAs.

We propose that monitors triggered in this first prong of the network
design must be sited in locations of expected maximum 1-hour
concentrations, at the appropriate spatial scale, within the boundaries
of a given CBSA. EPA also proposes that when state, local, or tribal
agencies make selections for monitoring sites from candidate site
locations, they shall prioritize monitoring where the maximum expected
hourly concentrations occur in greater proximity to populations. EPA
believes that states will likely need to use some form of quantitative
analysis, such as modeling, data analysis, or saturation studies to aid
in determining where ground-level SO2 maxima may occur in a given CBSA.
We also propose that the selection of these sites must be approved by
the EPA Regional Administrators, again based on objective quantitative
criteria which would necessarily vary case-by-case (and which therefore
would not be appropriate to further specify by rule). See further
discussion of this issue in section c. below.   EPA solicits comment on
1) the use of the Population Weighted Emissions Index (PWEI), 2) the
PWEI calculation method, 3) the PWEI breakpoints that correlate to a
number of required monitors, 4) requiring that the monitors shall be
sited in locations of expected maximum 1-hour concentration, and 5)
requiring that the monitoring sites must be approved by the EPA Regional
Administrators based on quantitative objective criteria.

b. 	State-level Emissions Triggered Monitoring

As the second prong of the SO2 network, we are proposing to require an
additional monitor or monitors in each state, allocated by state-level
SO2 emissions.  In this prong, EPA proposes to distribute approximately
114 sites, based on the corresponding percent contribution of each
individual state to the national anthropogenic, non-fire SO2 emission
inventory.  This prong of the network design is intended to allow a
portion of the overall required monitors to be placed where needed,
independent of the PWEI, inside or outside of CBSAs.  EPA proposes to
require monitors, using states as the geographic unit for allocation
purposes, in proportion to a State’s SO2 emissions, i.e., a State with
higher emissions will be required to have a proportionally higher number
of monitors.  The proposed percent contribution of individual states is
based on the most recent NEI, with SO2 emissions being aggregated by
state.  Each one percent out of one hundred percent (after rounding)
would correspond to one required monitor. For example, according to the
2005 NEI, the State of Ohio contributes 8.66% of the total
anthropogenic, non-fire SO2 inventory, which would correspond to
requiring nine monitors to be distributed within Ohio.  Further, EPA
proposes that each state have at least one monitor required as part of
this second prong, even if a particular state contributes less than 0.5%
of the total anthropogenic, non-fire national emissions inventory. As a
result, approximately 114 monitoring sites would be required and
distributed based on state-level SO2 emissions in the most recent NEI,
which in this case, is the 2005 NEI.  EPA solicits comment on the use of
state-level emission inventories based on the most recent NEI to
proportionally distribute approximately one third (~114 sites) of the
required monitoring network.

According to the most recent NEI, for this proposed second prong, we
estimate the state/percent contribution to the national
inventory/required monitor distribution to be:

State or Territory	Percent Contribution to the National SO2 Inventory
Proposed

Number of Required Monitors

Alabama	4.02%	4

Alaska	0.46%	1

Arizona	0.60%	1

Arkansas	0.77%	1

California	1.48%	1

Colorado	0.54%	1

Connecticut	0.23%	1

Delaware	0.58%	1

District of Columbia	0.03%	1

Florida	4.40%	4

Georgia	5.07%	5

Hawaii	0.08%	1

Idaho	0.16%	1

Illinois	3.51%	4

Indiana	7.10%	7

Iowa	1.50%	2

Kansas	1.33%	1

Kentucky	3.88%	4

Louisiana	2.40%	2

Maine	0.25%	1

Maryland	2.58%	3

Massachusetts	1.07%	1

Michigan	3.32%	3

Minnesota	1.05%	1

Mississippi	0.81%	1

Missouri	2.8%	3

Montana	0.26%	1

Nebraska	0.82%	1

Nevada	0.49%	1

New Hampshire	0.43%	1

New Jersey	0.69%	1

New Mexico	0.32%	1

New York	2.65%	3

North Carolina	4.40%	4

North Dakota	1.08%	1

Ohio	8.66%	9

Oklahoma	1.12%	1

Oregon	0.32%	1

Pennsylvania	7.96%	8

Puerto Rico	N/A	1

Rhode Island	0.06%	1

South Carolina	2.06%	2

South Dakota	0.19%	1

Tennessee	2.63%	3

Texas	6.34%	6

Utah	0.35%	1

Vermont	0.05%	1

Virgin Islands	N/A	1

Virginia	2.34%	2

Washington	0.45%	1

West Virginia	3.63%	4

Wisconsin	1.79%	2

Wyoming	0.83%	1

Table 5. State-level Emission Triggered Monitors.  This table shows
state and territory level contributions to the national SO2 inventory
and the corresponding number of monitors required for each state as
proposed in prong 2 of the proposed network design.

EPA proposes siting requirements for this second prong of required
monitors to be the same as those in the first prong: siting in locations
of expected maximum 1-hour concentrations, at the appropriate spatial
scale, within the boundaries of a given state, and prioritizing the
selection of candidate sites where the maximum expected hourly
concentrations occur in greater proximity to populations.  This again
would need to be determined case-by-case using quantitative analysis,
such as modeling, data analysis, or saturation studies to aid in
determining where ground-level SO2 maxima may occur in a given state.
We propose that these monitors can be located in or out of CBSAs, or on
tribal lands.  However, if a monitor required by the second prong is
placed inside a CBSA that already has a requirement for monitoring due
to the first prong of this network design, that monitor would not be
allowed to count towards satisfying the first prong requirements.  We
also propose that the selection of these sites must be approved by the
EPA Regional Administrators, as with the first prong.  The EPA solicits
comment on 1) the use of state-level emission inventories to
proportionally distribute required monitors, 2) requiring each state to
have at least one monitor under this prong of the network design, 3)
requiring all monitors to be sited in locations of expected
maximum1-hour concentration inside or outside of CBSAs or on tribal
land, and 5) requiring that monitoring sites must be approved by the EPA
Regional Administrators based on quantitative objective criteria.

EPA recognizes that emissions inventories change over time, suggesting a
need for periodic review of the monitoring network.  At the same time,
EPA recognizes the advantages of a stable monitoring network.
Therefore, while EPA currently provides for updates of the NEI every 3
years, EPA believes that the current network review requirements under
section 58.10 which requires an annual network plan and recurring 5-year
assessments provide a suitable schedule for planning and assessing the
monitoring network.  Through the 5-year assessments, EPA will be in a
position to review emissions distributions from updated NEIs to assess
whether the monitoring requirements remain appropriate.  EPA solicits
comment on whether such adjustments to the network should be required on
a 5-year cycle that matches the general frequency of network assessments
or the cycle of the next SO2 NAAQS review.

c. 	Monitor Placement and Siting

In the proposed network design, we have proposed that all sites are to
be placed in locations of expected maximum 1-hour concentrations, which
will also likely discern 5-minute peaks as well.  EPA expects that in
general, these locations will be in relative proximity to larger
emitting sources (in tons per year) and/or areas of relatively high
emissions densities where multiple sources may be contributing to peak
ground-level concentrations.  The variability in where such locations
exist relative to the responsible emission source(s) depends on multiple
factors including the tonnage emitted by a source (or group of sources),
stack height, stack diameter, emission exit velocity, emission
temperature, and meteorology.  Depending on these variables, plumes may
heavily fumigate areas immediately downwind of a source, or may never
truly touch down at all, dispersing into ambient air where SO2
concentrations continually decrease with increasing distance away from
the source.  Further, depending on meteorology, an emission plume from
an individual source may cause increased ground-level concentrations at
any heading, relative to the parent source, corresponding to the
prevailing winds.   For example, sources emitting from a relatively
higher stack compared to another source will have increased variability
in the location of where an emission plume may fumigate an area and in
the resultant ground level concentration.  The primary reason for this
variability is because the peak impacts of sources with higher stacks
will generally be farther downwind and may be more variably located than
is the case for sources with shorter stacks. This is illustrated in an
example where a relatively large source with a tall stack height may not
produce exceedingly high ground level concentrations anywhere along its
plume trajectory while a smaller source with a relatively short stack
may cause relatively higher ground level concentrations under the same
meteorological conditions at the same location.

When analyzing a particular source, a state may find multiple locations
where peak ground-level concentrations may occur around an individual
source.  EPA does not intend for multiple monitors to be sited around or
in proximity to one source.  Not siting multiple monitors around, or in
proximity, to one source ensures that more individual sources or groups
of sources will receive attention by the monitoring network.  States
always have the discretion to perform additional monitoring above the
minimum requirements to increase monitoring around a particular source
or group of sources.

Due to the variability of how, when, where, and to what degree a source
or group of sources can contribute to peak, ground-level SO2
concentrations, EPA expects that State and local monitoring agencies
will need to analyze all relevant information, including available
ambient and emissions data, and potentially use air quality modeling or
saturation studies to select appropriate monitoring site locations.
Further, due to the variability in where maximum ground-level
concentrations may occur, the appropriate spatial scales within which a
monitor might be placed include the microscale, middle, and neighborhood
scales, which are defined in 40 CFR Part 58 Appendix D.  EPA believes
that states, in evaluating a source (or group of sources) that
contribute to a peak ground-level SO2 concentration that varies with
space and time, should identify where the highest concentrations are
expected to occur in developing candidate site locations.  EPA proposes
that when state, local, or tribal agencies make selections for
monitoring sites from candidate site locations, they shall prioritize
monitoring where the maximum expected hourly concentrations occur in
greater proximity to populations.  EPA solicits comment on the role of
population exposure in the site selection process.

	d. 	Monitoring required by the Regional Administrator

	In addition to the two prongs of the proposed SO2 network design, we
propose that the Regional Administrator will have discretion to require
monitoring above these minimum requirements under prongs 1 and 2, as
necessary to address situations where the minimum monitoring
requirements are not sufficient to meet monitoring objectives noted
above.  EPA recognizes that the minimum required monitors in the
proposed network design under the two prongs described above are based
on more generalized indicators that may not provide for all the
monitoring that may be necessary in an area.  An example where EPA
envisions requiring an additional monitor might be a case where a source
having modest emissions still has high potential to cause a violation of
the NAAQS in a community or neighborhood.  This situation might occur
where a modest SO2 source has, for example, a low emission stack and/or
is in an area where meteorological conditions cause situations, such as
inversions or stagnation, that might lead to high ground-level
concentrations of SO2.  In this example, such a monitor might be needed
even though a state is fulfilling its monitoring requirements under the
first and second prongs of the proposed network design.  The purpose of
this provision is to monitor in and provide data for otherwise
non-monitored locations that have the potential to exceed the level of
the NAAQS or that are perceived to have higher exposure risks due to
proximity to a source or sources.  In such an example, the Regional
Administrators may make use of any available data including existing
model data, existing data analyses, or screening tools such as AERSCREEN
or SCREEN3, to inform a decision of whether or not a monitor should be
required for a given area or location.  Any such required monitor would
also be vetted, publicly, through a state or local monitoring agency’s
annual monitoring plan in accordance to 40 CFR Part 58, §58.10 (a)(1).
In any case, EPA encourages state, local, and tribal monitoring agencies
to provide input and information to the appropriate Regional
Administrators in determining whether additional monitors are needed and
the locations of such monitors.  We solicit comment on the proposal to
allow Regional Administrators the discretion to require monitoring above
the requirements under prongs 1 and 2 for any area or location where
those monitoring requirements are not sufficient to meet monitoring
objectives.

EPA notes that existing requirements restrict which existing monitors
can be shut down.  These requirements in 40 CFR Part 58 §58.14 (c)
indicate that any SO2 monitor that has not shown attainment during the
previous five years, or has a probability of greater than 10 percent of
exceeding 80 percent of the applicable NAAQS during the next three years
shall not be shut down.  EPA is not reopening or otherwise reconsidering
this provision.  However, this requirement is summarized here because
EPA does not intend for any monitor meeting the above requirements to be
moved by a state, local, or tribal agency in their efforts to meet the
monitoring requirements of prongs 1 or 2 proposed above.

e. 	Alternative Network Design

EPA solicits comments on an alternative network design, including
alternative methods to determine the minimum number of monitors per
state.  We are particularly interested in whether a screening approach
for assessing the likelihood of a NAAQS exceedance could be developed
and serve as a basis for determining the number and location of required
monitors.

More specifically, EPA requests comment on whether it should utilize
existing screening tools such as AERSCREEN or SCREEN 3, which use
parameters such as effective stack height and emissions levels to
identify facilities with the potential to cause an exceedance of the
proposed standard.  For that set of sources, EPA could then require
states to conduct more refined modeling (likely using the AMS EPA