concentrations may be greater during non-daylight periods than
during daylight hours.
---------------------------------------------------------------------------

    Some of the approaches used in these analyses focused on comparing
24-hour and 4-hour light extinction values in each of the 15 urban
areas, whereas other approaches focused on comparisons based on using
aggregated data across the urban areas. Two of these approaches, which
used regressions of city-specific annual 90th percentile light
extinction values or 3-year light extinction design values, gave nearly
identical results and were considered by staff to be most appropriate
for further consideration. These approaches (shown in U.S. EPA, 2011a,
Appendix G, Figures G-7 and G-8, referred to as Approaches A and B)
were preferred by staff based on the high R\2\ values of the
regressions and because the regressions were determined by data from
days with PM2.5 light extinction conditions in the range of
20 to 40 dv. This contrasted with the other approaches that were
influenced by PM2.5 light extinction conditions well below
this range. Based on these analyses (presented in Appendix G of the
Policy Assessment), the Policy Assessment notes that the single
approach thought by staff to be more appropriate for further
consideration (referred to as Approach B in Appendix G) yielded
adjusted 24-hour CPLs of 21, 25, and 28 dv as being levels that are
generally equivalent in an aggregate or central tendency sense to 4-
hour CPLs of 20, 25, and 30 dv.\178\
---------------------------------------------------------------------------

    \178\ To provide some perspective in considering these results
(U.S. EPA, 2011a, Appendix G, Table G-6), the Policy Assessment
notes that 1 dv is about the amount that persons can distinguish
when viewing scenic vistas, and that a difference of 1 dv is
equivalent to about a 10 percent difference in light extinction
expressed in Mm-1.
---------------------------------------------------------------------------

    Two of the approaches yielded not only estimates of generally
equivalent levels on an aggregated basis but also city-specific
estimates (referred to as Approaches C and E in Appendix G) that showed
greater variability than the aggregated estimates. In all cases, the
range of city-specific estimates of generally equivalent 24-hour levels
included the 4-hour level for each of the CPLs of 20, 25, and 30 dv (as
shown in Tables G-7 and G-8, Appendix G of the Policy Assessment, for
Approaches C and E, respectively). Looking more broadly at these
results could support consideration of using the same CPL for a 24-hour
standard as for a 4-hour standard, recognizing that there is no one
approach that can most closely identify a generally equivalent 24-hour
standard level in each urban area for each CPL. The use of such an
unadjusted CPL for a 24-hour standard would place more emphasis on the
relatively high degree of spatial and temporal variability in relative
humidity and fine particle composition observed in urban areas across
the country, so as to reduce the potential of setting a 24-hour
standard level that would require more than the intended degree of
protection in some areas.
    In more broadly considering alternative standard levels that would
be appropriate for a nationally applicable secondary standard focused
on protection from PM-related urban visibility impairment based on
either a 24-hour or multi-hour, sub-daily (e.g., 4-hour) averaging
time, the Policy Assessment was mindful of the important limitations in
the available evidence from public preference studies. While the Policy
Assessment concluded, consistent with CASAC advice, that it is
appropriate to consider a distinct secondary PM2.5 standard
to address PM-related visibility impairment focused primarily in urban
areas based on the evidence from public preference studies, it also
recognized that there are a number of uncertainties and limitations
associated with the preference studies that have served as a basis for
selecting an appropriate range of levels to consider, as discussed
above

in section VI.B.2. These uncertainties and limitations are due in part
to the small number of stated preference studies available for this
review; the relatively small number of study participants and the
extent to which the study participants may not be representative of the
broader study area population in some of the studies; and the
variations in the specific materials and methods used in each study
such as scene characteristics, the range of VAQ levels presented to
study participants, image presentation methods and specific wording
used to frame the questions used in the group interviews. In addition
the Policy Assessment was mindful that the scenic vistas available on a
daily basis in many urban areas across the country generally do not
have the inherent visual interest or the distance between viewer and
object of greatest intrinsic value as in the Denver and Phoenix
preference studies, and that there is the possibility that there could
be regional differences in individual preferences for VAQ.
    Given the uncertainties and limitations noted above, the EPA
broadly solicits comment on the strengths and limitations associated
with these preference studies and the use of these studies to inform
the selection of a range of levels that could be used to provide an
appropriate degree of public welfare protection when combined with the
other elements of the standard (i.e. indicator, form and averaging
time). In particular, the EPA solicits comment on the following
specific aspects of the public preference studies and on how these
studies should appropriately be considered in this review. Recognizing
that all of these studies evaluated a 50 percent acceptability
criterion as the basis for reaching judgments in the context of each
study, the EPA requests comment on the extent to which this criterion
is an appropriate basis for establishing target protection levels in
the context of establishing a distinct secondary NAAQS to address PM-
related visibility impairment in urban areas. Recognizing that these
studies vary in the extent to which the study participants may be
representative of the broader study area population, the EPA requests
comment on how this aspect of the study designs should appropriately be
weighed in the context of considering these studies in reaching
proposed conclusions on a distinct secondary NAAQS. The EPA also
solicits comment on the extent to which the ranges of VAQ levels
presented to participants in each of the studies may have influenced
study results and on how this aspect of the study designs should
appropriately be weighed in the context of considering these studies in
the context of this review.
    As in past reviews, the EPA is considering a national visibility
standard in conjunction with the Regional Haze Program as a means of
achieving appropriate levels of protection against PM-related
visibility impairment in urban, non-urban, and Federal Class I areas
across the country. The EPA recognizes that programs implemented to
meet a national standard focused primarily on the visibility problems
in urban areas can be expected to improve visual air quality in
surrounding non-urban areas as well, as would programs now being
developed to address the requirements of the Regional Haze Program
established for protection of visual air quality in Federal Class I
areas. The EPA also believes that the development of local programs,
such as those in Denver and Phoenix, can continue to be an effective
and appropriate approach to provide additional protection, beyond that
afforded by a national standard, for unique scenic resources in and
around certain urban areas that are particularly highly valued by
people living in those areas.
    Based on the above considerations, the Policy Assessment concludes
that it is appropriate to give primary consideration to alternative
standard levels toward the upper end of the ranges identified above for
24-hour and sub-daily standards, respectively (U.S. EPA, 2011a, p. 4-
63). Thus, the Policy Assessment concludes it is appropriate to
consider the following alternative levels: A level of 28 dv or somewhat
below, down to 25 dv, for a standard defined in terms of a calculated
PM2.5 light extinction indicator, a 90th percentile form,
and a 24-hour averaging time; and a standard level of 30 dv or somewhat
below, down to 25 dv, for a similar standard but with a 4-hour
averaging time (U.S. EPA, 2011a, p. 4-63). The Policy Assessment judges
that such standards would provide appropriate protection against PM-
related visibility impairment primarily in urban areas. The Policy
Assessment notes that CASAC generally supported consideration of the
20-30 dv range as CPLs and, more specifically, that support for
consideration of the upper part of the range of the CPLs derived from
the public preference studies was expressed by some CASAC Panel members
during the public meeting on the second draft Policy Assessment. The
Policy Assessment concludes that such a standard would be appropriate
in conjunction with the Regional Haze Program to achieve appropriate
levels of protection against PM-related visibility impairment in areas
across the country (U.S. EPA, 2011a, p. 4-63).
    Based on the above considerations, taking into account the
conclusions in the Policy Assessment and the extent to which those
conclusions reflected consideration of CASAC advice during the
development of the Policy Assessment, as an initial matter, the
Administrator provisionally concludes that it is appropriate to
establish a target level of protection--for a standard defined in terms
of a PM2.5 visibility index; a 90th percentile form averaged
over 3 years; and a 24-hour averaging time--equivalent to the
protection afforded by such a sub-daily (i.e., 4-hour) standard at a
level of 30 dv, which is the upper end of the range of CPLs identified
in the Policy Assessment and generally supported by CASAC. More
specifically, the Administrator provisionally concludes that a 24-hour
level of either 30 dv or 28 dv could be construed as providing such a
degree of protection, and that either level is supported by the
available information and is generally consistent with the advice of
CASAC. The option of setting such a 24-hour standard at a level of 30
dv would reflect recognition that there is considerable spatial and
temporal variability in the key factors that determine the value of the
PM2.5 visibility index in any given urban area, such that
there is a relatively high degree of uncertainty as to the most
appropriate approach to use in selecting a 24-hour standard level that
would be generally equivalent to a specific 4-hour standard level.
Selecting a 24-hour standard level of 30 dv would reflect a judgment
that such substantial degrees of variability and uncertainty should be
reflected in a higher standard level than would be appropriate if the
underlying information were more consistent and certain. Alternatively,
the option of setting such a 24-hour standard at a level of 28 dv would
reflect placing more weight on statistical analyses of aggregated data
from across the study cities and not placing as much emphasis on the
city-to-city variability as a basis for determining an appropriate
degree of protection on a national scale.
    In light of these provisional conclusions, the Administrator
proposes to set a new 24-hour standard (defined in terms of a
PM2.5 visibility index and a 90th percentile form, averaged
over 3 years) to provide appropriate protection from PM-related
visibility impairment based on one of two options. One option is to set
the level of such a standard at 30 dv and the other option is to set
the level at 28 dv. In so doing, the

Administrator solicits comment on each of these levels and on the
various approaches to identifying generally equivalent levels discussed
above upon which the alternative proposed levels are based. Recognizing
that there was some support for consideration of a broader range of
levels, the Administrator also solicits comment on a range of levels
down to 25 dv in conjunction with a 24-hour averaging time. Further,
having solicited comment on a sub-daily (e.g., 4-hour) averaging time,
the Administrator also solicits comment on a range of alternative
levels from 30 to 25 dv in conjunction with such a sub-daily averaging
time.
    Finally, as we have indicated, the information available for the
Administrator to consider when setting the secondary PM standard raises
a number of uncertainties. While CASAC supported moving forward with a
new standard on the basis of the available information, CASAC also
recognized these uncertainties, referencing the discussion of key
uncertainties and areas for future research in the second draft of the
Policy Assessment. In discussing areas of future research, CASAC stated
that: ``The range of 50% acceptability values discussed as possible
standards are based on just four studies (Figure 4-2), which, given the
large spread in values, provide only limited confidence that the
benchmark candidate protection levels cover the appropriate range of
preference values. Studies using a range of urban scenes (including,
but not limited to, iconic scenes--``valued scenic elements'' such as
those in the Washington DC study), should also be considered.'' (Samet,
2010d, p. 12). We invite comment on how the Administrator should weigh
those uncertainties as well as any additional comments and information
to inform her consideration of these uncertainties.

E. Other PM-Related Welfare Effects

    In the 2006 review, the Administrator concluded that there was
insufficient information to consider a distinct secondary standard
based on PM-related impacts to ecosystems, materials damage and
soiling, and climatic and radiative processes (71 FR 61144, October 17,
2006). Specifically, there was a lack of evidence linking various non-
visibility welfare effects to specific levels of ambient PM. To provide
a level of protection for welfare-related effects, the secondary
standards were set equal to the revised primary standards to
directionally improve the level of protection afforded vegetation,
ecosystems, and materials (71 FR 61210, October 17, 2006).
    In that review, the 2004 AQCD concluded that regardless of size
fraction, particles containing nitrates and sulfates have the greatest
potential for widespread environmental significance (U.S. EPA, 2004,
sections 4.2.2 and <IP_ADDRESS>). Considerable supporting evidence was
available that indicated a significant role of oxides of nitrogen and
sulfur, and their transformation products in acidification and nutrient
enrichment of terrestrial and aquatic ecosystems (71 FR 61209, October
17, 2006). The recognition of these ecological effects, coupled with
other considerations detailed below, led EPA to initiate a joint review
of the secondary NO2 and SO2 NAAQS that is
considering the gaseous and particulate species of oxides of nitrogen
and sulfur with respect to the ecosystem-related welfare effects that
result from the deposition of these pollutants and transformation
products.
    This section presents the Policy Assessment's conclusions with
regard to the current suite of secondary PM standards to protect
against non-visibility PM-related welfare effects. Specifically, the
Policy Assessment has assessed the relevant information related to
effects of atmospheric PM on the environment, including effects on
climate, ecological effects, and materials. Non-visibility welfare-
based effects of oxides of nitrogen and sulfur are divided between two
NAAQS reviews; (1) PM NAAQS review and, (2) the joint secondary NAAQS
review for oxides of nitrogen (NOX) and oxides of sulfur
(SOX).\179\ The scope of each document and the compounds of
nitrogen and sulfur considered in each review are summarized in this
section and in Table 5-1 of the Policy Assessment.
---------------------------------------------------------------------------

    \179\ For the purposes of this discussion, NOX and
SOX refers to all oxides of nitrogen and all oxides of
sulfur, respectively.
---------------------------------------------------------------------------

    In reviewing the current suite of secondary PM standards, the
Policy Assessment considers all PM-related effects that are not being
covered in the ongoing NOX/SOX review, including
visibility impairment (U.S. EPA, 2011a, chapter 4), climate forcing
effects (U.S. EPA, 2011a, section 5.2), ecological effects (U.S. EPA,
2011a, section 5.3), and materials damage (U.S. EPA, 2011a, section
5.4). By excluding the effects associated with deposited particulate
matter components of NOX and SOX and their
transformation products which are addressed fully in the
NOX/SOX secondary review, the discussion of
ecological effects of PM has been narrowed to focus on effects
associated with the deposition of metals and, to a lesser extent,
organics (U.S. EPA, 2011a, section 5.3). With regard to the materials
section, because the NOX/SOX review is not
considering materials, the discussion includes particles and gases that
are associated with the presence of ambient NOX and
SOX, as well as reduced forms of nitrogen such as ammonia
and ammonium ions for completeness.
    In contrast, the proposed rulemaking for the joint NOX/
SOX secondary review (76 FR 46084, August 1, 2011) focuses
on the welfare effects associated with exposures from deposited
particulate and gaseous forms of oxides of nitrogen and sulfur and
related nitrogen- and sulfur-containing compounds and transformation
products on ecosystem receptors, including effects of acidifying
deposition associated with particulate nitrogen and sulfur. In
addition, the NOX/SOX secondary review includes
evidence related to direct ecological effects of gas-phase
NOX and SOX.
1. Climate
    Information and conclusions about what is currently known about the
role of PM in climate is summarized in Chapter 9 of the Integrated
Science Assessment (U.S. EPA, 2009a). The Integrated Science Assessment
concludes ``that a causal relationship exists between PM and effects on
climate, including both direct effects on radiative forcing and
indirect effects that involve cloud feedbacks that influence
precipitation formation and cloud lifetimes'' (U.S. EPA, 2009a, section
9.3.10). The Policy Assessment summarizes and synthesizes the policy-
relevant science in the Integrated Science Assessment for the purpose
of helping to inform consideration of climate aspects in the review of
the secondary PM NAAQS (U.S. EPA, 2011a, section 5.2). This discussion
is summarized below.
    Atmospheric PM (referred to as aerosols \180\ in the remainder of
this section to be consistent with the Integrated Science Assessment)
affects multiple aspects of climate. These include absorbing and
scattering of incoming solar radiation, alterations in terrestrial
radiation, effects on the hydrological cycle, and changes in cloud
properties (U.S. EPA, 2009a, section 9.3.1). Major aerosol components
that contribute to climate processes include black carbon (BC),

organic carbon (OC), sulfates, nitrates, and mineral dusts. There is a
considerable ongoing research effort focused on understanding aerosol
contributions to changes in global mean temperature and precipitation
patterns. The Climate Change Research Initiative identified research on
atmospheric concentrations and effects of aerosols as a high research
priority (National Research Council, 2001) and the IPCC 2007 Summary
for Policymakers states that anthropogenic contributions to aerosols
remain the dominant uncertainty in radiative forcing (IPCC, 2007). The
current state of the science of climate alterations attributable to PM
is in flux as a result of continually updated information.
---------------------------------------------------------------------------

    \180\ In the sections of the Integrated Science Assessment
included from IPCC AR4 and CCSP SAP2.3 (U.S. EPA, 2009a, section
9.3), the term ``aerosols'' is more frequently used than ``PM'' and
that word is retained in the Policy Assessment (U.S. EPA, 2011a,
section 5.2) and in this section of the preamble.
---------------------------------------------------------------------------

    Global climate change has increasingly been the focus of intense
international research endeavors. As discussed in chapter 5 of the
Policy Assessment, major efforts are underway to understand the
complexities inherent in atmospheric aerosol interactions and to
decrease uncertainties associated with climate estimations.
    Aerosols have direct and indirect effects on climate processes. The
direct effects of aerosols on climate result mainly from particles
scattering light away from Earth into space, directly altering the
radiative balance of the Earth-atmosphere system. This reflection of
solar radiation back to space decreases the transmission of visible
radiation to the surface of the Earth and results in a decrease in the
heating rate of the surface and the lower atmosphere. At the same time,
absorption of either incoming solar radiation or outgoing terrestrial
radiation by particles, primarily BC, results in an increased heating
rate in the lower atmosphere. Global estimates of aerosol direct
radiative forcing (RF) were recently summarized using a combined model-
based estimate (Forster et al., 2007). The overall, model-derived
aerosol direct RF was estimated in the IPCC AR4 as -0.5 (-0.9 to -0.1)
watts per square meter (W/m\2\), with an overall level of scientific
understanding of this effect as ``medium low'' (Forster et al., 2007),
indicating a net cooling effect in contrast to greenhouse gases (GHGs)
which have a warming effect.
    The contribution of individual aerosol components to total aerosol
direct radiative forcing is more uncertain than the global average
(U.S. EPA, 2009a, section <IP_ADDRESS>). The direct effect of radiative
scattering by atmospheric particles exerts an overall net cooling of
the atmosphere, while particle absorption of solar radiation leads to
warming. For example, the presence of OC and sulfates decrease warming
from sunlight by scattering shortwave radiation back into space. Such a
perturbation of incoming radiation by anthropogenic aerosols is
designated as aerosol climate forcing, which is distinguished from the
aerosol radiative effect of the total aerosol (natural plus
anthropogenic). The aerosol climate forcing and radiative effect are
characterized by large spatial and temporal heterogeneities due to the
wide variety of aerosol sources, the spatial non-uniformity and
intermittency of these sources, the short atmospheric lifetime of
aerosols (relative to that of the greenhouse gases), and processing
(chemical and microphysical) that occurs in the atmosphere. For
example, OC can be warming (positive forcer) when deposited on or
suspended over a highly reflective surface such as snow or ice but, on
a global average, is a negative forcer in the atmosphere.
    More information has also become available on indirect effects of
aerosols. Particles in the atmosphere indirectly affect both cloud
albedo (reflectivity) and cloud lifetime by modifying the cloud amount,
and microphysical and radiative properties (U.S. EPA, 2009a, section
<IP_ADDRESS>). The RF due to these indirect effects (cloud albedo effect) of
aerosols is estimated in the IPCC AR4 to be -0.7 ( -1.8 to -0.3) W/m\2\
with the level of scientific understanding of this effect as ``low''
(Forster et al., 2007). Aerosols act as cloud condensation nuclei (CCN)
for cloud formation. Increased particulates in the atmosphere available
as CCN with no change in moisture content of the clouds have resulted
in an increase in the number and decrease in the size of cloud droplets
in certain clouds that can increase the albedo of the clouds (the
Twomey effect). Smaller particles slow the onset of precipitation and
prolong cloud lifetime. This effect, coupled with changes in cloud
albedo, increases the reflection of solar radiation back into space.
The altitude of the clouds also affects cloud radiative forcing. Low
clouds reflect incoming sunlight back to space but do not effectively
trap outgoing radiation, thus cooling the planet, while higher
elevation clouds reflect some sunlight but more effectively can trap
outgoing radiation and act to warm the planet (U.S. EPA, 2009a, section
<IP_ADDRESS>).
    The total negative RF due to direct and indirect effects of
aerosols computed from the top of the atmosphere, on a global average,
is estimated at -1.3 (-2.2 to -0.5) W/m\2\ in contrast to the positive
RF of +2.9 (+3.2 to +2.6) W/m\2\ for anthropogenic GHGs (IPCC 2007, p.
200).
    The understanding of the magnitude of aerosol effects on climate
has increased substantially in the last decade. Data on the atmospheric
transport and deposition of aerosols indicate a significant role for PM
components in multiple aspects of climate. Aerosols can impact
glaciers, snowpack, regional water supplies, precipitation, and climate
patterns (U.S. EPA, 2009a, section 9.3.9). Aerosols deposited on ice or
snow can lead to melting and subsequent decrease of surface albedo
(U.S. EPA, 2009a, section <IP_ADDRESS>). Aerosols are potentially important
agents of climate warming in the Arctic and other locations (U.S. EPA,
2009a, section 9.3.9). Carbonaceous aerosols emitted from intermittent
fires can occur at large enough scales to affect hemispheric aerosol
concentrations. In addition to incidental fires, routine biomass
burning, usually associated with agriculture in eastern Europe, has
also been shown to contribute to hemispheric concentrations of
carbonaceous aerosols and is therefore recognized as having a
significant impact on PM2.5 concentrations and climate
forcing (U.S. EPA, 2009a, section 9.3.7).
    A series of studies available since the last review examines the
role of aerosols on local and regional scale climate processes (U.S.
EPA, 2009a, section <IP_ADDRESS>). Studies on the South Coast Air Basin
(SCAB) in California indicate aerosols may reduce near-surface wind
speeds, which, in turn reduce evaporation rates and increase cloud
lifetimes. The overall impact can be a reduction in local precipitation
(Jacobson and Kaufmann, 2006). Conditions in the SCAB impact
ecologically sensitive areas including the Sierra Nevadas.
Precipitation suppression due to aerosols in California (Givati and
Rosenfield, 2004) and other similar studies in Utah and Colorado found
that mountain precipitation decreased by 15 to 30 percent downwind of
pollution sources. Evidence of regional-scale impacts of aerosols on
meteorological conditions in other regions of the U.S. is lacking.
    Advances in the understanding of aerosol components and how they
contribute to climate change have enabled refined global forcing
estimates of individual PM constituents. The global mean radiative
effect from individual components of aerosols was estimated for the
first time in the IPCC AR4 where they were reported to be (all in W/
m\2\ units): -0.4 (+0.2) for sulfate, -0.05 (+0.05) for fossil fuel-
derived OC, +0.2 (+0.15) for fossil fuel derived BC, +0.03 (+0.12) for
biomass burning, -0.1

(+0.1) for nitrates, and -0.1 (+0.2) for mineral dust (U.S. EPA, 2009a,
section 9.3.10). Sulfate and fossil fuel-derived OC cause negative
forcing whereas BC causes positive forcing because of its highly
absorbing nature (U.S. EPA, 2009a, <IP_ADDRESS>). Although BC comprises only
a small fraction of anthropogenic aerosol mass load and aerosol optical
depth (AOD), its forcing efficiency (with respect to either AOD or
mass) is an order of magnitude stronger than sulfate and particulate
organic matter (POM), so its positive shortwave forcing largely offsets
the negative direct forcing from sulfate and POM (IPCC, 2007; U.S. EPA,
2009a, <IP_ADDRESS>). Global loadings for nitrates and anthropogenic dust
remain very difficult to estimate, making the radiative forcing
estimates for these constituents particularly uncertain (U.S. EPA,
2009a, section 9.3.7).
    Improved estimates of anthropogenic emissions of some aerosols,
especially BC and OC, have promoted the development of improved global
emissions inventories and source-specific emissions factors useful in
climate modeling (Bond et al. 2004). Recent data suggests that BC is
one of the largest individual warming agents after carbon dioxide
(CO2) and perhaps methane (CH4) (Jacobson 2000;
Sato et al., 2003; Bond and Sun 2005). There are several studies
modeling BC effects on climate and/or considering emission reduction
measures on anthropogenic warming detailed in section 9.3.9 of the
Integrated Science Assessment. In the U.S., most of the warming
aerosols are emitted by biomass burning and internal engine combustion
and much of the cooling aerosols are formed in the atmosphere by
oxidation of SO2 or volatile organic compounds (VOCs) (U.S.
EPA, 2009a, section 3.3). Fires release large amounts of BC,
CO2, CH4 and OC (U.S. EPA, 2009a, section 9.3.7).
    Based on the above newly available scientific information on
climate-aerosol relationships, the Policy Assessment concludes that
aerosols alter climate processes directly through radiative forcing and
by indirect effects on cloud brightness, changes in precipitation, and
possible changes in cloud lifetimes (U.S. EPA, 2011a, p. 5-10).
Further, the Policy Assessment notes that the major aerosol components
that contribute to climate processes (i.e. BC, OC, sulfate, nitrate and
mineral dusts) vary in their reflectivity, forcing efficiencies and
even in the direction of climate forcing, though there is an overall
net climate cooling associated with aerosols in the global atmosphere
(U.S. EPA, 2009a, section 9.2.10). In light of this information, the
Policy Assessment considered the appropriateness of the current
secondary standards defined in terms of PM2.5 and
PM10 indicators, for providing protection against potential
climate effects of aerosols. The current standards that are defined in
terms of aggregate size mass cannot be expected to appropriately target
controls on components of fine and coarse particles that are related to
climate forcing effects. Thus, the Policy Assessment concludes that the
current mass-based PM2.5 and PM10 secondary
standards are not an appropriate or effective means of focusing
protection against PM-associated climate effects due to these
differences in components (U.S. EPA, 2011a, p. 5-11).
    Further, in light of the uncertainties associated with the spatial
and temporal heterogeneity of PM components that contribute to climate
forcing and the uncertainties associated with measurement of aerosol
components, the inadequate consideration of aerosol impacts in climate
modeling and the insufficient data on local and regional microclimate
variations and the heterogeneity of cloud formations, the Policy
Assessment concludes it is not currently feasible to conduct a
quantitative analysis for the purpose of informing revisions of the
current secondary PM standards based on climate (U.S. EPA, 2011a, p. 5-
11). Based on these considerations, the Policy Assessment concludes
that there is insufficient information at this time to base a national
ambient standard on climate impacts associated with current ambient
concentrations of PM or its constituents (U.S. EPA, 2011a, p. 5-11, -
12).\181\
---------------------------------------------------------------------------

    \181\ This conclusion would apply for both the secondary
(welfare-based) and the primary (health-based) standards.
---------------------------------------------------------------------------

2. Ecological Effects
    Information on what is currently known about ecological effects of
PM is summarized in Chapter 9 of the Integrated Science Assessment
(U.S. EPA, 2009a). Four main categories of ecological effects are
identified in the Integrated Science Assessment: Direct effects,
effects of PM-altered radiative flux, indirect effects of trace metals,
and indirect effects of organics. Exposure to PM for direct effects
occurs via deposition (e.g., wet, dry or occult) to vegetation
surfaces, while indirect effects occur via deposition to ecosystem
soils or surface waters where the deposited constituents of PM then
interact with biological organisms. Both fine and coarse-mode particles
may affect plants and other organisms; however, PM size classes do not
necessarily relate to ecological effects (U.S. EPA, 1996). More often,
the chemical constituents drive the ecosystem response to PM (Grantz et
al., 2003). The trace metal constituents of PM considered in the
ecological effects section of the Integrated Science Assessment are
cadmium (Cd), copper (Cu), chromium (Cr), mercury (Hg), nickel (Ni) and
zinc (Zn). Ecological effects of lead (Pb) in particulate form are
covered in the Air Quality Criteria Document for Lead (U.S. EPA, 2006).
The organics included in the ecological effects section of the PM
Integrated Science Assessment are persistent organic pollutants (POPs),
polyaromatic hydrocarbons (PAHs), and polybromiated diphenyl ethers
(PBDEs).
    Ecological effects of PM include direct effects to metabolic
processes of plant foliage; contribution to total metal loading
resulting in alteration of soil biogeochemistry and microbiology, and
plant and animal growth and reproduction; and contribution to total
organics loading resulting in bioaccumulation and biomagnification
across trophic levels.
    The Integrated Science Assessment states that overall, ecological
evidence is sufficient to conclude that a causal relationship is likely
to exist between deposition of PM and a variety of effects on
individual organisms and ecosystems based on information from the
previous review and limited new findings in this review (U.S. EPA,
2009a, sections 2.5.3 and 9.4.7). However the Integrated Science
Assessment also finds, in many cases, it is difficult to characterize
the nature and magnitude of effects and to quantify relationships
between ambient concentrations of PM and ecosystem response due to
significant data gaps and uncertainties as well as considerable
variability that exists in the components of PM and their various
ecological effects.
    Ecological effects of PM must then be evaluated to determine if
they are known or anticipated to have an adverse impact on public
welfare. Characterizing a known or anticipated adverse effect to public
welfare is an important component of developing any secondary NAAQS.
The most recent secondary NAAQS reviews have assessed changes in
ecosystem structure or processes using a weight-of-evidence approach
that uses both quantitative and qualitative data. A paradigm useful in
evaluating ecological adversity is the concept of ecosystem services.
Ecosystem services consist of the varied and numerous ways that
ecosystems are important to human welfare. Ecosystems provide many
goods and services that are of vital importance for the functioning of
the biosphere and

provide the basis for the delivery of tangible benefits to human
society. An EPA initiative to consider how ecosystem structure and
function can be interpreted through an ecosystem services approach has
resulted in the inclusion of ecosystem services in the NOX/
SOX Risk and Exposure Assessment (U.S. EPA, 2009h). The
Millennium Ecosystem Assessment (MEA) defines these to include
supporting, provisioning, regulating, and cultural services (Hassan et
al., 2005).
    An important consideration in evaluating biologically adverse
effects of PM and linkages to ecosystem services is that many of the
MEA categories overlap and any one pollutant may impact multiple
services. For example, deposited PM may alter the composition of soil-
associated microbial communities, which may affect supporting services
such as nutrient cycling. Changes in available soil nutrients could
result in alterations to provisioning services such as timber yield and
regulating services such as climate regulation. If enough information
is available, these alterations can be quantified based upon economic
approaches for estimating the value of ecosystem services. Valuation
may be important from a policy perspective because it can be used to
compare the benefits of altering versus maintaining an ecosystem.
Knowledge about the relationships linking ambient concentrations and
ecosystem services can be used to inform a policy judgment on a known
or anticipated adverse public welfare effect.
    The Policy Assessment seeks to build upon and focus this body of
science using the concept of ecosystem services to qualitatively
evaluate linkages between biologically adverse effects and particulate
deposition. This approach is similar to that taken in the
NOX/SOX Risk and Exposure Assessment in which the
relationship between air quality indicators, deposition of nitrogen and
sulfur, ecologically relevant indicators, and effects on sensitive
receptors are linked to changes in ecosystem structure and services
(U.S. EPA, 2009h). This approach considers the benefits received from
the resources and processes that are supplied by ecosystems. Several
ecosystem components (e.g., plants, soils, water, and wildlife) are
impacted by PM air pollution, which may alter the services provided by
the ecosystems in question. Key scientific evidence regarding PM
effects on plants, soil and nutrient cycling, wildlife, and water
available since the last review is summarized below to evaluate how
this information has improved understanding of ecosystem responses to
PM.
a. Plants
    As primary producers, plants play a pivotal role in energy flow
through ecosystems. Ecosystem services derived from plants include all
of the categories (supporting, provisioning, regulating, and cultural)
identified in the MEA (Hassan et al., 2005). Vegetation supports other
ecosystem processes by cycling nutrients through food webs and serving
as a source of organic material for soil formation and enrichment.
Trees and plants provide food, wood, fiber, and fuel for human
consumption. Flora help to regulate climate by sequestering
CO2, and control flooding by stabilizing soils and cycling
water via uptake and evapotranspiration. Plants are significant in
aesthetic, spiritual, and recreational aspects of human interactions.
    Particulate matter can adversely impact plants and ecosystem
services provided by plants by deposition to vegetative surfaces (U.S.
EPA, 2009a, section 9.4.3). Particulates deposited on the surfaces of
leaves and needles can block light, altering the radiation received by
the plant. PM deposition can obstruct stomata limiting gas exchange,
damage leaf cuticles, and increase plant temperatures. This level of PM
accumulation is typically observed near sources of heavy deposition
such as smelters and mining operations (U.S. EPA, 2009a, section
9.4.3). Plants growing on roadsides exhibit impact damage from near-
road PM deposition, having higher levels of organics and heavy metals,
and accumulate salt from road de-icing during winter months (U.S. EPA,
2009a, sections <IP_ADDRESS> and <IP_ADDRESS>).
    In addition to damage to plant surfaces, deposited PM can be taken
up by plants from soil or foliage. The ability of vegetation to take up
heavy metals and organics is dependent upon the amount, solubility, and
chemical composition of the deposited PM. Uptake of PM by plants from
soils and vegetative surfaces can disrupt photosynthesis, alter
pigments and mineral content, reduce plant vigor, decrease frost
hardiness, and impair root development. The Integrated Science
Assessment indicates that there are little or no effects on foliar
processes at ambient levels of PM (U.S. EPA, 2009a, sections 9.4.3 and
9.4.7). However, damage due to atmospheric pollution can occur near
individual point sources or under conditions where plants are subjected
to multiple stressors.
    Although all heavy metals can be directly toxic at sufficiently
high concentrations, only Cu, Ni, and Zn have been documented as being
frequently toxic to plants (U.S. EPA, 2004), while toxicity due to Cd,
Co, and Pb has been observed less frequently (Smith, 1990; U.S. EPA,
2009a, section <IP_ADDRESS>). In general, plant growth is negatively
correlated with trace metal and heavy metal concentration in soils and
plant tissue (Audet and Charest, 2007). Trace metals, particularly
heavy metals, can influence forest growth. Growth suppression of foliar
microflora has been shown to result from iron (Fe), aluminum (Al), and
Zn. These three metals can also inhibit fungal spore formation, as can
Cd, Cr, magnesium (Mg), and Ni (see Smith, 1990). Metals cause stress
and decreased photosynthesis (Kucera et al., 2008) and disrupt numerous
enzymes and metabolic pathways (Strydom et al., 2006). Excessive
concentrations of metals result in phytotoxicity.
    New information since the last review provides additional evidence
of plant uptake of organics (U.S. EPA, 2009a, section 9.4.6). An area
of active study is the impact of PAHs on provisioning ecosystem
services due to the potential for human and other animal exposure via
food consumption (U.S. EPA, 2009a, section 9.4.6 page 9-190). The
uptake of PAHs depends on the plant species, site of deposition,
physical and chemical properties of the organic compound, and
prevailing environmental conditions. It has been established that most
bioaccumulation of PAHs by plants occurs via leaf uptake, and to a
lesser extent, through roots. Differences between species in uptake of
PAHs confound attempts to quantify impacts to ecosystem provisioning
services.
    Plants as ecosystem regulators can serve as passive monitors of
pollution (U.S. EPA, 2009a, section <IP_ADDRESS>). Lichens and mosses are
sensitive to pollutants associated with PM and have been used with
limited success to show spatial and temporal patterns of atmospheric
deposition of metals (U.S. EPA, 2009a, section <IP_ADDRESS>). A limitation
to employing mosses and lichens to detect for the presence of air
pollutants is the difference in uptake efficiencies of metals between
species. Thus, quantification of ecological effects is not possible due
to the variability of species responses (U.S. EPA, 2009a, section
<IP_ADDRESS>).
    A potentially important regulating ecosystem service of plants is
their capacity to sequester contaminants (U.S. EPA, 2009a, section
<IP_ADDRESS>). Ongoing research on the application of plants to
environmental remediation efforts are

yielding some success in removing heavy metals and organics from
contaminated sites (phytoremediation) with tolerant plants such as the
willow tree (Salix spp.) and members of the family Brassicaceae (U.S.
EPA, 2009a, section <IP_ADDRESS>). Tree canopies can be used in urban
locations to capture particulates and improve air quality (Freer-Smith
et al., 2004). Plant foliage is a sink for Hg and other metals and this
regulating ecosystem service may be impacted by atmospheric deposition
of trace metals.
    An ecological endpoint (phytochelatin concentration) associated
with presence of metals in the environment has been correlated with the
ecological effect of tree mortality (Grantz et al., 2003). Metal stress
may be contributing to tree injury and forest decline in the
Northeastern U.S. where red spruce populations are declining with
increasing elevation. Quantitative assessment of PM damage to forests
potentially could be conducted by overlaying PM sampling data and
elevated phytochelatin levels. However, limited data on phytochelatin
levels in other species currently hinders use of this peptide as a
general biomarker for PM.
    The presence of PM in the atmosphere affects ambient radiation as
discussed in the Integrated Science Assessment which can impact the
amount of sunlight received by plants (U.S. EPA, 2009a, section 9.4.4).
Atmospheric PM can change the radiation reaching leaf surfaces through
attenuation and by converting direct radiation to diffuse radiation.
Diffuse radiation is more uniformly distributed in a tree canopy,
allowing radiation to reach lower leaves. The net effect of PM on
photosynthesis depends on the reduction of photosynthetically active
radiation (PAR) and the increase in the diffuse fraction of PAR.
Decreases in crop yields (provisioning ecosystem service) have been
attributed to regional scale air pollution, however, global models
suggest that the diffuse light fraction of PAR can increase growth
(U.S. EPA, 2009a, section 9.4.4).
b. Soil and Nutrient Cycling
    Many of the major indirect plant responses to PM deposition are
chiefly soil-mediated and depend on the chemical composition of
individual components of deposited PM. Major ecosystem services
impacted by PM deposition to soils include support services such as
nutrient cycling, products such as crops and regulating flooding and
water quality. Upon entering the soil environment, PM pollutants can
alter ecological processes of energy flow and nutrient cycling, inhibit
nutrient uptake to plants, change microbial community structure and,
affect biodiversity. Accumulation of heavy metals in soils depends on
factors such as local soil characteristics, geologic origin of parent
soils, and metal bioavailability. It can be difficult to assess the
extent to which observed heavy metal concentrations in soil are of
anthropogenic origin (U.S. EPA, 2009a, section <IP_ADDRESS>). Trace element
concentrations are higher in some soils that are remote from air
pollution sources due to parent material and local geomorphology.
    Heavy metals such as Zn, Cu, and Cd and some pesticides can
interfere with microorganisms that are responsible for decomposition of
soil litter, an important regulating ecosystem service that serves as a
source of soil nutrients (U.S. EPA, 2009a, sections <IP_ADDRESS> and
<IP_ADDRESS>). Surface litter decomposition is reduced in soils having high
metal concentrations. Soil communities have associated bacteria, fungi,
and invertebrates that are essential to soil nutrient cycling
processes. Changes to the relative species abundance and community
composition can be quantified to measure impacts of deposited PM to
soil biota. A mutualistic relationship exists in the rhizophere (plant
root zone) between plant roots, fungi, and microbes. Fungi in
association with plant roots form mycorrhizae that are essential for
nutrient uptake by plants. The role of mychorrizal fungi in plant
uptake of metals from soils and effects of deposited PM on soil
microbes is discussed in section <IP_ADDRESS> of the Integrated Science
Assessment.
c. Wildlife
    Animals play a significant role in ecosystem function including
nutrient cycling and crop production (supporting ecosystem service),
and as a source of food (provisioning ecosystem service). Cultural
ecosystem services provided by wildlife include bird and animal
watching, hunting, and fishing. Impacts on these services are dependent
upon the bioavailability of deposited metals and organics and their
respective toxicities to ecosystem receptors. Pathways of PM exposure
to fauna include ingestion, absorption and trophic transfer.
Bioindicator species (known as sentinel organisms) can provide evidence
of contamination due to atmospheric pollutants. Use of sentinel species
can be of particular value because chemical constituents of deposited
PM are difficult to characterize and have varying bioavailability (U.S.
EPA, 2009a, section <IP_ADDRESS>). Snails readily bioaccumulate contaminants
such as PAHs and trace metals. These organisms have been deployed as
biomonitors for urban pollution and have quantifiable biomarkers of
exposure including growth inhibition, impairment of reproduction,
peroxidomal proliferation, and induction of metal detoxifying proteins
(metallothioneins) (Gomet-de Vaufleury, 2002; Regoli, et. al, 2006).
Earthworms have also been used as sensitive indicators of soil metal
contamination.
    Evidence of deposited PM effects on animals is limited (U.S. EPA,
2009a, section <IP_ADDRESS>). Trophic transfer of pollutants of atmospheric
origin has been demonstrated in limited studies. PM may also be
transferred between aquatic and terrestrial compartments. There is
limited evidence for biomagnifications of heavy metals up the food
chain except for Hg which is well known to move readily through
environmental compartments (U.S. EPA, 2009a, section <IP_ADDRESS>).
Bioconcentration of POPs and PBDEs in the Arctic and deep-water oceanic
food webs indicates the global transport of particle-associated
organics (U.S. EPA, 2009a, section 9.4.6). Salmon migrations are
contributing to metal accumulation in inland aquatic systems,
potentially impacting the provisioning and cultural ecosystem service
of fishing (U.S. EPA, 2009a, section 9.4.6). Stable isotope analysis
can be applied to establish linkages between PM exposure and impacts to
food webs however, the use of this evaluation tool is limited for this
ecological endpoint due to the complexity of most trophic interactions
(U.S. EPA, 2009a, section <IP_ADDRESS>). Foraging cattle have been used to
assess atmospheric deposition and subsequent bioaccumulation of Hg and
trace metals and their impacts on provisioning services (U.S. EPA,
2009a, section <IP_ADDRESS>).
d. Water
    New limited information on impacts of deposited PM on receiving
water bodies indicate that the ecosystem services of primary
production, provision of fresh water, regulation of climate and floods,
recreational fishing and water purification are adversely impacted by
atmospheric inputs of metals and organics (U.S. EPA, 2009a, sections
<IP_ADDRESS> and <IP_ADDRESS>). Deposition of PM to surfaces in urban settings
increases the metal and organic component of storm water runoff (U.S.
EPA, 2009a, sections <IP_ADDRESS>). This atmospherically-associated
pollutant burden can then be toxic to aquatic biota.

    Atmospheric deposition can be the primary source of some organics
and metals to watersheds. The contribution of atmospherically deposited
PAHs to aquatic food webs was demonstrated in high elevation mountain
lakes with no other anthropogenic contaminant sources (U.S. EPA, 2009a,
section 9.4.6). Metals associated with PM deposition limit
phytoplankton growth, impacting aquatic trophic structure. Long-range
atmospheric transport of 47 pesticides and degradation products to the
snowpack in seven national parks in the Western U.S. was recently
quantified indicating PM-associated contaminant inputs to receiving
waters during spring snowmelt (Hageman et al., 2006).
    The recently completed Western Airborne Contaminants Assessment
Project (WACAP) is the most comprehensive database on contaminant
transport and PM depositional effects on sensitive ecosystems in the
U.S. In this project, the transport, fate, and ecological impacts of
anthropogenic contaminants from atmospheric sources were assessed from
2002 to 2007 in seven ecosystem components (air, snow, water, sediment,
lichen, conifer needles and fish) in eight core national parks (Landers
et al., 2008). The goals of the study were to identify where the
pollutants were accumulating, identify ecological indicators for those
pollutants causing ecological harm, and to determine the source of the
air masses most likely to have transported the contaminants to the
parks (U.S. EPA, 2009a, section 9.4.6). The study concluded that
bioaccumulation of semi-volatile organic compounds was observed
throughout park ecosystems (Landers et al., 2008). Findings from this
study included the observation of an elevational gradient in PM
deposition with greater accumulation at higher altitude areas of the
parks. Furthermore, specific ecological indicators were identified in
the WACAP that can be useful in assessing contamination on larger
spatial scales.
    In the WACAP study, bioaccumulation and biomagnification of
airborne contaminants were demonstrated on a regional scale in remote
ecosystems in the Western United States. Contaminants were shown to
accumulate geographically based on proximity to individual sources or
source areas, primarily agriculture and industry (Landers et al.,
2008). Although this assessment focuses on chemical species that are
components of PM, it does not specifically assess the effects of
particulates versus gas-phase forms; therefore, in most cases it is
difficult to apply the results to this assessment based on particulate
concentration and size fraction (U.S. EPA, 2009a, section 9.4.6). There
is a need for ecological modeling of PM components in different
environmental compartments to further elucidate links between PM and
ecological indicators.
    Europe and other countries are using the critical load approach to
assess pollutant effects at the level of the ecosystem. This type of
assessment requires site-specific data and information on individual
species responses to PM. In respect to trace metals and organics, there
are insufficient data for the vast majority of U.S. ecosystems to
calculate critical loads. However, a methodology is being presented in
the NOX/SOX Secondary Risk and Exposure
Assessment (U.S. EPA, 2010h) to calculate atmospheric concentrations
from deposition that may be applicable to other environmental
contaminants.
e. Effects Associated With Ambient PM Concentrations
    As reviewed above, there is considerable data on impacts of PM on
ecological receptors, but few studies that link ambient PM
concentrations to observed effect. This is due, in part, to the nature,
deposition, transport and fate of PM in ecosystems. PM is not a single
pollutant, but a heterogeneous mixture of particles differing in size,
origin and chemical composition (U.S. EPA, 2009a, section 9.4.1). The
heterogeneity of PM exists not only within individual particles or
samples from individual sites, but to even a greater extent, between
samples from different sites. Since vegetation and other ecosystem
components are affected more by particulate chemistry than size
fraction, exposure to a given mass concentration of airborne PM may
lead to widely differing plant or ecosystem responses, depending on the
particular mix of deposited particles. Many of the PM components
bioaccumulate over time in organisms or plants making correlations to
ambient concentrations of PM difficult.
    Bioindicator organisms demonstrated biological effects including
growth inhibition, metallothionein induction and reproductive
impairment when exposed to complex mixtures of ambient air pollutants
(U.S. EPA, 2009a, section <IP_ADDRESS>). Other studies quantify uptake of
metals and organics by plants or animals. However, due to the
difficulty in correlating individual PM components to a specific
physiological response, these studies are limited. Furthermore, there
may be differences in uptake between species such as differing
responses to metal uptake observed in mosses and lichens (U.S. EPA,
2009a, section <IP_ADDRESS>). PM may also biomagnify across trophic levels
confounding efforts to link atmospheric concentrations to physiological
endpoints (U.S. EPA, 2009a, section <IP_ADDRESS>).
    Evidence of PM effects that are linked to a specific ecological
endpoint can be observed when ambient levels are exceeded. Most direct
ecosystem effects associated with particulate pollution occur in
severely polluted areas near industrial point sources (quarries, cement
kilns, metal smelting) (U.S. EPA, 2009a, sections 9.4.3 and <IP_ADDRESS>).
Extensive research on biota near point sources provide some of the best
evidence of ecosystem function impacts and demonstrates that deposited
PM has the potential to alter species composition over long time
scales. The Integrated Science Assessment indicates at 4 km distance,
species composition of vegetation, insects, birds, and soil microbiota
changed, and within 1 km only the most resistant organisms were
surviving (U.S. EPA, 2009a, section <IP_ADDRESS>).
f. Conclusions in the Policy Assessment
    Based on the above discussions, the Policy Assessment made the
following observations:

    (1) A number of significant environmental effects that either
have already occurred or are currently occurring are linked to
deposition of chemical constituents found in ambient PM.
    (2) Ecosystem services can be adversely impacted by PM in the
environment, including supporting, provisioning, regulating and
cultural services.
    (3) The lack of sufficient information to relate specific
ambient concentrations of particulate metals and organics to a
degree of impairment of a specific ecological endpoint hinders the
identification of a range of appropriate indicators, levels, forms
and averaging times of a distinct secondary standard to protect
against associated effects.
    (4) Data from regionally-based ecological studies can be used to
establish probable local, regional and/or global sources of
deposited PM components and their concurrent effects on ecological
receptors.

    Taking into consideration the responses to specific questions
regarding the adequacy of the current secondary PM standards for
ecological effects, the Policy Assessment concludes that the available
information is insufficient to assess the adequacy of the protection
for ecosystems afforded by the current suite of PM secondary standards
(U.S. EPA, 2011a, p. 5-24). Ecosystem effects linked to PM are
difficult to determine because the changes may not be observed until

pollutant deposition has occurred for many decades. Because the high
levels necessary to cause injury occur only near a few limited point
sources and/or on a very local scale, protection against these effects
alone may not provide sufficient basis for considering a separate
secondary NAAQS based on the ecological effects of particulate metals
and organics. Data on ecological responses clearly linked with
atmospheric PM is not abundant enough to perform a quantitative
analysis although the WACAP study may represent an opportunity for
quantification at a regional scale. The Policy Assessment further
concludes that available evidence is not sufficient for establishing a
distinct national standard for ambient PM based on ecosystem effects of
particulates not addressed in the NOX/SOX
secondary review (e.g., metals, organics) (U.S. EPA, 2011a, p. 5-24).
    The Policy Assessment considered the appropriateness of continuing
to use the PM2.5 and PM10 size fractions as the
indicators for protection of ecological effects of PM. The chemical
constitution of individual particles can be strongly correlated with
size, and the relationship between particle size and particle
composition can be quite complex, making it difficult in most cases to
use particle size as a surrogate for chemistry. At this time it remains
to be determined as to what extent PM secondary standards focused on a
given size fraction would result in reductions of the ecologically
relevant constituents of PM for any given area. Nonetheless, in the
absence of information that provides a basis for specific standards in
terms of particle composition, the Policy Assessment concludes that
observations continue to support retaining an appropriate degree of
control on both fine and coarse particles to help address effects to
ecosystems and ecosystem components associated with PM (U.S. EPA,
2011a, p. 5-24).
3. Materials Damage
    Welfare effects on materials associated with deposition of PM
include both physical damage (materials damage effects) and impaired
aesthetic qualities (soiling effects). Because the effects of PM are
exacerbated by the presence of acidic gases and can be additive or
synergistic due to the complex mixture of pollutants in the air and
surface characteristics of the material, this discussion will also
include those particles and gases that are associated with the presence
of ambient oxides of nitrogen and oxides of sulfur, as well as reduced
forms of nitrogen (such as ammonia and ammonium ions) for completeness.
Building upon the information presented in the last PM Staff Paper
(U.S. EPA, 2005), and including the limited new information presented
in Chapter 9 of the PM Integrated Science Assessment (U.S. EPA, 2009a)
and Annex E. Effects of NOY, NHX, and
SOX on Structures and Materials of the Integrated Science
Assessment for Oxides of Nitrogen and Sulfur-Ecological Criteria (U.S.
EPA, 2008c) the following sections consider the policy-relevant aspects
of physical damage and aesthetic soiling effects of PM on materials
including metal and stone.
    The Integrated Science Assessment concludes that evidence is
sufficient to support a causal relationship between PM and effects on
materials (U.S. EPA, 2009a, sections 2.5.4 and 9.5.4). The deposition
of PM can physically affect materials, adding to the effects of natural
weathering processes, by potentially promoting or accelerating the
corrosion of metals, by degrading paints and by deteriorating building
materials such as stone, concrete and marble (U.S. EPA, 2009a, section
9.5). Particles contribute to these physical effects because of their
electrolytic, hygroscopic and acidic properties, and their ability to
sorb corrosive gases (principally sulfur dioxide). In addition, the
deposition of ambient PM can reduce the aesthetic appeal of buildings
and objects through soiling. Particles consisting primarily of
carbonaceous compounds cause soiling of commonly used building
materials and culturally important items such as statues and works of
art. Soiling is the deposition of particles on surfaces by impingement,
and the accumulation of particles on the surface of an exposed material
that results in degradation of its appearance (U.S. EPA, 2009a, section
9.5). Soiling can be remedied by cleaning or washing, and depending on
the soiled material, repainting.
    The majority of available new studies on materials effects of PM
are from outside the U.S., however, they provide limited new data for
consideration of the secondary standard.
    Metal and stone are also susceptible to damage by ambient PM.
Considerable research has been conducted on the effects of air
pollutants on metal surfaces due to the economic importance of these
materials, especially steel, Zn, Al, and Cu. Chapter 9 of the PM
Integrated Science Assessment and Annex E of the NOX/
SOX Integrated Science Assessment summarize the results of a
number of studies on the corrosion of metals (U.S. EPA, 2009a; U.S.
EPA, 2008c). Moisture is the single greatest factor promoting metal
corrosion, however, deposited PM can have additive, antagonistic or
synergistic effects. In general, sulfur dioxide is more corrosive than
oxides of nitrogen although mixtures of oxides of nitrogen, sulfur
dioxide and other particulate matter corrode some metals at a faster
rate than either pollutant alone (U.S. EPA, 2008c, Annex E.5.2).
Information from both the PM Integrated Science Assessment and
NOX/SOX Integrated Science Assessment suggest
that the extent of damage to metals due to ambient PM is variable and
dependent upon the type of metal, prevailing environmental conditions,
rate of natural weathering and presence or absence of other pollutants.
    The PM Integrated Science Assessment and NOX/
SOX Integrated Science Assessment summarize the results of a
number of studies on PM and stone surfaces. While it is clear from the
available information that gaseous air pollutants, in particular sulfur
dioxide, will promote the deterioration of some types of stones under
specific conditions, carbonaceous particles (non-carbonate carbon) and
particles containing metal oxides may help to promote the decay
process. Studies on metal and stone summarized in the Integrated
Science Assessment do not show an association between particle size,
chemical composition and frequency of repair.
    A limited number of new studies available on materials damage
effects of PM since the last review consider the relationship between
pollutants and biodeterioration of structures associated with microbial
communities that colonize monuments and buildings (U.S. EPA, 2009a,
section 9.5). Presence of air pollutants may synergistically enhance
microbial deterioration processes. The role of heterotrophic bacteria,
fungi and cyanobacteria in biodeterioration varied by local
meterological conditions and pollutant components.
    Particulate matter deposition onto surfaces such as metal, glass,
stone and paint can lead to soiling. Soiling results when PM
accumulates on an object and alters the optical characteristics
(appearance). The reflectivity of a surface may be changed or presence
of particulates may alter light transmission. These effects can impact
the aesthetic value of a structure or result in reversible or
irreversible damage to statues, artwork and architecturally or
culturally significant buildings. Due to soiling of building surfaces
by PM, the frequency and duration of cleaning may be increased. Soiling
affects the aesthetic appeal of painted surfaces. In addition to
natural

factors, exposure to PM may give painted surfaces a dirty appearance.
Pigments in works of art can be degraded or discolored by atmospheric
pollutants, especially sulfates (U.S. EPA, 2008c, Annex E-15).
    Formation of black crusts due to carbonaceous compounds and buildup
of microbial biofilms results in discoloration of surfaces. Black crust
includes a carbonate component derived from building material and OC
and EC. In limited new studies quantifying the organic carbon and
elemental contribution to soiling by black crust, organic carbon
predominated over elemental carbon at almost all locations (Bonazza et
al., 2005). Limited new studies suggest that traffic is the major
source of carbon associated with black crust formation (Putaud et al.,
2004) and that soiling of structures in Oxford, UK showed a
relationship with traffic and nitrogen dioxide concentrations (Viles
and Gorbushina, 2003). These findings attempt to link atmospheric
concentrations of PM to observed damage. However, no data on rates of
damage are available and all studies were conducted outside of the U.S.
    Based on the above discussion, the Policy Assessment makes the
following observations:

    (1) Materials damage and soiling that occur through natural
weathering processes are enhanced by exposure to atmospheric
pollutants, most notably sulfur dioxide and particulate sulfates.
    (2) While ambient particles play a role in the corrosion of
metals and in the weathering of materials, no quantitative
relationships between ambient particle concentrations and rates of
damage have been established.
    (3) While soiling associated with fine and course particles can
result in increased cleaning frequency and repainting of surfaces,
no quantitative relationships between particle characteristics and
the frequency of cleaning or repainting have been established.
    (4) Limited new data on the role of microbial colonizers in
biodeterioration processes and contributions of black crust to
soiling are not sufficient for quantitative analysis.
    (5) While several studies in the PM Integrated Science
Assessment and NOX/SOX Integrated Science
Assessment suggest that particles can promote corrosion of metals
there remains insufficient evidence to relate corrosive effects to
specific particulate levels or to establish a quantitative
relationship between ambient PM and metal degradation. With respect
to damage to calcareous stone, numerous studies suggest that wet or
dry deposition of particles and dry deposition of gypsum particles
can enhance natural weathering processes.

    Revisiting the overarching policy question as to whether the
available scientific evidence supports or calls into question the
adequacy of the protection for materials afforded by the current suite
of secondary PM standards, the Policy Assessment concludes that no new
evidence in this review calls into question the adequacy of the
protection for materials afforded by the current standard (U.S. EPA,
2011a, p. 5-29). PM effects on materials can play no quantitative role
in considering whether any revisions of the secondary PM NAAQS are
appropriate at this time. Nonetheless, in the absence of information
that provides a basis for establishing a different level of control,
the Policy Assessment concludes that observations continue to support
retaining an appropriate degree of control on both fine and coarse
particles to help address materials damage and soiling associated with
PM (U.S. EPA, 2011a, p. 5-29).
4. CASAC Advice
    Regarding the other non-visibility welfare effects, CASAC stated
that it ``concurs with the Policy Assessment's conclusions that while
these effects are important, and should be the focus of future research
efforts, there is not currently a strong technical basis to support
revisions of the current standards to protect against these other
welfare effects'' (Samet, 2010c). More specifically, with regard to
climate impacts, CASAC concludes that while there is insufficient
information on which to base a national standard, the causal
relationship is established and the risk of impacts is high, so further
research on a regional basis is urgently needed (Samet, 2010c, p. 5).
CASAC also notes that reducing certain aerosol components could lead to
increased radiative forcing and regional climate warming while having a
beneficial effect on PM-related visibility. As a consequence, CASAC
notes that a secondary standard directed toward reducing PM-related
visibility impairment has the potential to be accompanied by regional
warming if light scattering aerosols are preferentially targeted.
    With regard to ecological effects, CASAC concludes that the
published literature is insufficient to support a national standard for
PM effects on ecosystem services (Samet, 2010c, p.23). CASAC notes that
the best-established effects are related to particles containing
nitrogen and sulfur, which are being considered in the EPA's ongoing
review of the secondary NAAQS for NOX/SOX. With
regard to PM-related effects on materials, CASAC concludes that the
published literature, including literature published since the last
review, is insufficient either to call into question the current level
of the standard or to support any specific national standard for PM
effects on materials (Samet, 2010c, p.23). Nonetheless, with regard to
both types of effects, CASAC notes the importance of maintaining an
appropriate degree of control of both fine and coarse particles to
address such effects, even in the current absence of sufficient
information to develop a standard.
5. Administrator's Proposed Conclusions on Secondary Standards for
Other PM-related Welfare Effects
    Based on the above considerations and the advice of CASAC, the
Administrator provisionally concludes that it is not appropriate to
establish any distinct secondary PM standards to address other non-
visibility PM-related welfare effects. Nonetheless, the Administrator
concurs with the conclusions of the Policy Assessment and CASAC advice
that it is important to maintain an appropriate degree of control of
both fine and coarse particles to address such effects, including
ecological effects, effects on materials, and climate impacts. In the
absence of information that would support any different standards, the
Administrator proposes generally to retain the current suite of
secondary PM standards\182\ to address non-visibility welfare effects.
These secondary standards were set identical to the primary PM
standards in the last review. More specifically, the Administrator
proposes to retain all aspects of the current 24-hour PM2.5
and PM10 standards. With regard to the secondary annual
PM2.5 standard, the Administrator proposes to retain the
level of the current standard and to revise the form of the standard by
removing the option for spatial averaging for the reasons discussed
below in section VII.A. 2. In so doing, she notes that no areas in the
country are currently using the option for spatial averaging to
demonstrate attainment with the secondary annual PM2.5
standard.
---------------------------------------------------------------------------

    \182\ As summarized in section VI.A and Table 1 above, the
current suite of secondary PM standards includes annual and 24-hour
PM2.5 standards and a 24-hour PM10 standard.
---------------------------------------------------------------------------

F. Administrator's Proposed Decisions on Secondary PM Standards

    With regard to the secondary PM standards, the Administrator
proposes to revise the suite of secondary PM standards by adding a
distinct standard for PM2.5 to address PM-related visibility
impairment, focused primarily on visibility in urban areas. This
distinct secondary standard is defined

in terms of a calculated PM2.5 light extinction indicator,
translated into the deciview scale, which is referred to as a
PM2.5 visibility index; a 24-hour averaging time; a 90th
percentile form, averaged over 3 years; and a level set at one of two
options--either 30 dv or 28 dv. The Administrator solicits comment on a
range of levels for such a standard down to 25 dv, as well as on
alternative standards to address PM-related visibility impairment,
including a sub-daily averaging time (e.g., 4 hours) and associated
alternative levels in the range of 30 to 25 dv. To address other non-
visibility welfare effects, the Administrator proposes to revise the
form of the secondary annual PM2.5 standard to remove the
option for spatial averaging and to retain all other elements of the
current suite of secondary PM standards.

VII. Interpretation of the NAAQS for PM

    With regard to the NAAQS for PM2.5, this section
discusses EPA's proposed revisions to the data handling procedures in
40 CFR part 50, appendix N, for the proposed primary and secondary
annual and 24-hour standards for PM2.5 (referred to as
PM2.5 standards) and for the proposed distinct secondary
standard for PM2.5 to address PM-related visibility
impairment (referred to as the PM2.5 visibility index
standard).\183\ Appendix N describes the computations necessary for
determining when these standards are met and also addresses which
measurement data are appropriate for comparison to the proposed
standards, as well as data reporting protocols, data completeness
criteria, and rounding conventions.
---------------------------------------------------------------------------

    \183\ With regard to the PM10 NAAQS, as summarized in
sections IV.F and VI.F, the EPA is proposing to retain the current
primary and secondary PM10 standards. Data handling
procedures for these PM10 standards would remain as
presented in 40 CFR part 50, appendix K.
---------------------------------------------------------------------------

    As discussed in sections III and VI above, the EPA is proposing to:
(1) Revise the form and level of the primary annual PM2.5
standard, and retain the current primary 24-hour PM2.5
standard (section III.F); (2) retain the current secondary 24-hour
PM2.5 standard, and revise the form and retain the level of
the secondary annual PM2.5 standard for non-visibility-
related welfare protection (section VI.F); and (3) establish a distinct
secondary PM2.5 visibility index standard (section VI.F).
The EPA proposes to revise appendix N to conform to the proposed
revisions to the standards. The Agency also proposes to make additional
changes in the appendix N data handling provisions to codify existing
practices currently included in guidance documents or implemented as
EPA standard operating procedures as well as to provide greater clarity
and consistency in the application of these provisions. The proposed
revisions to appendix N are discussed in section VII.A below.
    Section 1(b) of appendix N refers to special considerations that
may be given to data resulting from exceptional events. An exceptional
event is defined in 40 CFR 50.1 as an event that affects air quality,
is not reasonably controllable or preventable, is an event caused by
human activity that is unlikely to recur at a particular location or a
natural event, and is determined by the Administrator in accordance
with 40 CFR 50.14 to be an exceptional event. Air quality data that are
determined to have been affected by an exceptional event under the
procedural steps, substantive criteria, and schedule specified in
section 50.14 may be excluded from consideration when EPA makes a
determination that an area is meeting or violating the associated
NAAQS. Proposed revisions to the schedule specified in section 50.14
for data flagging and submission of demonstrations for exceptional
events data considered for initial area designations under the proposed
revised primary and secondary PM standards are discussed in section
VII.B below.
    Several proposed updates and clarifications to the data handling
provisions associated with AQI reporting in 40 CFR part 58, Appendix G
are discussed in section VII.C below. These modifications reflect the
proposed changes to the AQI sub-index for PM2.5 as discussed
in section V above and harmonize reporting procedures for AQI sub-
indices for other criteria pollutants.

A. Proposed Amendments to Appendix N: Interpretation of the NAAQS for
PM2.5

    As discussed below, the proposed revisions to appendix N
corresponding to proposed changes in the standards addressed in
sections III and VI above are: (1) Modification of the level of the
primary annual PM2.5 standard (sections VII.A.1 and
VII.A.4); (2) modification of the form of the primary and secondary
annual PM2.5 standards to remove the option for spatial
averaging (sections VII.A.2 and VII.A.4); and (3) addition of data
handling procedures that detail how to make comparisons to the proposed
secondary standard for PM2.5 that addresses PM-related
visibility impairment (section VII.A.5), as well as to summarize
associated changes proposed in other sections of appendix N to
accommodate this proposed standard (sections VII.A.1, VII.A.2, and
VII.A.3). In addition to these three proposed appendix modifications
that are discussed in depth in sections III and VI above, the EPA also
proposes additional revisions to appendix N in order to: (1) Better
align appendix N language and requirements with proposed changes in the
PM2.5 ambient monitoring and reporting requirements as
discussed in section VIII below; (2) enhance consistency with recently
codified changes in data handling procedures for other criteria
pollutants; (3) codify existing practices currently included in
guidance documents or implemented as EPA standard operating procedures;
and (4) provide enhanced clarity and consistency in the articulation
and application of appendix N provisions. Key elements of the proposed
revisions to appendix N are summarized in sections VII.A.1 through
VII.A.5 below, where each of these preamble sections corresponds to the
similarly numbered section in appendix N.
1. General
    The EPA proposes to modify section 1.0 of appendix N to provide
additional clarity regarding the scope and interpretation of the NAAQS
for PM2.5. This section would reference the proposed
revisions to the primary annual PM2.5 standard and the
proposed revision to the form of the secondary annual PM2.5
standard (40 CFR 50.18) and the proposed addition of a distinct
secondary PM2.5 visibility index standard (40 CFR 50.19). As
summarized in section VI.F, the proposed secondary standard is defined
in terms of a calculated PM2.5 light extinction indicator,
which would use 24-hour average speciated PM2.5 mass
concentration data, along with associated relative humidity
information, to calculate light extinction, which would then be
translated to the deciview scale (referred to as a PM2.5
visibility index); a 24-hour averaging time; a 90th percentile form
averaged over 3 years; and a level of either 30 dv or 28 dv. The result
(i.e., the PM2.5 visibility index design value) would be
compared to the level of the standard. As noted earlier, the NAAQS
indicator and proposed data handling procedures are similar to those of
the Regional Haze Program. The EPA proposes to add to section 1.0 of
appendix N, a reference to section 2.9 of appendix C to 40 CFR part 58
which identifies the acceptable methods for the speciated
PM2.5 mass concentration data. With regard to the appendix N
term definitions which are delineated in this initial section, the EPA
proposes to

add, modify, or eliminate term definitions, as appropriate, in
accordance with the proposed data handling rule revisions such as the
addition of terms associated with the proposed secondary
PM2.5 visibility index standard and the modification of
terms that referenced spatial averaging. Additional term definitions
are also being added to reference otherwise unchanged appendix N logic
in an effort to streamline the appendix text, enhance clarity and thus
improve readability and understanding.
2. Monitoring Considerations
    The EPA proposes revisions to section 2.0 of appendix N consistent
with the proposed modification of the form of the primary annual
PM2.5 standard to remove the option for spatial averaging.
As described in more detail in section III.E.3.a above, the EPA is
proposing to remove this option as part of the form of the primary
annual PM2.5 standard. This proposed change is based on an
analysis that indicates the existing constraints on spatial averaging,
as modified in 2006, may be inadequate to avoid substantially greater
exposures in some areas, potentially resulting in disproportionate
impacts on susceptible populations (Schmidt 2011a, Analysis A).
    With respect to the form of the secondary annual PM2.5
standard, while, as discussed in section VI.E.5 above, the EPA is
proposing to retain the current secondary annual PM2.5
standard to provide protection for non-visibility welfare effects, the
EPA believes it would be reasonable and appropriate to align the data
handling procedures for the primary and secondary annual
PM2.5 standards. Therefore, the EPA proposes to remove the
option for spatial averaging for the secondary annual PM2.5
standard consistent with the proposed change in the form of the primary
annual PM2.5 standard. The EPA notes that no areas in the
country are currently using the option for spatial averaging to
demonstrate attainment with the secondary annual PM2.5
standard.
    Consistent with the proposed change to revise the forms of the
primary and secondary annual PM2.5 standards, the levels of
the standards would be compared to measurements from each appropriate
(i.e., ``eligible'') monitoring site in an area operated in accordance
with the network technical requirements specified in 40 CFR 58.11, the
operating schedule described in 40 CFR 58.12, and the special
considerations for data comparisons to the NAAQS specified in 40 CFR
58.30, with no allowance for spatial averaging. Thus, for an area with
multiple eligible monitoring sites, the site with the highest design
value would determine the attainment status for that area. As a result
of this proposed change, the EPA proposes to remove all references to
the spatial averaging option throughout appendix N.
3. Requirements for Data Use and Reporting for Comparisons With the
NAAQS for PM2.5
    The EPA proposes to make changes to section 3.0 of appendix N to
correspond with the proposed secondary PM2.5 visibility
index standard, to improve consistency with procedures used for other
NAAQS, and to improve consistency with current standard operating
procedures. Specifically, the EPA proposes revisions to this section
regarding: (1) Requirements for reporting monitored aggregated
PM2.5 and speciated PM2.5 mass concentration
data; (2) clarification of monitoring data appropriate to compare to
the PM2.5 and PM2.5 visibility index NAAQS; (3)
clarification of procedures for using hourly concentrations to
calculate 24-hour concentrations; and (4) clarification of procedures
for combining monitoring data from collocated instruments into a single
``combined site'' record. Further, the EPA proposes to codify, in this
same section, modifications to the PM2.5 data handling
provisions to make them consistent with recent changes made for other
criteria pollutants. For example, data for which the certification
deadline has passed, and the monitoring agency has not requested
certification of the data, can nevertheless be used to determine
compliance with the PM2.5 NAAQS and the PM2.5
visibility index NAAQS when EPA judges the data to be complete and
accurate.
    With regard to the criteria for reporting PM2.5
concentrations, section 3.0 of appendix N specifies that
PM2.5 mass concentrations used for NAAQS comparisons shall
be reported in units of [micro]g/m\3\ with the values truncated (not
rounded) to one digit to the right of the decimal point (i.e.,
truncated to one decimal place). Since, to date, appendix N has dealt
only with PM2.5 mass concentrations, intrinsically these
requirements have dealt only with that particular set of data.
    With regard to the proposed secondary PM2.5 visibility
index standard, the EPA already has a requirement in 40 CFR 58.16 to
report speciated PM2.5 mass concentration data. This
includes the nine required speciated PM2.5 mass
concentration inputs (i.e., sulfate, nitrate, OC (and related
PM2.5 OC which is reported OC with an adjustment for the
organic carbon artifact present on a filter), EC, Al, Si, Ca, Fe, and
Ti) used to calculate PM2.5 visibility index values as
described in section VII.A.5 below. Specifically, the EPA proposes to
require that all nine parameters be used in the appendix N procedures
in units of [micro]g/m\3\ with the values rounded to four decimal
places (or three significant digits if the value is 0.1 [micro]g/m\3\
or larger). These rounding conventions are consistent with the AQS
reporting protocols used in the CSN program, discussed in section
VIII.A.2 below, which is proposed to be a major source of ambient data
used in calculating PM2.5 visibility index design values to
compare to the level of proposed secondary NAAQS.
    Monitoring sites eligible for comparison to the NAAQS for
PM2.5 include those following the network technical
requirements specified in 40 CFR 58.11 as well as following the
eligibility criteria specified in 40 CFR 58.30.\184\ However, as
discussed in section VIII.A.1 below, an analysis of the quality of data
from two different methods used by FEMs has indicated that some sites
with continuous PM2.5 FEMs have an acceptable degree of
comparability with collocated FRMs, while other FEMs have less
acceptable data comparability that would not meet the performance
criteria originally used to approve the FEMs (Hanley and Reff, 2011).
Therefore, as explained in more detail in section VIII.B.3.b.ii below,
the EPA is proposing to allow monitoring agencies to identify
PM2.5 FEMs that are not providing data of sufficient
comparability to the FRM and, with EPA approval, to exclude the use of
these data in making comparisons to the NAAQS for
PM2.5.\185\
---------------------------------------------------------------------------

    \184\ As discussed in more detail in section VIII.B.2.b below,
the EPA is proposing to change the current presumption in 40 CFR
58.30 that micro- and middle-scale monitoring sites are ``unique''
and are comparable only to the 24-hour PM2.5 standards,
unless approved by the Regional Administrator to collectively
identify a larger region of localized high ambient PM2.5
concentrations. Today's proposal, if finalized, would change this
presumption, such that micro- and middle-scale monitoring sites
would not be presumed to be unique and, therefore, would be
comparable to the annual PM2.5 standards as well as the
24-hour PM2.5 standards, unless the Regional
Administrator determines that the micro- or middle-scale site is
unique.
    \185\ The EPA also allows use of alternative methods where
explicitly stated in the monitoring methodology requirements
(appendix C of 40 CFR part 58), such as PM2.5 Approved
Regional Methods (ARMs) which can be used to determine compliance
with the NAAQS. Monitoring agencies identifying ARMs that are not
providing data of sufficient quality would also be allowed to
exclude these data in making comparisons to the PM2.5 and
PM2.5 visibility index NAAQS. Currently, there are no
designated ARMs for PM2.5.

---------------------------------------------------------------------------

    With regard to data handling procedures for using hourly mass
concentrations to calculate 24-hour average mass concentrations,
current procedures are specific for handling aggregated
PM2.5 mass concentrations and are not currently relevant for
handling the speciated PM2.5 mass concentrations that would
be used for calculating PM2.5 visibility index design values
for the proposed secondary standard. In considering data handling
procedures for hourly speciated PM2.5 mass concentrations,
the EPA notes that the vast majority of speciation data collected
across the country are from filter-based sampling methods which
typically operate on a 24-hour sampling period. There are several
monitoring sites reporting hourly speciation data, but even in these
cases the methods employed only provide for a small number of
speciation parameters (e.g., EC, OC, sulfate) to be reported. However,
in anticipation that such continuous methods might be more widely
implemented for the speciated PM2.5 mass components in the
future, the EPA proposes to add clarifying language to section 3.0(a)
to indicate that the data handling procedures for using hourly
concentration data to calculate 24-hour average concentration data
would be applicable to both aggregated PM2.5 mass
concentrations and speciated PM2.5 mass concentrations.
    With respect to the procedures for combining monitored data from
collocated instruments into a single ``combined site'' data record, the
EPA proposes to revise the current methodology in situations where an
FRM monitor operating on a non-daily schedule is collocated with a
continuous FEM monitor (that has acceptable comparability with an FRM).
The EPA is not proposing to change the procedures for calculating a
combined site record \186\ but rather the subsequent evaluation of
whether the specific measurements are considered ``creditable'' or
``extra'' samples. Samples in the combined site record are deemed
``creditable'' or ``extra'' according to the required sampling
frequency for a specific monitoring site (i.e., ``site-level sampling
frequency'') which, by default, is defined to be the same as the
sampling frequency required of the primary monitor. Samples in the
combined site data record that correspond to scheduled days according
to the site-level sampling frequency are deemed ``creditable'' and,
thus, are considered for determining whether or not a specific
monitoring site meets data completeness requirements. These samples
also determine which daily value in the ranked list of daily values for
a year represents the annual 98th percentile concentration. Samples
that are not deemed ``creditable'' are classified as ``extra'' samples.
These samples do not count towards data completeness requirements and
do not affect which daily values represent the annual 98th percentile
concentration; ``extra'' samples, however, are candidates for selection
as the 98th percentile.
---------------------------------------------------------------------------

    \186\ Data for a combined site record originates by default from
the designated ``primary'' monitor at the site location and is then
augmented with data from collocated FRM or FEM monitors whenever
valid data are not generated by the primary monitor.
---------------------------------------------------------------------------

    Before the introduction of continuous PM2.5 FEMs, when
two or more samplers were collocated at the same site, monitoring
agencies typically identified the sampler that operated on the more
frequent sampling schedule as the ``primary'' monitor for developing a
single site record. However, due to concerns regarding the
comparability of continuous PM2.5 FEMs to FRMs operated in
some monitoring agency networks, and as briefly discussed above and in
more detail in section VIII.A.1 below, many monitoring agencies have
kept the FRM as the ``primary'' monitor while continuing to evaluate
the continuous FEM monitor. In cases where the FRM either does not have
a scheduled measurement or has a measurement that is invalidated and
the continuous FEM data are available for use, and the continuous FEM
data are not identified as not to be used (i.e., a special purpose
monitor (SPM) in its first 24 months of operation) the FEM data will be
substituted into the site record. In cases where the continuous FEM
measurements are reported on the FRM ``off'' days, these data are
technically considered ``extra'' samples.
    In light of this practice, the EPA modified standing operating
procedures and now proposes a conforming revision to section 3.0(e)
whereby collocated FEM samples reported on the FRM ``off'' days would
be considered ``scheduled'' and ``creditable.'' Thus, collocated FEM
samples would count towards data capture rates (actually, increasing
both the numerator and the denominator in the capture rate equation),
and also would count towards identifying annual 98th percentile
concentrations. Further, consistent with current practices, if data
from a collocated FEM are missing on an FRM ``off'' day (and no
unscheduled FRM data are reported that day), the EPA proposes not to
identify these as ``scheduled'' samples. Thus, reported data generated
from the collocated continuous FEMs can only help increase data capture
rates. The EPA specifically solicits comment on whether ``non-primary''
(i.e., collocated) FEM data should be combined with the primary data as
part of the comparison to the NAAQS for PM2.5.
    The EPA proposes to utilize the same general procedures for
combining speciated PM2.5 mass concentration data from
collocated monitors into a single ``combined site'' record as those
specified for the PM2.5 mass measurements.
4. Comparisons With the Annual and 24-Hour PM2.5 NAAQS
    Section 4.0 of appendix N specifies the procedures for comparing
monitored data to the annual and 24-hour PM2.5 standards.
The EPA proposes revisions to section 4.0 of appendix N to: (1) Provide
consistency with the proposed primary and secondary annual
PM2.5 standards; (2) expand the data completeness
assessments to be consistent with current guidance and standard
operating procedures; and (3) simplify the procedure for calculating
annual 98th percentile concentrations when using an approved seasonal
sampling schedule.
    Consistent with the proposed decisions to revise the level of the
primary annual PM2.5 standard (section III.F) and to retain
the current level of the secondary annual PM2.5 standard
(section VI.F), the EPA proposes to modify section 4.1(a) of appendix N
to separately list the levels of the primary and secondary annual
PM2.5 standards. Additionally, consistent with the proposed
decision to remove the option for spatial averaging for the primary
annual PM2.5 standard (section III.F) as well as for the
secondary annual PM2.5 standard (section VII.A.2), the EPA
proposes to amend section 4.4 of appendix N to remove equations and
associated instructions that relate to spatial averaging.
    With regard to assessments of data completeness, the EPA proposes
to include two additional data substitution tests \187\ (making a total
of three data substitution tests) for validating annual and 24-hour
PM2.5 design values otherwise deemed incomplete (via the 75
percent and 11 creditable sample minimum quarterly data completeness
checks). Data substitution tests are diagnostic in nature; that is;
they are only used in an illustrative manner to

show that the NAAQS status based on incomplete data is reasonable. If
an ``incomplete'' design value using substituted data passes the
diagnostic test, this ``incomplete'' design value (without the data
substitutions) is then considered the true actual ``complete'' design
value. If an incomplete design value does not pass any stipulated data
substitution test, then the original design value is still considered
incomplete.
---------------------------------------------------------------------------

    \187\ Data substitution tests are supplemental data completeness
assessments that use estimates of 24-hour average concentrations to
fill in for missing data (i.e., ``data substitution'').
---------------------------------------------------------------------------

    Currently, section 4.1(c) specifies one data substitution test for
validating an otherwise incomplete design value. This diagnostic test
is only applicable to the primary and secondary annual PM2.5
standard and only applies in instances of a violation. The EPA proposes
to modify the data completeness requirements by adding two additional
data substitution tests for handling incomplete data sets in order to
make the data handling procedures for PM2.5 more consistent
with the procedures used for other NAAQS pollutants and to codify
existing practices currently included in guidance documents (U.S. EPA,
1999) and implemented as EPA standard operating procedures. The
proposed additional data substitution tests would be applicable for
making comparisons to the primary and secondary annual and 24-hour
PM2.5 standards. One of these tests uses collocated
PM10 data to fill in ``slightly incomplete'' \188\ data
records, and the other uses quarter-specific maximum values to fill in
``slightly incomplete'' data records.
---------------------------------------------------------------------------

    \188\ ``Slightly incomplete'' is defined as less than 75 percent
but greater than or equal to 50 percent data capture.
---------------------------------------------------------------------------

    With regard to identifying annual 98th percentile concentrations
for comparison to the primary and secondary 24-hour PM2.5
standards, the EPA proposes to simplify the procedures used with an
approved seasonal sampling schedule. Specifically, the EPA proposes to
eliminate the use of a special formula for calculating annual 98th
percentile concentrations with a seasonal sampling schedule and
proposes to use only one method for calculating annual 98th percentile
concentrations at all sites.
    Currently, with an approved seasonal sampling schedule, a site
typically samples as required during periods of the year when the
highest concentrations are expected to occur, but less frequently
during periods of the year when lower concentrations are expected to
occur. This type of sampling schedule generally leads to an
``unbalanced'' data record; that is, a data record with proportionally
more ambient measurements (with respect to the total number of days in
the sampling period) in the ``high'' season and proportionally fewer
ambient measurements in the ``low'' season.
    In the last review, the EPA revised section 4.5 of appendix N to
include a special formula for computing annual 98th percentile values
when a site operates on an approved seasonal sampling schedule. This
special formula accounted for an unbalanced data record and was
consistent with guidance documentation (U.S. EPA, 1999), and, where
appropriate, with official OAQPS design value calculations (71 FR
61211, October 17, 2006). In cases where there is a balanced \189\ (or
near-balanced) data record, the special formula yields the same result
as the regular procedure for calculating annual 98th percentile
concentrations.
---------------------------------------------------------------------------

    \189\ A balanced data record has the same proportion of ambient
measurements (with respect to the total number of days in the
sampling period) in the ``high'' season as in the ``low'' season.
---------------------------------------------------------------------------

    To qualify for a seasonal sampling schedule, monitoring agencies
are required to collocate a continuous PM2.5 instrument with
the seasonal sampling FRM. Since the last review, there has been
considerable deployment of continuous PM2.5 FEM monitors. In
situations where a PM2.5 FRM monitor operating on a non-
daily periodic schedule (such as a 1-day-in-3 or a 1-day-in-6 schedule)
is collocated with a continuous PM2.5 FEM monitor, data are
combined based on procedures stated in section 3.0 of appendix N as
modified as discussed in section VII.A.3 above. The end result of
combining collocated FRM and FEM data is effectively an ``every day''
site-based sampling frequency, resulting in a balanced data record. In
such a case, if a site used a seasonal sampling schedule regime for the
FRM monitor, these data would be balanced by the ``every day'' FEM data
and there would be no need for the special formula for calculating
annual 98th percentile concentrations on the combined site data.
    The EPA notes that currently there are very few PM2.5
FRM monitors that actually operate on an approved seasonal sampling
schedule (only 15 sites out of approximately 1,000 total sites in 2010)
and that almost half of these sites have a collocated PM2.5
FEM monitor. For the most recent 3-year period (2008-2010), the annual
98th percentile concentrations calculated with the special formula at
these 15 sites were approximately five percent lower than if the
regular procedure was used. The EPA also notes that, in the last
review, the Agency modified the monitoring requirements for areas with
an FRM operating on a non-daily schedule such that, if the design
values were within five percent of the 24-hour PM2.5 NAAQS,
such areas are required to increase the frequency of sampling to every
day (40 CFR 58.12(d)(1); 71 FR 61165, October 17, 2006; 71 FR 61249,
October 17, 2006). Thus, the EPA proposes to simplify the data handling
procedures for sites operating on a seasonal sampling schedule by
eliminating the special formula and all references to it based on: (1)
The small difference between 98th percentile concentrations calculated
using the special formula versus the regular procedure and the small
number of sites currently using the special formula; (2) the EPA
requirements for every day sampling in areas with design values that
are within five percent of the 24-hour PM2.5 NAAQS; and (3)
the EPA requirement that FRMs operating on an approved seasonal
sampling schedule be collocated with a continuous PM2.5
instrument (and if that instrument were an FEM, the resulting combined
site record would tend to be balanced over the year and thus the
special formula would be superfluous). Thus, the EPA proposes to use
only one method for calculating annual 98th percentile concentrations
for all sites, that being the ``regular'' table look-up method
specified in section 4.5(a)(1) of appendix N. The EPA solicits comment
on the proposal to eliminate the special formula for sites operating on
a seasonal sampling schedule.
5. Data Handling Procedures for the Proposed Secondary PM2.5
Visibility Index NAAQS
    As summarized in section VI.F above, the EPA is proposing to
establish a distinct secondary standard for PM2.5 to address
PM-related visibility impairment. The EPA is proposing to define this
standard in terms of a PM2.5 visibility index (section
VI.D.1.c), which would use 24-hour average speciated PM2.5
mass concentration and historic monthly average relative humidity data
to calculate PM2.5 light extinction, translated into the
deciview scale, similar to the Regional Haze Program.
    The EPA proposes to add a new section 5.0 to appendix N to detail
the data handling procedures for calculating PM2.5
visibility index design values and comparing these design values to the
level of the proposed PM2.5 visibility index NAAQS. These
proposed procedures are drawn from and are generally consistent with
the original approach used in the Regional Haze Program [U.S. EPA,
2003] and discussed

in the Policy Assessment (U.S. EPA, 2011a, chapter 4, Appendix G).
    As discussed in section VI.B.1.a above, visibility impairment is
caused by the scattering and absorption of light by suspended particles
and gases in the atmosphere. The combined effect of light scattering
and absorption by both particles and gases is characterized as light
extinction. The amount of light extinction contributed by PM depends on
the particle size distribution and composition, as well as the
concentrations of speciated components of ambient PM. To make
estimation of light extinction more practical, visibility scientists
have developed simple algorithms, referred to as the IMPROVE algorithms
to relate speciated PM2.5 concentrations to light
extinction. These IMPROVE algorithms are routinely used to calculate
light extinction levels on a 24-hour basis in Federal Class I areas
under the Regional Haze Program.
    The EPA proposes to define the PM2.5 visibility index
using a PM2.5 light extinction indicator calculated on a 24-
hour basis using the original IMPROVE algorithm without the terms for
coarse mass and Rayleigh scatter. As discussed in section VI.D.1.c
above, using such an index appropriately reflects the relationship
between ambient PM and PM-related light extinction. When converting
PM2.5 light extinction values in Mm-\1\ to the
deciview scale, the Rayleigh scattering term must be included to avoid
the possibility of negative values.
    Consistent with the analyses and terminology used in the Policy
Assessment (U.S. EPA, 2011a, chapter 4, Appendix G), PM2.5
light extinction (PM2.5 bext) is defined as
[GRAPHIC] [TIFF OMITTED] TP29JN12.017

The above formula is implemented using 24-hr speciated PM2.5
concentration data together with monthly climatological relative
humidity factors as outlined below. The six steps involved in the
calculation of the PM2.5 visibility index values are as
follows:

    (1) As discussed in Section VI.B.1.a above, ``sulfate'' is
defined as ammonium sulfate and ``nitrate'' is defined as ammonium
nitrate. Multiply 24-hour average speciation measurements of sulfate
and nitrate ions by factors 1.375 and 1.29, respectively, to convert
the reported ion concentrations into sulfate and nitrate ammonium
concentrations (appendix N, equations 5a and 5b).
    (2) Convert artifact adjusted measured OC, which is termed
``PM2.5 OC'', into an estimate of organic mass (OM). The
PM2.5 OC is derived by subtracting the sampler-dependent
OC measurement artifact from the measured OC.\190\ The
PM2.5 OC is then multiplied by 1.4 to account for the
additional mass of hydrogen, oxygen and other elements associated
with the carbon in measured OC (appendix N, equation 5c).
---------------------------------------------------------------------------

    \190\ In the IMPROVE program, artifact adjusted OC (i.e.,
PM2.5 OC) is simply reported as OC. That is the value
used to produce OM for haze calculations. For the CSN measurements,
the OC artifact needed to convert measured OC into PM2.5
OC is estimated from sampler-specific network-wide field blanks
(Frank, 2012).
---------------------------------------------------------------------------

    (3) Calculate fine soil/crustal PM2.5 (FS) component
based on measurements of five soil derived elements (i.e., Al, Si,
Ca, Fe, and Ti) together with multipliers to account for their
normal oxides \191\ (appendix N, equation 5d).
---------------------------------------------------------------------------

    \191\ Fine Soil = 2.2[Al] + 2.49[Si] + 1.63[Ca] + 2.42[Fe] +
1.94[Ti]
---------------------------------------------------------------------------

    (4) Determine a representative long-term monthly average of
hourly relative humidity hygroscopic growth factors, referred to as
f(RH) values, at the speciation monitoring site, for each month of
the year. There will be 12 such values for any monitoring site. The
EPA proposes that the f(RH) values be selected using historical
data. A spatial interpolation of historical relative humidity data
is available which presents a gridded field of f(RH) values across
the U.S. at a resolution of 0.25 degrees (SAIC, 2001). As discussed
in section VI.D.2.a.ii above, these monthly average values were
developed to support the Regional Haze Program and are based on
considering any hour with relative humidity greater than 95 percent
as 95 percent. Because 10 years of hourly data were used to produce
a single humidity term for each month, the EPA believes that the
resulting monthly average of the humidity term is sufficient and
appropriate to reduce the effects of fog or precipitation. The EPA
proposes that the 10-year climatological data base be used to
specify the f(RH) value associated with the grid-point closest in
distance to the speciation monitoring site.\192\
---------------------------------------------------------------------------

    \192\ To facilitate the use of relative humidity data, the EPA
would make this ten-year climatological data base publically
available on its Web site.
---------------------------------------------------------------------------

    (5) Apply the original IMPROVE algorithm without the terms for
coarse mass and Rayleigh scatter (appendix N, equation 6) to
calculate a daily average PM2.5 light extinction
(PM2.5 bext, in units of Mm-\1\).
    (6) To translate PM2.5 light extinction to the
deciview scale for making comparisons to the level of the proposed
secondary PM2.5 visibility index standard, the following
equation, which includes the term for Rayleigh scattering term, is
used:
[GRAPHIC] [TIFF OMITTED] TP29JN12.018

The EPA solicits comment on all aspects of the calculation of the
PM2.5 visibility index, PM2.5 bext.
    As discussed in section VI.D.3 above, the EPA is proposing a 90th
percentile form, averaged over 3 years, for the proposed secondary
PM2.5 visibility index standard. Thus, 3 years of valid 24-
hr speciated PM2.5 mass concentration data would be required
to calculate PM2.5 visibility index design values. The
proposed new section 5.0 for appendix N addresses data completeness
requirements for speciated PM2.5 mass concentrations
(section 5.0(b)), specifically that PM2.5 visibility index
values be present for at least 11 creditable days of each quarter, for
each of the three consecutive years. The 11 sample minimum is
consistent with criteria specified for the current and proposed primary
and secondary annual PM2.5 standards (i.e., 40 CFR part 50,
appendix N 4.1(b)) and, furthermore, has been used extensively for
various PM characterization exercises (e.g., U.S. EPA, 2009a; U.S. EPA,
2011a). In addition, the proposed new section 5.0 outlines procedures
for identifying annual 90th percentile PM2.5 visibility
index values (section 5.0(d)(3)) similar to procedures used to identify
annual 98th percentile values for the primary

and secondary 24-hour PM2.5 standards. In situations where a
year does not contain the minimum 11 creditable samples in each
quarter, the EPA proposes (in section 5.0) to still consider the
identified 90th percentile index value to be valid if it, or a 3-year
average of 90th percentile index values (i.e., a visibility impairment
design value) including it, exceeds the level of the NAAQS. The EPA is
not proposing any data substitution tests for PM2.5
visibility index design values like those codified and proposed for the
aggregated PM2.5 mass standard design values; however, the
EPA solicits comment on the inclusion of such data substitution tests.
    With regard to rounding conventions, the EPA proposes that all
decimal digits be retained in the intermediate steps of the calculation
of the PM2.5 light extinction indicator and that the
PM2.5 visibility index values be rounded to the nearest
tenth deciview. Furthermore, the EPA proposes to round the 3-year
average 90th percentile PM2.5 visibility index design values
to the nearest 1 dv for comparison to the level of the proposed
secondary standard.
    Consistent with current procedures for PM and the other criteria
pollutants, the EPA plans to calculate design values for the proposed
secondary PM2.5 visibility index NAAQS using the procedures
described above. The EPA plans to post these design values on its Web
site.\193\
---------------------------------------------------------------------------

    \193\ Design values calculated by the EPA are computed and
published annually by EPA's OAQPS and reviewed in conjunction with
the EPA Regional Offices. These values are available at: http://www.epa.gov/airtrends/values.html.
---------------------------------------------------------------------------

B. Exceptional Events

    States \194\ are responsible for identifying air quality data that
they believe warrant special consideration, including data affected by
exceptional events. States identify such data by flagging (making a
notation in a designated field in the electronic data record) specific
values in the AQS database. States must flag the data and submit
supporting documentation showing that the data have been affected by
exceptional events if they wish the EPA to consider excluding the data
in regulatory decisions, including determining whether or not an area
is attaining the proposed revised PM NAAQS.
---------------------------------------------------------------------------

    \194\ References to ``state'' are meant to include state, local
and tribal agencies responsible for implementing the Exceptional
Events Rule.
---------------------------------------------------------------------------

    All states and areas of Indian country that include areas that
could exceed the proposed PM NAAQS and could therefore be designated as
nonattainment for the proposed PM NAAQS have the potential to be
affected by this rulemaking. Therefore, this action would apply to all
states; to local air quality agencies to which a state has delegated
relevant responsibilities for air quality management including air
quality monitoring and data analysis; and to tribal air quality
agencies where appropriate.
    The ``Treatment of Data Influenced by Exceptional Events; Final
Rule'' (72 FR 13560, March 22, 2007), known as the Exceptional Events
Rule and codified at 40 CFR 50.14, contains generic deadlines for a
state to submit to EPA specified information about exceptional events
and associated air pollutant concentration data. A state must initially
notify the EPA that data have been affected by an event by July 1 of
the calendar year following the year in which the event occurred. This
is done by flagging the data in AQS and providing an initial event
description. The state must also, after notice and opportunity for
public comment, submit a demonstration to justify any claim within
three years after the quarter in which the data were collected.
However, if a regulatory decision based on the data (for example, a
designation action) is anticipated, the schedule to flag data in AQS
and submit complete documentation to EPA for review may be shortened
and all information must be submitted to the EPA no later than one year
before the decision is to be made.
    These generic deadlines in the Exceptional Events Rule are suitable
after initial designations have been made under a NAAQS or when an area
is to be redesignated, either from attainment to nonattainment or from
nonattainment to attainment, and the redesignation status may depend on
the excluded data. However, these same generic deadlines may need to be
adjusted to accommodate the initial area designation process and
schedule under a newly revised NAAQS. Until the level and form of the
NAAQS have been promulgated, a state does not know whether the criteria
for excluding data (which are tied to the level and form of the NAAQS)
were met for a given event. In some cases, the generic deadlines,
especially the deadlines for flagging some relevant data, may have
already passed by the time the new or revised NAAQS is promulgated. In
addition, it may not be feasible for information on some exceptional
events that may affect final designations decisions to be collected and
submitted to EPA at least one year in advance of the final designation
decision. This scheduling constraint could have the unintended
consequence of the EPA designating an area nonattainment because of
uncontrollable natural or other qualified exceptional events.
    The Exceptional Events Rule at section 50.14(c)(2)(vi) indicates
``when EPA sets a NAAQS for a new pollutant or revises the NAAQS for an
existing pollutant, it may revise or set a new schedule for flagging
exceptional event data, providing initial data descriptions and
providing detailed data documentation in AQS for the initial
designations of areas for those NAAQS.''
    The EPA intends to promulgate the revised PM NAAQS in December
2012. State Governors (and tribes, if they choose) should submit
designations recommendations by December 2013, based on air quality
data from the years 2010 to 2012 or 2011 to 2013, if there are
sufficient data for these years. Initial designations under the revised
NAAQS would be made by December 2014 based on air quality data from the
years 2011 to 2013. (See section IX.A for a more detailed discussion of
the designation schedule.) Assuming this schedule, all events to be
considered during the designations process would need to be flagged and
fully documented by states one year prior to designations, or by
December 2013, under the existing generic deadline in the Exceptional
Events Rule. Without revision to 40 CFR 50.14, a state would not be
able to flag and submit documentation regarding events that occurred in
December 2013 by one year before designations are made in December
2014. The EPA believes this is not an appropriate restriction, and
therefore is proposing revisions to 40 CFR 50.14.
    The EPA proposes revisions to 40 CFR 50.14 only to change
submission dates for information supporting claimed exceptional events
affecting PM data for initial area designations under the proposed new
and revised PM NAAQS. The proposed rule language at the end of this
notice shows the changes that would apply assuming promulgation of the
new and revised PM NAAQS in December 2012 and initial area designations
by December 2014. For air quality data collected in 2010 or 2011, the
EPA proposes extending to July 1, 2013 the otherwise applicable generic
deadlines of July 1, 2011 and July 1, 2012, respectively, for flagging
data and providing an initial description of an event (40 CFR
50.14(c)(2)(iii)). The EPA proposes to retain the existing generic
deadline in the Exceptional Events Rule of July 1, 2013 for flagging
data and providing an initial description of events occurring in 2012.
Similarly, the EPA proposes to revise to December 12, 2013 the deadline
for submitting

documentation to justify PM-related exceptional events occurring in
2010 through 2012. The EPA believes these revisions/extensions will
provide adequate time for states to review the impact of exceptional
events from 2010 through 2012 on any revised standards, to notify the
EPA by flagging the relevant data and providing an initial description
in AQS, and to submit documentation to support claims for exceptional
events.
    If a state intends the EPA to consider in the PM designations
decisions whether PM data collected during 2013 have been affected by
exceptional events, the EPA proposes that these data must be flagged by
the generic Exceptional Event Rule deadline of July 1, 2014. The EPA
proposes to revise to August 1, 2014 the deadline for submitting
documentation to justify PM-related exceptional events occurring in
2013. The EPA believes that these deadlines provide states with
adequate time to review and identify potential exceptional events that
occur in calendar year 2013.
    Therefore, using the authority provided in CAA section 319(b)(2)
and in the Exceptional Events Rule at 40 CFR 50.14 (c)(2)(vi), the EPA
proposes to modify the schedule for data flagging and submission of
demonstrations for exceptional events data considered for initial area
designations under the proposed PM primary and secondary NAAQS as
presented in Table 3. If the promulgation date for a revised PM NAAQS
occurs on a different date than in December 2012, the EPA will revise
the final PM exceptional event flagging and documentation submission
deadlines accordingly, consistent with the logic of this proposal, to
provide states with reasonably adequate opportunity to review,
identify, and document exceptional events that may affect an area
designation under a revised NAAQS. The EPA invites comment on these
proposed changes, shown in Table 3, to the exceptional event data
flagging and documentation submission deadlines for the proposed
revised PM NAAQS.

  Table 3--Revised Schedule for Exceptional Event Flagging and Documentation Submission for Data To Be Used in
                                 Initial Area Designations for the 2012 PM NAAQS
----------------------------------------------------------------------------------------------------------------
                                   Air quality data
  NAAQS pollutant/standard/     collected for  calendar    Event flagging & initial     Detailed  documentation
 (level)/ promulgation date              year                description deadline         submission deadline
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hour Standard        2010 to 2011..............  July 1, 2013..............  December 12, 2013.
 (final level and             2012......................  \a\ July 1, 2013..........  December 12, 2013.
 promulgation date TBD).      2013......................  \a\ July 1, 2014..........  August 1, 2014.
PM2.5/Annual Standard (final  2010 to 2011..............  July 1, 2013..............  December 12, 2013.
 level and promulgation date  2012......................  \a\ July 1, 2013..........  December 12, 2013.
 TBD).                        2013......................  \a\ July 1, 2014..........  August 1, 2014.
Secondary PM (final level     2010 to 2011..............  July 1, 2013..............  December 12, 2013.
 and promulgation date TBD).  2012......................  \a\ July 1, 2013..........  December 12, 2013.
                              2013......................  \a\ July 1, 2014..........  August 1, 2014.
----------------------------------------------------------------------------------------------------------------
\a\ This date is the same as the general schedule in 40 CFR 50.14. Note: The table of revised deadlines only
  applies to data the EPA will use to establish the final initial area designations for revised NAAQS. The
  general schedule applies for all other purposes, most notably, for data used by the EPA for redesignations to
  attainment. TBD = to be determined.

C. Proposed Updates for Data Handling Procedures for Reporting the Air
Quality Index

    The EPA is proposing to update appendix G of 40 CFR part 58 to
clarify units, breakpoint precision, and truncation methods for AQI
sub-indices. These changes are intended to harmonize the AQI reporting
requirements with data handling provisions expressed elsewhere in 40
CFR part 50. Currently, the breakpoints for NO2 and
SO2 in Table 2 of appendix G of 40 CFR part 58 are expressed
in parts per million (ppm). The EPA proposes to change the sub-indices
for NO2 and SO2 to be based on parts per billion
(ppb) rather than ppm to be consistent with the units used for defining
the current levels of the primary NO2 and SO2
NAAQS (75 FR 6474, February 9, 2010; 75 FR 35520, June 22, 2010). In
addition, in modifying the sub-index for NO2 to express the
breakpoints in units of ppb, the EPA proposes to clarify the
breakpoints for NO2 in the Very Unhealthy and Hazardous
ranges to include four rather than three significant digits to increase
precision. Finally, the EPA proposes to modify appendix G to explicitly
identify truncation methods for using ambient measured concentrations
in AQI calculations.

VIII. Proposed Amendments to Ambient Monitoring and Reporting
Requirements

    The EPA proposes changes to the ambient air monitoring, reporting,
and network design requirements associated with the PM NAAQS. Ambient
PM monitoring data are used to meet a variety of monitoring objectives
including determining whether an area is in violation of the PM NAAQS.
Ambient PM 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. This
section discusses the monitoring changes that the EPA is proposing to
support the proposed PM NAAQS summarized in sections III.F, IV.F, and
VI.F above.

A. Issues Related to 40 CFR Part 53 (Reference and Equivalent Methods)

    To be used in a determination of compliance with the PM NAAQS, PM
data are typically collected using samplers or monitors employing an
FRM or FEM. The EPA also allows use of alternative methods where
explicitly stated in the monitoring methodology requirements (appendix
C of 40 CFR part 58), such as PM2.5 ARMs which can be used
to determine compliance with the NAAQS. The EPA prescribes testing and
approval criteria for FRM and FEM methods in 40 CFR part 53.
1. PM2.5 and PM10-2.5 Federal Equivalent Methods
    In 2006, the EPA finalized new testing and performance criteria for
Class II and Class III FEMs (71 FR 61281 to 61289, October 17, 2006).
Class II methods are equivalent methods for PM2.5 or
PM10-2.5

that utilize a PM2.5 sampler or PM10-2.5 sampler
in which integrated PM2.5 samples or PM10-2.5
samples are obtained from the atmosphere by filtration and are then
subjected to a filter conditioning process followed by gravimetric mass
determination. Class II equivalent methods are different from Class I
equivalent methods because of substantial deviations from the design
specifications of the sampler specified for reference methods in
appendix L or appendix O (as applicable) of 40 CFR part 50. Class III
refers to those methods for PM2.5 or PM10-2.5
that are employed to provide PM2.5 or PM10-2.5