with short-term exposures to PM10-2.5 in Phoenix (Mar et
al., 2003) and Coachella Valley, CA (Ostro et al., 2003), the reported
98th percentile values were approximately 70 and 107 [mu]g/m\3\,
respectively. These studies were conducted in areas with air quality
levels that did not meet the current PM10 standards. In
addition, a statistically significant association was reported between
PM10-2.5 and mortality in Steubenville as part of the
original Six Cities study (Schwartz et al., 1996), although in more
recent reanalyses, the association did not remain statistically
significant in most models (Schwartz, 2003a; Klemm and Mason, 2003)--
the PM10-2.5 concentrations in this eastern city were fairly
high, with a reported 98th percentile value of 53 [mu]g/m\3\. In
contrast to the statistically significant mortality associations with
PM10-2.5 reported in these studies, the Staff Paper notes
that no such associations were reported in a number of other studies,
including those in the five other cities that were part of the Six
Cities study (Boston, St. Louis, Knoxville, Topeka, and Portage), Santa
Clara County, CA, Detroit, Philadelphia, and Pittsburgh. With the
exception of Pittsburgh, these cities had much lower 98th percentile
PM10-2.5 values, ranging from 18 to 49 [mu]g/m\3\. Thus, in
mortality studies that reported statistically significant associations,
the reported 98th percentile PM10-2.5 values were all above
50 [mu]g/m\3\, whereas in the mortality studies that reported no
statistically significant associations, the reported 98th percentile
PM10-2.5 values were generally below 50 [mu]g/m\3\.
---------------------------------------------------------------------------

    \69\ As shown in air quality data trends reports: for Seattle,
1997 Air Quality Annual Report for Washington State, p. 17, at
http://www.ecy.wa.gov/pubs/97208.pdf; for Detroit, Michigan's 2003 Annual Air Quality Report, p. 46, at http://www.deq.state.mi.us/

documents/deq-aqd-air-reports-03AQReport.pdf.
---------------------------------------------------------------------------

    In looking more closely at air quality data used in the morbidity
and mortality studies discussed above, however, the Staff Paper
recognizes that the uncertainty related to exposure measurement error
associated with using ambient concentrations to represent area-wide
population exposure levels can be potentially quite large. For example,
in looking specifically at the Detroit study, the Staff Paper notes
that the PM10-2.5 air quality values were based on air
quality monitors located in Windsor, Canada. While the study authors
concluded that these monitors were appropriate for use in exploring the
association between air quality and hospital admissions in Detroit, a
close examination of air quality levels at Detroit and Windsor sites in
recent years led to the conclusion that the statistically significant,
generally robust association with hospital admissions in Detroit likely
reflects population exposures that may be appreciably higher in the
central city area, but not necessarily across the broader study area,
than would be estimated using data from the Windsor monitors (EPA,
2005a, p. 5-64).
    The EPA staff also looked more specifically at the Coachella Valley
mortality study (Ostro et al., 2003), in which data were used from a
single monitoring site in one city, Indio, within the study area where
daily measurements were available. A close examination of air quality
levels across the Coachella Valley suggests that while the association
of mortality with PM10-2.5 measurements made at the Indio
site was statistically significant, a portion of the study population
would have been expected to experience appreciably lower ambient
exposure levels. In contrast to the Detroit study, air quality data
used in the mortality

study conducted in Coachella Valley appear to represent concentrations
on the high end of PM10-2.5 levels for Coachella Valley
communities. On the other hand, a close examination of the air quality
data used in the other studies discussed above generally shows less
disparity between air quality levels at the monitoring sites used in
the studies and the broader pattern of air quality levels across the
study areas than that described above in the Detroit and Coachella
Valley studies.
    This close examination of air quality information generally
reinforces the view that exposure measurement error is potentially
quite large in these PM10-2.5 studies. As a consequence, the
air quality levels reported in these studies, as measured by ambient
concentrations at monitoring sites within the study areas, are not
necessarily good surrogates for population exposures that are likely
associated with the observed effects in the study areas or that would
likely be associated in other urban areas across the country. The
Detroit example suggests that population exposures were probably
appreciably underestimated in the Detroit morbidity study, such that
the observed effects are likely associated with higher
PM10-2.5 levels than reported. In contrast, the Coachella
Valley mortality study provides an example in which population levels
were probably appreciably overestimated, such that the observed effects
may well be associated with lower PM10-2.5 levels than
reported. At relatively low levels of air quality, population exposures
implied by these studies as being associated with the observed effects
likely become more uncertain, suggesting a high degree of caution in
interpreting the group of morbidity studies as a basis for identifying
a standard level that would protect against the observed effects.
    Taking into account this close examination of the studies, the
Staff Paper concludes that this evidence suggests that EPA could
consider a standard for urban thoracic coarse particles at a
PM10-2.5 level at least down to 50 [mu]g/m\3\, in
conjunction with a 98th percentile form. This view takes into account
the conclusion that this evidence is particularly uncertain as to
population exposures, especially from the morbidity studies reporting
effects at relatively low concentrations, as well as the general lack
of evidence of associations from the group of mortality studies with
reported concentrations below these levels.
    Another view that reflects a more cautious or restrained approach
to interpreting the limited body of PM10-2.5 epidemiologic
evidence would be to judge that the uncertainties in this whole group
of studies as to population exposures that are associated with the
observed effects are too large to use the reported air quality levels
directly as a basis for setting a specific standard level. Such a
judgment would be consistent with concluding that these studies,
together with other dosimetric and toxicologic evidence, provide
support for retaining standards for thoracic coarse particles at some
level to protect against the morbidity and mortality effects observed
in the studies, regardless of whether an associated population exposure
level can be clearly discerned from the studies.
    Based on this more cautious approach, the Staff Paper concludes
that it would be reasonable to interpret the available epidemiologic
evidence more qualitatively. Considering the available evidence in this
way leads to the following observations:
    (1) The statistically significant mortality associations with
short-term exposure to PM10-2.5 reported in the Phoenix and
Coachella Valley studies were observed in areas that did not meet the
current PM10 standards.
    (2) The statistically significant morbidity associations with
short-term exposure to PM10-2.5 reported in the Detroit and
Seattle studies were observed in areas that exceeded the level of the
current 24-hour PM10 standard on just a few occasions during
the time periods of the studies.
    (3) All but one of the statistically significant morbidity and
mortality associations with short-term exposure to PM10
reported in areas in which the thoracic coarse particle fraction of
PM10 was much greater than was the fine fraction (including
Reno/Sparks, NV, Tucson, AZ, Anchorage, AK, and the Utah Valley area)
were observed in areas that did not meet the current PM10
standards.
    Based on these considerations, the Staff Paper finds little basis
for concluding that the degree of protection afforded by the current
PM10 standards in urban areas is greater than warranted,
since potential mortality effects have been associated with air quality
levels not allowed by the current standards, but have not been
associated with air quality levels that would generally meet the
current standards, and morbidity effects have been associated with air
quality levels that exceeded the current standards only a few times.
Further, the Staff Paper finds little basis for concluding that a
greater degree of protection is warranted in light of the very high
degree of uncertainty in the relevant population exposures implied by
the morbidity studies. The Staff Paper concludes, therefore, that it is
reasonable to interpret the available evidence as supporting
consideration of a short-term standard for thoracic coarse particles,
so as to provide generally ``equivalent'' protection to that afforded
by the current PM10 standards, recognizing that no one
PM10-2.5 level will be strictly equivalent to a specific
PM10 level in all areas (EPA, 2005a, p. 5-67). Such a
standard would likely provide protection against morbidity effects
especially in urban areas where, unlike the study areas,
PM10 is generally dominated by coarse-fraction rather than
fine-fraction particles. Such a standard would also likely provide
protection against the more serious, but more uncertain,
PM10-2.5-related mortality effects generally observed at
somewhat higher air quality levels.
    To identify a range of levels for consideration for a 24-hour
PM10-2.5 standard, based on the indicator proposed above and
set so as to afford generally ``equivalent'' protection as the current
PM10 standards, the Staff Paper presents the results of
analyses of relevant data on PM10-2.5 and PM10
24-hour average concentrations.\70\ In one such analysis of 205
monitoring sites (Schmidt et al., 2005),\71\ a PM10-2.5
level of approximately 60 [mu]g/m\3\, in terms of a 98th percentile
form, would be roughly equivalent on average across the U.S. to the
current PM10 standard level of 150 [mu]g/m\3\, in terms of
the current one-expected-exceedance form.\72\ While noting appreciable
variability in the estimated point of equivalence across individual
sites, these levels of approximate average equivalence are quite
consistent across each of the five regions in which all of the areas
that do not meet the current PM10 standards are located
(including the southern California, southwest, northwest, upper mid-
west, and southeast regions). Notably different average equivalence

levels were observed in the other two regions, i.e., approximately 40
[mu]g/m\3\ in the northeast and over 70 [mu]g/m\3\ in the industrial
mid-west.
---------------------------------------------------------------------------

    \70\ Consistent with PM10-2.5 monitoring network
design criteria discussed in section <IP_ADDRESS> of the Staff Paper,
monitors included in this analysis are those in CBSAs with at least
100,000 population and in census block groups with a population
density of at least 500, and that also had 3 years of complete data
in each quarter for both PM10 and PM10-2.5
(EPA, 2005a, p. 5-67).
    \71\ These analyses were based on collocated PM10 and
PM10-2.5 data, and used linear regression methods to
predict PM10-2.5 concentrations (98th percentile form)
equivalent to the 24-hour PM10 standard level of 150
[mu]g/m\3\ (one expected exceedence form) at a national and at
regional levels.
    \72\ Across the U.S., the 95 percent confidence intervals around
these point estimates are approximately  3 [mu]g/m\3\,
while region-specific intervals are approximately  10
[mu]g/m\3\ in the five regions in which all of the areas that do not
meet the current PM10 standards are located (EPA, 2005a,
p. 5-68).
---------------------------------------------------------------------------

    Another such analysis was based on comparing the number of areas,
and the population in those areas, that would likely not meet a
specific PM10-2.5 standard, set at a given level and form,
with the same measures in areas that do not meet the current
PM10 standards. This analysis, based on 2001 to 2003 data,
provides some rough indication of the breadth of protection potentially
afforded by alternative standards. The results of this analysis
indicate that a PM10-2.5 standard of about 70 or 65 [mu]g/
m\3\, 98th percentile form, would impact approximately the same number
of counties or number of people, respectively, as would the current
PM10 standards.\73\
---------------------------------------------------------------------------

    \73\ As shown in Tables 5B-2(a) and (b) of the Staff Paper,
there are 585 counties with PM10 monitoring sites used in
determining compliance with the PM10 standards, whereas
only 309 of those counties have monitor sites that would be included
in the monitoring network design criteria discussed in section
<IP_ADDRESS> of the Staff Paper. Of these 309 counties, 259 have
PM10 and PM10-2.5 air quality data that meet
the data completeness criteria defined for this analysis, which are
somewhat less restrictive than the criteria that were applied in the
regression analysis described above.
---------------------------------------------------------------------------

    In considering the relevant dosimetric, toxicologic, and
epidemiologic evidence, related limitations and uncertainties, and
analyses of relevant air quality information, the Staff Paper concludes
that it is appropriate to consider a 24-hour PM10-2.5
standard in the range of 50 to 70 [mu]g/m\3\, with a 98th percentile
form.\74\ The lower end of this range is based on a close examination
of the air quality patterns related to the limited number of relevant
epidemiologic studies. The upper part of this range is based on a more
cautious approach to interpreting the available information and
reflects a generally ``equivalent'' degree of protection to that
afforded by the current PM10 standards. The upper end of
this range is also below the 98th percentile PM10-2.5
concentrations in the two mortality studies that reported statistically
significant associations. Consideration of a generally ``equivalent''
PM10-2.5 standard would reflect a judgment that while the
epidemiologic evidence supports establishing a short-term standard for
urban thoracic coarse particles at such a generally ``equivalent''
level, the evidence concerning air quality levels of thoracic coarse
particles in the studies is not strong enough to provide a basis for
changing the level of protection generally afforded by the current
PM10 standards.
---------------------------------------------------------------------------

    \74\ Beyond looking directly at the relevant epidemiologic
evidence and related air quality information, the Staff Paper also
considers the extent to which the PM10-2.5 risk
assessment, discussed above in section III.B, can help inform
consideration of alternative 24-hour PM10-2.5 standards.
The Staff Paper concludes that the nature and magnitude of the
uncertainties and concerns associated with this portion of the risk
assessment weigh against use of these risk estimates as a basis for
recommending specific standard levels (EPA, 2005a, p. 5-69).
---------------------------------------------------------------------------

    Based on its review of the Staff Paper, there was general agreement
among the CASAC Panel members that the Staff Paper-recommended range of
50 to 70 [mu]g/m\3\, with a 98th percentile form, for a 24-hour
PM10-2.5 standard was reasonably justified. Most CASAC Panel
members favored levels at the upper end of that range, while several
members supported the lower end of the range (Henderson, 2005b).
Because of the significant uncertainties resulting from the limited
number of studies to date in which PM10-2.5 has been
measured and the potentially large exposure measurement errors in such
studies, the CASAC Panel did not generally support a level below the
Staff Paper-recommended range.
    In considering an appropriate level for a 24-hour
PM10-2.5 standard intended to afford requisite protection of
public health from health effects associated with exposure to thoracic
coarse particles of concern, the Administrator has carefully considered
the rationale and recommendations contained in the Staff Paper, the
advice and recommendations of CASAC, and public comments to date on
this issue. Taking these considerations into account, the Administrator
proposes to set the level of the primary 24-hour PM10-2.5
standard at 70 [mu]g/m\3\. In the Administrator's provisional judgment,
based on the currently available evidence, a standard set at this level
would be requisite to protect public health with an adequate margin of
safety from the morbidity and possibly mortality effects that have been
associated with short-term exposures to thoracic coarse particles of
concern. This proposed standard is expected to have the most impact in
areas that do not meet the current 24-hour PM10 standard.
    In reaching this judgment, the Administrator recognizes that the
epidemiologic evidence on morbidity and possible mortality effects
related to PM10-2.5 exposure is very limited at this time,
and that there are potentially quite large uncertainties inherent in
interpreting the available evidence for PM10-2.5 as compared
with the evidence related to fine particles. For example,
PM10-2.5 concentrations can vary substantially across a
metropolitan area and thoracic coarse particles are less able to
penetrate into buildings than fine particles; thus, the ambient
concentrations reported in epidemiologic studies may not well represent
area-wide population exposure levels. It may also be difficult to
disentangle effects associated with PM10-2.5 and
PM2.5 in epidemiologic studies. Further, the Administrator
is mindful that considering what standard is requisite to protect
public health with an adequate margin of safety requires judgments that
neither overstate nor understate the strength and limitations of the
evidence or the appropriate inferences to be drawn from the evidence.
Thus, the Administrator provisionally concludes that the selection of a
level that provides generally equivalent protection to that provided by
the current PM10 standards is an appropriate policy response
to the very limited body of evidence that is available at this time.
The EPA intends to address the considerable uncertainties in the
currently available information on thoracic coarse particles as part of
the Agency's ongoing PM research program.
    The Administrator also recognizes that there is no one level for a
PM10-2.5 standard that would be equivalent to the current
PM10 standards in every area across the country, and that
there are likely additional approaches to identifying a generally
equivalent standard level beyond those approaches considered in the
Staff Paper upon which the proposed level is based. Thus, the
Administrator also solicits comment on alternative approaches to
identifying a generally ``equivalent'' standard level. While proposing
to set the PM10-2.5 standard at a level that is generally
equivalent to the 1987 PM10 standard, the Administrator
solicits comment on whether it would be more appropriate to set the
PM10-2.5 standard at a level that is generally equivalent to
the PM10 standard set in 1997.
    Having decided to propose the 24-hour PM10-2.5 standard
described above, the Administrator recognizes that there are important
views on the information relating to the effects of coarse fraction PM
that warrant consideration. For example, an alternative interpretation
of the available health evidence presented in the Criteria Document and
the Staff Paper questions the conclusions about PM10-2.5
associations drawn from one-pollutant models. This interpretation of
the available epidemiological evidence suggests that the results from
one-pollutant PM10-2.5 models are confounded by fine
particles and gaseous co-pollutants.
    The key PM10-2.5 epidemiologic results discussed in the
Criteria Document and

Staff Paper are drawn from one-pollutant models; i.e.,
PM10-2.5 is the only variable used in the statistical model
reflecting exposure to air pollution. There are four studies cited in
these documents as being suggestive of a statistically significant role
for PM10-2.5 in the reported associations: Ito (2003),
Burnett et al. (1997), Mar et al. (2003), and Ostro et al. (2003).
However, there is strong evidence that adverse health effects similar
to those observed in these studies, including both cardiovascular and/
or respiratory health effects are associated with exposure to
PM2.5. The authors of several of these studies focus on fine
particles (and in some cases one or more of the gaseous pollutants) as
playing an important role in ``explaining'' the association between PM
and various health endpoints. For example, in these key epidemiologic
studies, the correlation coefficients between PM2.5 and
PM10-2.5 concentrations range from moderate to high (i.e.,
0.4 to 0.7), which increases the likelihood that associations between
health effects and PM10-2.5 identified in one-pollutant
models may instead simply reflect the effects of exposure to
PM2.5 rather than independent health effects. With the
positive correlations between pollutants and similar health effects, it
generally would be appropriate for any assessment of the effect of
exposure to PM10-2.5 to control for exposure to the
PM2.5.
    In this light, it is important to review how the authors of the
four key PM10-2.5 epidemiology studies have accounted for
co-pollutants in their analysis. Ito (2003) noted significant estimates
of the health effects of associations in one-pollutant models, but in a
two-pollutant model with PM2.5 the PM10-2.5
associations lost statistical significance. Burnett et al. (1997)
concluded that the effect of PM10-2.5 in a one-pollutant
model could be explained by gaseous co-pollutants. Mar et al. (2003)
found PM10-2.5 to be positively associated with adverse
health effects in a one-pollutant model, but also found similar
associations with a range of other air pollutants. In addition, Mar et
al. (2003) noted that even though all PM mass metrics included in the
study were associated with an excess risk of cardiovascular death, the
strongest associations were with PM2.5, followed by
PM10 and PM10-2.5. Ostro et al. (2003) used a
one-pollutant model to estimate the association between
PM10-2.5 on mortality using an effectively linear construct
of PM10 (as observed in Indio, CA) to represent
PM10-2.5 for the entire study area. By using such a
construct of PM10, the estimated associations simply reflect
a PM10 association (i.e., the construct does not provide
additional information on the effect of PM10-2.5). Moreover,
roughly 75 percent of the cardiovascular mortality in this study
occurred in or near Palm Springs, CA and PM characteristics differ
significantly between Palm Springs and Indio (e.g., average
PM10 concentrations are roughly 30 percent lower in Palm
Springs and PM2.5 represents a higher fraction of
PM10, with a correlation coefficient between
PM2.5 and PM10-2.5 of 0.46 in Palm Springs).
Thus, the Ostro et al. (2003) study suggests a positive association
between PM10 monitored in Indio and mortality in Palm
Springs, but some view this study as offering little basis for
attributing significant mortality association to PM10-2.5 as
observed in either city.
    The Criteria Document and Staff Paper also present and discuss
other epidemiology studies in support of the proposal for both the
PM2.5 and PM10-2.5 standards (as shown in Figure
2 and discussed in Section III.A above): Burnett (1997), Fairley
(2003), Ito (2003), Lipfert et al (2000), Mar et al (2003), Moolgavkar
(2000), Sheppard et al (2003), Thurston et al (1994), Burnett (2000,
2003), Klemm and Mason (2003), and Schwartz and Neas (2000). However,
these studies report positive, statistically significant associations
with PM2.5 that are more consistent and robust than the
associations thus far identified for PM10-2.5. Indeed,
several of these and other studies that specifically considered
PM10-2.5, but did not find statistically significant
associations, including Schwartz et al (1996), Thurston et al. (1994),
Sheppard et al. (2003), Fairley (2003), Schwartz et al (1996) and
Lipfert et al. (2000). With respect to mortality effects in the Six-
City study, Schwartz et al. (1996) concluded that the PM associations
(in the six metropolitan areas--including Steubenville) were
specifically associated with PM2.5, with little additional
contribution from the PM10-2.5. Sheppard et al. (2003) noted
that bias in model selection and reporting can result in inflated
excess risk estimates for PM. Fairley (1999) noted that
PM10-2.5 effects become negative and insignificant when
modeled jointly with PM2.5. Lipfert et al. (2000) showed
insignificant effects for PM10-2.5 in one- and two-pollutant
models with O3. The authors also caution against drawing
causal interpretations from results when comparing health effects from
one region in a metropolitan area to air quality observations in
another region. In addition, several of these studies also report
positive, statistically significant associations with one or more of
the gaseous pollutants. Both Thurston et al. (1994) and Burnett et al.
(1997) reported substantial confounding with gaseous co-pollutants in
Toronto, and Thurston et al. (1994, p. 282) reported that ``it seems
clear that these apparent associations were merely a statistical by-
product of interpollutant confounding resulting from the shared day-to-
day variations in dispersion conditions.'' In addition, Burnett et al.
(2000) concluded that gaseous pollutants played an important role in
explaining the effect of urban air pollution on health. Similarly,
Moolgavkar (2000) concludes that gases were more strongly associated
with respiratory effects than PM in Los Angeles.
    Taken as a whole, evidence from PM10-2.5 epidemiologic
studies could be interpreted to suggest that one-pollutant
PM10-2.5 models suffer from bias due to omitting co-
pollutants in the statistical model, especially given the much stronger
evidence (discussed above) that these effects are associated with
exposure to PM2.5. As noted by many of the aforementioned
authors, while significant health associations may be noted for coarse
fraction PM in one-pollutant models, the actual association may be
insignificant from zero due to confounding co-pollutants. Of course,
the Administrator must conclude in the final rule that the evidence
about the health effects of PM10-2.5 is sufficiently robust
to finalize a standard for PM10-2.5.
    The Administrator, recognizing notably large uncertainties in the
underlying evidence and information that formed the basis for this
proposal as well as the challenges associated with moving toward a new
PM10-2.5 indicator and a related new monitoring network,
solicits comment on this and other alternative interpretations of the
available health evidence and alternative policy responses. Several
such alternative interpretations and policy responses are discussed
below.
    (1) In light of the large uncertainties in the evidence and the
challenges of moving to a new indicator, and provisionally recognizing
the need for a standard to provide a requisite level of protection from
the risks associated with thoracic coarse particles, the Administrator
also believes it appropriate to consider a policy option that would
retain the current 24-hour PM10 standard (with a one-
expected-exceedance form), while addressing issues such as the
appropriateness of the indicator and the level of the standard.
    As discussed in section I.D, in response to a challenge to the 1997
standards for thoracic coarse PM, the

U.S. Court of Appeals for the District of Columbia vacated the Agency's
1997 PM10 standards. In its decision the Court noted that
use of PM10 as an indicator to protect against the public
health risks associated with thoracic coarse particles resulted in
double regulation of PM2.5, since this size fraction is both
a component of PM10 and the subject of its own standard. The
Court further reasoned that, since PM2.5 concentrations vary
from area to area, use of PM10 as a thoracic coarse particle
indicator results in an arbitrary level of protection in public health
from the risks associated with thoracic coarse particles on a national
basis, as the level of protection would vary based on the concentration
of PM2.5 in an area. See American Trucking Associations v.
EPA, 175 F.3d at 1054-55.
    Under this option to retain the 24-hour PM10 standard,
EPA would modify the standard to exclude the double-counted
PM2.5 contribution in circumstances where this could present
a concern. First, there will be some areas that may be in nonattainment
with the PM10 standard because, and only because, they are
in nonattainment with the PM2.5 standard. To remedy the
double counting in this situation, EPA is requesting comment on
subtracting from a daily measured PM10 concentration the
value by which the concentration of PM2.5 measured at a
collocated monitor is in excess of 35 [mu]g/m3 (i.e., the
proposed level for the 24-hour PM2.5 standard). This
adjustment would need to be made only on days when a 24-hour average
PM10 concentration is measured in excess of 150 [mu]g/
m3. In such a case, the amount by which the PM2.5
concentration exceeds 35 [mu]g/m3 would be subtracted from
the measured PM10 concentration. The EPA would then use this
adjusted value in any comparison to the PM10 standard.
    The second situation where the overlap between the PM2.5
and PM10 standards may cause some concern is in areas where
a daily PM2.5 level is below 35 [mu]g/m3. In
those areas, the level of the PM10 standard would allow a
higher concentration of thoracic coarse particles before triggering an
exceedance than it would in other areas. The EPA is requesting comment
on not requiring any adjustment to the daily measured PM10
concentration in this situation, on the basis that any additional risk
to public health that may be associated with this higher allowable
concentration of thoracic coarse particles would reasonably be expected
to present less concern from a public health perspective than would the
otherwise allowable equivalent increase in the concentration of
PM2.5.
    The EPA also believes that it would be appropriate in this option
to focus the PM10 standard in a manner similar to that
proposed above for the PM10-2.5 standard. While the
indicator would remain specified as PM10, the focus would be
on including only the mix of ambient thoracic coarse particles that are
of concern to public health (and to exclude the mix for which
information is not sufficient to infer a public health concern) and
would be achieved in practice through the data handling requirements
associated with the standard, which are linked to the proposed
monitoring network design criteria (in the part 58 rule proposed
elsewhere in today's Federal Register).
    The EPA invites comment on whether this option would provide the
requisite level of public health protection from risks associated with
thoracic coarse particles. Given the difference in form between the 24-
hour PM10 standard (one-expected-exceedance form) and the
proposed PM10-2.5 standard (98th percentile form), and the
adjustments noted above, in practice there may not be an appreciable
difference in the degree of public health protection afforded by this
option relative to that afforded by the proposed PM10-2.5
standard. The EPA invites comment on whether this approach addresses
one of the concerns about use of a PM10 indicator for
thoracic coarse particles noted by the Court in its ATA decision,
namely that the level of public health protection from thoracic coarse
particles in an area would vary depending on the relative proportions
of fine and thoracic coarse particles, by recognizing that the
PM10 indicator and standard would cover both fine and
thoracic coarse particles.
    With respect to revocation of the 1987 24-hour PM10
standard, under this option EPA would apply the same approach to
revocation as that proposed below in section III.H. in conjunction with
the proposed PM10-2.5 standard. Since the 24-hour
PM10 standard would be focused in basically the same manner
as the proposed PM10-2.5 standard, it would be appropriate
to follow the same approach to revocation of the current 24-hour
PM10 standard under this option as well.
    The EPA solicits comment on all aspects of this approach, including
views on whether a 24-hour PM10 standard revised as noted
above would be requisite to protect public health from the risks
associated with thoracic coarse particles, with an adequate margin of
safety, as well as views on any legal, scientific, or policy issues
associated with this alternative, and including comments on the
consistency of this option with CASAC's recommendations. The EPA also
solicits comment on whether a 98th percentile form should be considered
for a 24-hour PM10 standard and on the appropriate level of
such a standard.
    (2) The Administrator recognizes that some commenters hold the view
that the uncertainties that exist at the present time are so great that
no standards for thoracic coarse particles are warranted. Some such
commenters point to conclusions reached in the Staff Paper in part as a
basis for their view, including, for example, the conclusion that the
``substantial uncertainties associated with this limited body of
epidemiological evidence on health effects related to
PM10-2.5 * * * suggests a high degree of caution in
interpreting this evidence * * *.'' (EPA 2005, pp. 5-50). This view
generally places significant weight on the issue of confounding between
PM2.5 and PM10-2.5 (discussed above in section
III.A), with some commenters stating that the correlation coefficients
between fine and thoracic coarse particle levels are modest to high for
all studies for which such data are available, increasing the
possibility that the positive association identified in the
PM10-2.5 one-pollutant models may instead reflect the
effects of fine particles. Noting that the Staff Paper puts little
weight on the health risk assessment because of the significant
uncertainties in the underlying health studies, some commenters suggest
that the risk assessment therefore does not provide a basis for
determining whether the health effects possibly associated with
PM10-2.5 constitute a meaningful public health risk. Some
commenters take the view that, based either on the studies or the risk
assessment, the magnitude of the health effects possibly associated
with PM10-2.5 do not constitute a meaningful risk to public
health. These commenters also maintain that significant uncertainty
remains as to an appropriate level of a standard, even assuming that a
meaningful public health risk exists. In consideration of these views,
the Administrator also solicits comment on revoking the current 24-hour
PM10 standard at this time (as well as the current annual
PM10 standard, as proposed above), not adopting a thoracic
coarse particle standard at this time, and taking into account any new
relevant research that becomes available as a basis for considering a
more targeted standard for thoracic coarse particles in the next
periodic review of the PM NAAQS.

    (3) In sharp contrast to the views noted above, another view that
the Administrator takes note of would place greater weight on the
available epidemiologic evidence as a basis for selecting a level down
to 50 [mu]g/m3 or below and/or for selecting an unqualified
PM10-2.5 indicator. While recognizing that important
uncertainties are present in the available evidence, this view would
support incorporating a larger margin of safety consistent with a more
highly precautionary policy response. In soliciting comments on a wide
array of views, the Administrator solicits comment on this view and on
standard levels that are consistent with this view.

H. Proposed Decisions on Primary PM10-2.5 Standard

    For the reasons discussed above, and taking into account the
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and public
comments to date, the Administrator proposes to revise the current
primary PM10 standards. In particular, to provide more
targeted protection from thoracic coarse particles that are of concern
to public health, the Administrator proposes to establish a new
indicator for thoracic coarse particles in terms of
PM10-2.5, the definition of which includes qualifications
that identify both the mix of such particles that are of concern to
public health, and are thus included in the indicator, and those for
which currently available information is not sufficient to infer a
public health concern, and are thus excluded. More specifically, the
proposed PM10-2.5 indicator is qualified so as to include
any ambient mix of PM10-2.5 that is dominated by particles
generated by high-density traffic on paved roads, industrial sources,
and construction sources, and to exclude any ambient mix of particles
dominated by rural windblown dust and soils and agricultural and mining
sources. The Administrator proposes to replace the current primary 24-
hour PM10 standard with a 24-hour standard defined in terms
of this new PM10-2.5 indicator and set at a level of 70
[mu]g/m3, which would generally maintain the degree of
public health protection afforded by the current PM10
standards from short-term exposure to thoracic coarse particles of
concern. The proposed new standard would be met at an ambient air
quality monitoring site \75\ when the 3-year average of the annual 98th
percentile 24-hour average PM10-2.5 concentration is less
than or equal to 70 [mu]g/m3.\76\ The Administrator also
proposes to revoke and not replace the annual PM10 standard.
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    \75\ Monitoring sites that are appropriate for determining
compliance with this standard are those that are consistent with the
proposed indicator. Guidance on this can be found in the proposed
monitoring network design criteria published elsewhere in today's
Federal Register.
    \76\ Data handling conventions are specified in a new proposed
Appendix P, as discussed in Section V below, and the reference
method for monitoring PM as PM10-2.5 is specified in a
new proposed Appendix L, as discussed in Section VI below.
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    In recognition of alternative views of the currently available
scientific information and the appropriate policy response to this
information, the Administrator also solicits comments on (1)
alternative approaches to selecting the level of a 24-hour
PM10-2.5 standard or to selecting an unqualified
PM10-2.5 indicator, and (2) alternative approaches to
providing continued protection from thoracic coarse particles based on
retaining the current 24-hour PM10 standard. Alternatively,
the Administrator also solicits comment on revoking and not replacing
the 24-hour PM10 standard. Based on the comments received
and the accompanying rationale, the Administrator may adopt other
standards within the range of the alternatives identified above in lieu
of the standard he is proposing today.
    The Administrator is also proposing to revoke the current annual
PM10 standard upon promulgation of this rule. Further, if
EPA finalizes a 24-hour primary PM10-2.5 standard, the
Administrator is proposing to revoke the current 24-hour
PM10 standard everywhere except in areas where there is at
least one monitor that is located in an urbanized area \77\ with a
minimum population of 100,000 people and that violates the 24-hour
PM10 standard based on the most recent three years of data.
---------------------------------------------------------------------------

    \77\ As defined by the U.S. Bureau of the Census, an urbanized
area has ``a minimum residential population of at least 50,000
people'' and generally includes ``core census block groups or blocks
that have a population density of at least 1,000 people per square
mile and surrounding census blocks that have an overall density of
at least 500 people per square mile.'' The Census Bureau notes that
``under certain conditions, less densely settled territory may be
part of each UA.'' See http://www.census.gov/ geo/www/ua/ua--

2k.html.
---------------------------------------------------------------------------

    EPA specifically proposes that the 24-hour PM10 standard
would be revoked in this rulemaking in all areas except the following:

1. Birmingham urban area (Jefferson County, AL)
2. Maricopa and Pinal Counties; Phoenix planning area (AZ)
3. Riverside, Los Angeles, Orange and San Bernardino Counties; South
Coast Air Basin (CA)
4. Fresno, Kern, Kings, Tulare, San Joaquin, Stanislaus, Maderia
Counties; San Joaquin Valley planning area (CA)
5. San Bernardino County (part); excluding Searles Valley Planning Area
and South Coast Air Basin (CA)
6. Riverside County; Coachella Valley Planning Area (CA)
7. Simi Valley urban area (CA)
8. Lake County; Cities of East Chicago, Hammond, Whiting, and Gary (IN)
9. Wayne County (part) (MI)
10. St. Louis urban area (MO)
11. Albuquerque urban area (NM)
12. Clark County; Las Vegas planning area (NV)
13. Columbia urban area (SC)
14. El Paso urban area (including those portions in TX and those
portions in NM)
15. Salt Lake County (UT)

    A separate memorandum explaining the factual basis for our proposed
determinations regarding each PM10 area where we are
proposing to retain the current 24-hour standard is part of the
administrative record for this proposed rule (Rosendahl, 2005).
    In essence, we are proposing to retain the current 24-hour
PM10 standard only in areas which could be in violation of
the proposed PM10-2.5 standard. While it is possible that
some existing PM10 monitors may not be sited in accordance
with all of the criteria for PM10-2.5 monitor siting
proposed elsewhere in today's Federal Register (see section IV.E.2.b.ii
of the preamble to the proposed changes to Part 53/58), it is not
possible for EPA to make a case-by-case assessment of monitor placement
within each area at this time. Therefore, EPA believes that all areas
with violating PM10 monitors located in urbanized areas with
a minimum population of 100,000 people should be considered areas that
may violate the PM10-2.5 standard.
    For those areas where we propose to retain the 24-hour
PM10 standard which were previously designated nonattainment
for PM10 or which are currently designated nonattainment for
PM10, EPA proposes, in the alternative, either that the
standard would continue to apply in the entire attainment/nonattainment
area, or that the area to which the standard would continue to apply
should be limited to the urbanized area containing the violating
monitor(s). For areas with violating monitor(s) which were never
designated nonattainment, EPA proposes that the boundaries of the area
to which the standard would continue to apply should be limited to the
urbanized area containing the violating monitor(s). For

all areas in which the 24-hour PM10 standard would be
retained, EPA invites comments on the appropriate boundaries within
which the standard should continue to apply.
    Consistent with our request for comment in the Part 53/58 proposal,
section IV.E.2.b.ii, on whether we should establish criteria for
locating discretionary monitors appropriate for comparison with the
proposed 24-hour PM10-2.5 standard in locations other than
urbanized areas with population of at least 100,000 people, we also
request comment on whether the 24-hour PM10 standard should
be retained in areas that are either urbanized areas with a population
less than 100,000 people or non-urbanized areas (i.e. population less
than 50,000) but where the majority of the ambient mix of
PM10-2.5 is generated by high density traffic on paved
roads, industrial sources, and construction activities, and which have
at least one monitor that violates the 24-hour PM10
standard. The EPA requests comment on the criteria that should be used
to determine whether such an area with a violating monitor must retain
the 24-hour PM10 standard. Such criteria could include
whether the area has one (or more) industrial source(s) listed in
either the National Emissions Inventory or the Toxics Release Inventory
located within a certain radius of the violating monitor, and whether
these sources are in industrial categories that do not include
agricultural or mining sources. One approach to defining such
categories would be to utilize the U.S. Census Bureau's North American
Industry Classification System,\78\ which defines separate
classifications for agricultural and mining activities such as Crop
Production (111), Animal Production (112), and Mining (112). The EPA
requests comments on how this or another classification system,
combined with information on the location of sources relative to the
violating PM10 monitor, could be used to identify additional
areas to which the 24-hour PM10 standard should continue to
apply due to the presence of industrial sources. The EPA also requests
comments on which areas would meet these criteria or other criteria
that may be appropriate to determine in which, if any, areas the 24-
hour PM10 standard should be retained, and the appropriate
boundaries within which the standard should continue to apply for these
areas. A more detailed example of criteria that could be used to
identify areas to which the standard should continue to apply, along
with a list of all areas with violating PM10 monitors that
meet these criteria, are part of the administrative record for this
proposed rule (Rosendahl, 2005). For all areas where the 24-hour
PM10 standard would be retained under this proposal, we
contemplate that the 24-hour PM10 standard would be revoked
after designations are completed under a final 24-hour
PM10-2.5 standard.
---------------------------------------------------------------------------

    \78\ http://www.census.gov/epcd/naics02/naicod02.htm#N21.

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

    The EPA also recognizes that it is possible that some areas for
which we are proposing to retain the PM10 daily standard
would, upon a case-specific investigation (see section IV.E.2.c of the
Part 53/58 preamble), warrant revocation as not being an area where the
ambient coarse PM mix is dominated by the type of coarse PM described
by the proposed indicator. The EPA is not in a position to conduct such
case-by-case evaluation for this proposal, but could address revocation
in such situations in a future rulemaking. The EPA invites comment on
this issue.
    To address issues related to the transition from the current
PM10 standards to a new PM10-2.5 standard, the
Administrator intends to seek public comment on EPA's plans for
assuring an effective transition as part of an ANPR that EPA intends to
issue by the end of January 2006. In the forthcoming ANPR dealing with
transition issues, EPA intends to address, among other things, the
timing for revocation of the PM10 standard in areas in which
we are proposing to retain that standard, and the consequences of
revoking the PM10 standards on the PM10 PSD
program (including PM10 increments), on the PM10
nonattainment New Source Review (NSR) program, and on our existing
policy of using PM10 as a surrogate for the PM2.5
NSR program.

IV. Rationale for Proposed Decisions on Secondary PM Standards

    The Criteria Document and Staff Paper examined the effects of PM on
such aspects of public welfare as visibility, vegetation and
ecosystems, materials damage and soiling, and climate change. The
existing suite of secondary PM standards, which is identical to the
suite of primary PM standards, includes annual and 24-hour
PM2.5 standards and annual and 24-hour PM10
standards. This existing suite of secondary standards is intended to
address visibility impairment associated with fine particles and
materials damage and soiling related to both fine and coarse particles.
The following discussion of the rationale for the proposed decisions on
secondary PM standards focuses on those considerations most influential
in the Administrator's proposed decisions, first addressing visibility
impairment followed by the other welfare effects considered in this
review.\79\
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    \79\ As noted in section I.A above, in establishing secondary
standards that are requisite to protect the public welfare from any
known or anticipated adverse effects, EPA may not consider the costs
of implementing the standards.
---------------------------------------------------------------------------

A. Visibility Impairment

    This section presents the rationale for the Administrator's
proposed revision of the current secondary PM2.5 standard to
address PM-related visibility impairment. As discussed below, the
rationale includes consideration of: (1) The latest scientific
information on visibility effects associated with PM; (2) insights
gained from assessments of correlations between ambient
PM2.5 and visibility impairment prepared by EPA staff; and
(3) specific conclusions regarding the need for revisions to the
current standards (i.e., indicator, averaging time, form, and level)
that, taken together, would be requisite to protect the public welfare
from adverse effects on visual air quality.
1. Visibility Impairment Related to Ambient PM
    This section outlines key information contained in the Criteria
Document and Staff Paper on: (1) The nature of visibility impairment,
including trends in visual air quality and the characterization of
current visibility conditions; (2) quantitative relationships between
ambient PM and visibility; (3) the impacts of visibility impairment on
public welfare; and (4) approaches to evaluating public perceptions and
attitudes about visibility impairment.
a. Nature of Visibility Impairment
    Visibility can be defined as the degree to which the atmosphere is
transparent to visible light. Visibility conditions are determined by
the scattering and absorption of light by particles and gases, from
both natural and anthropogenic sources. Visibility is often described
in terms of visual range, light extinction, or deciviews.\80\ The
classes of fine particles principally responsible for visibility
impairment are sulfates, nitrates, organic matter, elemental carbon,
and soil dust. Fine

particles are more efficient per unit mass at scattering light than
coarse particles. The scattering efficiency of certain classes of fine
particles, such as sulfates, nitrates, and some organics, increases as
relative humidity rises because these particles can absorb water and
grow to sizes comparable to the wavelength of visible light. In
addition to limiting the distance that one can see, the scattering and
absorption of light caused by air pollution can also degrade the color,
clarity, and contrast of scenes.
---------------------------------------------------------------------------

    \80\ Visual range can be defined as the maximum distance at
which one can identify a black object against the horizon sky. It is
typically described in kilometers or miles. Light extinction is the
sum of light scattering and absorption by particles and gases in the
atmosphere. It is typically expressed in terms of inverse megameters
(Mm-1), with larger values representing poorer
visibility. The deciview metric describes perceived visual changes
in a linear fashion over its entire range, analogous to the decibel
scale for sound.
---------------------------------------------------------------------------

    Visibility impairment is manifested in two principal ways: As local
visibility impairment and as regional haze. Local visibility impairment
may take the form of a localized plume, a band or layer of
discoloration appearing well above the terrain that results from
complex local meteorological conditions. Alternatively, local
visibility impairment may manifest as an urban haze, sometimes referred
to as a ``brown cloud.'' A ``brown cloud'' is predominantly caused by
emissions from multiple sources in the urban area and is not typically
attributable to a single nearby source or to long-range transport from
more distant sources. The second type of visibility impairment,
regional haze, generally results from pollutant emissions from a
multitude of sources located across a broad geographic region. Regional
haze impairs visibility in every direction over a large area, in some
cases over multi-state regions. It is regional haze that is principally
responsible for impairment in national parks and wilderness areas
across the country (NRC, 1993).
    While visibility impairment in urban areas at times may be
dominated by local sources, it often may be significantly affected by
long-range transport of haze due to the multi-day residence times of
fine particles in the atmosphere. Fine particles transported from urban
and industrialized areas, in turn, may, in some cases, be significant
contributors to regional-scale impairment in Class I areas \81\ and
other rural areas.
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    \81\ There are 156 mandatory Class I Federal areas protected by
the visibility provisions in sections 169A and 169B of the Act.
These areas are defined in section 163 of the Act as those national
parks exceeding 6000 acres, wilderness areas and memorial parks
exceeding 5000 acres, and all international parks which were in
existence on August 7, 1977.
---------------------------------------------------------------------------

    As discussed in the Staff Paper (EPA, 2004, section 6.2), in Class
I areas, visibility levels on the 20 percent haziest days in the West
are about equal to levels on the 20 percent best days in the East.
Despite improvement through the 1990's, visibility in the rural East
remains significantly impaired, with an average visual range of
approximately 20 km on the 20 percent haziest days (compared to the
naturally occurring visual range in the eastern U.S. of about 150
 45 km). In the rural West, the average visual range showed
little change over this period, with an average visual range of
approximately 100 km on the 20 percent haziest days (compared to the
naturally occurring visual range in the western U.S. of about 230
 40 km).
    In urban areas, visibility levels show far less difference between
eastern and western regions. For example, the average visual ranges on
the 20 percent haziest days in eastern and western urban areas are
approximately 20 km and 27 km, respectively (Schmidt et al., 2005).
Even more similarity is seen in considering 4-hour (12 to 4 p.m.)
average PM2.5 concentrations, for which the average visual
ranges on the 20 percent haziest days in eastern and western urban
areas are approximately 26 km and 31 km, respectively (Schmidt et al.,
2005).
    Data on visibility conditions indicate that urban areas generally
have higher loadings of PM2.5 and, thus, higher visibility
impairment than monitored Class I areas. Since efforts are now underway
to address all human-caused visibility in Class I areas through the
regional haze program (EPA, 1999; 65 FR 35713), implemented under
sections 169A and 169B of the CAA, and since the Clean Air Interstate
Rule (CAIR) (70 FR 25162) is expected to result in improvements to
visual air quality, particularly in eastern Class I and non-urban
areas, new assessments included in the Staff Paper were primarily
focused on visibility impairment in urban areas.
b. Correlations Between Urban Visibility and PM2.5 Mass
    Direct relationships exist between measured ambient pollutant
concentrations and their contributions to light extinction and thus to
visibility impairment. The contribution of each PM constituent to total
light extinction is derived by multiplying the constituent
concentration by its extinction efficiency to calculate a
``reconstructed'' light extinction.\82\ For certain fine particle
constituents, extinction efficiencies increase significantly with
increases in relative humidity. As a consequence, while higher
PM2.5 mass concentrations generally indicate higher levels
of visibility impairment, it is not as precise a metric as the light
extinction coefficient. Nonetheless, by using historic averages,
regional estimates, or actual day-specific measurements of the
component-specific percentage of total mass, one can develop reasonable
estimates of light extinction from PM mass concentrations. As discussed
below, the Staff Paper concludes that fine particle mass concentrations
can be used as a general surrogate for visibility impairment (EPA,
2005a, p. 2-74).
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    \82\ Extinction efficiencies vary by type of constituent and
have been obtained for typical atmospheric aerosols by a combination
of empirical approaches and theoretical calculations. As discussed
in the Staff Paper, EPA's guidance for tracking progress under the
regional haze program specifies an algorithm for calculating total
light extinction as a function of the major fine particle components
(EPA, 2005a, section 2.8.1). ``Reconstructed'' light extinction
simply refers to the calculation of PM-related light extinction by
the use of that formula.
---------------------------------------------------------------------------

    In an effort to better characterize urban visibility, the Staff
Paper presents results of analyses of the extensive new data now
available on PM2.5 primarily in urban areas. This rapidly
expanding national database includes federal reference method (FRM)
\83\ measurements of PM2.5 mass, continuous measurements of
hourly PM2.5 mass, and PM2.5 chemical speciation
measurements. These data allowed for analyses that explored factors
that have historically complicated efforts to address visibility
impairment nationally, including regional differences related to levels
of primarily fine particles and to relative humidity. These analyses
show a consistently high correlation between visibility, in terms of
reconstructed light extinction, and hourly PM2.5
concentrations for urban areas in a number of regions across the U.S.
and, more generally, in the eastern and western U.S. These correlations
in urban areas are generally similar in the East and West, in sharp
contrast to the East/West differences observed in rural areas.
---------------------------------------------------------------------------

    \83\ The PM2.5 Federal Reference Method (FRM)
monitoring network provides 24-hour average PM2.5
concentrations.
---------------------------------------------------------------------------

    While the average daily relative humidity levels are generally
higher in the East than in the West, in both regions relative humidity
levels are appreciably lower during daylight as compared to night time
hours. The reconstructed light extinction coefficient, for a given mass
and concentration, increases sharply as relative humidity rises. Thus,
with lower relative humidity levels, visibility impacts related to
East/West differences in average relative humidity are minimized during
daylight hours, when relative humidity is generally lower.
    Both 24-hour and shorter-term daylight hour averaging periods were

considered in evaluations of correlations between PM2.5
concentrations in urban areas and visibility in eastern and western
areas, as well as nationwide. Clear and similarly strong correlations
are found between visibility and 24-hour average PM2.5 in
eastern, western, and all urban areas (EPA, 2005a, Figure 6-3).
Somewhat stronger correlations are observed between visibility and
PM2.5 concentrations averaged over a 4-hour time period
(EPA, 2005a, Figure 6-5). The correlations between visibility and
PM2.5 concentrations during daylight hours in urban areas
are relatively more reflective of PM2.5 mass rather than
relative humidity effects, in comparison to correlations based on a 24-
hour averaging time.
c. Impacts of Urban Visibility Impairment on Public Welfare
    EPA has long recognized that impairment of visibility is an
important effect of PM on public welfare, and that it is experienced
throughout the U.S. in urban areas as well as in remote Class I areas
(62 FR 38680). Visibility is an important welfare effect because it has
direct significance to people's enjoyment of daily activities in all
parts of the country. Individuals value good visibility for the sense
of well-being it provides them directly, both in places where they live
and work, and in places where they enjoy recreational opportunities.
    Survey research on public awareness of visual air quality using
direct questioning typically reveals that 80 percent or more of the
respondents are aware of poor visual air quality (Cohen et al., 1986).
The importance of visual air quality to public welfare across the
country has been demonstrated by a number of studies designed to
quantify the benefits (or willingness to pay) associated with potential
improvements in visibility (Chestnut and Dennis, 1997; Chestnut and
Rowe, 1991). Economists have performed many studies in an attempt to
quantify the economic benefits associated with improvements in current
visibility conditions both in national parks and in urban areas
(Chestnut and Dennis, 1997). These economic benefits may include the
value of improved aesthetics during daily activities (e.g., driving or
walking, daily recreations), for special activities (e.g., visiting
parks and scenic vistas, hiking, hunting), and for viewing scenic
photography. They may also include the value of improved road and air
safety, and/or preservation of the resource for its own sake. As
discussed in the Staff Paper and below, the value placed on protecting
visual air quality is further demonstrated by the existence of a number
of programs, goals, standards, and planning efforts that have been
established in the U.S. and abroad to address visibility concerns in
urban and non-urban areas.
    Protection against visibility impairment in special areas is
provided for in sections 169A, 169B, and 165 of the CAA, in addition to
that provided by the secondary NAAQS. Section 169A, added by the 1977
CAA Amendments, established a national visibility goal to ``remedy
existing impairment and prevent future impairment'' in 156 national
parks and wilderness areas (Class I areas). The Amendments also called
for EPA to issue regulations requiring States to develop long-term
strategies to make ``reasonable progress'' toward the national goal.
EPA issued initial regulations in 1980 focusing on visibility problems
that could be linked to a single source or small group of sources. The
1990 CAA Amendments placed additional emphasis on regional haze issues
through the addition of section 169B. In accordance with this section,
EPA established the Grand Canyon Visibility Transport Commission
(GCVTC) in 1991 to address adverse visibility impacts on 16 Class I
national parks and wilderness areas on the Colorado Plateau. The GCVTC
issued its recommendations to EPA in 1996, triggering a requirement in
section 169B for EPA issuance of regional haze regulations.
    EPA accordingly promulgated a final regional haze rule in 1999
(U.S. EPA, 1999; 65 FR 35713). Under the regional haze program, States
are required to establish goals for improving visibility on the 20
percent most impaired days in each Class I area, and for allowing no
degradation on the 20 percent least impaired days. Each state must also
adopt emission reduction strategies which, in combination with the
strategies of contributing States, assure that Class I area visibility
improvement goals are met. The first State implementation plans are to
be adopted in the 2003-2008 time period, with the first implementation
period extending until 2018. Five multi-state planning organizations
are evaluating the sources of PM2.5 contributing to Class I
area visibility impairment to lay the technical foundation for
developing strategies, coordinated among many States, in order to make
reasonable progress in Class I areas across the country.
    A number of other programs, goals, standards, and planning efforts
have also been established in the U.S. and abroad to address visibility
concerns in urban and non-urban areas. These regulatory and planning
activities are of interest because they are illustrative of the
significant value that the public places on improving visibility, and
because they have developed and applied methods for evaluating public
perceptions and judgments about the acceptability of varying degrees of
visibility impairment, as discussed below in the next section.
    Several state and local governments have developed programs to
improve visual air quality in specific urban areas, including Denver,
CO; Phoenix, AZ; and, Lake Tahoe, CA. At least two States have
established statewide standards to protect visibility. In addition,
interest in visibility protection in other countries, including Canada,
Australia, and New Zealand has resulted in various studies, surveys,
and programs. Examples of these efforts are highlighted below.
    In 1990, the State of Colorado adopted a visibility standard for
the city of Denver. The Denver standard is a short-term standard that
establishes a limit of a four-hour average light extinction level of 76
Mm-1 (equivalent to a visual range of approximately 50 km)
during the hours between 8 a.m. and 4 p.m. (Ely et al., 1991). In 2003,
the Arizona Department of Environmental Quality created the Phoenix
Region Visibility Index, which focuses on an averaging time of 4 hours
during actual daylight hours. This visibility index establishes visual
air quality categories (i.e., excellent to very poor) and establishes
the goals of moving days in the poor/very poor categories up to the
fair category, and moving days in the fair category up to the good/
excellent categories (Arizona Department of Environmental Quality,
2003). This approach results in a focus on improving visibility to a
visual range of approximately 48-36 km. In 1989, the state of
California revised the visibility standard for the Lake Tahoe Air Basin
and established an 8-hour visibility standard equal to a visual range
of 30 miles (approximately 48 km) (California Code of Regulations).
    California and Vermont each have standards to protect visibility,
though they are based on different measures. Since 1959, the state of
California has had an air quality standard for particle pollution where
the ``adverse'' level was defined as the ``level at which there will be
* * * reduction in visibility or similar effects.'' California's
general statewide visibility standard is a visual range of 10 miles
(approximately 16 km) (California Code of Regulations). In 1985,
Vermont established a state visibility standard that is expressed as a
summer seasonal sulfate concentration of 2 [mu]g/m\3\, that equates to
a visual range

of approximately 50 km. This standard was established to represent
``reasonable progress'' toward attaining the congressional visibility
goal for the Class 1 Lye Brook National Wilderness Area, and applies to
this Class 1 area and to all other areas of the state with elevations
greater than 2500 ft.
    Outside of the U.S., efforts have also been made to protect
visibility. The Australian state of Victoria has established a
visibility objective (State Government of Victoria, 1999 and 2000), and
a visibility guideline is under consideration in New Zealand (New
Zealand National Institute of Water & Atmospheric Research, 2000a and
2000b; New Zealand Ministry of Environment, 2000). A survey was
undertaken for the Lower Fraser Valley in British Columbia, with
responses from this pilot study being supportive of a standard in terms
of a visual range of approximately 40 km for the suburban township of
Chilliwack and 60 km for the suburban township of Abbotsford, although
no visibility standard has been adopted for the Lower Fraser Valley at
this time.
d. Approaches to Evaluating Public Perceptions and Attitudes
    New methods and tools have been developed to communicate and
evaluate public perceptions of varying visual effects associated with
alternative levels of visibility impairment relative to varying
pollution levels and environmental conditions. New survey methods have
been applied and evaluated in various studies, such as those done in
Denver, Phoenix, and the Lower Fraser Valley in British Columbia. These
methods are intended to assess public perceptions as to the
acceptability of varying levels of visual air quality, considered in
these studies to be an appropriate basis for developing goals and
standards for visibility protection. A pilot study was also conducted
in Washington, DC by EPA staff.\84\ Even with variations in each
study's approaches, the public perception survey methods used for the
Denver, Phoenix, and British Columbia studies produced reasonably
consistent results from location to location, with each study
indicating that a majority of participants find visual ranges within
about 40 to 60 km to be acceptable.
---------------------------------------------------------------------------

    \84\ This small pilot study was briefly discussed in the
preliminary draft staff paper (Abt Associates, 2001).
---------------------------------------------------------------------------

    These public perception studies use images of urban and distant
scenic views under different visibility conditions together with survey
techniques designed to elicit judgments from members of the public
about the acceptability of differing levels of visual air quality.
Images used are either photographs or computer simulations using the
WinHaze program.\85\ Examples of images that illustrate visual air
quality in Denver, Phoenix, Washington, DC, and Chicago under a range
of visibility conditions associated with a range of PM2.5
concentrations are available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_sp.html
 (labeled as Appendix 6A: Images of Visual Air

Quality in Selected Urban Areas in the U.S.). These examples include
simulated images for Denver, Phoenix, and Washington, DC, and
photographs of Chicago.
---------------------------------------------------------------------------

    \85\ The Criteria Document discusses methods available to
represent different levels of visual air quality (EPA, 2004, p. 4-
174). In particular, Molenar et al. (1994) describe a sophisticated
visual air quality simulation technique, incorporated into the
WinHaze program developed by Air Resources Specialists, Inc., which
combined various modeling systems under development for the past 20
years to produce images that standardize non-pollution related
effects on visibility so that perceptions of these images are not
biased due to these other factors.
---------------------------------------------------------------------------

    Survey techniques were developed in conjunction with the Denver
study and relied on citizen judgments of acceptable and unacceptable
levels of visual air quality (Ely et al., 1991; EPA, 2005a, section
<IP_ADDRESS>). The studies in Phoenix and British Columbia, and the pilot
study in Washington, DC used survey approaches based on that used in
Denver. This approach involves conducting a series of meetings with
civic and community groups to elicit individual ratings of a number of
images of well-known local vistas having varying levels of visual air
quality. Participants are told that the results are intended to provide
input on setting a visibility standard, and they are asked to base
their judgments on three factors: (1) The standard is for an urban
area, not a pristine national park area where the standards might be
more strict; (2) standard violations should be at visual air quality
levels considered to be unreasonable, objectionable, and unacceptable
visually; and (3) judgments of standard violations should be based on
visual air quality only, not on any health effects that some may
perceive as being linked with poor visual air quality. The Denver
visibility survey process produced the following findings: (1)
Individuals' judgments of an images's visual air quality and whether
the image should be considered to violate a visibility standard are
highly correlated with the group average; (2) when participants judged
duplicate slides, group averages of the first and second ratings were
highly correlated; and (3) group averages of visual air quality ratings
and ``standard violations'' were highly correlated. The strong
relationship of standard violation judgments with the visual air
quality ratings is cited as the best evidence available from this study
for the validity of this approach as input to a standard setting
process (Ely et al., 1991).
    The Denver visibility standard was established based on a 50
percent acceptability criterion. That is, under this approach, the
standard was identified as the light extinction level that divides the
images into two groups: those found to be acceptable and those found to
be unacceptable by a majority of study participants. In fact, when
researchers evaluated all citizen judgments made on all the
photographic images at this level and above as a single group, more
than 85 percent of the participants found visibility impairment at and
above the level of the selected standard to be unacceptable.
    Generally consistent results were found in the Phoenix study, which
used simulated images from the WinHaze program. The study carefully
selected participants to be demographically representative of the
Phoenix population. The Phoenix survey demonstrates that the rating
methodology developed for gathering citizen input for establishing the
Denver visibility standard can be reliably transferred to another city
while relying on updated imaging technology to simulate a range of
visibility impairment levels. Similarly, the British Columbia study
reinforces the conclusion that the methodology originally developed for
the Denver standard setting process is a sound and effective one for
obtaining public participation in a standard setting process (EPA,
2005a, p. 6-22).
2. Need for Revision of the Current Secondary PM Standards for
Visibility Protection
    The initial issue to be addressed in the current review of the
secondary PM standards is whether, in view of the information now
available, the existing secondary standards should be revised to
provide requisite protection from PM-related adverse effects on visual
air quality. As discussed in the Criteria Document and Staff Paper,
while new research has led to improved understanding of the optical
properties of particles and the effects of relative humidity on those
properties, it has not changed the fundamental characterization of the
role of PM, especially fine particles, in visibility impairment from
the last review. However, extensive new information

now available from visibility and fine particle monitoring networks has
allowed for updated characterizations of visibility trends and current
levels in urban areas, as well as Class I areas. As discussed above,
these new data are a critical component of analyses that better
characterize visibility impairment in urban areas and the relationships
between visibility and PM2.5 concentrations, finding that
PM2.5 concentrations can be used as a general surrogate for
visibility impairment in urban areas.
    Taking into account the most recent monitoring information and
analyses, and recognizing that efforts are now underway to address all
human-caused visibility impairment in Class I areas through the
regional haze program implemented under sections 169A and 169B of the
CAA, as discussed above, this review focuses on visibility impairment
primarily in urban areas. In so doing, consideration is first given to
the question of whether visibility impairment in urban areas allowed by
the current 24-hour secondary PM2.5 standard can be
considered adverse to public welfare.
    As discussed above, studies in the U.S. and abroad have provided
the basis for the establishment of standards and programs to address
specific visibility concerns in a number of local areas. These studies
(e.g., in Denver, Phoenix, British Columbia) have produced reasonably
consistent results in terms of the visual ranges found to be generally
acceptable by the participants in the various studies, which ranged
from approximately 40 to 60 km in visual range. Standards targeting
protection within this range have also been set by the State of Vermont
and by California for the Lake Tahoe area, in contrast to the statewide
California standard that targets a visual range of approximately 16 km.
    In addition to the information available from such programs,
photographic representations (simulated images and actual photographs)
of visibility impairment are available, as discussed above, to help
inform judgments about the acceptability of varying levels of visual
air quality in urban areas across the U.S. In considering these images
for Phoenix, Washington, DC, and Chicago (for which PM2.5
concentrations are reported), the Staff Paper observes that:
    (1) At concentrations at or near the level of the current 24-hour
PM2.5 standard (65 [mu]g/m3), which equates to
visual ranges roughly around 10 km, scenic views (e.g., mountains,
historic monuments), as depicted in these images around and within the
urban areas, are significantly obscured from view.
    (2) Appreciable improvement in the visual clarity of the scenic
views depicted in these images occurs at PM2.5
concentrations below 35 to 40 [mu]g/m3, which equate to
visual ranges generally above 20 km for the urban areas considered
(EPA, 2005a, p. 7-6).
    (3) Visual air quality appears to be good in these images at
PM2.5 concentrations generally below 20 [mu]g/m3,
corresponding to visual ranges of approximately 25 to 35 km (EPA,
2005a, p. 7-8).
    While being mindful of the limitations in using visual
representations from a small number of areas as a basis for considering
national visibility-based secondary standards, the Staff Paper
nonetheless concludes that these observations, together with
information from the analyses and other programs discussed above,
support revising the current secondary PM2.5 standards to
improve visual air quality, particularly in urban areas. As discussed
in the following sections, the Staff Paper recommends the establishment
of a new short-term secondary PM2.5 standard to provide
increased and more targeted protection primarily in urban areas from
visibility impairment related to fine particles (EPA, 2005a, p. 7-12).
Based on its review of the Staff Paper, the CASAC advised the
Administrator that most CASAC PM Panel members strongly supported the
Staff Paper recommendation to establish a new, secondary
PM2.5 standard to protect urban visibility (Henderson,
2005a).\86\ Most Panel members considered such a standard to be a
reasonable complement to the Regional Haze Rules that protect Class I
areas.
---------------------------------------------------------------------------

    \86\ A dissenting view was expressed in one Panel member's
invididual review comments to the effect that any urban visibility
standard should be voluntary and locally adopted (Henderson, 2005a).
---------------------------------------------------------------------------

    In considering whether the secondary PM standards should be revised
to target PM-related visibility impairment primarily in urban areas,
the Administrator has carefully considered the rationale and
recommendation in the Staff Paper, the advice and recommendations from
CASAC, and public comments to date on this issue. In so doing, the
Administrator first recognizes that PM-related visibility impairment is
principally related to fine particle levels, such that it is
appropriate to focus in this review on the current secondary
PM2.5 standards to provide such targeted protection. The
Administrator also recognizes that visibility is most directly related
to instantaneous levels of visual air quality, such that it is
appropriate to focus on a standard with a short-term averaging time
(e.g., 24-hours or less). Thus, the Administrator has considered
whether the current 24-hour secondary PM2.5 standard should
be revised to provide a requisite level of protection from visibility
impairment, principally in urban areas, in conjunction with the
regional haze program for protection of visual air quality in Class I
areas. The Administrator observes that at concentrations at or near the
level of the current 24-hour PM2.5 standard (65 [mu]g/
m3), corresponding to visual ranges of about 10 km, images
of scenic views (e.g., mountains, historic monuments, urban skylines)
around and within a number of urban areas are significantly obscured
from view. Further, the Administrator notes the various State and local
standards and programs that have been established protect visual air
quality beyond the degree of protection that would be afforded by the
current 24-hour secondary PM2.5 standard. Based on all of
the above considerations, the Administrator provisionally concludes
that it is appropriate to revise the current 24-hour secondary
PM2.5 standard to provide requisite protection from
visibility impairment principally in urban areas.
3. Indicator of PM for Secondary Standard To Address Visibility
Impairment
    As discussed in the Staff Paper, fine particles contribute to
visibility impairment directly in proportion to their concentration in
the ambient air. Hygroscopic components of fine particles, in
particular sulfates and nitrates, contribute disproportionately to
visibility impairment under high humidity conditions. Particles in the
coarse mode generally contribute only marginally to visibility
impairment in urban areas. In analyzing how well PM2.5
concentrations correlate with visibility in urban locations across the
U.S. (see EPA, 2005a, section 6.2.3), the Staff Paper concludes that
the observed correlations are strong enough to support the use of
PM2.5 as the indicator for such standards. More
specifically, clear correlations exist between 24-hour average
PM2.5 concentrations and reconstructed light extinction,
which is directly related to visual range. These correlations are
similar in the eastern and western regions of the U.S.. Further, these
correlations are less influenced by relative humidity and more
consistent across regions when PM2.5 concentrations are
averaged over shorter, daylight time periods (e.g., 4 to

8 hours). Thus, the Staff Paper concludes that it is appropriate to use
PM2.5 as an indicator for standards to address visibility
impairment in urban areas, especially when the indicator is defined for
a relatively short period of daylight hours. Based on its review of the
Staff Paper, most CASAC PM Panel members endorsed a PM2.5
indicator for a secondary standard to address visibility impairment.
    The Administrator concurs with the EPA staff and CASAC
recommendations, and concludes that PM2.5 should be retained
as the indicator for fine particles as part of a secondary standard to
address visibility protection. In the Administrator's view,
PM2.5 is the appropriate indicator for any such standard,
whether averaged over 24-hours or over a shorter, sub-daily time
period.
4. Averaging Time of a Secondary PM2.5 Standard for
Visibility Protection
    As discussed in the Staff Paper, averaging times from 24 to 4 hours
have been considered for a standard to address visibility impairment.
Within this range, as noted above, clear and similarly strong
correlations are found between visibility and 24-hour average
PM2.5 concentrations in eastern and western areas, while
somewhat stronger correlations are found with PM2.5
concentrations averaged over a 4-hour time period. In general,
correlations between PM2.5 concentrations and light
extinction are generally less influenced by relative humidity and more
consistent across regions as shorter, sub-daily averaging times, within
daylight hours from approximately 10 a.m. to 6 p.m., are considered.
The Staff Paper concludes that an averaging time from 4 to 8 hours,
generally within this daylight time period, should be considered for a
standard to address visibility impairment.
    In reaching this conclusion, the Staff Paper recognizes that the
PM2.5 Federal Reference Method (FRM) monitoring network
provides 24-hour average concentrations, and, in some cases, on a
third- or sixth-day sample schedule, such that implementing a standard
with a less-than-24-hour averaging time would necessitate the use of
continuous monitors that can provide hourly time resolution. Given that
the data used in the analysis discussed above are from commercially
available PM2.5 continuous monitors, such monitors clearly
could provide the hourly data that would be needed for comparison with
a potential visibility standard with a less-than-24-hour averaging
time.\87\
---------------------------------------------------------------------------

    \87\ Decisions as to which PM2.5 continuous monitors
are providing data of sufficient quality to be used in a sub-daily
visibility standard would follow protocols for approval of Federal
equivalent methods (FEMs) that can provide data in at least hourly
intervals, as proposed in the revisions to Part 53, published
elsewhere in today's Federal Register.
---------------------------------------------------------------------------

    Most CASAC PM Panel members supported the Staff Paper
recommendation of a sub-daily (4 to 8 daylight hours) averaging time,
finding it to be an innovative approach that strengthens the quality of
the PM2.5 indicator by targeting the driest part of the day
(Henderson, 2005a). In its advice to the Administrator, CASAC noted an
indirect but important benefit to advancing EPA's monitoring program
goals that would come from the direct use of hourly data from a network
of continuous PM2.5 mass monitors.
    In considering the Staff Paper recommendation and CASAC's advice,
the Administrator provisionally concludes that averaging times from 24
hours to 4 daylight hours would represent a reasonable range of choices
for a standard to address urban visibility impairment. A 24-hour
averaging time could be selected and applied based on the extensive
data base currently available from the existing PM2.5 FRM
monitoring network, whereas a sub-daily averaging time would
necessarily depend upon an expanded network of continuous
PM2.5 mass monitors. While the Administrator agrees that
broader deployment of continuous PM2.5 mass monitors is a
desirable goal, working toward that goal does not depend upon nor
provide a basis for setting a sub-daily standard. The Administrator
believes that it is appropriate to evaluate averaging time in
conjunction with reaching decisions on the form and level of a
standard, as discussed below.
5. Elements of a Secondary PM2.5 Standard for Visibility
Protection
    In considering PM2.5 standards that would provide
requisite protection against PM-related impairment of visibility
primarily in urban areas, the Administrator has taken into account the
results of public perception and attitude surveys in the U.S. and
Canada, State and local visibility standards within the U.S., and
visual inspection of photographic representations of several urban
areas across the U.S. In the Administrator's judgment, these sources
provide useful but still quite limited information on the range of
levels appropriate for consideration in setting a national visibility
standard primarily for urban areas, given the generally subjective
nature of the public welfare effect involved. In considering
alternative forms for such standards, the Administrator has also taken
into account the same general factors that were considered in selecting
an appropriate form for the 24-hour primary PM2.5 standard,
as well as additional information on the percent of areas not likely to
meet various alternative PM2.5 standards, consistent with
CASAC advice to consider such information (Henderson, 2005a).
    In considering elements of a secondary PM2.5 standard,
the Administrator has looked to the rationale presented in the Staff
Paper and to CASAC's advice and recommendations for such a standard.
Based on photographic representations of varying levels of visual air
quality, public perception studies, and local and State visibility
standards, as discussed above, the Staff Paper concludes that 30 to 20
[mu]g/m3 PM2.5 represents a reasonable range for
a national visibility standard primarily for urban areas, based on a
sub-daily averaging time. The upper end of this range is below the
levels at which the illustrative scenic views are significantly
obscured, and the lower end is around the level at which visual air
quality generally appears to be good based on observation of the
illustrative views. Analyses of 4-hour average PM2.5
concentrations indicate that this concentration range can be expected
generally to correspond to median visual ranges in urban areas within
regions across the U.S. of approximately 25 to 35 km (see EPA, 2005a,
Figure 7-1).\88\ This range of visual range values is bounded above by
the visual range targets selected in specific areas where State or
local agencies placed particular emphasis on protecting visual air
quality.
---------------------------------------------------------------------------

    \88\ The Staff Paper notes that a standard set at any specific
PM2.5 concentration will necessarily result in visual
ranges that vary somewhat in urban areas across the country,
reflecting the variability in the correlations between
PM2.5 concentrations and light extinction (EPA, 2005a, p.
7-8).
---------------------------------------------------------------------------

    In considering a reasonable range of forms for a PM2.5
standard within this range of levels, the Staff Paper concludes that a
concentration-based percentile form is appropriate for the same reasons
as discussed above in section II.F.1 (on the form of the 24-hour
primary PM2.5 standard). The Staff Paper also concludes that
the upper end of the range of concentration percentiles should be
consistent with the percentile used for the primary standard, which is
proposed to be the 98th percentile, and that the lower end of the range
should be the 92nd percentile, which represents the mean of the
distribution

of the 20 percent worst day, as targeted in the regional haze program
(EPA, 2005a, p. 7-11 to 12).
    In its letter to the Administrator (Henderson, 2005a), the CASAC PM
Panel recognizes that it is difficult to select any specific level and
form based on currently available information. Some Panel members felt
that the range of levels recommended in the Staff Paper was on the high
side, but recognized that developing a more specific (and more
protective) level in future reviews would require updated and refined
public visibility valuation studies, which CASAC strongly encouraged
the Agency to support prior to the next review. With regard to the form
of the standard, the recommendations in the final Staff Paper reflected
CASAC's advice to consider percentiles in the range of the 92nd to the
98th percentile. Some Panel members recommend considering a percentile
within this range in conjunction with a level toward the upper end of
the range recommended in the Staff Paper.\89\
---------------------------------------------------------------------------

    \89\ Some CASAC Panel members also recommend that such a
standard be implemented in conjunction an ``exceptional events''
policy so as to avoid having non-compliance with the standard be
driven by natural source influences such as dust storms and wild
fires (Henderson, 2005a).
---------------------------------------------------------------------------

    Based on the above considerations, the Administrator believes that
it is appropriate to first consider the level of protection that would
be afforded by the suite of primary PM2.5 standards proposed
today. The limited and uncertain evidence currently available for use
in evaluating the appropriate level of protection suggests that a
cautious approach is warranted in establishing a secondary standard.
While significantly more information is available since the last review
concerning the relationship between fine PM levels and visibility
across the country, there is still little available information for use
in making the relatively subjective value judgment needed in setting
the secondary standard. Given this, it is appropriate to first evaluate
the level of protection that the proposed primary standards would
likely provide, and then determine whether the available evidence
warrants adopting a standard with a different level, form, or averaging
time. In comparing the extent to which the proposed suite of primary
standards would require areas across the country to improve visual air
quality with the extent of increased protection likely to be afforded
by a standard based on a sub-daily averaging time, the Administrator
has looked to information on the predicted percent of areas not likely
to meet various alternative secondary and primary PM2.5
standards (EPA, 2005a, Tables 7A-1 and 5B-1(a) \90\). In so doing, the
Administrator observes that the predicted percent of counties with
monitors not likely to meet the proposed suite of primary
PM2.5 standards (i.e., a 24-hour standard set at 35 [mu]g/
m3, with a 98th percentile form, and an annual standard of
15 [mu]g/m3) is somewhat higher (27 percent) than the
predicted percent of counties with monitors not likely to meet a sub-
daily secondary standard with an averaging time of 4 to 8 daylight
hours, a level toward the upper end of the range recommended in the
Staff Paper (e.g., up to 30 [mu]g/m3), and a form within the
recommended range (e.g., around the 95th percentile) (24 percent). A
similar comparison is seen in considering the predicted percentages of
the population living in such areas.
---------------------------------------------------------------------------

    \90\ The information in these Tables is based on analysis of
2001-2003 air quality data, including 562 counties with FRM monitors
that met specific data completeness criteria for developing
predicted percentages of counties not likely to meet the suite of
primary PM2.5 standards and 168 counties with continuous
PM2.5 monitors that met less restrictive data
completeness criteria for developing predicted percentages for a 4-
hour secondary PM2.5 standard.
---------------------------------------------------------------------------

    The Administrator provisionally concludes that revising the current
secondary PM2.5 to be identical to the proposed suite of
primary PM2.5 standards is a reasonable policy approach to
addressing visibility protection primarily in urban areas. Such an
approach would result in improvements in visual air quality in as many
or more urban areas across the country as would the alternative
approach of setting a sub-daily standard consistent with that generally
recommended by CASAC. Such an approach also takes into account the
substantial limitations in the available hourly air quality data and in
available studies of public perception and attitudes with regard to the
acceptability of various degrees of visibility impairment in urban
areas across the country. Given these limitations, the Administrator
concludes, subject to consideration of public comment, that a secondary
standard with a different averaging time, level, or form is not
warranted, because the available evidence does not support a decision
to achieve a level of protection different from that provided by the
current primary standards, and because no change in averaging time,
level, or form appears needed to achieve a comparable level of
protection.
    The Administrator believes that a secondary NAAQS should be
considered 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 Class I areas across the
country. Programs implemented to meet a national standard focused
primarily on 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 rule
established for protection of visual air quality in Class I areas. The
Administrator further believes that the development of local programs
continues to be an effective and appropriate approach to provide
additional protection for unique scenic resources in and around certain
urban areas that are particularly highly valued by people living in
those areas. Based on these considerations, and taking into account the
observations, analyses, and recommendations discussed above, the
Administrator proposes to revise the current secondary PM2.5
standards by making them identical in all respects to the proposed
suite of primary PM2.5 standards.
    As discussed above, most CASAC PM Panel members strongly supported
a sub-daily (4- to 8-hour averaging time) PM2.5 standard.
The Administrator places great importance on the advice of CASAC, and
therefore solicits public comment on such a standard.

B. Other PM-Related Welfare Effects

    This section presents the rationale for the Administrator's
proposed revision of the current secondary PM standards to address PM-
related effects other than visibility impairment, including vegetation
and ecosystems, materials damage and soiling, and climate change. In
considering the currently available evidence on each of these types of
PM-related welfare effects, the Staff Paper notes that there is much
information linking ambient PM to potentially adverse effects on
materials and ecosystems and vegetation, and on characterizing the role
of atmospheric particles in climatic and radiative processes. However,
given the evaluation of this information in the Criteria Document and
Staff Paper which highlighted the substantial limitations in the
evidence, especially the lack of evidence linking various effects to
specific levels of ambient PM, the Administrator provisionally
concludes that the available evidence does not provide a sufficient
basis for establishing distinct secondary standards for PM based on any
of these effects alone.

    The Administrator has also addressed the question of whether
reductions in PM likely to result from the current secondary PM
standards, or from the range of proposed revisions to the primary PM
standards, would provide requisite protection against any of these PM-
related welfare effects. As discussed below, these considerations
include the latest scientific information characterizing the nature of
these PM-related effects and judgments as to whether revision of the
current secondary standards are appropriate based on that information.

1. Nature of Effects

    Particulate matter contributes to adverse effects on a number of
welfare effects categories other than visibility impairment, including
vegetation and ecosystems, soiling and materials damage and climate.
These welfare effects result predominantly from exposure to excess
amounts of specific chemical species, regardless of their source or
predominant form (particle, gas or liquid). Reflecting this fact, the
Criteria Document concludes that regardless of size fraction, particles
containing nitrates and sulfates have the greatest potential for
widespread environmental significance, while effects are also related
to other chemical constituents found in ambient PM, such as trace
metals and organics.\91\ The following characterizations of the nature
of these welfare effects are based on the information contained in the
Criteria Document and Staff Paper.
---------------------------------------------------------------------------

    \91\ The Staff Paper notes that some of these other components
are regulated under separate statutory authorities, e.g., section
112 of the CAA.
---------------------------------------------------------------------------

a. Effects on Vegetation and Ecosystems
    Potentially adverse PM-related effects on vegetation and ecosystems
are principally associated with particulate nitrate and sulfate
deposition. In characterizing such effects, it is important to
recognize that nitrogen and sulfur are necessary and beneficial
nutrients for most organisms that make up ecosystems, with optimal
amounts of these nutrients varying across organisms, populations,
communities, ecosystems and time scales. Therefore, it is impossible to
generalize to all species in all circumstances as to the amount at
which inputs of these nutrients or acidifying compounds become
stressors. The Staff Paper recognizes that the public welfare benefits
from the use of nitrogen (N) and sulfur (S) nutrients in fertilizers in
managed agricultural and commercial forest settings. The focus of this
review, therefore, is on identifying risks to sensitive species and
ecosystems where unintentional additions of these atmospherically
derived nutrient and acidifying compounds may be contributing to
undesired change in the nation's ecosystems and resulting in adverse
impacts on essential ecological attributes such as species shifts, loss
of species richness and diversity, impacts on threatened and endangered
species, and alteration of native fire cycles. In these cases,
deposited particulate nitrate and sulfate are appropriately termed
ecosystem ``stressors.''
i. Vegetation Effects
    At current ambient levels, risks to vegetation from short-term
exposures to dry deposited particulate nitrate or sulfate are low.
However, when found in acid or acidifying deposition, such particles do
have the potential to cause direct foliar injury. Specifically, the
responses of forest trees to acid precipitation (rain, snow) include
accelerated weathering of leaf cuticular surfaces, increased
permeability of leaf surfaces to toxic materials, water, and disease
agents; increased leaching of nutrients from foliage; and altered
reproductive processes--all which serve to weaken trees so that they
are more susceptible to other stresses (e.g., extreme weather, pests,
pathogens). Acid deposition with levels of acidity associated with the
foliar effects described above are currently found in some locations in
the eastern U.S. (EPA, 2003). Even higher concentrations of acidity can
be present in occult deposition (e.g. fog, mist or clouds) which more
frequently impacts higher elevations. Thus, the risks of foliar injury
occurring from acid deposition in some areas of the eastern U.S. is
high. However, based on currently available information, the
contribution of particulate sulfates and nitrates to the total acidity
found at these locations is not clear.
ii. Ecosystem Effects
    The N- and S-containing components of PM have been associated with
a broad spectrum of terrestrial and aquatic ecosystem impacts that
result from either the nutrient or acidifying characteristics of the
deposited compounds.
    Reactive nitrogen (Nr) is the form of N that is available to
support the growth of plants and microorganisms. Since the mid-1960's,
Nr creation through natural terrestrial processes has been overtaken by
Nr creation as a result of human processes, and is now accumulating in
the environment on all spatial scales--local, regional and global. Some
Nr emissions are transformed into ambient PM and deposited onto
sensitive ecosystems. Some of the most significant detrimental effects
associated with excess Nr deposition are those associated with a
syndrome known as ``nitrogen saturation.'' These effects include: (1)
Decreased productivity, increased mortality, and/or shifts in
terrestrial plant community composition, often leading to decreased
biodiversity in many natural habitats wherever atmospheric Nr
deposition increases significantly and critical thresholds are
exceeded; (2) leaching of excess nitrate and associated base cations
from terrestrial soils into streams, lakes and rivers and mobilization
of soil aluminum; and (3) alteration of ecosystem processes such as
nutrient and energy cycles through changes in the functioning and
species composition of beneficial soil organisms (Galloway and Cowling
2002). Thus, through its effects on habitat suitability, genetic
diversity, community dynamics and composition, nutrient status, energy
and nutrient cycling, and frequency and intensity of natural
disturbance regimes (fire), excess Nr deposition is having profound and
adverse impact on the essential ecological attributes associated with
terrestrial ecosystems. In the U.S., numerous forests now show severe
symptoms of nitrogen saturation. For other forested locations, ongoing
expansion in nearby urban areas will increase the potential for
nitrogen saturation unless there are improved emission controls.
    Excess nutrient inputs into aquatic ecosystems (e.g., streams,
rivers, lakes, estuaries or oceans) either from direct atmospheric
deposition, surface runoff, or leaching from nitrogen saturated soils
into ground or surface waters can contribute to conditions of severe
water oxygen depletion (hypoxia); eutrophication and algae blooms;
altered fish distributions, catches, and physiological states; loss of
biodiversity; habitat degradation; and increases in the incidence of
disease. Estuaries are among the most intensely fertilized systems on
Earth.
    Reactive nitrogen moves from one environmental reservoir to another
through a number of sequential environmental processes. Though strong
correlation between the stressor and adverse environmental response
exists in many locations, and N-addition studies have confirmed the
relationship between stressor and response, the ability to determine
the temporal and spatial distribution of environmental effects for a
given input of Nr are extremely limited by the large uncertainties
associated with the rates at which Nr cascades through and

accumulates in various environmental reservoirs.
    Acid and acidifying deposition is another significant source of
stress to forest and aquatic ecosystems. It changes the chemical
composition of soils by depleting the content of available plant
nutrient cations such as calcium (Ca2+), increasing the
mobility of aluminum (Al), and increasing the S and N content (Driscoll
et al., 2001).
    Leaching of soil nutrients is often of major importance in cation
cycles, and many forest ecosystems show a net loss of base cations. In
sensitive forest soils, acid deposition leads to a shift in chemical
speciation of Al from organic to inorganic forms that are toxic to
terrestrial and aquatic biota, and increases inorganic Al mobilization
and transport into surface waters. The toxic effect of Al on forest
vegetation is attributed to its interference with plant uptake of
essential nutrients, such as Ca and Mg. There are large variations in
Al sensitivity among ecotypes, between and within species, due to
differences in nutritional demands and physiological status, that are
related to age and climate, and which change over time.
    Acid deposition has been firmly implicated as a causal factor in
the decline of red spruce in high elevation sites in the Northeast. Red
spruce is valued commercially, for recreation and aesthetics, and as
habitat for unique and endangered species. Dieback of red spruce trees
has also been observed in mixed hardwood-conifer stands at relatively
low elevations in the western Adirondack Mountains, where inputs of
acid deposition are high. Exposure to acidic mist or cloud water
reduces foliar calcium levels in red spruce needles, leading to
increased susceptibility to freezing (winter injury). There is also the
strong possibility that acid deposition altering of foliar calcium
levels leading to reduced cold tolerance is not unique to red spruce
but has been demonstrated in many other northern temperate forest tree
species including yellow birch, white spruce, red maple, eastern white
pine, and sugar maple. Less sensitive forests throughout the U.S. are
experiencing gradual losses of base cation nutrients, which in many
cases will reduce the quality of forest nutrition in the future
(National Science and Technology Council, 1998).
    Inputs of acid deposition to regions with base-poor soils have also
resulted in the acidification of soil waters, shallow ground waters,
streams, and lakes in a number of locations within the U.S.
Acidification has marked effects on the trophic structure of surface
waters. Decreases in pH and increases in Al concentrations contribute
to declines in species richness and in the abundance of zooplankton,
macroinvertebrates, and fish. Numerous studies have shown that
decreases in pH result in decreases in fish species richness (the
number of fish species in a water body) by eliminating acid-sensitive
species including important recreational fishes plus ecologically
important minnows that serve as forage for sport fishes.
    Though significant decreases in sulfur emissions have occurred in
the U.S. and Europe in recent decades, these decreases have not been
accompanied by equivalent declines in net acidity related to sulfate in
precipitation, and may have, to varying degrees, been offset by steep
declines in atmospheric base cation concentrations over the past 10 to
20 years (Hedin et al., 1994; Driscoll et al. 2001). Projections made
using an acidification model (PnET-BGC) \92\ indicate that full
implementation of the 1990 CAA Amendments will not afford substantial
chemical recovery at Hubbard Brook Experimental Forest and at many
similar acid-sensitive locations (Driscoll et al., 2001). Model
calculations indicate that the magnitude and rate of recovery from acid
deposition in the northeastern U.S. are directly proportional to the
magnitude of emissions reductions. Model evaluations of policy
proposals calling for additional reductions in utility SO2
and NOX emissions, year round emissions controls, and early
implementation indicate greater success in facilitating the recovery of
sensitive ecosystems (Driscoll et al., 2001).
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    \92\ PnET-BGC is designed to simulate element cycling in forest
and interconnected aquatic ecosystems. The model PnET is a simple,
generalized, and well validated model that provides estimates of
forest net primary productivity, nutrient uptake by vegetation, and
water balances. Recently, PnEt was coupled with a soil model that
simulates abiotic soil processes, resulting in a comprehensive
forest-soil-water model, PnET-BGC (Driscoll et al., 2001).
---------------------------------------------------------------------------

    Driscoll et al. (2001) envision a recovery process that will
involve two phases: chemical and biological. Initially, a decrease in
acid deposition following emissions controls will facilitate a phase of
chemical recovery in forest and aquatic ecosystems. Recovery time for
this phase will vary widely across ecosystems and will be a function of
a number of factors. In most cases, it seems likely that chemical
recovery will require decades, even with additional controls on
emissions. The second phase in ecosystem recovery is biological
recovery, which can occur only if chemical recovery is sufficient to
allow survival and reproduction of plants and animals. The time
required for biological recovery is uncertain. For terrestrial
ecosystems, it is likely to be at least decades after soil chemistry is
restored because of the long life of tree species and the complex
interactions of soil, roots, microbes, and soil biota. For aquatic
systems, research suggests that stream macroinvertebrate populations
may recover relatively rapidly (approximately 3 years), whereas lake
populations of zooplankton are likely to recover more slowly
(approximately 10 years) (Gunn and Mills, 1998). Some fish populations
may recover in 5 to 10 years after the recovery of zooplankton
populations, perhaps sooner with fish stocking (Driscoll et al., 2001).
iii. Ecosystem Exposure to PM Deposition
    In order to establish exposure-response profiles useful in
ecological risk assessments, two types of monitoring networks need to
be in place. First, a deposition network is needed that can track
changes in deposition rates of PM stressors (nitrates/sulfates)
occurring in sensitive or symptomatic areas/ecosystems. Secondly, a
network or system of networks should be established that measures the
response of key sensitive ecological indicators over time to changes in
atmospheric deposition of PM stressors.
    Data from existing deposition networks in the U.S. demonstrate that
N and S compounds are being deposited in amounts known to be sufficient
to affect sensitive terrestrial and aquatic ecosystems over time.
Though the percentages of N and S containing compounds in PM vary
spatially and temporally, nitrates and sulfates make up a substantial
portion of the chemical composition of PM. In the future, speciated
data from these networks may allow better understanding of the specific
components of total deposition that are most strongly influencing PM-
related ecological effects.
    At this time, however, there are only a few sites where long-term
monitoring of sensitive indicators of ecosystem response to excess
nitrogen and/or acidic and acidifying deposition is taking place within
the U.S. Because the complexities of ecosystem response make
predictions of the magnitude and timing of chemical and biotic recovery
uncertain, it is important that this type of long-term monitoring
network be continued, and that biological monitoring be enhanced to
support future evaluations of the response of forested watersheds and
surface waters to a host of research and regulatory issues related to
nutrient and acid and acidifying deposition.

iv. Critical Loads
    The critical load (CL) has been defined as a ``quantitative
estimate of an exposure to one or more pollutants below which
significant harmful effects on specified sensitive elements of the
environment do not occur according to present knowledge'' (Lokke et
al., 1996). The concept is useful for estimating the amounts of
pollutants that ecosystems can absorb on a sustained basis without
experiencing measurable degradation. The estimation of ecosystem
critical loads requires an understanding of how an ecosystem will
respond to different loading rates in the long term and is a direct
function of the level of sensitivity of the ecosystem to the pollutants
in question and its ability to ameliorate pollutant stress.
    The CL approach is very data-intensive, and, at the present time,
there is a paucity of ecosystem-level data for most sites. However, for
a limited number of areas which already have a long-term record of
ecosystem monitoring, (e.g., Rocky Mountain National Park in Colorado
and the Lye Brook Wilderness in Vermont), Federal Land Managers may be
able to develop site specific CLs. More specifically, with respect to
PM deposition, there are insufficient data for the vast majority of
U.S. ecosystems that differentiate the PM contribution to total N or S
deposition to allow for practical application of this approach as a
basis for developing national standards to protect sensitive U.S.
ecosystems from adverse effects related to PM deposition. Though
atmospheric sources of Nr and acidifying compounds, including ambient
PM, are clearly contributing to the overall excess load or burden
entering ecosystems annually, insufficient data are available at this
time to quantify the contribution of ambient PM to total Nr or acid
deposition as its role varies both temporally and spatially along with
a number of other factors. Thus, at the present time, a CL could not be
developed that would address the portion of the total N or S input that
is contributed by ambient PM.
b. Effects on Materials Damage and Soiling
    As discussed in the Staff Paper, the effects of the deposition of
atmospheric pollution, including ambient PM, on materials are related
to both physical damage and impaired aesthetic qualities. The
deposition of PM (especially sulfates and nitrates) 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 concrete and limestone. As noted in the last review, only
chemically active fine-mode or hygroscopic coarse-mode particles
contribute to these physical effects. In addition, the deposition of
ambient PM can reduce the aesthetic appeal of buildings and culturally
important articles 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. Available data indicate that particle-related soiling can result
in increased cleaning frequency and repainting, and may reduce the
useful life of the soiled materials. However, to date, no quantitative
relationships between particle characteristics (e.g., concentrations,
particle size, and chemical composition) and the frequency of cleaning
or repainting have been established. Thus, the Administrator concludes
that PM effects on materials can play no quantitative role in
considering whether any revisions of the secondary PM standards are
appropriate at this time.
c. Effects on Climate
    As discussed in the Staff Paper, atmospheric particles can alter
the earth's energy balance by both scattering and absorbing radiation
transmitted through the earth's atmosphere. Most components of ambient
PM (especially sulfates) scatter and reflect incoming solar radiation
back into space, thus tending to have a cooling effect on climate. In
contrast, some components of ambient PM (especially black carbon)
absorb incoming solar radiation or outgoing terrestrial radiation, thus
tending to have a warming effect on climate. Other impacts of
atmospheric particles are associated with their role in affecting the
radiative properties of clouds, through changes in the number and size
distribution of cloud droplets (which can have an effect on the climate
in either direction), and by altering the amount of ultraviolet solar
radiation (especially UV-B) penetrating through the atmosphere to
ground level, where it can exert a variety of effects on human health,
plant and animal biota, and other environmental components.
    The available information, however, provides no basis for
estimating how localized changes in the temporal, spatial, and
composition patterns of ambient PM likely to occur as a result of
expected future emissions of particles and their precursor gases across
the U.S., would affect local, regional, or global changes in climate or
UV-B radiation penetration. Even the direction of such effects on a
local scale remains uncertain. Moreover, similar concentrations of
different particle components can produce opposite net effects,
depending on other atmospheric parameters such as humidity. The
Administrator thus concludes that, given this uncertainty, the
potential indirect effects of ambient PM on public health and welfare,
secondary to potential PM-related changes in climate and UV-B
radiation, can play no quantitative role in considering whether any
revisions of the primary or secondary PM standards are appropriate at
this time.
2. Need for Revision of Current Secondary PM Standards To Address Other
PM-Related Welfare Effects
    In considering the currently available evidence on each type of PM-
related welfare effects discussed above, the Administrator notes that
there is much information linking the S- and N-containing components of
ambient PM to potentially adverse effects on ecosystems and vegetation,
materials damage and soiling, and on climatic and radiative processes.
However, after reviewing the extent of relevant studies and other
information provided since the 1997 review of the PM standards, which
highlighted the substantial limitations in the evidence, especially
with regard to the lack of evidence linking various effects to specific
levels of ambient PM, the Administrator concurs with conclusions
reached in the Staff Paper and by CASAC (Henderson, 2005a) that the
available data do not provide a sufficient basis for establishing
separate and distinct secondary PM standards based on any of these non-
visibility PM-related welfare effects.
    While recognizing that PM-related impacts on vegetation and
ecosystems and PM-related soiling and materials damage are associated
with chemical components in both fine and coarse-fraction PM, the
Administrator provisionally concludes that sufficient information is
not available at this time to consider either an ecologically based
indicator or an indicator based distinctly on soiling and materials
damage, in terms of specific chemical components of PM. Further,
consistent with the rationale and recommendations in the Staff Paper,
the Administrator agrees that it is appropriate to continue control of
ambient fine and coarse-fraction particles, especially long-term
deposition of particles such as particulate nitrates and sulfates that
contribute to adverse impacts on vegetation and ecosystems and/or to

materials damage and soiling. The Administrator also agrees with the
Staff Paper that the available information does not provide a
sufficient basis for the development of distinct national secondary
standards to protect against such effects beyond the protection likely
to be afforded by the proposed suite of primary PM standards. In
considering those proposed standards in combination, including the
proposed more protective 24-hour standard for PM2.5 and the
proposed 24-hour standard for PM10-2.5, which is intended to
provide an equivalent degree of protection to the current
PM10 standards in areas where the proposed
PM10-2.5 indicator applies (which tend to be more densely
populated areas where materials damage would be of greater concern),
the Administrator believes that this proposed suite of standards would
afford at least the degree of protection as that afforded by the
current secondary PM standards.
    Finally, the Administrator believes, as noted above, that such
standards should be considered in conjunction with the protection
afforded by other programs intended to address various aspects of air
pollution effects on ecosystems and vegetation, such as the Acid
Deposition Program and other regional approaches to reducing pollutants
linked to nitrate or acidic deposition. Based on these considerations,
and taking into account the information and recommendations discussed
above, the Administrator therefore proposes to revise the current
secondary PM2.5 and PM10 standards to address
these other welfare effects by making them identical in all respects to
the proposed suite of primary PM2.5 and PM10-2.5
standards.

C. Proposed Decisions on Secondary PM Standards

    For the reasons discussed above, and taking into account the
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and public
comments to date, the Administrator proposes to revise the current
secondary PM2.5 and PM10 standards by making them
identical in all respects to the proposed primary PM2.5 and
PM10-2.5 standards to address PM-related welfare effects
including visibility impairment, effects on vegetation and ecosystems,
materials damage and soiling, and effects on climate change. In
recognition of an alternative view expressed by most members of the
CASAC PM Panel, the Administrator also solicits comments on a sub-daily
(4- to 8-hour averaging time) PM2.5 standard to address
visibility impairment, within the range of 20 to 30 [mu]g/m3
and with a form within the range of the 92nd to 98th percentile. Based
on the comments received and the accompanying rationale, the
Administrator may adopt other standards within the range of
alternatives identified above in lieu of the standards he is proposing
today.

V. Interpretation of the NAAQS for PM

A. Proposed Amendments to Appendix N--Interpretation of the National
Ambient Air Quality Standards for PM2.5

    The EPA is proposing to revise the data handling procedures for the
annual and 24-hour primary PM2.5 standards in appendix N to
40 CFR part 50. The proposed amendments to appendix N would detail the
computations necessary for determining when the proposed primary and
secondary PM2.5 national ambient air quality standards
(NAAQS) are met. The proposed amendments also would address data
reporting, monitoring considerations, and rounding conventions. Key
elements of the proposed revisions to appendix N are summarized below
in sections V.A.1 through V.A.5 of this preamble.
1. General
    Several new definitions would be added to section 1.0 and utilized
throughout the appendix, most notably ones for ``design values''. Also,
the 24-hour time would be clarified as representing ``local standard
(word inserted) time''. This proposal reflects EPA's previous intent as
well as majority practice, and also avoids ambiguity since local clock
time varies according to daylight savings periods.
2. PM2.5 Monitoring and Data Reporting Considerations
    Two new sections would be added to appendix N to more specifically