discontinue operating installed controls, or to operate those controls
less effectively, except to the extent non-CAIR legal requirements
mandate emission reductions or to the extent that sources would find it
economic to operate the controls for non-CAIR market-based emission
control programs. EPA properly treats the costs of operating controls
installed to meet CAIR requirements as costs of meeting Transport Rule
requirements.\17\ EPA's base case accounts for non-CAIR requirements
and does not make the unreasonable assumption that installed controls
would be operated to achieve emission reductions that are not necessary
to meet non-CAIR requirements. For all of these reasons, EPA rejects
commenters' claims that the base case is ``unrepresentative'' or lacks
``a rational relationship to the real world.''
---------------------------------------------------------------------------

    \17\ For more details on how EPA models economic operation of
existing pollution control equipment in the Transport Rule base
case, please see Section 6 (``Dispatchable Controls'') in ``Updates
to EPA Base Case v3.02 EISA Using the Integrated Planning Model''
Technical Support Document (TSD) for the Transport Rule Docket ID
No. EPA-HQ-OAR-2009-0491, U.S. EPA, July 2010 (available at http://www.epa.gov/airmarkets/progsregs/epa-ipm/IPM Update
Documentation.pdf).
---------------------------------------------------------------------------

C. Air Quality Modeling To Identify Downwind Nonattainment and
Maintenance Receptors

1. Emission Inventories
    To inform air quality modeling for the development of the final
Transport Rule, EPA developed emission

inventories for a 2005 base year and for 2012 and 2014 projections. The
inventories for all years include emission estimates for EGUs, non-EGU
point sources, stationary nonpoint sources, onroad mobile sources,
nonroad mobile sources, and biogenic (non-human) sources. EPA's air
quality modeling relies on this comprehensive set of emission
inventories because emissions from multiple source categories are
needed to model ambient air quality and to facilitate comparison of
model outputs with ambient measurements. In addition, EPA considers all
relevant emissions (regardless of source category) when determining
whether a state is found to be significantly contributing to or
interfering with maintenance of a particular NAAQS in another state.
    The emission inventories were processed through the Sparse Matrix
Operator Kernel Emissions (SMOKE) Modeling System version 2.6 to
produce the gridded, hourly, speciated, model-ready emissions for input
to the CAMx air quality model. Additional information on the
development of the emission inventories and related data sets for
emissions modeling are provided in the Emission Inventory Final
Transport Rule TSD.
    On October 27, 2010, EPA issued a NODA on ``Revisions to Emission
Inventories.'' The NODA's primary purpose was to notify the public
about changes to emission inventories made since the proposal modeling.
The affected emission sectors were non-EGU stationary point sources,
nonpoint sources, and Category 3 commercial marine vessel sources. The
NODA also presented a newly released model for developing onroad mobile
source emissions for use in air quality modeling for the final
Transport Rule.
    The major comments received in response to the emission inventories
and modeling included in the proposed Transport Rule and the October 27
NODA are summarized in the following subsections. EPA agreed with the
comments summarized below and adopted technical corrections or updates
to the emission inventories and modeling accordingly. For EPA to be
able to take appropriate action, comments on the emission inventories
needed to be specific enough to allow for credible alternative data
sources to be located. EPA adopted corrections from comments on in-
place control programs or devices where the controls were enforceable
and quantifiable.
a. Foundation Emission Inventory Data Sets
    EPA developed emission data representing the year 2005 to support
air quality modeling of a base year from which future air quality could
be forecasted. EPA used the 2005 National Emission Inventory (NEI),
version 2 from October 6, 2008, as the chief basis for the U.S.
inventories supporting the 2005 air quality modeling. This inventory
includes 2005-specific data for point and mobile sources, while most
nonpoint data were carried forward from version 3 of the 2002 NEI. The
future base case scenarios modeled for 2012 and 2014 represent
predicted emission reductions primarily from already promulgated
federal measures.
    EPA used a 2006 Canadian inventory and a 1999 Mexican inventory for
the portions of Canada and Mexico within the air quality modeling
domains for all modeled scenarios. Emissions from Canada and Mexico for
all source sectors (including EGUs) in these countries were held
constant for all base- and future-year cases. EPA made this assumption
because it does not currently have sufficient data to support
projections of future-year emissions from Canada and Mexico.
b. Development of Emission Inventories for EGUs
    The annual NOX and SO2 emissions for EGUs in
the 2005 NEI v2 are based primarily on data from continuous emissions
monitoring systems (CEMS), with other EGU pollutants estimated using
emission factors and annual heat input data reported to EPA. Although
only NOX and SO2 are considered for control in
this rule, emissions for all criteria air pollutants are necessary to
model air quality. For EGUs without CEMS, EPA used data submitted to
the NEI by the states. For more information on the details of how the
2005 EGU emissions were developed, see the Emissions Inventory Final
Rule TSD.
    Commenters stated that some point sources that were classified as
non-EGUs in the proposal modeling were actually EGUs, resulting in
double counting of emissions in future-year modeling. EPA reviewed its
assignment of EGUs and non-EGUs and reclassified EGU sources found to
be in the non-EGU inventory for the updated 2005 EGU inventory to
prevent double counting of future-year emissions.
    The future base case scenarios for EGUs reflect projected changes
to fuel usage and economics, as described in the Emission Inventory
Final Rule TSD. Future year base case EGU emissions that predict
SO2, NOX, and PM2.5 were obtained from
version 4.10--FTransport of the Integrated Planning Model (IPM) outputs
(http://www.epa.gov/airmarkt/progsregs/epa-ipm/index.html). The IPM is
a multi-regional, dynamic, deterministic linear programming model of
the U.S. electric power sector; version 4.10--FTransport reflects state
rules and consent decrees through December 1, 2010, and incorporates
public comments on existing controls submitted to EPA through both the
Transport Rule-related notice and comment process as well as the
proposed Mercury and Air Toxics Standards Information Collection
Request (ICR). The operation of existing SO2 or
NOX advanced controls (e.g., scrubber, SCR) on units that
were not required to operate those controls for compliance with Title
IV, New Source Review (NSR), state settlements, or state-specific rules
was projected by IPM on the basis of providing least cost operation of
the power generation system subject to existing regulatory requirements
except CAIR (see baseline discussion in section V.B).
    Additionally, IPM v.4.10--FTransport incorporates comments received
during the rulemaking process. Fuel-related updates include comment-
driven unit-specific limitations on 2012 coal rank selection, limiting
unrestricted switching from bituminous to subbituminous coal by
imposing boiler modification costs for those units shifting from
bituminous to subbituminous coal without historical precedent, and a
correction of waste coal prices. Pollution control-related updates
include keying the performance assumptions for FGD and SCR more closely
to historic performance data, and the inclusion of dry sorbent
injection (DSI), a SO2 removal technology. Other notable
updates include revised assumptions on the heat rate and consequent
dispatching of cogenerating units and incorporation of additional
planned retirements. Further details on these updates are available in
the IPM Documentation, available in the docket and at: http://www.epa.gov/airmarkets/progsregs/epa-ipm/index.html.
c. Development of Emission Inventories for Non-EGU Point Sources
    Details on the development of emission inventories are available in
the Emission Inventory Final Rule TSD. In both the proposal and final
modeling, controls on industrial boilers installed under the
NOX SIP call were assumed to have been implemented by 2005
and captured in the 2005 NEI v2. The non-EGU point source emissions
were updated from the 2005 NEI and the

emissions used for the proposal modeling through the incorporation of
comments on the proposal emissions values, previously unknown facility
closures, and through other data improvements as identified by EPA
analyses.
    EPA does not factor in economic growth to develop non-EGU point
source emission projections because analysis of historical emission
trends and economic data did not support using economic growth to
project non-EGU emissions. More details on the rationale for not
applying economic growth to non-EGU industrial sources can be found in
Appendix D of the Regulatory Impact Assessment (RIA) for the PM NAAQS
rule (http://www.epa.gov/ttn/ecas/regdata/RIAs/Appendix%20D_
Inventory.pdf). Although projections based on economic growth were not
included, EPA did include reductions resulting from plant and unit
closures, local and federal consent decrees, and several Maximum
Achievable Control Technology (MACT) standards.
    For non-EGU point sources, local control programs that may be
necessary for areas to attain the annual PM2.5 NAAQS and the
ozone NAAQS are only included in the future base case projections when
specific information about existing enforceable local controls was
provided.
    Since aircraft at airports were treated as point emissions sources
in the 2005 NEI v2, we applied projection factors based on activity
growth projected by the Federal Aviation Administration Terminal Area
Forecast (TAF) system, published in December 2008.
    A number of comments were received on the stationary non-EGU point
source inventories. Below is a summary of the major comments that
impacted the stationary non-EGU point source inventories for the final
modeling:
    Comment: Commenters stated that EPA did not properly represent some
point source emissions in base-year and future-year inventories due to
facility and unit closures, consent decrees, emission caps, control
programs, and alternative emission estimates.
    Response: EPA reviewed the sources referenced in the individual
comments regarding the base-year and future-year inventories. In cases
where credible alternative data were available, EPA revised the
emission inventories to incorporate additional facility and unit
closures, consent decrees, emission caps, control programs, enforceable
local controls, and alternative emission estimates.
    Comment: Commenters stated that EPA should include controls from
the National Emission Standards for Hazardous Air Pollutants for
Reciprocating Internal Combustion Engines (RICE NESHAP) in our
modeling.
    Response: EPA included reductions expected to be achieved by the
RICE NESHAP across the United States in our final modeling of
stationary non-EGU and nonpoint sources.
    Comment: Commenters stated that EPA was not properly representing
existing or planned controls for cement plants.
    Response: EPA updated control and projection information for cement
plants based on the latest available data and cement sector-specific
modeling results.
    Comment: EPA specifically requested comments on whether to
incorporate emission reduction estimates from the NESHAP for Major
Sources: Industrial, Commercial, and Institutional Boilers and Process
Heaters (75 FR 32006). Commenters stated that emission reduction
estimates should not be included until the rule became final.
    Response: EPA did not incorporate emission reduction estimates from
the NESHAP for Major Sources: Industrial, Commercial, and Institutional
Boilers and Process Heaters (75 FR 32006) into the proposal or final
modeling because the rule was not final at the time the modeling was
performed. Note that reductions from this rule would not have impacted
the 2012 base case due to its implementation schedule, and only the
2014 emissions would have been affected.
d. Development of Emission Inventories for Onroad Mobile Sources
    The onroad emissions in the proposal modeling were primarily based
on the National Mobile Inventory Model (NMIM) monthly, county, and
process level emissions along with gasoline exhaust emissions from a
fall 2008 draft version of the Motor Vehicle Emission Simulator
(MOVES). A major comment on the proposal modeling for onroad mobile
sources was the following:
    Comment: Commenters stated that EPA should use a publicly released
version of MOVES for its final modeling.
    Response: EPA updated the final modeling to use data from the
publicly released version of the MOVES 2010 model because the model
became available in time for inclusion of its results in the final
modeling. It was not used for the proposal modeling because it was not
available at the time the modeling was performed.
    In the final Transport Rule modeling, EPA used MOVES 2010 state-
month level emissions for all criteria pollutants and all modes
(evaporative, exhaust, brake wear and tire wear) and allocated those
emissions to counties according to state-county NMIM emissions ratios.
For California (the emissions for which are included to support the
coarse modeling domain), the onroad mobile emissions data were derived
from data provided by the state. These data were augmented with MOVES
2010 outputs for NH3 because data for that pollutant had not
been provided. Additional information on the approach to onroad mobile
source emissions is available in the Emission Inventory Final Rule TSD.
    In the future-year base modeling for mobile sources, all national
measures available at the time of modeling were included. The future
scenarios for mobile sources reflect projected changes to fuel usage,
as described in the Emission Inventory Final Rule TSD. Emissions for
these years reflect onroad mobile control programs including the Light-
Duty Vehicle Tier 2 Rule, the Onroad Heavy-Duty Rule, the Light-Duty
Vehicle Greenhouse Gas Rule, the Renewable Fuel Standards Rule, and the
Mobile Source Air Toxics (MSAT) final rule.
e. Development of Commercial Marine Category 3 Vessel Emission
Inventories
    For the 2005 modeling, the commercial marine category 3 (C3) vessel
emissions, a portion of nonroad mobile emissions, were augmented with
gridded 2005 emissions from the previous modeling efforts for the rule
called ``Control of Emissions from New Marine Compression-Ignition
Engines at or Above 30 Liters per Cylinder.'' Emissions out to 200
nautical miles from the coastline were allocated to states in the
proposal modeling. A major comment on the proposal modeling was the
following:
    Comment: Commenters stated that emissions from commercial marine
sources (a component of the nonroad emissions in the summaries that
were provided for the NPR) were too high.
    Response: EPA reviewed the approach used for commercial marine
C3 emissions in the proposal. In the final modeling, instead
of using the boundary of 200 nautical miles from the coast as was used
in the proposal, EPA adopted the Mineral Management Service state-
federal water boundaries that assign state waters 3-10 nautical miles
from the coast. This approach is consistent with the approach used in
the 2005 and 2008 National Emission Inventories. In addition, the
category 3 commercial marine emissions were adjusted to reflect a
coordination between the Emissions Control Area proposal to the
International Maritime Organization

(EPA-420-F-10-041, August 2010) control strategy; reductions of
NOX, VOC, and CO emissions for new C3 engines
starting in 2011; and fuel sulfur limits that go into effect as early
as 2010.
f. Development of Emission Inventories for Other Nonroad Mobile Sources
    The nonroad mobile source emissions for sources other than
C3 marine were primarily based on NMIM monthly, county, and
process level emissions from the 2005 NEI v2. These emissions were
unchanged from proposal modeling, except for PM emissions in California
that were updated to correct for missing emissions in a few counties
and source categories.
    Nonroad mobile emissions were created for future years with NMIM
using an approach consistent with that used for 2005. The nonroad
emissions for 2012 and 2014 were calculated using NMIM future-year
equipment population estimates and control programs. Nonroad mobile
emission reductions for 2012 and 2014 include reductions to
locomotives, various nonroad engines including diesel engines and
various marine engine types, fuel sulfur content, and evaporative
emissions standards. A more comprehensive list of control programs
included for mobile sources is available in the Emission Inventory
Final Rule TSD.
    The 2012 and 2014 nonroad mobile emissions for locomotives and
category 1 and 2 (C1 and C2) commercial marine vessels were based on
emissions published in EPA's Locomotive Marine Rule, Regulatory Impact
Assessment, Chapter 3.
g. Development of Nonpoint Emission Inventories
    For the proposal Transport Rule modeling, EPA augmented the 2002
NEI nonpoint emission inventory with a non-California Western Regional
Air Partnership (WRAP) oil and gas exploration inventory, which
includes emissions in several states within the eastern U.S. 12 km
modeling domain and additional states within the national 36 km
modeling domain. For the final Transport Rule modeling, EPA updated the
nonpoint emission estimates for oil and gas sources. EPA continued to
use the same WRAP inventory from the proposal, emissions in Texas and
Oklahoma were updated but for the final modeling with data from the
Texas Commission on Environmental Quality (TCEQ) and the Oklahoma
Department of Environmental Quality (DEQ), respectively.
    The average-year county-based inventories for wildfire and
prescribed burning emissions were unchanged between the proposal and
final modeling.
    For stationary nonpoint sources, local control programs that may be
necessary for areas to attain the annual PM2.5 NAAQS and the
ozone NAAQS are not included in the future base case projections unless
specific information about existing enforceable controls was available
(e.g., ozone SIP controls from Ozone Transport Commission rules that
impact source categories such as Consumer Products, Solvent Cleaning,
Adhesives and Sealants). EPA specifically requested comment on local
control data as part of the proposal and the October 27 NODA, and
incorporated any usable data that was provided into the final
inventories.
    For stationary nonpoint sources, refueling emissions were projected
using the refueling results from the NMIM runs performed for the onroad
mobile sector.
    Portable fuel container emissions were projected to future years
using estimates from previous OTAQ rulemaking inventories. Emissions of
ammonia and dust from animal operations were projected based on animal
population data from the Department of Agriculture and EPA. Residential
wood combustion was projected by replacement of obsolete wood stoves
with new wood stoves and a 1 percent annual increase in fireplaces.
Landfill emissions were projected using MACT controls. All other
nonpoint sources were held constant between 2005 and the future years.
    Some specific adjustments to the inventories were made in the final
modeling to address comments that were received as described below.
Area source MACT programs and controls from the RICE NESHAP were
included in the final modeling to address submitted comments, as were
fuel sulfur controls that were enforceable and that take effect by
2014.
    The major comments that impacted the nonpoint sectors are as
follows:
    Comment: Commenters stated that the SO2 emissions from
industrial fuel combustion in Nebraska EPA are too high.
    Response: EPA reviewed the NEI 2002-based data that had been used
for the proposal modeling and determined that emissions from the 2005
inventory compiled for the Central Regional Air Planning Association
(CENRAP) were more up to date for this source category and based on
more localized data sources. The 2005 CENRAP emissions for industrial
fuel combustion were used in the final modeling.
    Comment: Commenters stated that EPA should include sulfur rule
controls that take effect prior to the future years that were modeled.
    Response: EPA included quantifiable sulfur rule controls in 2014
modeling for those states that had implemented the rules (New Jersey
and Maine).
    Comment: A commenter stated that emissions for Delaware were
overestimated for several nonpoint categories in base-year and future-
year inventories and provided alternative estimates for these
categories.
    Response: EPA reviewed the alternative estimates provided and found
them to be credible and based on more detailed local scale information
than were available in the national inventories. EPA incorporated the
alternative emission estimates for Delaware into the final modeling.
    Comment: A commenter stated that residual oil is not used as an
industrial fuel in South Carolina.
    Response: EPA analyzed the emissions from residual oil industrial
fuel combustion in South Carolina and all other states, and analyzed
preliminary regional planning office inventories and the 2008 NEI
submittals. The South Carolina residual oil industrial fuel emissions
were determined to be anomalously large in comparison to the near zero
emissions in other submittals and were therefore removed from the
nonpoint inventory.
2. Air Quality Basis for Identifying Receptors
a. Introduction
    In this section, we describe the final approach to identify
downwind nonattainment and maintenance receptors. We briefly summarize
the modeling platform, the proposed approach to identify receptors,
comments received, and the results of the final analysis.
    In the Transport Rule, EPA has explicitly given independent meaning
to the ``interfere with maintenance'' prong of section
110(a)(2)(D)(i)(I) by evaluating contributions to identified
maintenance receptors as well as contributions to identified
nonattainment receptors. EPA identified maintenance receptors as those
receptors that would have difficulty maintaining the relevant NAAQS in
a scenario that takes into account historic variability in air quality
at that receptor. Specifically, EPA projects future air quality design
values based on measured data during the period 2003 to 2007. In
determining the downwind receptors of concern, EPA

does not solely rely on the projection of an average design value based
on measured data from the relevant period (in this case 2003 to 2007)
to make a determination of ``attainment'' or ``nonattainment.''
Instead, EPA also evaluates the maximum future design value at that
receptor based on measured data over the relevant period. Receptors for
which this latter analysis projects design values higher than the NAAQS
are identified as maintenance receptors.
    EPA believes it is appropriate and reasonable to use this approach
to identify receptors that may have maintenance problems in the future.
This approach uses measured data in order to establish potential air
quality outcomes at each receptor that take into account the variable
meteorological conditions present across the entire period of measured
data (2003 to 2007). EPA interprets the maximum future design value to
be a potential future air quality outcome consistent with the
meteorology that yielded maximum measured concentrations in the ambient
data set analyzed for that receptor. In other words, the average design
value gives a reasonable projection of future air quality at the
receptor under ``average'' conditions. However, EPA also recognizes
that previously experienced meteorological conditions (e.g., dominant
wind direction, temperatures, air mass patterns) promoting ozone or
fine particle formation that led to maximum concentrations in the
measured data may reoccur in the future. The maximum design value gives
a reasonable projection of future air quality at the receptor under a
scenario in which such conditions do, in fact, reoccur. It also
identifies upwind emissions that under those circumstances could
interfere with the downwind area's ability to maintain the NAAQS.
    Per the court's opinion in North Carolina, it is necessary for the
Agency to evaluate ``interference with maintenance'' separately from
``significant contribution to nonattainment'' in order to give
independent meaning to that phrase in the statute. The approach
described above does so and provides a reasonable basis for identifying
upwind emissions that interfere with maintenance of the NAAQS at
downwind receptors.
    Because the methodology is based on actual variations in design
values measured at the receptors, EPA believes that the application of
this design value methodology for identifying maintenance receptors
reasonably anticipates possible future air quality outcomes based on
meteorological conditions independent of emission reduction
requirements occurring between 2005 (the base year for air quality
analysis) and 2012 (the future year for air quality analysis of the
base case without CAIR or the Transport Rule in place). EPA uses air
quality modeling to properly account for changes in air quality from
2005 to 2012 due to emission control requirements and trends in
emission source fleet turnover (such as increasingly cleaner motor
vehicle fleets). The air quality modeling process allows EPA to
effectively adjust measured data to project design values in 2012 based
on the forecast changes in emissions. For a given receptor, the
forecast change in emissions from 2005 to 2012 is a constant factor
applied across all of the design values from the period 2003 to 2007.
Thus, a comparison of the projected (future-year) design values
themselves is equivalent to comparing the base period design values
from the data set to consider how pollution concentrations are affected
by non-modeled factors such as environmental and meteorological
variability independent of the forecast emission reductions that stem
from successful imposition of emission limitations and controls on
various sources between the base and future modeling years. EPA
believes it is reasonable to anticipate that these year-to-year
meteorological fluctuations may reoccur at any time in the future and
are relevant to determining receptors that are at risk of having a
problem in the future with maintenance of the NAAQS. Therefore, EPA
assesses the relationship of the maximum projected design value for
2012 at each receptor to the relevant NAAQS, and where such a value
exceeds the NAAQS, EPA determines that receptor to be a ``maintenance''
receptor for purposes of defining interference with maintenance under
the Transport Rule.
    To provide an illustrative example, consider a hypothetical
receptor ``Y'' whose measured data for 2003-2007 yields three design
values for annual fine particles: 17 for 2003-05; 14 for 2004-06; and
12 [micro]g/m\3\ for 2005-07. Thus, the maximum measured design value
for this period is 17 and the average design value is 14.3. To
determine whether the receptor is a nonattainment or maintenance
receptor, EPA projects a corresponding future-year (2012) design value
for each measured design value. These projections are based on the
results of air quality modeling, which demonstrates predicted changes
in pollution concentrations for each receptor from 2005 to 2012. For
this example, assume that the projected future-year design values that
correspond with the measured design values, are 16 (corresponds with
the 2003-05 design value of 17), 13 (corresponds with the 2004-06
design value of 14), and 11 [micro]g/m\3\ (corresponds with the 2005-07
design value of 12). The average future-year design value is 13.3
(corresponds with the average measured design value from 2003-2007 of
14.3). The projected future design values are all lower than the
measured design values because air quality is projected to improve
between 2005 and 2012. In this example, the analysis establishes that
the average projected future design value is 13.3 and the maximum
projected future design value is 16.
    The average future (2012) projected design value of 13.3 based on
the average design value for the period 2003-07 does not exceed the
1997 annual PM2.5 NAAQS. For this reason, EPA would conclude
that receptor Y will most likely have attainment air quality in the
future year. Therefore, it would not be identified as a nonattainment
receptor.
    However, the future projected design value of 16 based on the
maximum design value for the period 2003-07 does exceed the NAAQS. For
this reason, EPA would conclude that the receptor may have difficulty
maintaining attainment with the NAAQS under future potential
meteorological conditions. EPA therefore would identify the receptor as
a maintenance receptor and evaluate whether upwind state emissions
interfere with maintenance of the NAAQS at that receptor.
    EPA's methodology accounts for the range of meteorological
conditions reflected by design values from the measured 2003-2007 data
at receptor Y and also accounts for the projected changes in emissions
from 2005 to 2012 at receptor Y. The range of meteorological conditions
is accounted for by using data from three different 3-year periods as
described above. The projected changes in emissions are accounted for
by applying to the measured design values the forecasted change in
PM2.5 concentrations, as determined through air quality
modeling of the 2005 and 2012 emissions. In this example, the maximum
measured design value for receptor Y is 17. This design value
represents measured data from 2003 to 2005. EPA applies to this design
value the modeled 2005-to-2012 change in concentrations at receptor Y
to obtain a 2012 maximum design value for that

receptor, which is 16. In this way, this maximum 2012 design value
takes into consideration the air quality impacts of all known and
legally applicable emission limitations taking effect after the 2003 to
2005 base period. Therefore, each of the projected future-year design
values provide a fair representation of future air quality at receptor
Y under different conditions while accounting for the emissions
projected to remain in 2012. EPA thus believes that if one of these
future-year design values for a particular receptor exceeds the NAAQS,
it is reasonable to conclude that the area may have difficulty
maintaining that NAAQS. For this reason, EPA identifies such receptors
as maintenance receptors. In this example, EPA would find that while
receptor Y's average future-year design value would not exceed the
NAAQS, its maximum future-year design value (16) would exceed the
NAAQS, and it would thus be designated as a ``maintenance'' receptor
for purposes of the Transport Rule analyses.
    In the proposed rule we used air quality modeling to (1) Identify
locations where we expected there to be nonattainment and/or
maintenance problems for annual average PM2.5, 24-hour
PM2.5, and/or 8-hour ozone in 2012, (2) quantify the impacts
(i.e., air quality contributions) of SO2 and NOX
emissions from upwind states on downwind annual average and 24-hour
PM2.5 concentrations at monitoring sites projected to be
nonattainment or have maintenance problems in 2012 for the 1997 annual
and 2006 24-hour PM2.5 NAAQS, respectively, and (3) quantify
the impacts of NOX emissions from upwind states on downwind
8-hour ozone concentrations at monitoring sites projected to be
nonattainment or have maintenance problems in 2012 for the 1997 ozone
NAAQS.
    To support the proposal, air quality modeling was performed for
four emission scenarios: a 2005 base year, a 2012 ``no CAIR'' base
case, a 2014 ``no CAIR'' base case, and a 2014 control case that
reflects the emission reductions expected from the FIPs. The modeling
for 2005 was used as the base year for projecting air quality for each
of the 3 future-year scenarios. The 2012 base case modeling was used to
identify future nonattainment and maintenance locations and to quantify
the contributions of emissions in upwind states to annual average and
24-hour PM2.5 and 8-hour ozone. The 2012 ozone and
PM2.5 concentrations were derived by projecting 2003 through
2007 based ambient ozone and/or PM2.5 data to the future
using the relative (percent) change in modeled concentrations between
2005 and 2012. The 2014 base case and 2014 control case modeling were
used to quantify the benefits of this proposal.
    In the proposed rule, EPA used the Comprehensive Air Quality Model
with Extensions (CAMx) version 5.20 \18\ to simulate ozone and
PM2.5 concentrations for the 2005 base year and the 2012 and
2014 future year scenarios. The CAMx model applications were designed
to cover states in the central and eastern U.S. using a horizontal
resolution of 12 x 12 km.\19\
---------------------------------------------------------------------------

    \18\ Comprehensive Air Quality Model with Extensions Version 5
User's Guide. Environ International Corporation. Novato, CA. March
2009.
    \19\ The 12 km domain was nested within a coarse grid, 36 x 36
km modeling domain which covers the lower 48 states and adjacent
portions of Canada and Mexico. Predictions from this Continental
U.S. (CONUS) domain were used to provide initial and boundary
concentrations for simulations in the 12 km domain.
---------------------------------------------------------------------------

    CAMx contains ``source apportionment'' tools that are designed to
quantify the contribution of emissions from various sources and areas
to ozone and PM2.5 component species in other downwind
locations. The source apportionment tools were used to quantify the
downwind contributions of ozone and PM2.5 from upwind
states.
    In the proposed rule, EPA used a 2005-based air quality modeling
platform which included 2005 base year emissions and 2005 meteorology
for modeling ozone and PM2.5 with CAMx.
    We received comments related to several aspects of the air quality
modeling platform.
    Comment: There was wide support from commenters for the use of CAMx
as an appropriate, state-of-the science air quality tool for use in the
Transport Rule. There were no comments that suggested that EPA should
use an alternative model for quantifying interstate transport. Many
commenters requested that EPA update the emission inventories used for
the Transport Rule and then remodel the 2005 base year and future year
emissions using the updated emissions and the most recent version of
CAMx to reassess interstate transport for the final rule.
    Response: For the final rule we have updated our modeling using the
latest public release of CAMx (version 5.30) and associated
preprocessors. We have also made numerous improvements to the emission
inventories for the 2005 base year as well as the 2012 and 2014 future
year base cases in response to public comments. The emissions changes
are described in section V.C.1. The projection of future year
nonattainment and maintenance sites and the quantification of ozone and
PM2.5 transport for the final rule are based on modeling
with CAMx v5.30 using the updated emission inventories. The final rule
air quality projections of 2012 nonattainment and maintenance are
described below. The final rule interstate contributions are presented
in section V.D.
    Comment: The performance evaluation of the 2005 base year model
predictions for the proposed rule was too cursory and did not provide
sufficient detail on model performance. Commenters requested additional
analyses and spatial resolution describing how well base year model
predictions compare to the corresponding measured values.
    Response: For the final rule we have expanded the scope of the
model evaluation for 2005 to include a broader suite of statistics to
characterize performance for individual subregions of the eastern U.S.
modeling domain. The results of the performance evaluation for the
final rule 2005 base year air quality modeling are described in the Air
Quality Modeling Final Rule TSD.
    Comment: The 2005 based modeling platform should be updated to a
more recent year. There were several different aspects of this comment.
Some commenters stated that EPA should be using a more recent emission
inventory as a base year, due to identified changes and updates to the
inventories. Other commenters stated that EPA should use a more recent
base year, due to a trend of improvement in air quality over the past
few years. The commenters claim that the 2005-based EPA modeling does
not account for large emission reductions and air quality improvements
that have occurred over the last several years.
    Response: There are several reasons why the use of a 2005 modeling
base case is both reasonable and, in fact, necessary for the Transport
Rule. As explained in section V.B, above, because the Transport Rule
will replace CAIR, EPA cannot consider reductions associated with CAIR
in the analytical baseline emissions scenario. Thus, the base year for
the air quality projections should be a year that represents emissions
before CAIR was in place (i.e. 2005). We are projecting emissions to a
future 2012 ``no CAIR'' case and therefore want to best represent the
air quality change between 2005 and 2012, without CAIR. To do this, we
projected emissions that existed before CAIR was in effect and modeled
the air quality change that occurs between 2005 and 2012 without CAIR.

    A key consideration in our projection methodology is the use of
ambient data to anchor the design value projections to the future. The
modeling is used in a relative sense by multiplying the modeled percent
change in ozone or PM2.5 species concentrations by the base
year ambient data. The ozone and PM2.5 modeling guidance
recommends projecting design values based on 5 years \20\ of monitoring
data that is centered on the base model year. Using 2005 as a base
emissions and meteorological year entailed the use of 2003-2007 ambient
air quality data (5 years of data centered about 2005). This was a
reasonable choice because the majority of the ambient data from this
period was not impacted by CAIR emission reductions.
---------------------------------------------------------------------------

    \20\ The modeling guidance recommends using a five year weighted
average design value. This is calculated by averaging the three
consecutive design value periods of 2003-2005, 2004-2006, and 2005-
2007.
---------------------------------------------------------------------------

    After 2005, early emission reductions of SO2 and
NOX in response to CAIR began to impact the measured air
quality concentrations. Since the modeling projection methodology uses
both modeled and observed data, 2005 is the latest base year that we
deemed appropriate (before CAIR emission reductions took place) for use
in projecting the measured air quality to a 2012 future year. The early
years of the 5 year period (2003, 2004, and 2005) were not impacted by
CAIR.\21\ The last 2 years in the period (2006 and 2007) were slightly
impacted by CAIR emission reductions. But the 5 year average is
weighted towards the middle year of the period (2005), so the impact of
the years after CAIR promulgation should be minimal.
---------------------------------------------------------------------------

    \21\ The CAIR final rule was published on May 12, 2005.
---------------------------------------------------------------------------

    The 2005 base year was also chosen because it was an appropriate
meteorological year. In the eastern U.S. there was relatively high
ozone during the summer of 2005 and relatively high PM2.5
periods during the year. The modeled attainment tests for both ozone
and 24-hour PM2.5 depend on having a sufficient number of
``high'' modeled days to project to the future. Modeling a year that is
not meteorologically conducive to ozone and/or PM2.5
formation is discouraged by the modeling guidance because a
meteorological year that is not conducive to ozone or PM2.5
formation may be less responsive to changes in emissions in the future.
Therefore, projecting the relative change in ozone or PM2.5
for a non-conducive base year may underestimate the future change in
ozone and/or PM2.5 concentrations.
    Additionally, all enforceable emission reductions that occurred
between 2005 and 2012 (other than those required under CAIR) are
captured by the modeling system. Any enforceable non-EGU emission
reductions due to existing rules or the installation of emissions
controls after 2005 were included in the 2012 base case inventory. As
explained above in section V.B, to capture changes in EGU emissions
between 2005 and 2012, EPA did not assume operation of all controls
installed during that time period, as many of those controls were built
in response to CAIR. EPA used IPM to project 2012 EGU emissions
incorporating all non-CAIR enforceable emission constraints; operation
of existing pollution controls was taken into account only where non-
CAIR constraints made it economic or legally necessary to operate them.
We also accounted for permanent source shutdowns that occurred after
2005. Where possible, we incorporated reported emission changes based
on comments to the proposed rule and a subsequent emission inventory
NODA.
    Comment: Several commenters stated that we used a ``modeled +
monitored'' test in CAIR to identify future year nonattainment
receptors, but we only used a modeled test in the Transport Rule
proposal. They suggest that we should either go back to the ``modeled +
monitored'' test or explain why we should not use monitoring data in
the identification of nonattainment and maintenance receptors. They say
that we should not base nonattainment and maintenance receptors solely
on modeled violations. They also say that we if we had looked at the
most recent ambient data we would see that most of the modeled
nonattainment and maintenance receptors are already attaining the ozone
and/or PM2.5 NAAQS.
    Response: In the identification of future year nonattainment
receptors for CAIR, EPA used what was called the ``modeled + monitored
test''. The most recent ambient data (2001-2003 design values at the
time) were examined to further verify that nonattainment was still
being measured at potential future year nonattainment receptors. In the
proposed Transport Rule, EPA identified future year nonattainment and
maintenance receptors based on modeled projections of ambient data from
the 2003-2007 time period. The future year receptors were not compared
to most recent ambient data to verify that nonattainment still existed.
    For the final Transport Rule, there are several reasons that EPA
did not examine the most recent ambient data to verify that receptors
were still measuring nonattainment. The main reason for dropping the
``monitored'' part of the modeled + monitored test is the fact that the
most recent monitoring data (2007-2009 design values) include large
emission reductions from CAIR. As explained in section V.B, above,
because the Transport Rule will replace CAIR, we must model a future
year base case which does not assume that CAIR is in place (a ``no-
CAIR'' case). It is simply not appropriate to examine the current
monitoring data, which represent air quality with CAIR emission
reductions in place, and compare the values to 2012 projected air
quality that is based on a no-CAIR modeling case. As discussed above,
we modeled a 2005 base case with pre-CAIR emissions and a 2012 future
``no CAIR'' case. The change in modeled air quality is due to the non-
CAIR enforceable emission changes between 2005 and 2012 and therefore
explicitly does not take CAIR into account. As a consequence, the 2012
projected design values represent a unique case (necessary for
analyzing future air quality without either CAIR or its replacement
Transport Rule in effect) that cannot be represented by current ambient
data.
    It is also important to note that all of the projected 2012 design
values are based on projections of measured ambient data. They are a
combination of measured data and modeled response factors. Therefore,
it is inaccurate to imply that future year nonattainment and
maintenance receptors are solely based on modeled projections. The
future year concentrations are firmly rooted in base year measured
ambient data that have been projected to the future using modeled data.
    There are additional reasons for not verifying the nonattainment
and maintenance receptors against the most recent ambient data. In CAIR
we did not explicitly identify maintenance receptors. In the Transport
Rule proposal we identified maintenance receptors based on 2012
projections of maximum design values from the 2003-2007 period. Even
though receptors may be measuring attainment based on recent data, they
may still be at risk for falling back into nonattainment. Therefore,
even if commenters argue that recent data show that monitoring sites
should not be nonattainment receptors (with which we disagree), the
same argument cannot be made regarding maintenance receptors. Clearly,
receptors with recent ``clean'' ambient data may still experience
higher PM2.5 and/or ozone concentrations in the future
(based on

meteorological and emission variability) and therefore may be
appropriate maintenance receptors.
    Comment: Several commenters claim that the maintenance receptor
methodology overstates actual future design values. They also recommend
an alternative methodology which takes into account the downward trend
in observed PM2.5 concentrations over the last 5+ years. The
methodology would remove the trend in the data where air quality is
improving over the period by applying a linear fit to the data,
calculating the residuals and then adding the residuals back to the
average of the data. Given a site with a downward trend, this has the
effect of decreasing the calculated maximum values from the early years
in the period and increasing the values from the end years in the
period.
    Response: EPA continues to believe that our approach to identify
maintenance receptors is reasonable and appropriate. For the final
rule, we continue to identify maintenance receptors by projecting the
maximum design value from the 2003-2007 period to the future. The
methodology assumes that the combination of emissions and meteorology
that occurred in the base period (which led to relatively high ambient
design values) could happen again in the future (albeit at lower
emissions levels). There is no information presented by the commenters
which explains why the magnitude of base year design value variability
could not occur in the same way in the future. The commenters cite the
downward trend in ambient data as the reason why the EPA methodology is
not reasonable. However, in most cases, the recent downward trend in
ambient data is due to a combination of ongoing emission reductions
(which includes CAIR), variability in meteorology, and depressed
emissions due to the recession. In fact, the most recent ambient design
value period (2007-2009) is heavily influenced by extremely low ozone
and PM2.5 concentrations measured in 2009. The 2009 data are
marked by relatively low emissions due to cool summer weather and
ongoing effects of the recession. The preliminary \22\ 2010 ambient
data in the eastern U.S. show that ozone and PM2.5 values
were considerably higher in 2010 compared to 2009. In the states that
are included in the final Transport Rule region, there were 158 ozone
monitor days that exceeded 84 ppb in 2009 compared to 412 monitor
exceedance days in 2010. For PM2.5, there were 251 monitor
days that exceeded 35 [mu]g/m \3\ in 2009 compared to 417 monitor
exceedance days in 2010. Even though the SO2 and
NOX emissions were generally lower in 2010, the observed
ozone and PM2.5 concentrations were higher. This shows the
important influence of meteorology on ambient concentrations. Clearly,
the year to year variability due to meteorology can be large. We
acknowledge the downward trend in ambient data over the last few years.
But this does not mean that conditions that led to high ozone and/or
PM2.5 in the 2003-2007 period could not occur again in the
future. The 2010 ambient data show that meteorology can cause
concentrations to go back up, even though there is a downward trend in
emissions.
---------------------------------------------------------------------------

    \22\ The 2010 data is preliminary. Exceptional event data has
not been flagged and removed from the reported data.
---------------------------------------------------------------------------

    We also believe that the alternate maintenance methodology
presented by the commenter is inappropriate. The EPA modeling for 2012
(and 2014) appropriately accounts for emission reductions that occur
after 2005 except for those that should not be considered, as explained
in section V.B., because they were required only by CAIR. Therefore,
the starting point design values used to project to the future should
not be lowered to account for emission reduction trends that occur
after 2005. Doing so would give ``double credit'' to the more recent
emission reductions and provides an inappropriate downward adjustment
to the early design value periods of the 2003-2007 period.
    Comment: One commenter claims that EPA did not follow our own
modeling guidance by not doing local scale modeling in urban areas with
high PM2.5 concentration gradients. They suggested that the
methodology to calculate future year design values should have included
dispersion modeling to calculate the change in concentration over time
of primary PM2.5 emissions.
    Response: EPA modeling guidance for PM2.5 attainment
demonstrations recommends photochemical grid modeling to examine future
year changes in PM2.5 concentrations. There are several
optional aspects of the modeling which are recommended in specific
cases. This includes a recommendation for a ``local area analysis''
using a dispersion model. An area with relatively large local primary
PM2.5 concentration gradients may want to do additional
modeling to examine the impacts of local controls on its future year
PM2.5 concentrations. This is particularly important when
local controls of primary PM2.5 are included as part of the
attainment demonstration.
    As noted above, a ``local area analysis'' is recommended as part of
the local attainment demonstration process in specific situations. It
is impractical for EPA to perform this type of analysis for each local
area in the regional Transport Rule. National rulemakings are not
attainment demonstrations. We are not able to perform fine scale
analyses for each area. For the final rule modeling, we have attempted
to address all emissions and modeling related comments. We have updated
the modeling platform to use the latest version of CAMx and are
continuing to model ozone and PM2.5 at 12km grid resolution,
which for PM2.5 is a more refined grid resolution compared
to the CAIR modeling.
    Additionally, there is no evidence presented by the commenter that
would indicate that the future year PM2.5 concentrations
from the Transport Rule are biased high. In fact, depending on the
circumstances, local fine scale grid or dispersion modeling may result
in lower or higher future year design values. In a fine scale analysis,
the dominant local primary PM2.5 emissions become a larger
percentage of the PM2.5 concentrations. Therefore, if the
local emissions are forecast to decrease, fine scale modeling may lead
to lower future design values. However, if the local emissions are
forecast to increase or stay the same between the base and future
years, local modeling will likely show higher future year design values
compared to a regional analysis. This points to the fact that perceived
biases in modeling results may not always be correct.
    In sum, fine scale modeling of local areas may lead to either
higher or lower future year design values. There is no indication that
EPA's regional modeling is biased in either direction. EPA's Transport
Rule modeling generally followed EPA's modeling guidance and is
appropriate for the purpose of this rulemaking.
    Comment: One commenter completed and submitted a detailed CAMx
based modeling analysis with a 2008 base year and future years of 2014
and 2018. The analysis shows that the majority of the proposed rule
2012 nonattainment and maintenance sites are already attaining based on
either 2006-2008 or 2007-2009 ambient data. Based on this, the
commenter claims that air quality has improved more rapidly than
predicted by EPA's proposed rule modeling. Also, based on the
commenter's 2014 modeling of CAIR emissions (including utility consent
decrees and state programs), the commenter concludes that no additional
controls are needed

beyond CAIR to bring most or all sites into attainment by 2014.
    Response: As an initial matter, we note that the basic question
addressed by the commenter, ``whether additional controls beyond CAIR
are necessary,'' is not on point. As explained previously, the D.C.
Circuit remanded CAIR to EPA and it remains in place only temporarily.
The question EPA must answer in this rulemaking, therefore, is not what
controls in addition to CAIR are necessary but what, if any,
restrictions on emissions must be put in place to replace CAIR in order
to satisfy the requirements of section 110(a)(2)(D)(i)(I) of the CAA.
For this reason, and as explained in greater detail in section V.B of
this preamble, any analysis of whether beyond CAIR controls are
necessary is irrelevant to this rulemaking. Nonetheless, we have
carefully reviewed different aspects of the commenter's analysis. We
previously addressed comments related to the use of more recent ambient
data to examine future year nonattainment and maintenance receptors. As
noted above, the 2006-2008 and 2007-2009 ambient data is heavily
influenced by several factors. Among them are the emissions reductions
from CAIR, the relatively low recent observed ozone and
PM2.5 concentrations at least partially due to non-conducive
meteorology (particularly in 2009), and the atypical suppression of
emissions due to the sharp recession. For all of these reasons, we
believe it is not possible to directly compare the most recent design
values to the predicted future year 2012 and 2014 design values from
the Transport Rule. In particular, it is inappropriate to compare
current design values to EPA's no-CAIR 2012 future year modeling
results. As noted in the comment summary, the commenter's modeling
analysis assumed that CAIR was in place in both 2008 and the future
years. This is a fundamentally different assumption than the modeling
EPA used to define the Transport Rule nonattainment and maintenance
receptors in 2012 and is inappropriate for purposes of the Transport
Rule for reasons described above and in section V.B.
    Additionally, EPA's maintenance methodology chooses the highest of
three base year design value periods projected to the future. The
commenter only used a single design value period in their analysis and
therefore did not fully examine maintenance issues. In fact, the 2014
nonattainment modeling receptors in the final Transport Rule and the
commenter's modeling analysis are similar. As documented in section
VI.D, in the 2014 final rule remedy case, there is only one remaining
nonattainment area for ozone and one remaining nonattainment area for
24-hour PM2.5. This is similar to the modeling results
presented in the comments.\23\ However, EPA modeling identifies
additional maintenance receptors in 2012 that continue to have
maintenance issues in 2014.
---------------------------------------------------------------------------

    \23\ The purpose of this comparison is to note that the modeling
analyses are actually more similar than the commenter implies.
However, the Transport Rule differs from the commenter's modeling
due to the assumption that CAIR was in place. CAIR and the Transport
Rule differ in state coverage and emission budgets. They are
therefore not directly comparable.
---------------------------------------------------------------------------

    EPA also examined our ozone and PM2.5 projection
procedures to see if there might be additional reasons for the
relatively lower current ambient design values (and modeled design
values in the commenter's analysis) compared to the 2014 remedy modeled
values. Upon further analysis of EPA's 24-hour attainment test
methodology, we noted certain discrepancies between the methodology and
the calculation of the ambient 24-hour design values. In the proposed
rule 24-hour attainment test, for each PM2.5 monitor, we
projected the measured 98th percentile concentrations from the 2003-
2007 period to the future. A basic assumption in this methodology is
that the distribution of high measured days in the base period will be
the same in the future. For example, if the observed 98th percentile
day is the 3rd high day for a particular year, we assume that the 1st,
2nd, and 3rd high days (and subsequent high days) in the future remain
in the same basic distribution. Further examination of the proposed
rule modeling found that this is not always the case. In situations
where there are large summer PM2.5 concentration reductions,
some of the high days may switch from the summer in the base period to
the winter in the future period.
    In order to better account for the complicated future response in
24-hour design values, we have updated the 24-hour attainment
demonstration methodology to more closely reflect the way 24-hour
design values are calculated. In the revised methodology, we do not
assume that the temporal distribution of high days in the base and
future periods will remain the same. We project a larger set of ambient
days from the base period to the future and then re-rank the entire set
of days to find the new future 98th percentile value (for each year).
More specifically, we project the highest 8 days per quarter (32 days
per year) to the future and then re-rank the 32 days to derive the
future year 98th percentile concentrations. In the case of the
Transport Rule model results, this has the effect of lowering the
future year 24-hour design values compared to the old methodology. The
2012 base case design values for all nonattainment and maintenance
receptors were either unchanged or lower with the revised methodology.
3. How did EPA project future nonattainment and maintenance for annual
PM2.5, 24-hour PM2.5, and 8-hour ozone?
    Final Rule: In general, the methodology to project ozone and
PM2.5 concentrations to the future year(s) remains the same
for the final rule. The proposal modeling followed the modeling
guidance procedures for projecting ambient design values to future
years. For the final rule, we continue to follow the basic procedures
outlined in the guidance. The 8-hour ozone and annual PM2.5
methodology are unchanged from the proposal. However, the 24-hour
PM2.5 methodology has been updated in the final rule to be
more consistent with the calculation of 24-hour PM2.5 design
values. There were also additional minor updates to the ambient
data.\24\ The methodology to identify maintenance receptors is also
unchanged from the proposal. We continue to use the maximum design
value (projected from the 5 year base period) to calculate future year
maintenance receptors.
---------------------------------------------------------------------------

    \24\ The base year design values were updated based on the
latest official data. See http://www.epa.gov/airtrends/values.html.
---------------------------------------------------------------------------

    As noted in the proposal, EPA considers that the maintenance
concept has two components: Year-to-year variability in emissions and
air quality, and continued maintenance of the air quality standard over
time. The way that EPA defined maintenance based on year-to-year
variability (as discussed in detail here) directly affects the
requirements of this final rule. EPA also considered whether further
reductions were necessary to ensure continued lack of interference with
maintenance of the NAAQS over time (e.g., after 2014). EPA concluded
that in light of projected emission trends, and also considering the
emission reductions from this proposed rule, no further reductions are
required solely for this purpose at PM2.5 and ozone
receptors for which we are partially or fully determining significant
contribution for the current NAAQS. (See discussion of emission trends
in Chapter 7 of TSD entitled ``Emission Inventories,'' included in the
docket for the Transport Rule proposal.)

a. Which ambient ozone and PM2.5 data did EPA use for the
purpose of projecting future year concentrations?
    The final rule modeling continues to use a 2005 base case inventory
and 2005 meteorology. Therefore, we continue to use ambient data from
the 2003-2007 period. For each monitoring site, all valid design values
(up to 3) from this period were averaged together. Since 2005 is
included in all three design value periods, this has the effect of
creating a 5-year weighted average, where the middle year is weighted 3
times, the 2nd and 4th years are weighted twice, and the 1st and 5th
years are weighted once. We refer to this as the 5-year weighted
average value. The 5-year weighted average values were then projected
to the future years that were analyzed for this final rule. The 2003-
2005, 2004-2006, and 2005-2007 design values are accessible at http://www.epa.gov/airtrends/values.html. The design values have been updated
based on the latest official values. The official values have
exceptional events removed from the calculations if they are flagged by
states and concurred with by EPA Regional offices.
    The procedures for projecting annual average PM2.5 and
8-hour ozone conform to the methodology in the current attainment
demonstration modeling guidance.\25\
---------------------------------------------------------------------------

    \25\ U.S. EPA, 2007: Guidance on the Use of Models and Other
Analyses for Demonstrating Attainment of Air Quality Goals for
Ozone, PM2.5, and Regional Haze; Office of Air Quality
Planning and Standards, Research Triangle Park, NC.
---------------------------------------------------------------------------

b. Projection of Future Annual and 24-Hour PM2.5
Nonattainment and Maintenance
(1) Methodology for Projecting Future Annual PM2.5
Nonattainment and Maintenance
    For the final rule, annual PM2.5 modeling was performed
for the 2005 base year emissions and for the 2012 base case as part of
the approach for projecting which locations are expected to be in
nonattainment and/or have difficulty maintaining the PM2.5
standards in 2012. We refer to these areas as nonattainment sites and
maintenance sites respectively.
    Concentrations of PM2.5 in 2012 were estimated by
applying the modeled 2005-to-2012 relative change in PM2.5
species to each of the 3-year ambient monitoring data periods (i.e.,
2003-2005, 2004-2006, and 2005-2007) to obtain up to 3 future-year
PM2.5 design values for each monitoring site. We used the
highest of these projections at each monitoring site to determine which
sites are expected to have maintenance problems in 2012. We used the 5
year weighted average of those projections to determine which
monitoring sites are expected to be nonattainment in this future year.
    For the analysis of both nonattainment and maintenance, monitoring
sites were included in the analysis if they had at least one complete
design value in the 2003-2007 period.\26\ There were 721 monitoring
sites in the 12 km modeling domain which had at least one complete
design value period for the annual PM2.5 NAAQS, and 722
sites which met this criterion for the 24-hour NAAQS.\27\
---------------------------------------------------------------------------

    \26\ If there is only one complete design value, then the
nonattainment and maintenance design values are the same.
    \27\ Design values were only used if they were deemed to be
officially complete based on CFR 40 Part 50 Appendix N. The
completeness criteria for the annual and 24-hour PM2.5
NAAQS are different. Therefore, there are fewer complete sites for
the annual NAAQS.
---------------------------------------------------------------------------

    EPA followed the procedures recommended in the modeling guidance
for projecting PM2.5 by projecting individual
PM2.5 component species and then summing these to calculate
the concentration of total PM2.5. EPA's Modeled Attainment
Test Software (MATS) was used to calculate the future year design
values. The software (including documentation) is available at: http://www.epa.gov/scram001/modelingapps_mats.htm. Additional details on the
annual PM2.5 nonattainment and maintenance projections
methodology can be found in the Air Quality Modeling Final Rule TSD.
    The 2012 annual PM2.5 design values were calculated for
each of the 721 sites. The calculated annual PM2.5 design
values are truncated after the second decimal place.\28\ This is
consistent with the ambient monitoring data truncation and rounding
procedures for the annual PM2.5 NAAQS. Any value that is
greater than or equal to 15.05 [micro]g/m\3\ is rounded to 15.1
[micro]g/m\3\ and is considered to be violating the NAAQS. Thus, sites
with projected 5-year weighted average (``average'') annual
PM2.5 design values of 15.05 [micro]g/m\3\ or greater are
predicted to be nonattainment sites. Sites with projected maximum
design values of 15.05 [micro]g/m\3\ or greater are predicted to be
maintenance sites. Note that nonattainment sites are also maintenance
sites because the maximum design value is always greater than or equal
to the 5-year weighted average. For ease of reference we use the term
``nonattainment sites'' to refer to those sites that are projected to
exceed the NAAQS based on both the average and maximum design values.
Those sites that are projected to be attainment based on the average
design value, but exceed the NAAQS based on the maximum design value,
are referred to as maintenance sites. The monitoring sites that we
project to be nonattainment and/or maintenance for the annual
PM2.5 NAAQS in the 2012 base case are the nonattainment/
maintenance receptors used for assessing the contribution of emissions
in upwind states to downwind nonattainment and maintenance of the
annual PM2.5 NAAQS.
---------------------------------------------------------------------------

    \28\ For example, a calculated annual average concentration of
14.94753 * * * becomes 14.94 when digits beyond two places to the
right of the decimal are truncated.
---------------------------------------------------------------------------

    Table V.C-1 contains the 2003-2007 base case period average and
maximum annual PM2.5 design values and the corresponding
2012 base case average and maximum design values for sites projected to
be nonattainment of the annual PM2.5 NAAQS in 2012. Table
V.C-2 contains this same information for projected 2012 maintenance
sites.

        Table V.C-1--Average and Maximum 2003-2007 and 2012 Base Case Annual PM2.5 Design Values ([micro]g/m\3\) at Projected Nonattainment Sites
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Final rule        Final rule
          Monitor ID                     State                    County           Average  design   Maximum  design   average design    maximum design
                                                                                value 2003-2007   value 2003-2007     value 2012        value 2012
--------------------------------------------------------------------------------------------------------------------------------------------------------
010730023....................  Alabama.................  Jefferson..............             18.57             18.94             16.15             16.46
010732003....................  Alabama.................  Jefferson..............             17.15             17.69             15.16             15.64
131210039....................  Georgia.................  Fulton.................             17.43             17.47             15.07             15.10
171191007....................  Illinois................  Madison................             16.72             17.01             15.46             15.73
261630033....................  Michigan................  Wayne..................             17.50             18.16             15.73             16.32

390350038....................  Ohio....................  Cuyahoga...............             17.37             18.10             15.99             16.66
390350045....................  Ohio....................  Cuyahoga...............             16.47             16.98             15.14             15.61
390350060....................  Ohio....................  Cuyahoga...............             17.11             17.66             15.67             16.18
390610014....................  Ohio....................  Hamilton...............             17.29             17.53             15.76             15.98
390610042....................  Ohio....................  Hamilton...............             16.85             17.25             15.40             15.77
390618001....................  Ohio....................  Hamilton...............             17.54             17.90             16.01             16.33
420030064....................  Pennsylvania............  Allegheny..............             20.31             20.75             17.94             18.33
--------------------------------------------------------------------------------------------------------------------------------------------------------

        Table V.C-2--Average and Maximum 2003-2007 and 2012 Base Case Annual PM2.5 Design Values ([mu]g/m\3\) at Projected Maintenance-Only Sites
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Final rule        Final rule
          Monitor ID                     State                    County           Average  design   Maximum  design   average design    maximum design
                                                                                value 2003-2007   value 2003-2007     value 2012        value 2012
--------------------------------------------------------------------------------------------------------------------------------------------------------
180970081....................  Indiana.................  Marion.................             16.05             16.36             14.86             15.16
180970083....................  Indiana.................  Marion.................             15.90             16.27             14.71             15.06
390350065....................  Ohio....................  Cuyahoga...............             15.97             16.44             14.67             15.10
390617001....................  Ohio....................  Hamilton...............             16.17             16.56             14.74             15.10
--------------------------------------------------------------------------------------------------------------------------------------------------------

 (2) Methodology for Projecting Future 24-Hour PM2.5
Nonattainment and Maintenance
    The procedures for calculating the future year 24-hour
PM2.5 design values have been updated for the final
rule.\29\ The revised procedures are in response to comments which
noted relatively high future year 24-hour PM2.5 design
values in EPA's modeling of the proposed Transport Rule. The updates
are intended to make the projection methodology more consistent with
the procedures for calculating ambient design values.
---------------------------------------------------------------------------

    \29\ There were no updates to the ozone and annual
PM2.5 attainment test methodology.
---------------------------------------------------------------------------

    As noted above, for the proposed Transport Rule EPA projected for
each PM2.5 monitor the measured 98th percentile
concentrations from the 2003-2007 period to the future. As an
additional check, we also projected the next highest concentrations
from the three calendar quarters in each year when the 98th percentile
did not occur in the 2003-2007 base period, to ensure that the future
year 98th percentile did not switch seasons in the future year compared
to the base year. A basic assumption in this methodology is that the
distribution of high measured days in the base period will be the same
in the future.
    In other words, EPA assumed at proposal that the 98th-percentile
day could only be displaced ``from below'' in the instance that a
different day's future concentration exceeded the original 98th-
percentile day's future concentration. In that case, the original 98th-
percentile day may become the 97th- or 96th-percentile day in the
future year; EPA accounted for this possibility at proposal. EPA did
not, however, consider that the 98th-percentile day could also be
displaced ``from above'' in the instance that higher-concentration days
in the base period were projected to have future concentrations lower
than the original 98th-percentile day's future concentration. In that
case, the original 98th-percentile day may become the 99th- or 100th-
percentile day. Because EPA continued to use that day's future
concentration to determine the monitor's future design value at
proposal, this sometimes resulted in overstatement of future-year
design values for 24-hour PM2.5 monitoring sites whose
seasonal distribution of highest-concentration 24-hour PM2.5
days changed between the 2003-2007 period and the future year modeling.
Examination of the proposed rule remedy modeling (2014 remedy case)
showed that many of the highest PM2.5 days switched from the
summer in the base period to the winter in the future period. This is
especially true in areas of the upper Midwest which experience both
high summer and winter PM2.5 episodes.
    In the revised methodology, we do not assume that the seasonal
distribution of high days in the base period years and future years
will remain the same. We project a larger set of ambient days from the
base period to the future and then re-rank the entire set of days to
find the new future 98th percentile value (for each year). More
specifically, we project the highest 8 days per quarter (32 days per
year) to the future and then re-rank the 32 days to derive the future
year 98th percentile concentrations. In the case of the Transport Rule
model results, this has the effect of lowering the future year 24-hour
design values compared to the old methodology.
    The modeling guidance recommendations for state attainment
demonstrations have been updated to reflect the changes outlined above.
Further details on the 24-hour PM2.5 design value
calculations can be found in the Air Quality Modeling Final Rule TSD.
The above procedures for determining future year 24-hour
PM2.5 concentrations were applied for each site. The 24-hour
PM2.5 design values are truncated after the first decimal
place. This approach is consistent with the ambient data truncation and
rounding procedures for the 24-hour PM2.5 NAAQS. Any value
that is greater than or equal to 35.5 [micro]g/m\3\ is rounded to 36
[mu]g/m\3\ and is violating the NAAQS. Sites with future year 5-year
weighted average design values of 35.5 [mu]g/m\3\ or greater, based on
the projection of 5-year weighted average concentrations, are predicted
to be nonattainment. Sites with future year maximum design values of
35.5 [micro]g/m\3\ or greater are predicted to be maintenance sites.
Note that nonattainment sites for the 24-hour NAAQS are also
maintenance sites because the maximum design value is always greater
than or equal to the 5-year weighted average. The monitoring

sites that we project to be nonattainment and/or maintenance for the
24-hour PM2.5 NAAQS in the 2012 base case are the
nonattainment/maintenance receptors used for assessing the contribution
of emissions in upwind states to downwind nonattainment and maintenance
of 24-hour PM2.5 NAAQS as part of this final rule.
    Table V.C-3 contains the 2003-2007 base period average and maximum
24-hour PM2.5 design values and the 2012 base case average
and maximum design values for sites projected to be 2012 nonattainment
of the 24-hour PM2.5 NAAQS in 2012. Table V.C-4 contains
this same information for projected 2012 24-hour maintenance sites.

          Table V.C-3--Average and Maximum 2003-2007 and 2012 Base Case 24-Hour PM2.5 Design Values ([mu]g/m3) at Projected Nonattainment Sites
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Final rule        Final rule
          Monitor ID                     State                    County           Average  design   Maximum  design   average design    maximum design
                                                                                value 2003-2007   value 2003-2007     value 2012        value 2012
--------------------------------------------------------------------------------------------------------------------------------------------------------
010730023....................  Alabama.................  Jefferson..............              44.0              44.2              36.9              37.3
170311016....................  Illinois................  Cook...................              43.0              46.3              37.5              40.4
171191007....................  Illinois................  Madison................              39.1              40.1              36.5              36.8
180970043....................  Indiana.................  Marion.................              38.4              39.9              35.7              37.1
180970066....................  Indiana.................  Marion.................              38.3              39.6              35.7              36.9
180970081....................  Indiana.................  Marion.................              38.2              39.2              35.8              36.9
261470005....................  Michigan................  St Clair...............              39.6              40.6              36.2              37.1
261630015....................  Michigan................  Wayne..................              40.1              40.6              35.5              36.0
261630016....................  Michigan................  Wayne..................              42.9              45.4              38.9              41.2
261630019....................  Michigan................  Wayne..................              40.9              41.4              37.3              37.8
261630033....................  Michigan................  Wayne..................              43.8              44.2              39.4              39.8
390350038....................  Ohio....................  Cuyahoga...............              44.2              47.0              39.4              41.8
390350060....................  Ohio....................  Cuyahoga...............              42.1              45.7              37.7              40.8
420030064....................  Pennsylvania............  Allegheny..............              64.2              68.2              56.7              59.9
420030093....................  Pennsylvania............  Allegheny..............              45.6              51.5              39.1              44.3
420030116....................  Pennsylvania............  Allegheny..............              42.5              42.5              35.5              35.5
420070014....................  Pennsylvania............  Beaver.................              43.4              44.6              36.2              37.4
420710007....................  Pennsylvania............  Lancaster..............              40.8              44.0              35.9              38.3
540090011....................  West Virginia...........  Brooke.................              43.9              44.9              37.5              38.3
550790043....................  Wisconsin...............  Milwaukee..............              39.9              40.8              36.2              37.1
--------------------------------------------------------------------------------------------------------------------------------------------------------

      Table V.C-4--Average and Maximum 2003-2007 and 2012 Base Case 24-Hour PM2.5 Design Values ([micro]g/m\3\) at Projected Maintenance-Only Sites
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Final rule        Final rule
          Monitor ID                     State                    County           Average  design   Maximum  design   average design    maximum design
                                                                                value 2003-2007   value 2003-2007     value 2012        value 2012
--------------------------------------------------------------------------------------------------------------------------------------------------------
010732003....................  Alabama.................  Jefferson..............              40.3              40.8              35.3              35.9
170310052....................  Illinois................  Cook...................              40.2              41.4              34.9              36.0
170312001....................  Illinois................  Cook...................              37.7              40.6              33.6              36.1
170313301....................  Illinois................  Cook...................              40.2              43.3              34.9              37.6
170316005....................  Illinois................  Cook...................              39.1              41.8              34.1              36.4
171190023....................  Illinois................  Madison................              37.3              38.1              35.1              35.8
180890022....................  Indiana.................  Lake...................              38.9              44.0              34.9              39.5
180890026....................  Indiana.................  Lake...................              38.4              41.3              34.0              37.0
261610008....................  Michigan................  Washtenaw..............              39.4              40.8              35.0              36.3
390170003....................  Ohio....................  Butler.................              39.2              41.1              34.4              36.5
390350045....................  Ohio....................  Cuyahoga...............              38.5              41.5              34.7              38.1
390350065....................  Ohio....................  Cuyahoga...............              38.6              41.0              34.9              37.6
390618001....................  Ohio....................  Hamilton...............              40.6              40.9              35.2              35.8
390811001....................  Ohio....................  Jefferson..............              41.9              45.5              34.5              37.8
391130032....................  Ohio....................  Montgomery.............              37.8              40.0              33.6              35.6
420031008....................  Pennsylvania............  Allegheny..............              41.3              42.8              35.0              36.3
420031301....................  Pennsylvania............  Allegheny..............              40.3              42.4              33.9              35.6
420033007....................  Pennsylvania............  Allegheny..............              37.5              43.1              32.3              37.3
421330008....................  Pennsylvania............  York...................              38.2              40.7              33.3              36.0
550790010....................  Wisconsin...............  Milwaukee..............              38.6              40.0              35.4              36.7
550790026....................  Wisconsin...............  Milwaukee..............              37.3              41.3              33.6              37.2
--------------------------------------------------------------------------------------------------------------------------------------------------------

(3) Methodology for Projecting Future 8-Hour Ozone Nonattainment and
Maintenance
    The final rule methodology to calculate 8-hour ozone nonattainment
and maintenance receptors is identical to the proposed rule. The May-
to-September 24-hour maximum 8-hour average concentrations from the
2005 base case and the 2012 base case were used to project ambient
design values to 2012. The following is a brief summary of the future
year 8-hour average ozone calculations. Additional details are provided
in the Air Quality Modeling Final Rule TSD.
    We are using the base period 2003-2007 ambient ozone design value
data for projecting future year design values. Relative response
factors (RRF) for each monitoring site were calculated as the

percent change in ozone on days with modeled ozone greater than 85
ppb.\30\
---------------------------------------------------------------------------

    \30\ As specified in the attainment demonstration modeling
guidance, if there are less than 10 modeled days > 85 ppb, then the
threshold is lowered in 1 ppb increments (to as low as 70 ppb) until
there are 10 days. If there are less than 5 days > 70 ppb, then an
RRF calculation is not completed for that site.
---------------------------------------------------------------------------

    The maximum future design value is calculated by projecting design
values for each of the three base periods (2003-2005, 2004-2006, and
2005-2007) separately. The highest of the three future values is the
maximum design value. This maximum value is used to identify the 8-hour
ozone maintenance receptors.
    The future year design values are truncated to integers in units of
ppb. This approach is consistent with the ambient data truncation and
rounding procedures for the 8-hour ozone NAAQS. Future year design
values that are greater than or equal to 85 ppb are considered to be
violating the NAAQS. Sites with future year 5-year weighted average
design values of 85 ppb or greater are predicted to be nonattainment.
Sites with future year maximum design values of 85 ppb or greater are
predicted to be future year maintenance sites. Note that, as described
previously for the annual and 24-hour PM2.5 NAAQS,
nonattainment sites for the ozone NAAQS are also maintenance sites
because the maximum design value is always greater than or equal to the
5-year weighted average. The monitoring sites that we project to be
nonattainment and/or maintenance for the 8-hour ozone NAAQS in the 2012
base case are the nonattainment/maintenance receptors used for
assessing the contribution of emissions in upwind states to downwind
nonattainment and maintenance of ozone NAAQS.
    Table V.C-5 contains the 2003-2007 base period average and maximum
8-hour ozone design values and the 2012 base case average and maximum
design values for sites projected to be 2012 nonattainment of the 8-
hour ozone NAAQS in 2012. Table V.C-6 contains this same information
for projected 2012 8-hour ozone maintenance sites.

             Table V.C-5--Average and Maximum 2003-2007 and 2012 Base Case 8-Hour Ozone Design Values (ppb) at Projected Nonattainment Sites
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Final rule        Final rule
          Monitor ID                     State                    County           Average  design   Maximum  design   average design    maximum design
                                                                                value 2003-2007   value 2003-2007     value 2012        value 2012
--------------------------------------------------------------------------------------------------------------------------------------------------------
220330003....................  Louisiana...............  East Baton Rouge.......              92.0                96              85.6              89.3
480391004....................  Texas...................  Brazoria...............              94.7                97              86.7              88.8
482010051....................  Texas...................  Harris.................              93.0                98              86.1              90.8
482010055....................  Texas...................  Harris.................             100.7               103              93.3              95.4
482010062....................  Texas...................  Harris.................              95.7                99              88.8              91.8
482010066....................  Texas...................  Harris.................              92.3                96              87.1              90.6
482011039....................  Texas...................  Harris.................              96.3               100              88.8              92.2
--------------------------------------------------------------------------------------------------------------------------------------------------------

           Table V.C-6--Average and Maximum 2003-2007 and 2012 Base Case 8-Hour Ozone Design Values (ppb) at Projected Maintenance-Only Sites
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Average  design   Maximum  design   Average design    Maximum design
          Monitor ID                     State                    County           value 2003-2007   value 2003-2007     value 2012        value 2012
--------------------------------------------------------------------------------------------------------------------------------------------------------
090011123....................  Connecticut.............  Fairfield..............              92.3                94              83.9              85.5
090093002....................  Connecticut.............  New Haven..............              90.3                93              82.7              85.1
240251001....................  Maryland................  Harford................              92.7                94              84.4              85.6
260050003....................  Michigan................  Allegan................              90.0                93              82.4              85.1
482010024....................  Texas...................  Harris.................              88.0                92              83.4              87.2
482010029....................  Texas...................  Harris.................              91.7                93              84.2              85.4
482011015....................  Texas...................  Harris.................              89.0                96              82.4              88.9
482011035....................  Texas...................  Harris.................              86.3                95              79.9              88.0
482011050....................  Texas...................  Harris.................              89.3                92              82.8              85.4
--------------------------------------------------------------------------------------------------------------------------------------------------------

D. Pollution Transport From Upwind States

1. Choice of Air Quality Thresholds
a. Thresholds
    In this action, EPA uses air quality thresholds to identify
linkages between upwind states and downwind nonattainment and
maintenance receptors. States whose contributions to a specific
receptor meet or exceed the thresholds identified are considered linked
to that receptor; those states' emissions (and available emission
reductions) are analyzed further in the second step of EPA's
significant contribution analysis. States whose contributions are below
the thresholds are not included in the Transport Rule for that NAAQS.
In other words, we are finding that states whose contributions are
below these thresholds do not significantly contribute to nonattainment
or interfere with maintenance of the relevant NAAQS.
    We use separate air quality thresholds for annual PM2.5,
24-hour PM2.5, and 8-hour ozone. Each air quality threshold
is calculated as 1 percent of the NAAQS. Specifically, we use an air
quality threshold of 0.15 [mu]g/m\3\ for annual PM2.5, 0.35
[mu]g/m\3\ for 24-hour PM2.5, and 0.8 ppb for 8-hour ozone.
These are the same air quality thresholds we proposed.
    EPA received a number of comments on the thresholds we proposed,
and those comments and EPA's responses are discussed below.
b. General Comments on the Overall Stringency and Use of 1 Percent of
the NAAQS
    EPA received numerous comments supporting and opposing the proposed
thresholds. A number of commenters cited support for EPA's approach.
Some

commenters believed that use of a 1 percent threshold was too
stringent, and recommended that EPA should use a threshold greater than
1 percent. Others believed that 1 percent was not stringent enough, and
they recommended using a lower value such as 0.5 percent. EPA believes
that for both PM2.5 and for ozone, it is appropriate to use
a threshold of 1 percent of the NAAQS for identifying states whose
contributions do not significantly contribute to nonattainment or
interfere with maintenance of the relevant NAAQS; therefore, EPA has
retained the 1 percent threshold for the reasons described below.
    As we found at the time of CAIR, EPA's analysis of base case
PM2.5 transport shows that, in general, PM2.5
nonattainment problems result from the combined impact of relatively
small contributions from many upwind states, along with contributions
from in-state sources and, in some cases, substantially larger
contributions from a subset of particular upwind states. (See section
II of the January 2004 CAIR proposal, 69 FR 4575-87).
    In the 1998 NOX SIP Call (63 FR 57456, October 27, 1998)
and in CAIR, EPA also found important contributions from multiple
upwind states. As a result of the upwind ``collective contributions,''
EPA determined that it is appropriate to use a low air quality
threshold when analyzing upwind states' contributions to downwind
states' attainment and maintenance problems for ozone as well as
PM2.5.
    Low threshold values are also warranted, as EPA discussed in the
notices for CAIR, due to adverse health impacts associated with ambient
PM2.5 and ozone even at low concentrations (See relevant
portions of the CAIR proposal notice (63 FR 4583-84) and the CAIR final
rule notice (70 FR 25189-25192)).
    To aid in responding to comments, EPA has compiled the contribution
modeling results to analyze the impact of different possible
thresholds. This analysis demonstrates the reasonableness of using the
1 percent threshold to account for the combined impact of relatively
small contributions from many upwind states (see Air Quality Modeling
Final Rule TSD). In this analysis, EPA identifies for annual
PM2.5 (sulfate and nitrate), 24-hour PM2.5
(sulfate and nitrate), and 8-hour ozone receptors: (1) Total upwind
state contributions, and (2) the amount of the total upwind state
contribution that is captured at thresholds of 1 percent, 5 percent and
0.5 percent of the NAAQS. EPA continues to find that the total
``collective contribution'' from upwind sources represents a large
portion of PM2.5 and ozone at downwind locations and that
the total amount of transport is composed of the individual
contribution from numerous upwind states.
    The analysis shows that the 1 percent threshold captures a high
percentage of the total pollution transport affecting downwind states
for both PM2.5 and ozone. In response to commenters who
advocated a higher threshold, EPA observes that higher thresholds would
exclude increasingly large percentages of total transport, which we do
not believe would be appropriate. For example, a 5 percent threshold
would exclude the majority--and for annual PM, more than 80 percent--of
interstate pollution transport affecting the downwind state receptors
analyzed (based on the average percentage of total interstate transport
across all receptors captured at the 5 percent threshold).
    In response to commenters who advocated a lower threshold, EPA
observes that the analysis shows that a lower threshold such as 0.5
percent would result in relatively modest increases in the overall
percentages of PM2.5 and ozone pollution transport captured
relative to the amounts captured at the 1 percent level. A 0.5 percent
threshold could lead to emission reduction responsibilities in
additional states that individually have a very small impact on those
receptors--an indicator that emission controls in those states are
likely to have a smaller air quality impact at the downwind receptor.
We are not convinced that selecting a threshold below 1 percent is
necessary or desirable. A strong indication that the amount of
pollution transport being excluded from consideration is not excessive
is that the controls required under this rule are projected to
eliminate nonattainment and maintenance problems with air quality
standards at most downwind state receptors.
    Considering the combined downwind impact of multiple upwind states,
the health effects of low levels of PM2.5 and ozone
pollution, and EPA's previous use of a 1 percent threshold for
PM2.5 in CAIR, EPA's judgment is that the 1 percent
threshold is a reasonable choice.
    Some commenters noted that the PM2.5 thresholds used for
this rule are less than the ``significant impact levels'' (SILs) used
for permitting programs. As EPA stated at the time of CAIR, since the
thresholds referred to by the commenters serve different purposes than
the CAIR threshold for significant contribution, it does not follow
that they should be made equivalent (70 FR 25191; May 12, 2005).
c. Comments on the Rounding Conventions for PM2.5
    In the final Transport Rule, EPA is using two-digit values for the
PM2.5 thresholds. Some commenters suggested that EPA should
use the same rounding convention for annual PM2.5 used in
CAIR; that is, the threshold should be 0.2 [mu]g/m\3\ rather than 0.15
[mu]g/m\3\. The reasons for EPA's decision are below.
    The rationale for the single digit value for the final CAIR rule
was that a single digit is consistent with the EPA monitoring data
reporting requirements in Part 50, Appendix N, section 4.3. These
reporting requirements specify that design values for the annual
PM2.5 standard shall be rounded to the tenths place
(decimals 0.05 and greater are rounded up to the next 0.1, and any
decimal lower than 0.05 is rounded down to the nearest 0.1).
    Because the design value is to be reported only to the nearest 0.1
[mu]g/m\3\, EPA deemed it preferable for the final CAIR to select the
threshold value at the nearest 0.1 [mu]g/m\3\ as well, and hence one
percent of the 15 [mu]g/m\3\, rounded to the nearest 0.1 [mu]g/m\3\
became 0.2 [mu]g/m\3\.
    The reporting requirements in section Part 50, Appendix N, section
4.3 for the 24-hour PM2.5 standard state that design values
for this standard shall be rounded to the nearest 1 [mu]g/m\3\
(decimals 0.5 and greater are rounded up to the nearest whole number,
and any decimal lower than 0.5 is rounded down to the nearest whole
number).
    If the approach used in CAIR were to be used to establish an air
quality threshold for the 24-hour PM2.5 NAAQS (which CAIR
did not address), the resulting threshold would be zero. One percent of
the 24-hour standard is 0.35 [mu]g/m\3\, and rounding to the nearest
whole number would yield an air quality threshold of zero. Thus if we
were to apply the same rationale used to develop the annual
PM2.5 threshold for the final CAIR, there would be no air
quality threshold for 24-hour PM2.5, which EPA believes to
be counter-intuitive and unworkable as an approach for assessing
interstate contributions.
    Therefore, for this rule, EPA proposed and is now finalizing an
approach that decouples the precision of the air quality thresholds
from the monitoring reporting requirements, and uses 2-digit values
representing one percent of the PM2.5 NAAQS; that is, 0.15
[mu]g/m\3\ for the annual standard, and 0.35 [mu]g/m\3\ for the 24-hour
standard. EPA believes there are a number of considerations favoring
this approach. First, it provides for a consistent approach for the
annual and 24-hour standards. Second, the

approach is readily applicable to any current and future NAAQS and
would automatically adjust the stringency of the transport threshold to
maintain a constant relationship with the stringency of the relevant
NAAQS as they are revised. The CAIR approach would not allow for this
continuity: For example, if EPA were to retain the CAIR approach for
the annual standard, any future lowering of the PM2.5 NAAQS
to below 15 [mu]g/m\3\ would reduce the air quality threshold to the
same outcome: 0.1 [mu]g/m\3\. This would occur because any value less
than 0.15 [mu]g/m\3\ would round to 0.1 [mu]g/m\3\ (assuming EPA would
not round down to zero for the reasons described above), which means
that the air quality threshold would have a different relative
stringency to each possible future NAAQS value. For the above reasons,
EPA believes the use of two-digit thresholds for both annual
PM2.5 and 24-hour PM2.5 in the final rule is both
reasonable and appropriate. The departure from the approach used for
annual PM2.5 in CAIR is appropriate given the additional
considerations that were not in existence at the time of the final
CAIR, and the importance of using a consistent approach to developing
air quality thresholds for all NAAQS addressed by this rule as well as
future NAAQS considered in future transport-related actions.
    Some of these commenters suggested using the CAIR rounding
conventions coupled with use of a 1-digit threshold of 0.4 [mu]g/m\3\
for 24-hour PM2.5. EPA considered the approach suggested by
commenters, but determined that the proposed approach is more
appropriate. First, adhering to the rounding conventions used for CAIR
for annual PM2.5 is not workable for the 24-hour standard
because the rounding convention would yield a threshold of zero.
Rounding alternatively to 0.4 [mu]g/m\3\ would require EPA to find a
basis for rounding the threshold to the nearest 0.1 [mu]g/m\3\ instead
of using a strict application of 1 percent; we do not see any basis for
such rounding at this time.
d. Comments Related to the Multi-Factor Test EPA Used for Ozone in CAIR
    Some commenters suggested that, for ozone, EPA should use the
multiple-metric test we used for CAIR, and not a simple threshold based
on 1 percent of the NAAQS. With respect to ozone, EPA proposed in the
Transport Rule to take a more straightforward approach to air quality
thresholds than the multi-factor approaches used for the NOX
SIP Call and the CAIR. As proposed, EPA is using a contribution metric
that is calculated based on the multi-day average contribution. This
metric is compared to one percent of the 1997 8-hour ozone standard of
0.08 ppm. Under this approach, one percent of the NAAQS is a value of
0.8 ppb. Contributions of 0.8 ppb and higher are above the threshold;
ozone contributions less than 0.8 ppb are below the threshold. In past
rulemakings (e.g., CAIR) EPA used multiple ozone metrics, including the
average contribution and maximum single day contribution to downwind
nonattainment. EPA believes the average contribution (calculated over
multiple high ozone days) is a robust metric compared to the maximum
contribution on a single day. EPA believes that this approach is
preferable because it uses a robust metric, it is consistent with the
approach for PM2.5, and it provides for a consistent
approach that takes into account, and is applicable to, any future
ozone standards below 0.08 ppm.
    One of these commenters suggested that the 0.8 ppb threshold value
was substantially more stringent than the 2 ppb screening test which
was a part of the approach used for CAIR. The 1 percent threshold (0.8
ppb) is not substantially more stringent than the previous 2 ppb test
because of differences in the metrics used to evaluate contributions
against these two levels. The 2 ppb test was evaluated using the
highest single day absolute model-predicted downwind contribution from
an upwind state. The 1 percent threshold is evaluated based on the
average relative downwind impact calculated over multiple days.
Therefore, it is appropriate to set a lower concentration threshold for
use with the average contribution metric calculated for the Transport
Rule. More details on the calculation of the contribution metric can be
found in the Air Quality Modeling Final Rule TSD. As noted above, EPA
believes that the approach used for the proposed rule provides for a
simplified, yet robust approach compared to CAIR. Accordingly, for the
final rule we have retained the approach used for the proposal.
    One commenter suggested that EPA retain the CAIR multiple-factor
approach for ozone, and to apply that same approach to 24-hour
PM2.5. As noted above, EPA is not retaining this approach
for ozone, and for similar reasons we believe a multi-factor approach
is not needed for 24-hour PM2.5. The approach based on 1
percent of the NAAQS is consistent with the form of the 24-hour
standard. In addition, this approach is based on contributions on days
with high 24-hour PM2.5 predictions and therefore is
relevant for characterizing transport during short-term high
PM2.5 episodic conditions.
e. Comments on the Relationship to Measurement Precision
    Other commenters suggested that, as did commenters on the
thresholds used in CAIR, EPA should take into consideration the
measurement precision of existing PM2.5 monitors in setting
the thresholds for the Transport Rule. EPA disagrees that monitoring
precision is relevant to determining the amount of modeled
PM2.5 or ozone that should be considered to be a
``contribution'' from upwind states since states are not required to,
nor would it be possible for them to, measure their individual state
impacts on downwind receptors. The approach for eliminating significant
contribution is based on the implementation of enforceable emissions
budgets and not on a measurement of ambient air quality. Thus, EPA
believes it is a reasonable exercise of its discretion to de-couple
monitoring precision from the choice of contribution states.
f. Comments Related to the CAIR Court Decision
    Commenters recommended that EPA should have retained the criteria
used for CAIR because those values were upheld by the Court. As noted
above, EPA could not have used the approach for annual PM2.5
that was used in CAIR to develop a 24-hour PM2.5 threshold,
as that approach would have yielded a threshold value of zero 24-hour
PM2.5.
    Further, nothing in the North Carolina opinion suggests that the
thresholds and methods used in CAIR were the only possible approaches
EPA could have used, that they were preferable to other approaches, or
that other alternatives would not be acceptable. Instead, the Court
upheld the 0.2 [micro]g/m\3\ threshold used for PM2.5 on the
grounds that it was not ``wholly unsupported by the record'' (North
Carolina, 531 F.3d at 915). EPA has determined for reasons explained in
the record that the thresholds used in this final rule are both
reasonable and appropriate for use in this final rule.
2. Approach for Identifying Contributing Upwind States
    This section documents the procedures used by EPA to quantify the
contribution of emissions in specific upwind states to air quality
concentrations in projected 2012 downwind nonattainment and maintenance
locations for annual PM2.5, 24-hour PM2.5, and 8-
hour ozone. In the

proposed rule EPA used CAMx photochemical source apportionment modeling
to quantify the impact of emissions in specific upwind states on
projected downwind nonattainment and maintenance receptors for both
PM2.5 and 8-hour ozone. In this modeling we tracked the
ozone and PM2.5 formed from 2012 base case emissions from
anthropogenic sources in each upwind state in the 12 km modeling
domain. The CAMx Particulate Source Apportionment Technique (PSAT) was
used to calculate downwind contributions to nonattainment and
maintenance of PM2.5. In the PSAT simulation NOX
emissions are tracked to particulate nitrate concentrations,
SO2 emissions are tracked to particulate sulfate
concentrations, and primary particulates (organic carbon, elemental
carbon, and other PM2.5) are tracked as primary
particulates. As described earlier in section V.A, the nitrate and
sulfate contributions were combined and used to evaluate interstate
contributions of PM2.5.
    The CAMx Ozone Source Apportionment Technique (OSAT) was used to
calculate downwind 8-hour ozone contributions to nonattainment and
maintenance. OSAT tracks the formation of ozone from NOX and
VOC emissions.
    Comment: Three commenters stated that the CAMx source apportionment
techniques used for the proposed rule reflect state-of-the science
technologies and are appropriate for evaluating interstate transport.
One commenter asked that EPA do more to demonstrate that the PSAT and
OSAT techniques give reliable answers, although no suggestions were
provided on how this might be done. Another commenter said that the
results of the contribution analyses were consistent with the results
of their scientific research.
    Response: EPA is not changing its conclusion that the CAMx source
apportionment techniques are appropriate for quantifying interstate
transport. The strength of the source apportionment technique is that
all modeled ozone and/or PM2.5 mass at a given location in
the modeling domain is tracked back to specific sources of emissions
and boundary conditions to fully characterize culpable sources. No
commenters provided technically valid analyses indicating that EPA's
use of CAMx source apportionment techniques are inappropriate for the
purposes of the Transport Rule.
    Comment: We received comments that certain states included in the
proposed rule should be excluded from the final rule because EPA had
overstated the 2012 emissions in these states. Commenter requested that
we redo the contribution modeling using 2012 base case emission
inventories that are revised based on proposed rule comments. Several
commenters also asked that EPA update the contribution modeling
analyses using the latest version of CAMx.
    Response: In response to these comments, we have rerun our source
apportionment modeling for PM2.5 and ozone for the 2012 base
case using the updated emission inventories described above in section
V.C.1 and the latest version of CAMx, version 5.30.
    The states EPA analyzed for interstate contributions for ozone and
for PM2.5 for the final rule are: Alabama, Arkansas,
Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Iowa,
Kansas, Kentucky, Louisiana, Maine, Maryland,\31\ Massachusetts,
Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Hampshire,
New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma,
Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee,
Texas, Vermont, Virginia, West Virginia, and Wisconsin.\32\ These are
the same states that EPA analyzed for the proposed rule.
---------------------------------------------------------------------------

    \31\ As in the proposal, EPA has combined the contributions from
Maryland and the District of Columbia as a single entity in our
contribution analysis for the final rule. EPA believes that this is
a fair representation of emissions for transport analysis because of
the small size of the District of Columbia and its close proximity
to Maryland. However, the District of Columbia is not included in
the Transport Rule due to the significant contribution analysis
findings in section VI.D.
    \32\ There were also several other states that are only
partially contained within the 12 km modeling domain (i.e.,
Colorado, Montana, New Mexico, and Wyoming). However, EPA did not
individually track the emissions or assess the contribution from
emissions in these states.
---------------------------------------------------------------------------

    For the proposed rule, we used a relative approach for calculating
the contributions to downwind nonattainment and maintenance receptors
from the outputs of the source apportionment modeling. As part of this
approach, the source apportionment predictions are combined with
measurement-based concentrations to calculate the contributions from
each state to nonattainment and/or maintenance receptors. This is
similar to the approach used to calculate future year design values, as
described in section V.C.2.
    Comment: One commenter said that using the source apportionment
modeling predictions in a relative sense strengthens the determination
of contributions and addresses an important source of uncertainty.
There were no comments that suggested an alternative approach.
    Response: For the final Transport Rule we are applying the relative
approach developed for the proposed rule to calculate contributions
from each state to downwind nonattainment and maintenance receptors.
    As noted above, for the final rule we modeled the updated 2012 base
case emissions using CAMX v5.30 to determine the
contributions from emissions in upwind states to nonattainment and
maintenance sites in downwind states. Contributions to nonattainment
and maintenance receptors are evaluated independently for each state to
determine if the contributions are at or above the threshold criteria.
    For each upwind state, the maximum contribution to nonattainment is
calculated based on the single largest contribution to a future year
(2012) downwind nonattainment receptor. The maximum contribution to
maintenance is calculated based on the single largest contribution to a
future year (2012) downwind maintenance receptor. Since the
contributions are calculated independently for each receptor, the
upwind contribution to maintenance can sometimes be larger than the
contribution to nonattainment, and vice versa. This also means that
maximum contributions to nonattainment can be below the threshold while
maximum contributions to maintenance may be at or above the threshold,
or vice versa.
V.D.2.a. Estimated Interstate Contributions to Annual PM2.5
and 24-Hour PM2.5
    In this section, we present the interstate contributions from
emissions in upwind states to downwind nonattainment and maintenance
sites for the annual PM2.5 NAAQS and the 24-hour
PM2.5 NAAQS based on modeling updated for the final rule. As
described previously in section V.D.1, states which contribute 0.15
[mu]g/m\3\ or more to annual PM2.5 nonattainment or
maintenance in another state are identified as states with
contributions large enough to warrant further analysis. For 24-hour
PM2.5, states which contribute 0.35 [mu]g/m \3\ or more to
24-hour PM2.5 nonattainment or maintenance in another state
are identified as states with contributions to downwind nonattainment
and maintenance sites large enough to warrant further analysis.
    For annual PM2.5, we calculated each state's
contribution to each of the 12 monitoring sites that are projected to
be nonattainment and each of the 4 sites that are projected to have
maintenance problems for the annual PM2.5 NAAQS in the 2012
base case. A detailed

description of the calculations can be found in the Air Quality
Modeling Final Rule TSD. The largest contribution from each state to
annual PM2.5 nonattainment in downwind sites is provided in
Table V.D-1. The Largest Contribution from Each State to Annual
PM2.5 maintenance in downwind sites is also provided in
Table V.D-1. The contributions from each state to all projected 2012
nonattainment and maintenance sites for the annual PM2.5
NAAQS are provided in the Air Quality Modeling Final Rule TSD.

 Table V.D-1--Largest Contribution to Downwind Annual PM2.5 ([mu]g/m\3\)
           Nonattainment and Maintenance for Each of 37 States
------------------------------------------------------------------------
                                   Largest downwind    Largest downwind
                                    contribution to     contribution to
          Upwind state             nonattainment for    maintenance for
                                     annual PM2.5        annual PM2.5
                                     ([mu]g/m\3\)        ([mu]g/m\3\)
------------------------------------------------------------------------
Alabama.........................                0.51                0.19
Arkansas........................                0.10                0.04
Connecticut.....................                0.00                0.00
Delaware........................                0.00                0.00
Florida.........................                0.08                0.01
Georgia.........................                0.46                0.13
Illinois........................                0.50                0.65
Indiana.........................                1.34                1.27
Iowa............................                0.26                0.14
Kansas..........................                0.09                0.04
Kentucky........................                0.94                0.81
Louisiana.......................                0.09                0.03
Maine...........................                0.00                0.00
Maryland........................                0.15                0.06
Massachusetts...................                0.00                0.00
Michigan........................                0.64                0.64
Minnesota.......................                0.14                0.09
Mississippi.....................                0.05                0.01
Missouri........................                1.22                0.27
Nebraska........................                0.06                0.03
New Hampshire...................                0.00                0.00
New Jersey......................                0.02                0.01
New York........................                0.21                0.21
North Carolina..................                0.20                0.06
North Dakota....................                0.06                0.04
Ohio............................                1.34                0.94
Oklahoma........................                0.08                0.03
Pennsylvania....................                0.54                0.54
Rhode Island....................                0.00                0.00
South Carolina..................                0.24                0.04
South Dakota....................                0.03                0.01
Tennessee.......................                0.32                0.32
Texas...........................                0.18                0.07
Vermont.........................                0.00                0.00
Virginia........................                0.12                0.06
West Virginia...................                0.95                0.40
Wisconsin.......................                0.22                0.19
------------------------------------------------------------------------

    Based on the state-by-state contribution analysis, there are 18
states \33\ which contribute 0.15 [mu]g/m\3\ or more to downwind annual
PM2.5 nonattainment. These states are: Alabama, Georgia,
Illinois, Indiana, Iowa, Kentucky, Maryland, Michigan, Missouri, New
York, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee,
Texas, West Virginia, and Wisconsin. In Table V.D-2, we provide a list
of the downwind nonattainment sites to which each upwind state
contributes 0.15 [mu]g/m\3\ or more (i.e., the upwind state to downwind
nonattainment ``linkages'').
---------------------------------------------------------------------------

    \33\ As in the proposal, EPA has combined the contributions from
Maryland and the District of Columbia as a single entity in our
contribution analysis for the final rule. EPA believes that this is
a fair representation of emissions for transport analysis because of
the small size of the District of Columbia and its close proximity
to Maryland. However, the District of Columbia is not included in
the Transport Rule due to the significant contribution analysis
findings in section VI.D.
---------------------------------------------------------------------------

    There are 12 states which contribute 0.15 [mu]g/m\3\ or more to
downwind annual PM2.5 maintenance. These states are:
Alabama, Illinois, Indiana, Kentucky, Michigan, Missouri, New York,
Ohio, Pennsylvania, Tennessee, West Virginia, and Wisconsin. In Table
V.D-3, we provide a list of the downwind maintenance sites to which
each upwind state contributes 0.15 [mu]g/m\3\ or more (i.e., the upwind
state to downwind maintenance ``linkages'').

             Table V.D-2--Upwind State to Downwind Nonattainment Site ``Linkages'' for Annual PM2.5
----------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------
        Upwind state                                        Downwind receptor sites
----------------------------------------------------------------------------------------------------------------
Alabama.....................  Fulton, GA           Hamilton, OH         Hamilton, OH         Hamilton, OH
                               (131210039).         (390610014).         (390610042).         (390618001).
Georgia.....................  Jefferson, AL        Jefferson, AL
                               (10730023).          (10732003).
Illinois....................  Jefferson, AL        Fulton, GA           Wayne, MI            Cuyahoga, OH
                               (10732003).          (131210039).         (261630033).         (390350038).
                              Cuyahoga, OH         Cuyahoga, OH         Hamilton, OH         Hamilton, OH
                               (390350045).         (390350060).         (390610014).         (390610042).
                              Hamilton, OH         Allegheny, PA
                               (390618001).         (420030064).
Indiana.....................  Jefferson, AL        Jefferson, AL        Fulton, GA           Madison, IL
                               (10730023).          (10732003).          (131210039).         (171191007).
                              Wayne, MI            Cuyahoga, OH         Cuyahoga, OH         Cuyahoga, OH
                               (261630033).         (390350038).         (390350045).         (390350060).
                              Hamilton, OH         Hamilton, OH         Hamilton, OH         Allegheny, PA
                               (390610014).         (390610042).         (390618001).         (420030064).
Iowa........................  Madison, IL
                               (171191007).
Kentucky....................  Jefferson, AL        Jefferson, AL        Fulton, GA           Madison, IL
                               (10730023).          (10732003).          (131210039).         (171191007).
                              Wayne, MI            Cuyahoga, OH         Cuyahoga, OH         Cuyahoga, OH
                               (261630033).         (390350038).         (390350045).         (390350060).
                              Hamilton, OH         Hamilton, OH         Hamilton, OH         Allegheny, PA
                               (390610014).         (390610042).         (390618001).         (420030064).
Maryland....................  Allegheny, PA
                               (420030064).
Michigan....................  Madison, IL          Cuyahoga, OH         Cuyahoga, OH         Cuyahoga, OH
                               (171191007).         (390350038).         (390350045).         (390350060).
                              Hamilton, OH         Hamilton, OH         Hamilton, OH         Allegheny, PA
                               (390610014).         (390610042).         (390618001).         (420030064).
Missouri....................  Madison, IL          Cuyahoga, OH         Cuyahoga, OH         Cuyahoga, OH
                               (171191007).         (390350038).         (390350045).         (390350060).