[Federal Register Volume 77, Number 124 (Wednesday, June 27, 2012)]
[Proposed Rules]
[Pages 38399-38420]
[FR Doc No: 2012-15544]

Vol. 77

Wednesday,

No. 124

June 27, 2012

Part II

Environmental Protection Agency

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

40 CFR Part 52

Approval and Promulgation of Implementation Plans; Arizona; Nogales
PM10 Nonattainment Area Plan; Proposed Rule

Federal Register / Vol. 77 , No. 124 / Wednesday, June 27, 2012 /
Proposed Rules

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

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R09-OAR-2012-0458; FRL-9693-6]

Approval and Promulgation of Implementation Plans; Arizona;
Nogales PM10 Nonattainment Area Plan

AGENCY: Environmental Protection Agency.

ACTION: Proposed rule.

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

SUMMARY: EPA is proposing to approve a state implementation plan
revision submitted by the Arizona Department of Environmental Quality
to address the moderate area PM10, particulate matter with
an aerodynamic diameter of less than or equal to a nominal ten
micrometers, planning requirements for the Nogales nonattainment area.
Consistent with this proposal, EPA is also proposing to approve the
following plan elements as meeting the requirements of the Clean Air
Act: the Nogales nonattainment area 2008 and 2011 emission inventories;
the demonstration that the Nogales nonattainment area is attaining the
National Ambient Air Quality Standard for PM10, but for
international emissions sources in Nogales, Mexico; the demonstration
that reasonably available control measures sufficient to meet the
standard have been implemented in the nonattainment area; the
reasonable further progress demonstration; the demonstration that
implementation of measures beyond those needed for attainment meet the
contingency measure requirement; and, the motor vehicle emissions
budget for the purposes of determining the conformity of transportation
plans, programs, and projects with this PM10 plan.

DATES: Written comments must be received on or before July 27, 2012.

ADDRESSES: Submit comments, identified by docket number EPA-R09-OAR-
2012-0458, using one of the following methods: Via the Federal
eRulemaking Portal, at www.regulations.gov, please follow the on-line
instructions; via Email to<EMAIL_ADDRESS>via mail or delivery
to Jerry Wamsley, Air Planning Office, AIR-2, Environmental Protection
Agency (EPA) Region IX, 75 Hawthorne Street, San Francisco, CA 94105-
3901.
    Instructions: All comments will be included in the public docket
without change and may be made available online at www.regulations.gov,
including any personal information provided, unless the comment
includes Confidential Business Information (CBI) or other information
whose disclosure is restricted by statute. Information you consider to
be CBI or otherwise protected should be clearly identified as such and
should not be submitted through www.regulations.gov or email.
www.regulations.gov is an ``anonymous access'' system, and EPA will not
know your identity or contact information unless you provide it in the
body of your comment. If you send an email directly to EPA, your email
address will be automatically captured and included as part of the
public comment. If you submit an electronic comment, EPA recommends
that you include your name and other contact information in the body of
your comment and with any disk or CD-ROM you submit. If EPA cannot read
your comment due to technical difficulties and cannot contact you for
clarification, EPA may not be able to consider your comment. Electronic
files should avoid the use of special characters, any form of
encryption, and be free of any defects or viruses.
    Docket: The index to the docket for this action is available
electronically at www.regulations.gov and in hard copy at EPA Region
IX, 75 Hawthorne Street, San Francisco, California. While all documents
in the docket are listed in the index, some information may be publicly
available only at the hard copy location (e.g., copyrighted material),
and some may not be publicly available at either location (e.g., CBI).
To inspect the hard copy materials, please schedule an appointment
during normal business hours with the contact listed in the FOR FURTHER
INFORMATION CONTACT section.

FOR FURTHER INFORMATION CONTACT: Jerry Wamsley, Air Planning Office,
AIR-2, EPA Region IX, 75 Hawthorne Street, San Francisco, CA 94105-
3901, telephone number:<PHONE_NUMBER>, or email address,
<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION: Throughout this document, wherever ``we'',
``us'' or ``our'' are used, we mean EPA. We are providing the following
outline to help locate information in this proposal.

Table of Contents

I. The PM10 National Ambient Air Quality Standard and the
Nogales PM10 Nonattainment Area
    A. PM10 National Ambient Air Quality Standard
    B. Designation and Classification of PM10 Nonattainment Areas,
Including the Nogales Nonattainment Area
    C. Clean Air Act Plan Requirements for Moderate PM10
Nonattainment Areas
II. Arizona's State Implementation Plan Submittal To Address PM10
Attainment in the Nogales Nonattainment Area
    A. Arizona's Submittal and Clean Air Act Procedural Requirements
    B. Description of the Nogales Nonattainment Area
III. CAA and Regulatory Requirements for Moderate Area
PM10 Attainment Plans and Nonattainment Areas Influenced
by International Transport
    A. Moderate PM10 Area Planning Requirements
    B. Clean Air Act Provisions and EPA Guidance Concerning
International Border Areas
    1. Section 179B of the Clean Air Act
    2. The 1994 General Preamble Addendum
    3. Statutory Requirements and Guidance for Determining
Attainment of the PM10 NAAQS
IV. Review of the Nogales 2012 Plan
    A. Emissions Inventories
    1. Requirements for Emissions Inventories
    2. Review of the Nogales Nonattainment Area Emissions Inventory
    3. Proposed Action on the Nogales Nonattainment Area 2008 and
2011 Emissions Inventories
    B. Section 179B Analysis and Demonstration of Attainment but for
International Sources of PM10 Emissions
    1. Review of Statute and Guidance Applied to the Nogales Section
179B Analysis and Demonstration of Attainment but for International
Sources of PM10 Emissions
    2. Review of Arizona's Section 179B Analysis and Demonstration
of Attainment but for International Sources of PM10
Emissions
    a. Population Growth in the Ambos Nogales Region
    b. Review and Comparison of U.S./Mexico Emission Inventories
    c. Review and Analysis of Regional Meteorology, Topography and
Ambient PM10 Monitoring Data
    (i) Ambos Nogales Regional Meteorology and Topography
    (ii) Ambient PM10 Monitoring Network, Data, Analyses,
and Findings
    d. Findings From Reviews of Emission Inventories, and Studies of
Ambient PM10 Data, and Meteorological Data
    e. Arizona's Demonstration of Attainment for the Nogales
Nonattainment Area but for International Sources of PM10
Emissions
    (i) Daily Analysis To Demonstrate Attainment but for
International Sources of PM10 Emissions
    (ii) Hourly Analysis To Demonstrate Attainment but for
International Sources of PM10 Emissions
    3. Proposed Action on the Nogales Nonattainment Area Section
179B Analysis and Demonstration of Attainment but for International
Sources of PM10 Emissions
    C. Reasonably Available Control Measures (RACM)/Reasonably
Available Control Technology (RACT) and Adopted Control Strategy
    1. Requirement for RACM/RACT
    2. RACM/RACT in the Nogales Nonattainment Area
    D. Reasonable Further Progress Demonstration and Contingency
Measures in the Nogales Nonattainment Area

    1. Reasonable Further Progress
    2. Contingency Measures
    E. Motor Vehicle Emissions Budgets for Transportation Conformity
    1. Requirements for Transportation Conformity
    2. Motor Vehicle Emissions Budget for the Nogales Nonattainment
Area
    3. Proposed Action on the Motor Vehicle Emissions Budget for the
Nogales Nonattainment Area
VI. EPA's Proposed Action and Request for Comment
VII. Statutory and Executive Order Reviews

I. The PM10 National Ambient Air Quality Standard and the
Nogales PM10 Nonattainment Area

A. PM10 National Ambient Air Quality Standard

    The EPA sets the National Ambient Air Quality Standard (NAAQS) for
certain ambient air pollutants at levels required to protect human
health and the environment. Particulate matter with an aerodynamic
diameter less than or equal to a nominal ten micrometers, or
PM10, is one of these ambient air pollutants for which EPA
has established health-based standards. On July 1, 1987, EPA
promulgated two primary standards for PM10: A 24-hour
standard of 150 micrograms per cubic meter ([micro]g/m\3\); and, an
annual PM10 standard of 50 [micro]g/m\3\. EPA also
promulgated secondary PM10 standards that were identical to
the primary standards. 52 FR 24634; (July 1, 1987). Because they are
identical, we refer to the primary and secondary standards using the
singular term, ``standard.'' Effective December 18, 2006, EPA revoked
the annual PM10 standard but retained the 24-hour
PM10 standard. 71 FR 61144; (October 17, 2006).
    An area attains the 24-hour PM10 standard when the
expected number of days per calendar year with a 24-hour concentration
in excess of the standard (referred to herein as an ``exceedance''), is
equal to or less than one,\1\ as determined in accordance with 40 CFR
part 50, appendix K. See 40 CFR 50.6 and 40 CFR part 50, appendix K.
Conversely, a violation of the PM10 NAAQS occurs when the
number of expected annual exceedances of the 24-hour standard is
greater than one.
---------------------------------------------------------------------------

    \1\ An exceedance is defined as a daily value that is above the
level of the 24-hour standard, 150 [micro]g/m\3\, after rounding to
the nearest 10 [micro]g/m\3\ (i.e., values ending in five or greater
are to be rounded up). Thus, a recorded value of 154 [micro]g/m\3\
would not be an exceedance since it would be rounded to 150
[micro]g/m\3\; whereas, a recorded value of 155 [micro]g/m\3\ would
be an exceedance since it would be rounded to 160 [micro]g/m\3\. See
40 CFR part 50, appendix K, section 1.0.
---------------------------------------------------------------------------

B. Designation and Classification of PM10 Nonattainment Areas,
Including the Nogales Nonattainment Area

    Areas meeting the requirements of section 107(d)(4)(B) of the Clean
Air Act (CAA or ``Act'') were designated nonattainment for
PM10 by operation of law and classified ``moderate'' upon
enactment of the 1990 Clean Air Act Amendments. These areas included
all former Group I PM10 planning areas identified in 52 FR
29383, (August 7, 1987), as further clarified in 55 FR 45799, (October
31, 1990), and any other areas violating the NAAQS for PM10
prior to January 1, 1989. A Federal Register notice announcing the
areas designated nonattainment for PM10 upon enactment of
the 1990 Amendments, known as ``initial'' PM10 nonattainment
areas, was published on March 15, 1991, (56 FR 11101); and, a
subsequent Federal Register document correcting the description of some
of these areas was published on August 8, 1991, (56 FR 37654).
    As a former ``Group I'' area, the Nogales nonattainment area (NA)
was included in the March 1991 list of initial moderate PM10
nonattainment areas. Later, we codified the PM10
nonattainment designations and moderate area classifications in 40 CFR
part 81 (56 FR 56694; November 6, 1991). For ``moderate'' nonattainment
areas, such as the Nogales NA, CAA section 188(c) of the 1990 Amended
Act established an attainment date of December 31, 1994. On January 11,
2011, pursuant to section 188(b)(2) of the CAA, we determined that the
Nogales NA met the PM10 NAAQS as of the applicable
attainment date, December 31, 1994. See 76 FR 1532; (January 11, 2011).
The designation, classification, and boundaries of the Nogales NA are
codified at 40 CFR 81.303.

C. Clean Air Act Plan Requirements for Moderate PM10 Nonattainment
Areas

    Along with the new designations, classifications, and attainment
dates, the CAA as amended in 1990 also established new planning
requirements. States were required to develop and submit state
implementation plan (SIP) revisions providing for, among other
elements, implementation of reasonably available control measures
(RACM) for control of PM10, a demonstration that the plan
would provide for attainment by the applicable attainment date
(``attainment demonstration''), and contingency measures, for all
moderate PM10 nonattainment areas. See CAA sections 172(c)
and 189(a). As discussed later, CAA section 179B(a) allows a State to
submit a demonstration that the plan would be adequate to attain and
maintain the standard but for emissions emanating from outside the
United States in lieu of an attainment demonstration. CAA section
179B(a) does not, however, relieve qualifying moderate PM10
nonattainment areas of the other SIP requirements, including but not
limited to RACM and contingency measures.
    In response, on June 14, 1993, the Arizona Department of
Environmental Quality (referred to herein as ``ADEQ,'' ``Arizona,'' or
``the State'') submitted the ``Final State Implementation Plan for the
Nogales PM10 Nonattainment Area,'' June 1993 (``1993 Nogales
PM10 Plan''). The 1993 Nogales PM10 Plan
identifies emissions sources located in Mexico as the principal sources
affecting ambient PM10 concentrations in the area. EPA has
not taken action on the 1993 Nogales PM10 Plan. Today's
action relates to an updated plan for the Nogales PM10
nonattainment area that is intended by ADEQ, once submitted in final
form, to supersede the 1993 Nogales PM10 Plan.

II. Arizona's State Implementation Plan Submittal To Address
PM10 Attainment in the Nogales Nonattainment Area

A. Arizona's Submittal and Clean Air Act Procedural Requirements

    Today's proposed action concerns the Proposed State Implementation
Plan for the Nogales PM10 Nonattainment Area (``Nogales 2012 Plan''),
submitted by ADEQ on May 29, 2012. ADEQ concurrently requested that EPA
``parallel process'' our review and proposed action on the Nogales 2012
Plan addressing the CAA's PM10 moderate area requirements
for the Nogales NA.2 3 We have agreed to parallel process
the Nogales 2012 Plan concurrently with the ADEQ's public hearing and
submittal process using our authority under 40 CFR part 51, appendix V.
ADEQ's parallel processing request and the Nogales 2012 Plan consist of
the following documents:

``Proposed State Implementation Plan for the Nogales PM10
Nonattainment Area'' with Appendices A-J, May 17, 2012. The Nogales
2012 Plan, supporting documents, and public hearing information can
also be found at ADEQ's Web site, http://www.azdeq.gov/environ/air/plan/notmeet.html#nog.
---------------------------------------------------------------------------

    \2\ Under EPA's ``parallel processing'' procedure, EPA proposes
rulemaking action on a proposed SIP revision concurrently with the
State's public review process. If the State's proposed SIP revision
is changed, EPA will evaluate that subsequent change and may publish
another notice of proposed rulemaking. If no significant change is
made, EPA will propose a final rulemaking on the SIP revision after
responding to any submitted comments. Final rulemaking action by EPA
will occur only after the final SIP revision has been fully adopted
by ADEQ and submitted formally to EPA for approval as part of the
Arizona SIP. See 40 CFR part 51, appendix V.
    \3\ Letter from Eric Massey, Director, Air Quality Division,
Arizona Department of Environmental Quality, to Jared Blumenfeld,
Regional Administrator, EPA, dated May 29, 2012.
---------------------------------------------------------------------------

    We have reviewed the ADEQ's May 29, 2012 parallel processing
submittal against the completeness criteria at 40 CFR part 51, appendix
V, section 2.3.1. and find that the submittal is complete. These
completeness criteria are used specifically for parallel processing
submittals. Once we have received ADEQ's supplemental submittal after
the State concludes their public hearing process, we will use the
general completeness criteria at 40 CFR part 51, appendix V, 2.0 to
determine completeness of that submittal. Our completeness finding on
this supplemental submittal will be made as part of our final action on
this proposal.

B. Description of the Nogales Nonattainment Area

    Covering 76.1 square miles, the Nogales NA is located within Santa
Cruz County, Arizona, with the southernmost boundary of the Nogales NA
and Santa Cruz County being the United States (U.S.)/Mexico border.
Adjacent to the U.S./Mexico border, the city of Nogales, Arizona is 60
miles south of Tucson, Arizona. The city of Nogales, Arizona is the
largest city and population center in the Nogales NA.
    The Nogales NA is located within the Sonoran Desert. This desert
covers 120,000 square miles with a minimum elevation of 2,500 feet
above sea level and is in the Basin and Range topographic province.
This topography is characterized by north-south elongated valleys
surrounded by mountain ranges. Nogales is located in such a north-south
valley created by the Nogales Wash running north to the Santa Cruz
River. The mean elevation in Nogales, Arizona is 3,865 feet above sea
level. Major highways in the Nogales, Arizona area are U.S. Interstate
19 which connects Tucson, Arizona to Nogales, Arizona and continues
south into Mexico, where it becomes Federal Highway 15, and Arizona
State Route 82, which connects Nogales, Arizona with Patagonia, Arizona
(19 miles) and Sonoita (31 miles) to the northeast.
    Nogales, Mexico lies directly south of Nogales, Arizona across the
U.S./Mexico border. Taken together and referred to as Ambos Nogales,
the communities of Nogales, Arizona and Nogales, Mexico comprise the
largest international border community in Arizona, with a combined
population of 232,550 inhabitants in 2010, approximately 91 percent of
whom live in Nogales, Mexico.\4\ The mean elevation in Nogales, Mexico
is 4,265 feet above sea level.\5\
---------------------------------------------------------------------------

    \4\ In 2010, Nogales, Arizona had 20,017 inhabitants and
Nogales, Mexico had 212,533 inhabitants. U.S. Census Bureau 2010 and
Instituto Nacional de Estadistica Geografia e Informatica, (INEGI)
2010.
    \5\ ``Statistical Municipal Workbook for Nogales, Sonora,'' 2005
edition, INEGI.
---------------------------------------------------------------------------

III. CAA and Regulatory Requirements for Moderate Area PM10
Attainment Plans and Nonattainment Areas Influenced by International
Transport

A. Moderate PM10 Area Planning Requirements

    The air quality planning requirements for moderate PM10
nonattainment areas are set out in subparts 1 and 4 of the CAA,
including sections 110, 172, and 189 of the statute. These sections
will be discussed further during the review for each plan element,
later in this proposal. Also, we have issued guidance in a General
Preamble describing how we will review state submittals under Title I
of the CAA, including moderate PM10 nonattainment areas. See
57 FR 13498; (April 16, 1992) and 57 FR 18070; (April 28, 1992). In
general, moderate area PM10 plans must include the following
elements: a current, comprehensive emissions inventory of emissions
sources in the nonattainment area; provisions to ensure that reasonably
available control measures and/or reasonably available control
technologies (RACM/RACT) have been implemented in the nonattainment
area; provisions demonstrating attainment of the PM10 NAAQS
with quantitative milestones which show reasonable further progress
(RFP) towards attainment of the NAAQS as expeditiously as practicable;
contingency measures for RFP and attainment; and, a motor vehicle
emissions budget for the purpose of determining the conformity of
transportation programs and plans developed by State transportation
agencies.\6\ Because the Nogales NA lies along the international border
with Mexico, the CAA allows Arizona to submit a demonstration that the
area would have attained the PM10 NAAQS but for
international transport from Mexico in lieu of a demonstration that the
area has attained the PM10 NAAQS. The statutory requirements
and guidance for such a demonstration under section 179B of the CAA are
discussed next. Under CAA section 179B, however, other SIP
requirements, such as RACM and contingency measures, among other
requirements, continue to apply to PM10 nonattainment areas
even if they quality for relief from the attainment demonstration
requirement.
---------------------------------------------------------------------------

    \6\ The Nogales PM10 nonattainment area is subject to
the ``moderate'' area, not the ``serious'' area, SIP planning
requirements under the CAA. This is because the mandatory ``bump-
up'' from ``moderate'' to ``serious'' under CAA section 188(b)(2) is
only triggered if any area fails to attain the standard by the
applicable attainment date (in this case, 1994), and the Nogales
area, which was originally designated nonattainment for
PM10 based on exceedances measured in the late 1980's,
attained the standard by 1994. Several years after 1994, the Nogales
area once again began to experience exceedances but such post-
attainment date exceedances do not trigger the mandatory ``bump-up''
provision in CAA section 188(b)(2). The issue of the applicability
of the ``bump-up'' provision in CAA section 188(b)(2) to the Nogales
area was addressed fully in EPA's final determination that the
Nogales area attained the PM10 standard by the applicable
attainment date. See 76 FR 1532; (January 11, 2011).
---------------------------------------------------------------------------

B. Clean Air Act Provisions and EPA Guidance Concerning International
Border Areas

    Because the southern boundary of the Nogales NA lies along the
international border with Mexico and transport of PM10
emissions from Mexico affects air quality in Nogales, Arizona, there
are specific statutory requirements in the CAA that apply to the
Nogales NA. With a demonstration from Arizona showing that the Nogales
NA would have attained the PM10 NAAQS, but for international
sources of PM10, EPA may approve an attainment plan provided
by the State, even if the attainment plan does not demonstrate
attainment of the NAAQS. The PM10 attainment plan, however,
must meet other requirements of the CAA, contingent upon meeting the
NAAQS but for international transport. Such a ``but for'' attainment
demonstration, however, must be consistent with statutory and
regulatory requirements. First, we will review the statutory basis for
a ``but for'' attainment demonstration. Secondly, we will review EPA's
published guidance on how such an analysis may be structured. Lastly,
we will review how EPA determines whether an area's air quality is
meeting the PM10 NAAQS using air quality data gathered at
monitoring sites in the nonattainment area and our application of 40
CFR part 50, appendix K.
1. Section 179B of the Clean Air Act
    For international border areas like the Nogales NA, CAA section
179B(a) provides that notwithstanding any other provision of law, an
implementation plan or plan revision shall be approved by the
Administrator if such plan or

revision meets all the requirements applicable to it other than a
requirement that such plan or revision demonstrate attainment and
maintenance of the relevant national ambient air quality standards by
the attainment date specified under the applicable provision, or in a
regulation promulgated under such provision, and the submitting State
establishes to the satisfaction of the Administrator that the
implementation plan of such State would be adequate to attain and
maintain the relevant national ambient air quality standards by the
attainment date specified under the applicable provision, or in a
regulation promulgated under such provision, but for emissions
emanating from outside of the United States.
    As stated above, notwithstanding any other provision of law, should
Arizona establish to the satisfaction of the EPA Administrator that the
Nogales NA would have attained the PM10 NAAQS by the
applicable attainment date but for emissions emanating from outside the
U.S., then the Nogales NA is not subject to the provisions of CAA
section 189(a)(1)(b), requiring a demonstration of attainment of the
PM10 standards by the applicable attainment date.\7\ The
underlying purpose of section 179B is to balance the requirements of
the CAA in nonattainment areas adjacent to international borders
affected by transport of pollution from foreign sources with the
consideration that the State does not have the jurisdiction to control
these foreign sources of pollution affecting attainment of the NAAQS in
that State.
---------------------------------------------------------------------------

    \7\ As discussed earlier, we determined that the Nogales NA met
the PM10 NAAQS as of the applicable attainment date for
moderate nonattainment areas, December 31, 1994; consequently, we
did not reclassify the area to ``serious.'' See 76 FR 1532; (January
11, 2011).
---------------------------------------------------------------------------

2. The 1994 General Preamble Addendum
    As part of guidance relating to serious PM10
nonattainment areas (General Preamble Addendum), EPA included a
discussion of the requirements applicable to international border
areas.\8\ The General Preamble Addendum reviews the information and
methods that may be used to determine if an international border area
qualifies for treatment under CAA section 179B and to demonstrate that
the area would attain the relevant NAAQS but for emissions emanating
from outside the U.S.
---------------------------------------------------------------------------

    \8\ ``State Implementation Plans for Serious PM10
Nonattainment Areas, and Attainment Date Waivers for PM10
Nonattainment Areas Generally; Addendum to the General Preamble for
the Implementation of Title I of the Clean Air Act Amendments of
1990,''; 59 FR 41998, August 16, 1994.
---------------------------------------------------------------------------

    The General Preamble Addendum provides that ``several types of
information may be used to evaluate the impact of emissions emanating
from outside the U.S.'' The EPA will consider the information ``for
individual nonattainment areas on a case-by-case basis in determining
whether an area may qualify for treatment under section 179B.'' See 59
FR 42001; (August 16, 1994). The General Preamble Addendum suggests
five methods that may be used to determine the impact of emissions
emanating from outside the U.S. Below, we describe the five methods in
general terms and later, when reviewing Arizona's section 179B analysis
and demonstration, we will discuss the particular applicability of
these five methods to the analysis done for the Nogales NA.
    Method 1. Place several ambient PM10 monitors and a
meteorological station measuring wind speed and direction in the U.S.
nonattainment area near the international border. Evaluate and quantify
any changes in monitored PM10 concentrations with a change
in the predominant wind direction.
    Method 2. Comprehensively inventory PM10 emissions
within the U.S. in the vicinity of the nonattainment area and
demonstrate that those sources, after application of reasonably
available controls, do not cause the NAAQS to be exceeded. This
analysis must include an influx of background PM10 in the
area. Background PM10 levels could be based on
concentrations measured in a similar area not influenced by emissions
from outside the U.S.
    Method 3. Analyze ambient sample filters for specific types of
particles emanating from across the border. Although not required,
characteristics of emissions from sources may be helpful so as to
better demonstrate the causal relationship with and contribution to
exceedances in the U.S. nonattainment area due to domestic and
international emissions.
    Method 4. Inventory the sources on both sides of the border and
compare the magnitude of PM10 emissions originating within
the U.S. to those emanating from outside the U.S.
    Method 5. Perform air dispersion and/or receptor modeling to
quantify the relative impacts on the nonattainment area of sources
located within the U.S., and of foreign sources of PM10
emissions.
    As stated in the General Preamble Addendum, the EPA will consider
the information ``for individual nonattainment areas on a case-by-case
basis in determining whether an area may qualify for treatment under
section 179B.'' Because the individual circumstances surrounding a
nonattainment area may differ widely whether by data, resources, or
emissions sources, EPA anticipates that ``the State may use one or more
of these types of information or other techniques, depending on their
feasibility and applicability, to evaluate the impact of emissions
emanating from outside the U.S. on the nonattainment area.'' See 59 FR
42001; (August 16, 1994). Therefore, the analysis Arizona has provided
for the Nogales NA is specific to this nonattainment area only and the
timeframe, data, and circumstances therein, and EPA is evaluating the
analysis as such.
    As explained earlier, the underlying purpose of section 179B is to
balance the requirements of the CAA in nonattainment areas adjacent to
international borders affected by transport of pollution from foreign
sources with the consideration that the State does not have the
jurisdiction to control these foreign sources of pollution affecting
attainment of the NAAQS in that State. In this light, the General
Preamble Addendum discusses several attainment plan requirements as
applied to nonattainment areas affected by international transport.
    The 1994 General Preamble Addendum discusses the requirements for
RACM as applied to nonattainment areas affected by international
transport. In international border areas, ``RACM/RACT must be
implemented to the extent necessary to demonstrate attainment by the
applicable attainment date if emissions emanating from outside the U.S.
were not included in the analysis.'' See 59 FR 42001; (August 16,
1994). As set forth in section 179B(a)(2), a State's moderate area
PM10 plan must be ``adequate'' to attain and maintain the
PM10 NAAQS, but for emissions from outside the U.S.
Therefore, nothing in section 179B relieves a State from the
requirement to address and implement RACM. Nonetheless, States are not
required to implement control measures that go beyond what the plan
demonstrates would otherwise be adequate for timely attainment and
maintenance of the PM10 NAAQS but for emissions from outside
the U.S. Furthermore, to the degree that the State can satisfactorily
demonstrate that implementation of a control measure clearly would not
advance the area's attainment date, EPA may conclude that these control
measures are unreasonable and do not constitute RACM for the
nonattainment area. See 59 FR 42001; (August 16, 1994).

    The 1994 General Preamble Addendum also discusses the requirements
for reasonable further progress (RFP) and contingency measures as
applied to nonattainment areas affected by international transport.
Section 179B(a)(1) does not relieve a nonattainment area of the CAA
requirements for RFP and contingency measures. In international border
areas, however, ``EPA will not require the contingency measures for
PM10 to be implemented after the area fails to attain if EPA
determines that the area would have attained the NAAQS, but for
emissions emanating from outside the U.S.'' Conversely, to the degree
that contingency measures are needed to control U.S. sources of
PM10 to meet RFP or attainment contingency measure
requirements but for PM10 emissions emanating from outside
of the U.S., then the statutory requirements for RFP and contingency
measures still apply. See 59 FR 42001, 42002; (August 16, 1994).
3. Statutory Requirements and Guidance for Determining Attainment of
the PM10 NAAQS
    EPA determines whether an area's air quality is meeting the
PM10 NAAQS based upon air quality data gathered at
monitoring sites in the nonattainment area. Then, EPA reviews the data
to determine the area's air quality status according to 40 CFR part 50,
appendix K. Three consecutive years of clean air quality data (i.e., no
more than one expected exceedance per year) is generally needed to show
attainment of the 24-hour PM10 standard. As defined by 40
CFR part 50, appendix K, a complete year of air quality data is
composed of all four calendar quarters with each quarter containing
data from at least 75 percent of the scheduled sampling days.
    Under 40 CFR part 50, appendix K, a nonattainment area meets the
24-hour PM10 NAAQS when the expected number of days per
calendar year with a 24-hour average concentration above 150 micrograms
per cubic meter ([mu]g/m\3\) is equal to or less than one. In general,
the number of expected exceedances at a site which samples every day is
determined by recording the number of exceedances in each calendar year
and then averaging them over the most recent three calendar years. For
sites which do not sample every day, EPA requires adjusting the
observed exceedances to account for days not sampled. The procedures
for making this data adjustment are specified in 40 CFR part 50,
appendix K.
    For this review of the Nogales NA and the contribution of
international emissions, the standard we will use to demonstrate
attainment of the PM10 NAAQS, ``but for'' international
emissions, is similar to the one described above: The expected number
of days per calendar year with a 24-hour average concentration above
150 [mu]g/m\3\ must be equal to or less than one. To demonstrate that
the Nogales NA has met the PM10 standard ``but for''
emissions from Mexico, the State's analysis must show that no more than
three exceedances, based on data completeness and every day sampling,
over the specific three-year analysis period, would have occurred on
the U.S. side of the border, setting aside any contributions from
Mexican sources of PM10.

IV. Review of the Nogales 2012 Plan

    In this section, according to the statutory requirements and
guidance discussed above in section III, we will review Arizona's
submitted Nogales 2012 Plan and section 179B analysis and demonstration
that the Nogales NA is attaining the PM10 NAAQS but for
international emissions sources from Nogales, Mexico.

A. Emissions Inventories

1. Requirements for Emissions Inventories
    Section 172(c)(3) of the CAA requires plan submittals to include a
comprehensive, accurate, and current inventory of actual emissions from
all sources in the nonattainment area.
2. Review of the Nogales Nonattainment Area Emissions Inventories
    Arizona submitted emissions inventories for the Nogales NA for the
years 2008 and 2011. These emissions inventories were calculated using
information from version 1.5 of EPA's 2008 National Emission Inventory
(NEI) and the NEI emissions estimates for Santa Cruz County, Arizona. A
Nogales NA 2008 emissions inventory was scaled from the larger Santa
Cruz County emissions inventory using a combination of population and
land allocation ratios. A specific point source's location was the
basis for assigning point sources to the Nogales NA emissions
inventory. On-road motor vehicle PM10 emissions for 2008 and
2011 were calculated using County-level data for 2008 and 2011 and the
MOVES2010a model.\9\ The larger and remaining portions of the 2011
emissions inventory, particularly area sources, were calculated from
the 2008 emissions inventory according to estimates of population and
economic growth. An overview of the Nogales NA 2008 and 2011 emissions
inventories is provided here; for detailed results and a complete
discussion of the methodology used to produce the emission inventories,
see ``PM10 Emission Inventories for 2008 and 2011, Nogales
Non-Attainment Area, Santa Cruz County, Arizona'', in Appendix B of the
Nogales 2012 Plan.
---------------------------------------------------------------------------

    \9\ On March 2, 2010, EPA approved the availability of the Motor
Vehicle Emissions Simulator model (MOVES2010a) in official SIP
submissions to EPA regarding air quality and for certain
transportation conformity analyses outside the state of California;
see 75 FR 9411. Also see EPA's Web site for more information, http://www.epa.gov/otaq/models/moves/index.htm.
---------------------------------------------------------------------------

    EPA's NEI database contains information about sources that emit
criteria air pollutants and their precursors, and hazardous air
pollutants. The database includes estimates of annual air pollutant
emissions, including PM10, from point, nonpoint, and mobile
sources in the 50 states, including Arizona, and specifically Santa
Cruz County. Collaborating with the states, EPA develops the emissions
inventory and releases an updated version of the NEI database every
three years. A complete description of the development of the 2008 NEI
may be found at the following URL: http://www.epa.gov/ttn/chief/net/2008inventory.html.
    In calculating PM10 emissions from on-road mobile
sources in Santa Cruz County, Arizona used the MOVES2010a version dated
September 23, 2010 (hereafter referred to as ``MOVES''). This is the
current version of the MOVES model. MOVES allows the use of county-
specific data concerning factors such as the average speed distribution
of on-road vehicles, daily vehicle miles traveled, and road types among
others in place of national default values. The MOVES model requires
the use of county-specific data for SIP purposes. In this instance, the
MOVES calculation was performed using input data from the 2008 NEI for
Santa Cruz County. Similar MOVES model runs were completed to estimate
2011 on-road mobile source PM10 emissions.
    Although EPA has no specific guidance on assigning emissions
sources from a county level of analysis to a smaller area within that
county, for the Nogales NA emissions inventory, Arizona used a
combination of population ratios, land area ratios, and point source
locations within the Nogales NA to determine the appropriate allocation
of county-wide emissions to the Nogales NA. See Table

1 for the specific population and land allocation ratios used to scale
PM10 emissions from the County to the Nogales NA level.

                       Table 1--Summary of Land Area and 2008 Population Allocation Ratios
----------------------------------------------------------------------------------------------------------------
                                                                                Allocation
                                                                   Santa Cruz       Nogales NA         ratio
                                                                     County                          (percent)
----------------------------------------------------------------------------------------------------------------
Land Area (square miles)......................................     \10\ 1,237.6             76.1            6.15
2008 Population...............................................    \11\ 43,091      \12\ 23,735             55.1
----------------------------------------------------------------------------------------------------------------

    The State used data from the U.S. Census Bureau to estimate the
2008 population of the Nogales NA population and Santa Cruz County. A
land area-weighted emission ratio was developed using U.S. Census
geographic data and confirmed with Arizona Commerce Authority data.\13\
Some source categories, such as agricultural emissions, are likely to
be proportional to land area; consequently, they are logically
allocated by the land area ratio. To confirm whether specific point
sources in the Santa Cruz County emissions inventory should be included
in the Nogales NA emissions inventory, ADEQ and EPA used visual
inspections with location information, such as satellite photography
using Google Earth.
---------------------------------------------------------------------------

    \10\ U.S. Census, Quickfacts, Santa Cruz County, Arizona.
    \11\ 2010 U.S. Census population estimates.
    \12\ Ibid.
    \13\ Arizona Department of Commerce Profile: Santa Cruz County
Arizona, May 10, 2011, http://www.azcommerce.com/doclib/commune/SantaCruzpercent20county.pdf.
---------------------------------------------------------------------------

    As shown in Table 2, in 2008, the majority of PM10
emissions in the Nogales NA came from fugitive dust from four source
categories: Unpaved road dust, road construction, commercial/
industrial/institutional construction, and paved road dust. The
estimated emissions inventory for 2011 only differed slightly as total
emissions decreased from 1,524 tons per year (tpy) in 2008 to 1,521 tpy
in 2011, due primarily to implementation of new and cleaner engine
standards for diesel engines. Little or no growth in population or
economic activity occurred from 2008 to 2011. From 2008 to 2011, the
emissions estimated for five of the top six source categories remain
unchanged, except for residential wood burning which increased by two
tons per year. Again, in 2011 as in 2008, these six source categories
account for approximately 95 percent of all PM10 emissions
in the Nogales NA.

                          Table 2--2008 and 2011 Nogales NA PM10 Emissions Inventories
                                                 [Tons per year]
----------------------------------------------------------------------------------------------------------------
                               Source category                                      2008              2011
----------------------------------------------------------------------------------------------------------------
Dust--Unpaved Road Dust.....................................................               865               865
Dust--Road Construction.....................................................               267               267
Dust--Commercial/Industrial/Institutional Construction......................               143               143
Dust--Paved Road Dust.......................................................               121               121
Fuel Combustion--Residential--Wood..........................................                24                26
Dust--Residential Construction..............................................                24                24
Waste Disposal--Residential Garbage Burning.................................                23                25
All other sources...........................................................                57                50
                                                                             -----------------------------------
    Total...................................................................             1,524             1,521
----------------------------------------------------------------------------------------------------------------
Note: All other sources include emissions from source categories such as all on-road mobile and off-road mobile,
  all commercial and industrial fuel combustion, agriculture, land clearing and burning activities.
Source: Table 5 in ``PM10 Emission Inventories for 2008 and 2011, Nogales Non-Attainment Area, Santa Cruz
  County, Arizona,'' Appendix B of the Nogales 2012 Plan. Table 5 also provides a detailed listing of all source
  categories. Due to rounding, totals may not reflect exactly the sum of each source category.

3. Proposed Action on the Nogales Nonattainment Area 2008 and 2011
Emissions Inventories
    We propose to find that the Nogales NA emissions inventories for
2008 and 2011 are comprehensive, accurate, and current inventories of
actual emissions from all sources in the nonattainment area and that
they meet the requirements of section 172(c)(3) of the CAA. The State
has provided a 2008 base year and 2011 future year emissions inventory
comprehensively addressing all source categories in the Nogales NA. The
State also used the most recent iteration of mobile source emissions
modeling tool, MOVES2010a, in developing its emissions inventories.
Consequently, we are proposing to find that the emissions inventories
provided by Arizona meet the requirements of section 172(c)(3) and
provide an adequate basis for the attainment demonstration under
section 179B, and the State's RACM/RACT and RFP demonstrations.

B. Section 179B Analysis and Demonstration of Attainment but for
International Sources of PM10 Emissions

1. Review of Statute and Guidance Applied to the Nogales Section 179B
Analysis and Demonstration of Attainment but for International Sources
of PM10 Emissions
    As discussed earlier, the General Preamble Addendum provides that
``several types of information may be used to evaluate the impact of
emissions emanating from outside the U.S.'' The EPA will consider the
information ``for individual nonattainment areas on a case-by-case
basis in determining

whether an area may qualify for treatment under section 179B.'' See 59
FR 42001; (August 16, 1994). The General Preamble Addendum suggests
five methods that may be used to determine the impact of emissions
emanating from outside the U.S. and explains that ``the State may use
one or more of these types of information or other techniques,
depending on their feasibility and applicability, to evaluate the
impact of emissions emanating from outside the U.S. on the
nonattainment area.'' See 59 FR 42001; (August 16, 1994). Below, we
discuss these five methods for evaluating the effects from transport of
international pollution and the applicability of these methods to the
Nogales NA, as presented in the Nogales 2012 Plan.
    Method 1. Place several ambient PM10 monitors and a
meteorological station measuring wind speed and direction in the U.S.
nonattainment area near the international border. Evaluate and quantify
any changes in monitored PM10 concentrations with a change
in the predominant wind direction.
    The State reviewed the ambient PM10 data, meteorology,
and topography in the Ambos Nogales area. Arizona maintains a monitor
in Nogales, Mexico, as well as three monitors in Nogales, Arizona. The
Nogales, Arizona monitors are divided as follows: Two monitors measure
ambient PM10 levels; and one monitor measures ambient
PM2.5 levels.\14\ Arizona also has two reference monitors at
increasing distances from the Nogales NA. Arizona's complete analysis
of the ambient data, meteorology, and topography is provided in
Appendix D of the Nogales 2012 Plan and is discussed below in section
IV.B.2.c of this proposal. This method provided useful information to
understand emissions sources and PM10 concentrations in the
Nogales NA.
---------------------------------------------------------------------------

    \14\ PM2.5, also called fine particulate, refers to
particulate matter with an aerodynamic diameter less than or equal
to 2.5 micrometers. PM10 includes both PM2.5
and the particulates with aerodynamic diameter between 2.5 and 10
micrometers, which is referred to as PM10-2.5. This
larger fraction is called ``coarse'' particulate. While fine
particles originate mostly from combustion sources and secondary
aerosol generation processes, coarse particles usually originate
from mechanical activities and fugitive source categories.
---------------------------------------------------------------------------

    Method 2. Comprehensively inventory PM10 emissions
within the U.S. in the vicinity of the nonattainment area and
demonstrate that those sources, after application of reasonably
available controls, do not cause the NAAQS to be exceeded. This
analysis must include an influx of background PM10 in the
area. Background PM10 levels could be based on
concentrations measured in a similar area not influenced by emissions
from outside the U.S.
    This method implies the use of an air quality model to demonstrate
that emissions within the U.S. do not create a violation of the NAAQS.
Although a comprehensive, area-wide inventory of PM10
emissions is available for Nogales, Arizona, information about the
spatial and temporal distribution of those emissions required to
support air quality modeling is not readily available and would require
significant effort to develop. Furthermore, given the complex
topography of the Ambos Nogales area, it is not feasible to develop an
adequate demonstration using available modeling tools.
    Method 3. Analyze ambient sample filters for specific types of
particles emanating from across the border. Although not required,
characteristics of emissions from foreign sources may be helpful so as
to better demonstrate the causal relationship with and contribution to
exceedances in the U.S. nonattainment area due to international
emissions.
    This method is unlikely to produce useful information for the
Nogales NA because the large proportion of crustal PM sources on either
side of the international border far outweigh any specific stationary
or combustion-based PM source that could be identified by a filter-
based analysis, and differentiating between Arizona and Mexican sources
of crustal material is not feasible. Also, specific local and
international point source emissions information, such as source-
specific signature emissions compounds, was not available with which to
correlate the filter analyses results.
    Method 4. Inventory the sources on both sides of the border and
compare the magnitude of PM10 emissions originating within
the U.S. to those emanating from outside the U.S.
    Arizona provided two emissions inventories: The first emissions
inventory, discussed above, describes the PM10 sources and
estimates PM10 emissions in and around the Nogales NA,
Arizona; and, the second inventory describes the PM10
sources and estimates PM10 emissions in and around Nogales,
Mexico. The Nogales NA PM10 emissions inventory is provided
in Appendix B and the Nogales Municipality, Mexico emissions inventory
is provided in Appendix C of the Nogales 2012 Plan. The results of both
inventories are discussed below in section IV.B.2.b. of this proposal.
Also, as a basis for these analyses, Arizona reviewed population
estimates and relative population differences for these areas, which is
further discussed in section IV.B.2.a. of this proposal.
    Method 5. Perform air dispersion and/or receptor modeling to
quantify the relative impacts on the nonattainment area of U.S. and
foreign sources of PM10 emissions.
    As discussed above, the information necessary to support air
dispersion or receptor modeling is not readily available for the
Nogales, Arizona area, nor is it available for the Nogales, Mexico
area. For example, neither ADEQ, nor EPA, had available a gridded
emissions inventory or a data set from an extensive monitoring array of
ambient PM10 values and meteorological data derived from
observations on multiple exceedance days.
    Backward wind trajectory analysis using the HYSPLIT model was
considered, based on Eta Data Assimilation System (EDAS) gridded
meteorological data, but again, neither Arizona nor EPA pursued this
analysis.\15\ Previously, EPA performed such an analysis for the
Nogales, Arizona area and found the resulting wind trajectories to be
inconclusive. The EDAS has a 40-kilometer grid resolution; in contrast,
the valley containing Nogales is 20 kilometers wide at its widest
point. As a result, the EDAS data were not of a fine enough resolution
to portray the south-to-north valley air drainage flows that are a key
feature of local Nogales meteorology; consequently, further use of
HYSPLIT model results for purposes of this section 179B analysis was
rejected by the State and EPA.
---------------------------------------------------------------------------

    \15\ HYSPLIT is the ``Hybrid Single Particle Lagrangian
Integrated Trajectory'' Model, developed and maintained by the
National Oceanic and Atmospheric Administration; see
www.arl.noaa.gov/HYSPLIT_info.php for more information.
---------------------------------------------------------------------------

    To summarize, the State analyzed ambient PM10 levels in
and around the Nogales NA, the local meteorology associated with
exceedances of the PM10 standards, and sources of
PM10 emissions on either side of the international border.
These analyses are consistent with Methods 1 and 4 described by the
General Preamble Addendum. The State examined method 3, but did not
pursue this avenue of investigation because it was unlikely that
definitive results could be produced given the large crustal source
emissions on either side of the international border.
    Initially, the State did not pursue Methods 2 and 5 because it did
not have the data and the models required for this type of analysis.
Instead, the State used the available information consistent with
methods 1 and 4, to

demonstrate if the Nogales NA would have attained the standard, but for
international emissions.
    As stated in the General Preamble Addendum, EPA will consider the
information ``for individual nonattainment areas on a case-by-case
basis in determining whether an area may qualify for treatment under
section 179B.'' See 59 FR 42001; (August 16, 1994). Because the
individual circumstances surrounding a nonattainment area may differ
widely whether by data, resources, or emissions sources, EPA
anticipates that ``the State may use one or more of these types of
information or other techniques, depending on their feasibility and
applicability, to evaluate the impact of emissions emanating from
outside the U.S. on the nonattainment area.'' See 59 FR 42001; (August
16, 1994). The analysis the State has provided for the Nogales NA is
specific to this nonattainment area only and the timeframe, data, and
circumstances therein, and EPA evaluated the analysis as such.
2. Review of Arizona's Section 179B Analysis and Demonstration of
Attainment but for International Sources of PM10 Emissions
a. Population Growth in the Ambos Nogales Region
    In producing emissions inventories, Arizona reviewed recent 2010
population information from the U.S. Census Bureau and Mexican Census
data from the Instituto Nacional de Estadistica Geografia e Informatica
(INEGI). While population estimates, by themselves, are not direct
indicators of emissions activity, they provide an indication of
relative human activity and resulting PM10 emissions on
either side of the international border. Table 3 provides a comparison
of the populations residing in the Nogales NA and the Nogales
Municipality, Mexico. The Nogales NA population estimate includes
persons residing in the city of Nogales, Arizona, and the surrounding
community of Rio Rico within the Santa Cruz County portion of the
nonattainment area.

 Table 3--2010 Population: Nogales NA, Arizona and Nogales Municipality,
                                 Mexico
------------------------------------------------------------------------
                  Area                      Population        Percent
------------------------------------------------------------------------
Nogales NA, Arizona.....................          24,059             9.8
Nogales Municipality, Mexico............         220,292            90.2
                                         -------------------------------
    Total...............................         244,351             100
------------------------------------------------------------------------
Source: INEGI & U.S. Census.

    Although the Nogales Municipality is a larger land area than the
Nogales NA, a large proportion of the Municipality's population is
concentrated within the city of Nogales, Mexico and the surrounding
area. In sum, 90.2 percent of the 2010 population in the Ambos Nogales
area can be attributed to the Mexican side of the international border.
    It is also instructive to examine population change since 1995,
when the Nogales NA met the PM10 NAAQS along with the
subsequent observed exceedances of the PM10 NAAQS.\16\ Table
4 shows population estimates for 1995, 2000, 2005, and 2010, while
Table 5 shows the annual number of expected exceedances of the
PM10 NAAQS since 1998, the first year the Nogales NA
recorded exceedances after meeting the PM10 standard in
1994. The Nogales NA did not record exceedances of the PM10
standard from 1995 to 1997.
---------------------------------------------------------------------------

    \16\ See 76 FR 1532; (January 11, 2011) for our determination
that the Nogales NA attained the PM10 NAAQS by December
31, 1994.

     Table 4--Nogales, Arizona and Nogales Municipality, Mexico Populations: 1995, 2000, 2005 and 2010 \17\
----------------------------------------------------------------------------------------------------------------
                                                       1995            2000            2005            2010
----------------------------------------------------------------------------------------------------------------
Nogales, Arizona................................          20,184          20,878          20,421          20,837
Nogales Municipality, Mexico....................         133,491         159,787         193,517         220,292
----------------------------------------------------------------------------------------------------------------
Source: INEGI & U.S. Census.

    Between  1995 and 2010, Nogales, Arizona population increased
approximately three percent, and has fallen slightly since 2000. The
2010 Nogales NA population at 24,059 persons is marginally larger than
the city of Nogales because the nonattainment area estimate includes
portions of the Rio Rico communities in the northernmost portion of the
nonattainment area. In contrast, the Nogales Municipality, Mexico
population has increased 65 percent in the 1995 to 2010 timeframe. With
the exceptions of 2000 and 2004, exceedances of the PM10
standard have been recorded since 1998 in the Nogales NA. The largest
number of expected exceedances, 47.9, was recorded in 2006. See Table
5.
---------------------------------------------------------------------------

    \17\ The 1995 Nogales, Arizona population estimate was
interpolated from 1990 and 2000 U.S. Census figures; the 1990
population estimate was 19,489.

                                   Table 5--Nogales, Arizona Expected Exceedances of 24-Hour PM10 NAAQS From 1998-2010
--------------------------------------------------------------------------------------------------------------------------------------------------------
                Monitor frequency                  1998    1999    2000    2001    2002    2003    2004    2005    2006    2007    2008    2009    2010
--------------------------------------------------------------------------------------------------------------------------------------------------------
1 in 6 day......................................    13.5    15.5     0.0     6.9     6.1    12.3     0.0    17.9    20.0     6.1     6.6     0.0     0.0
Continuous......................................  ......  ......  ......  ......  ......  ......  ......    29.6    47.9    14.0    13.2     2.0   * 8.5
--------------------------------------------------------------------------------------------------------------------------------------------------------
* There were no quarters in 2010 where there was a complete data set per 40 CFR part 50, appendix K; see section IV.B.2.c. for a discussion of 2010
  data.
Source for expected exceedance data: EPA Air Quality System Database.

    To summarize, population estimates since 1995 show the Nogales NA
population remaining relatively constant while the Nogales
Municipality, Mexico population has steadily increased to the present
where 9 of 10 people in the Ambos Nogales area reside in Mexico. Over
the same timeframe, after attaining the PM10 NAAQS in 1994
through 1997, expected exceedances of the PM10 NAAQS in the
Nogales NA increased to a high of 47.9 in 2006 and the area does not
meet the NAAQS today. The dramatic differential population increase in
Nogales, Mexico compared to Nogales, Arizona and the surrounding
nonattainment area supports the inference that a large and growing
proportion of PM10 emissions in the Ambos Nogales area
emanates from outside of the Nogales NA and the U.S.
b. Review and Comparison of U.S./Mexico Emissions Inventories
    Both the Nogales NA and the Nogales Municipality, Mexico have
similar contributing sources of PM10, primarily fugitive
dust from unpaved and paved roads, as well as combustion sources and
construction. The Nogales NA emissions inventories were presented above
in section IV.A.2 of this proposal. While less detailed than the
Nogales NA emissions inventories, the Nogales Municipality, Mexico
emissions inventories shows that the largest contributing sources of
PM10 emissions are from unpaved and paved road dust followed
by residential wood combustion and other area sources. Because Nogales
Municipality, Mexico specific data could not be found to calculate
unpaved and paved road emissions, the State reviewed other U.S./Mexico
border emissions inventories to identify data for use in these
calculations. Given the range of data generated and used by these U.S./
Mexico border emissions inventories, low and high estimates were
calculated for the unpaved and paved road source categories. Much of
the difference between the low and high estimates of Nogales
Municipality emissions is attributed to the low and high estimates of
unpaved and paved road emissions. A high estimate for point sources was
included because the State did not have readily available source-
specific information providing a precise estimate for stationary point
sources of PM10 in the Nogales Municipality, Mexico.\18\ The
methods for calculating these estimates are discussed in ``2008 and
2011 PM10 Emission Inventories, Nogales Municipality,
Sonora, Mexico'' in Appendix C of the Nogales 2012 Plan. The Nogales
Municipality, Mexico emissions inventories for 2008 and 2011 are
presented in Table 6.
---------------------------------------------------------------------------

    \18\ Two methods were used to scale point source emissions from
1999 to 2008 and 2011 generating the high and low estimates for
point source PM10: For the low estimate, National point
source emissions growth; and, for the high estimate, population
based allocation ratio. The starting 1999 baseline for point source
emission was 0.9 tpy and the high estimate, therefore, assumes an
increase of three orders of magnitude compared to the low estimate.
No point sources in the Nogales Municipality, Mexico have been
identified as operating at a level of emissions consistent with the
high estimate, but lacking source specific data to adjudicate the
difference in estimates, the high estimate was reported as an upper
bound. See Appendix C of the Nogales 2012 Plan for the Nogales
Municipality Emissions Inventory for a complete discussion.

             Table 6--PM10 Emissions Inventories for Nogales Municipality, Mexico for 2008 and 2011
                                                 [Tons per year]
----------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------
Source category                                                        Range              2008          2011
----------------------------------------------------------------------------------------------------------------
Point Sources.......................  ......................  Low Estimate..........           1.1           1.1
                                                              High Estimate.........           305           390
Area Sources........................  Unpaved Road..........  Low Estimate..........         2,144         2,308
                                                              High Estimate.........         5,521         5,944
                                      Paved Road............  Low Estimate..........            53            57
                                                              High Estimate.........           646           696
                                      Agricultural Tilling..  ......................           0.8           0.8
                                      Agricultural Burning..  ......................           1.6           1.6
                                      Residential Wood        ......................           176            47
                                       Combustion.
                                      Open Burning of Waste.  ......................            55            56
                                      Construction            ......................            23            24
                                       Activities.
                                      Remaining Area Sources  ......................           159           150
Mobile Sources......................  ......................  ......................            80            85
Nonroad Sources.....................  ......................  ......................            20            27
                                                                                ---------------------------
    Total...........................  ......................  Low Estimate..........         2,713         2,757
                                                                                ---------------------------
    Total...........................  ......................  High Estimate.........         6,987         7,420
----------------------------------------------------------------------------------------------------------------
Emissions are rounded to the nearest ton/year, or to the nearest tenth of a ton/year for emissions less than 10
  tons/year.
Source: Table 18 from ``2008 and 2011 p.m.10 Emission Inventories, Nogales Municipality, Sonora, Mexico'' in
  Appendix C of the Nogales 2012 Plan.

    A review of the emissions inventory data by relative percentage and
relative ratio provides two ways of considering the data. A comparison
of 2008 and 2011 Nogales Municipality, Mexico low emission inventory
estimates with the Nogales NA 2008 and 2011 emission inventory
estimates shows a 36/64 percent split in total combined U.S./Mexico
emissions inventories between emissions from the Nogales NA, Arizona
and Nogales Municipality, Mexico areas, respectively. To characterize
the relative difference by ratio using the low emissions estimate for
the Nogales Municipality, Mexico, for every one ton of PM10
emissions produced annually in Nogales NA, there is an estimated 1.8
tons produced in Nogales Municipality. Similarly, a comparison of 2008
and 2011 Nogales Municipality high emission inventory estimates
suggests that there is an 18/82 percent split in total combined U.S./
Mexico emissions inventories between emissions from the Nogales NA,
Arizona and Nogales Municipality, Mexico areas,

respectively. Again, to characterize the relative difference by ratio
using the high emissions estimate for the Nogales Municipality, Mexico,
for every one ton of PM10 emissions produced annually in
Nogales NA, there is an estimated 4.6 tons produced in Nogales
Municipality, Mexico.\19\
---------------------------------------------------------------------------

    \19\ See Tables 6-9 from ``Clean Air Act, Section 179B
Attainment Determination for the Nogales, Arizona PM10
Nonattainment Area'' in Appendix A of the Nogales 2012 Plan for the
presentation of the data underlying this relative percentage and
relative ratio presentation.
---------------------------------------------------------------------------

    In summary, a comparison of the State's 2008 and 2011 emissions
inventory data shows for every one ton of PM10 produced in
the Nogales NA, there was between 1.8 and 4.6 tons of PM10
emissions produced annually in the Nogales Municipality, Mexico,
depending on the choice of either the low or the high estimate of
Nogales Municipality, Mexico emissions. The emission sources appear to
be similar, with the majority of emissions from fugitive dust sources,
such as reentrained unpaved and paved road dust.
c. Review and Analysis of Regional Meteorology, Topography and Ambient
PM10 Monitoring Data
    In its review of the ambient PM10 data, meteorological
data, and through its analyses, Arizona found that the Ambos Nogales
area's meteorology and topography influence the observed exceedances of
PM10 NAAQS and there is a definite south-to-north
directional component to the ambient air quality data underlying the
exceedances of the PM10 NAAQS. Over the 2007-2009 timeframe,
there were 29 exceedances at the Nogales, Arizona Post Office (Model:
Met One BAM 1020) monitor.\20\
---------------------------------------------------------------------------

    \20\ For a listing of the 29 exceedance days by year and
observed 24-hour concentrations, see Tables 1-3 in ``Analysis of
Ambient PM10 Levels, Topography, and Meteorological Data
in Nogales, Arizona: 2007-2009'' in Appendix D of the Nogales 2012
Plan.
---------------------------------------------------------------------------

(i) Ambos Nogales Regional Meteorology and Topography
    The State's analysis of ambient concentration and meteorological
data identified 26 of the 29 exceedances as having nearly identical
diurnal patterns; the three exceptions were January 1, 2007, May 22,
2008, and January 1, 2009.\21\ For each of the 26 days, there is a
strong pattern of decreasing PM10 concentrations in the
early morning. Generally, the wind speeds are low and variable
overnight and wind direction starts southerly but becomes increasingly
variable into the daylight morning hours. The majority of days have a
pronounced PM10 increase and drop-off between 6:00 a.m. and
9:00 a.m., suggesting a reproducible direct PM10 source,
noting the times correspond to a morning commute pattern. The
PM10 concentrations reach their lowest points between 10:00
a.m. and 4:00 p.m., with corresponding increases in ambient temperature
and wind speed observed during those times. Usually, northerly winds
accompany these increases in temperature and wind speed. As
temperatures and wind speeds drop in the evening hours, a pronounced
spike in PM10 concentration is then observed beginning
between 4:00 p.m. and 6:00 p.m., with concentrations remaining high for
several hours and gradually dropping off towards midnight. The
afternoon spike in PM10 concentrations correlates with a
significant drop in temperature and wind speed, and generally a shift
to low and variable southerly (from the south) winds.
---------------------------------------------------------------------------

    \21\ See, in particular, Section 3 of ``Analysis of Ambient
PM10 Levels, Topography, and Meteorological Data in
Nogales, Arizona: 2007-2009'', in Appendix D of the Nogales 2012
Plan.
---------------------------------------------------------------------------

    Looking at the topography from south to north, the highest
elevation of a primary roadway transect is at 4,331 feet above sea
level at the southern edge of Nogales, Mexico, falling to the
international border at 3,933 feet, continuing to the northern edge of
the Nogales NA at 3,425 feet, and elevation continues to fall along the
Santa Cruz River watershed to the north to approximately 3,100
feet.\22\ Across this largest 48.5-mile local transect, the elevation
falls approximately 1,200 feet from south to north, i.e., from Nogales,
Mexico, through the Nogales NA, and to the north towards Tucson,
Arizona.
---------------------------------------------------------------------------

    \22\ See Figure 18, Long Aerial and Elevation Transect of
Nogales Arizona and Nogales, Sonora, in ``Analysis of Ambient
PM10 Levels, Topography, and Meteorological Data in
Nogales, Arizona: 2007-2009'', in Appendix D of the Nogales 2012
Plan.
---------------------------------------------------------------------------

    In examining a smaller 14.8-mile transect along a similar primary
roadway route, the State found that elevation declines on a south-to-
north axis across two sub-transects centering on the international
border. The Nogales, Mexico sub-transect shows an elevation drop of 201
feet over 4.8 miles to the international border where there is a slight
leveling; starting at 4,134 feet above sea level at the Nogales, Mexico
urban boundary and dropping to 3,933 feet at the international border.
The Nogales, Arizona sub-transect shows an elevation drop of 508 feet
over 10 miles, from the international border to the northern boundary
of the Nogales NA; starting at 3,933 feet and dropping to 3,425
feet.\23\ In sum, looking at a south-to-north transect along the
Nogales Wash, elevations fall from south to north with the highest
elevations occurring in the Nogales, Mexico area. Looking at the
general topography of the Ambos Nogales area from a northwest
perspective in Arizona to the southeast into Mexico, there is a funnel
created as the Nogales Wash falls from higher southern elevations to
the international border along the route of the Alvaro Obreg[oacute]n
Boulevard and into Nogales, Arizona.\24\ Small side canyons extend off
of the Nogales Wash bottom and into the surrounding hills between the
international border and south of the Nogales, Mexico city center, and
to a lesser extent into Nogales, Arizona as elevations drop moving to
the north.
---------------------------------------------------------------------------

    \23\ See Figure 19, Short Aerial and Elevation Transect of
Nogales, Arizona and Nogales, Sonora, from ``Analysis of Ambient
PM10 Levels, Topography, and Meteorological Data in
Nogales, Arizona: 2007-2009'', in Appendix D of the Nogales 2012
Plan.
    \24\ See Figure 17, Elevated Topographical View of Ambos Nogales
Area from Northwest Perspective with Nogales, Sonora Highlighted and
International Border in Red Line, from ``Analysis of Ambient
PM10 Levels, Topography, and Meteorological Data in
Nogales, Arizona: 2007-2009'', in Appendix D of the Nogales 2012
Plan.
---------------------------------------------------------------------------

(ii) Ambient PM10 Monitoring Network, Data, Analyses, and
Findings
    As suggested by method 1 from the General Preamble Addendum, the
State analyzed hourly observations of PM10 concentrations,
wind direction, wind speed and temperature.\25\ First, we will provide
an overview and review of the Nogales, Arizona monitoring network.
Second, we will examine the State's review of the ambient
PM10 data for 2007-2009. Finally, we will review the
findings from the State's analyses of the ambient PM10 and
meteorological data.
---------------------------------------------------------------------------

    \25\ Observations of PM10 concentrations, wind
direction, wind speed and temperature were taken at the Nogales,
Arizona Post Office site; hourly temperature observations were taken
at the Nogales International Airport, 7.6 miles from the Nogales
Post Office monitoring site and within the Nogales NA.
---------------------------------------------------------------------------

    Ambient PM10 and Meteorological Monitoring Network. There are five
ambient air monitors in the vicinity of Ambos Nogales that the State
considered for this analysis.\26\ Within the nonattainment area, the
Nogales, Arizona Post Office is the primary violating monitor location
for PM10. Arizona operates two PM10 monitors
there, along with a PM2.5 monitor. The Nogales, Arizona Post
Office monitoring site is 0.3 miles north of the border and this
monitoring site is 0.9 miles northeast of the Nogales, Mexico Fire

Station monitoring site. The Green Valley and Corona de Tucson
monitoring sites are approximately 35 and 45 miles away from the U.S./
Mexico border, respectively. The Nogales Post Office and the Nogales,
Mexico Fire Station monitors are operated by ADEQ. The Corona de Tucson
and the Green Valley monitors, located near Tucson, Arizona, are
operated by the Pima County Department of Environmental Quality (PDEQ).
---------------------------------------------------------------------------

    \26\ These monitors are described in detail in Section 2 of
``Analysis of Ambient PM10 Levels, Topography, and
Meteorological Data in Nogales, Arizona: 2007-2009'', in Appendix D
of the Nogales 2012 Plan. Also, see Figure 2 of the same document
for a map of their locations.
---------------------------------------------------------------------------

    Also, Arizona operates a meteorological data collection station at
the Nogales, Arizona Post Office monitoring site. Wind speed
observations discussed in its analyses were collected at that location.
Temperature observations were collected at the Nogales International
Airport, located approximately six miles northeast of the Nogales,
Arizona Post Office monitoring site and within the nonattainment area.
    EPA performed independent Technical System Audits (TSAs) of ADEQ's
ambient air monitoring program in December 2004, September 2009, and
April 2012 and TSAs of PDEQ's ambient monitoring program in June 2008
and September 2011, per requirements in 40 CFR part 58, appendix A,
section 2.5.\27\ We assessed ADEQ and PDEQ's compliance with
established regulations governing the collection, analysis, validation,
and reporting of ambient air quality data and concluded that ADEQ and
PDEQ have a robust ambient air monitoring program, with an appropriate
quality system in place for collecting ambient air monitoring data. EPA
reviewed and subsequently approved the 2011 ADEQ annual monitoring
network plan on December 1, 2011.\28\ We found that ADEQ's 2011
monitoring network plan was complete and met the requirements for
annual network plans described in 40 CFR 58.10.
---------------------------------------------------------------------------

    \27\ See EPA's ``Arizona Department of Environmental Quality
Technical System Audit'' final October 2005; ``Technical System
Audit Report, Arizona Department of Environmental Quality, Air
Quality Division, Ambient Air Quality Monitoring Program,'' final
September 2010; and ``Pima County Department of Environmental
Quality, Technical System Audit'' final February 2009. Final reports
for the April 2012 TSA of ADEQ and September 2011 TSA of PDEQ are
not yet complete.
    \28\ See ADEQ's ``State of Arizona Air Monitoring Network Plan
For the Year 2011, Final Report'' dated August 2, 2011 and EPA's
approval letter from Matthew Lakin, Manager of EPA Region 9's Air
Quality Analysis Office, to Eric Massey, Director of the Air Quality
Division of Arizona Department of Environmental Quality, dated
December 1, 2011.
---------------------------------------------------------------------------

    Ambient PM10 Data for 2007-2009. The 24-hour PM10 NAAQS
is based on the number of expected exceedances greater than 150 [mu]g/
m\3\ averaged over three years.\29\ For this analysis, the State
considered the most recent and most complete three-year data range
available: 2007-2009. There was a large period of missing data at the
Nogales, Arizona Post Office PM10 federal equivalency method
(FEM)/special purpose monitor between March 16 and October 27, 2010.
Consequently, we concur with the State that 2007 to 2009 is the most
appropriate timeframe for this section 179B analysis and attainment
demonstration. At the Nogales, Arizona Post Office monitors,
PM10 data completeness for each quarter within the 2007-2009
timeframe is greater than 75 percent.
---------------------------------------------------------------------------

    \29\ The NAAQS for all pollutants can be found at www.epa.gov/air/criteria.html.
---------------------------------------------------------------------------

    In the 2007-2009 period, there were 29 exceedances at the Nogales,
Arizona Post Office, FEM/special purpose monitor.\30\ \31\ Of those
exceedances, 14 occurred in 2007, 13 in 2008, and two in 2009. Twenty-
seven of the twenty-nine exceedances were observed in the October
through March annual timeframe. Twenty-four hour PM10
concentrations on exceedance days varied between 155 and 238 [mu]g/
m\3\, with some hourly measurements reaching 900 [mu]g/m\3\. Arizona
has not flagged any of these 2007, 2008, or 2009 exceedance days for
potential exclusion from air quality planning considerations under
EPA's Exceptional Events Rule.\32\ The State focused on the data from
the Nogales, Arizona Post Office FEM/Met One BAM 1020 monitor for the
following reasons: it is comparable to the NAAQS; it has recorded all
the exceedances in the area; it has recorded hourly ambient values;
and, it has a sufficiently complete dataset for comparison to the
NAAQS.
---------------------------------------------------------------------------

    \30\ This monitor is formally designated as AQS ID: 04-023-0004,
POC 3.
    \31\ For a list of the 29 exceedance days by year and observed
24-hour concentrations at all five Nogales area monitors, see Tables
1-3 in ``Analysis of Ambient PM10 Levels, Topography, and
Meteorological Data in Nogales, Arizona: 2007-2009'' in Appendix D
of the Nogales 2012 Plan.
    \32\ For the Exceptional Events Rule see ``Treatment of air
quality monitoring data influenced by exceptional events''; 40 CFR
50.14.
---------------------------------------------------------------------------

    The State did not use 2010 and 2011 data for its detailed
meteorological analysis and attainment demonstration for two reasons.
First, the 2010 dataset did not meet the completeness criteria
specified in 40 CFR part 50, appendix K; no quarter in 2010 had
complete data. This was due to a large data gap from March 16 to
October 27 resulting from poor quality assurance and control results.
Second, at the time of this analysis, the 2011 dataset had yet to be
entered completely into the EPA's Air Quality System (AQS) database and
certified by Arizona. As stated earlier, a complete year of air quality
data, as defined by 40 CFR part 50, appendix K, comprises all four
calendar quarters with each quarter containing data from at least 75
percent of the scheduled sampling days. While the 2010 and 2011 ambient
data do not provide the basis for the State's attainment demonstration,
the State examined this data and found no information to contradict its
conclusions using the 2007-2009 data set.\33\
---------------------------------------------------------------------------

    \33\ See Section 4.5 in ``Analysis of Ambient PM10
Levels, Topography, and Meteorological Data in Nogales, Arizona:
2007-2009'', in Appendix D of the Nogales 2012 Plan.
---------------------------------------------------------------------------

    The State reviewed the 2010 and 2011 data to see how ambient
PM10 levels compared to the 2007-2009 dataset. In 2010, the
Nogales, Arizona Post Office (FRM/Met One BAM 1020) monitor recorded
six exceedances of the 24-hour PM10 NAAQS; these 24-hour
average ambient values ranged from 159 [mu]g/m\3\ to 191 [mu]g/m\3\.
There was one exceedance of the PM10 standard in 2011.
Arizona has not flagged any of these 2010 or 2011 exceedances for
potential exclusion from air quality planning considerations under
EPA's Exceptional Events Rule.
    Analyses of 2007-2009 Ambient PM10 Data, Meteorological Data and
Findings. To understand and characterize the ambient PM10
data and meteorological data from the Nogales NA on the 29 exceedance
days chosen for this analysis, the State conducted two initial studies:
an examination of hourly ambient PM10 concentrations, hourly
wind speed observations, and hourly temperatures; and, several analyses
of hourly wind direction observations and hourly ambient
PM10 concentrations.
    The first study of hourly observations of ambient PM10
concentrations, wind speeds, and temperatures on the 29 exceedance days
involved line plots of these three variables over the 24 hour
exceedance day.\34\ These line plots showed a relatively tight grouping
among the three subject variables across 29 exceedance days except for
three days that were distinct from the rest. The line plot of hourly
PM10 concentrations versus time of day for all exceedance
days identified January 1, 2007, May 22, 2008, and January 1, 2009 as
having a significantly different diurnal pattern.\35\ The remaining 26
of

the 29 observed exceedances have nearly identical diurnal patterns.\36\
Line plots of hourly wind speed versus time of day for all exceedance
days show wind speeds were eight miles per hour (mph) or below for all
exceedance days, with the exception of May 22, 2008, when elevated wind
speeds were observed.\37\ Line plots of hourly temperatures versus time
of day for all exceedance days show a distinct diurnal heating and
cooling pattern with no particular day deviating substantially from the
others.\38\
---------------------------------------------------------------------------

    \34\ See Figures 4, 5, 6, and 7 in ``Analysis of Ambient
PM10 Levels, Topography, and Meteorological Data in
Nogales, Arizona: 2007-2009'', in Appendix D of the Nogales 2012
Plan.
    \35\ See Figure 4 in ``Analysis of Ambient PM10
Levels, Topography, and Meteorological Data in Nogales, Arizona:
2007-2009'', in Appendix D of the Nogales 2012 Plan.
    \36\ See Figure 5 in ``Analysis of Ambient PM10
Levels, Topography, and Meteorological Data in Nogales, Arizona:
2007-2009'', in Appendix D of the Nogales 2012 Plan.
    \37\ See Figure 6 in ``Analysis of Ambient PM10
Levels, Topography, and Meteorological Data in Nogales, Arizona:
2007-2009'', in Appendix D of the Nogales 2012 Plan.
    \38\ See Figure 7 in ``Analysis of Ambient PM10
Levels, Topography, and Meteorological Data in Nogales, Arizona:
2007-2009'', in Appendix D of the Nogales 2012 Plan.
---------------------------------------------------------------------------

    In a second set of analyses of ambient PM10
concentrations and wind direction on exceedance days, the State found
that high PM10 concentrations are associated with wind
direction from a southerly quadrant, or southerly air flows, more often
than what is typically observed on non-exceedance days. Also, the State
found that the largest number of hourly ambient values above 150 [mu]g/
m\3\ and the highest ambient values, including those markedly above 150
[mu]g/m\3\, originated from a southerly wind direction quadrant.\39\
These observations suggest a greater influence on ambient
PM10 concentrations from sources in Mexico during these
hours of southerly wind direction.
---------------------------------------------------------------------------

    \39\ Throughout these analyses and this document, the term
``southerly wind direction quadrant'' refers to wind originating
from between 135 and 224 degrees on a compass rose. Similarly, the
term ``all other wind direction quadrants'' refers to the remaining
270 degrees of wind direction between 225 and 134 degrees on a
compass rose.
---------------------------------------------------------------------------

    Beginning with wind rose analyses, the State determined that the
prevailing wind direction was from the south, and to a lesser degree,
from the west southwest directions on non-exceedance days, but almost
primarily from the south on exceedance days.\40\ Following with
pollution rose studies that link hourly ambient PM10
concentration and wind direction observations, these studies showed a
significant percentage of values greater than 150 [mu]g/m\3\
originating from the southerly wind direction quadrant.\41\ A
presentation of the Figure 11 pollution rose data in tabular form is
provided in Table 7. The largest proportion of hourly values above 150
[mu]g/m\3\ and the highest hourly concentrations were found in the
southerly wind direction quadrant. When ambient PM10 values
above 150 [mu]g/m\3\ were sorted by 100 [mu]g/m\3\ increments to 550
[mu]g/m\3\ and greater, the analysis showed that within each increment
above 150 [mu]g/m\3\, 71 to 92 percent of the ambient PM10
observations were from the southerly wind quadrant. Again, these
observations suggest a greater influence on ambient PM10
concentrations from sources in Mexico during these hours of southerly
wind direction.
---------------------------------------------------------------------------

    \40\ See Figures 8 and 9 in ``Analysis of Ambient
PM10 Levels, Topography, and Meteorological Data in
Nogales, Arizona: 2007-2009'', in Appendix D of the Nogales 2012
Plan.
    \41\ See Figures 11 and 12 in ``Analysis of Ambient
PM10 Levels, Topography, and Meteorological Data in
Nogales, Arizona: 2007-2009'', in Appendix D of the Nogales 2012
Plan.

                    Table 7--Hourly Ambient PM10 Concentrations Sorted by Concentration and Wind Direction, 2007-2009 Exceedance Days
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                               Range of ambient concentration values (microgram/m\3\)
                                                          ----------------------------------------------------------------------------------------------
                 Wind direction quadrant                                                                                Share of all
                                                               <150       150-250      250-350      350-450      450-550       >=550      wind direction
                                                            (percent)    (percent)    (percent)    (percent)    (percent)    (percent)     observations
--------------------------------------------------------------------------------------------------------------------------------------------------------
Northerly NW to NNE......................................           27            6            3            3            3            0               17
Easterly NE to ESE.......................................           15           16           16           11            3            8               14
Southerly SE to WSW......................................           41           71           72           84           92           92               57
Westerly SW to WNW.......................................           18            6            8            3            3            0               12
                                                          ----------------------------------------------------------------------------------------------
    Total................................................          100          100          100          100          100          100              100
--------------------------------------------------------------------------------------------------------------------------------------------------------
Source: Table 11 in ``Clean Air Act, Section 179B Attainment Determination for the Nogales PM10 Nonattainment Area'' in Appendix A of the Nogales 2012
  Plan.

    Finally, in a third analysis, the State examined the wind direction
and hourly PM10 concentrations on each exceedance day to
determine two average ambient values for each exceedance day: one value
for the southerly wind quadrant and a second value representing all
other wind direction quadrants.\42\ The results showed that two of the
29 exceedance days, January 1, 2007 and January 26, 2008, have an
average ambient concentration greater than 150 [mu]g/m\3\ for the ``all
other wind direction'' quadrants. The ratio of the southerly quadrant
concentration to the ``all other direction'' quadrant concentration
ranges from 0.86 to one to 11 to one, with an average ratio value of
3.83 to one. Only one day, January 1, 2007, has a ratio value less than
1.0 to one; i.e., the ``all other direction'' quadrants' share exceeds
the southerly quadrant share. This analysis also suggests a greater
influence on ambient PM10 concentrations from sources in
Mexico during these hours of southerly wind direction.
---------------------------------------------------------------------------

    \42\ See Table 12 for all estimated values on all exceedance
days in ``Clean Air Act, Section 179B Attainment Determination for
the Nogales PM10 Nonattainment Area'' in Appendix A of
the Nogales 2012 Plan.
---------------------------------------------------------------------------

    To summarize, the State analyzed hourly ambient concentrations on
exceedance days and found that high PM10 concentrations are
associated with wind direction from a southerly quadrant, or southerly
air flows, more often than what is typically observed on non-exceedance
days. The State found that the largest number of hourly ambient values
above 150 [mu]g/m\3\ and the highest ambient values, including those
markedly above 150 [mu]g/m\3\, originated from a southerly wind
direction quadrant. These studies of hourly ambient data confirm these
general findings; however, the January 1, 2007 and January 26, 2008
exceedance days may be exceptions. Also, due to the differing
meteorology exhibited on May 22, 2008 and January 1, 2009, these days
are marked for further study. All four of these exceedance days are
reviewed and discussed further, below.

d. Findings From Reviews of Emission Inventories, and Studies of
Ambient PM10 Data, and Meteorological Data
    From the State's analyses, the Nogales NA emissions inventories,
the Nogales Municipality, Mexico emissions inventories, and the 2007-
2009 ambient data and meteorological analyses, the State made the
findings listed below.
     The majority of exceedances, 79 percent, occurred in the
October to January timeframe, mostly in November.\43\ Also, given the
high desert environment and winter light regime, temperatures usually
drop dramatically, 20 degrees Fahrenheit over the 3-4 hours after
sunset.\44\
---------------------------------------------------------------------------

    \43\ See Figure 3 in ``Analysis of Ambient PM10
Levels, Topography, and Meteorological Data in Nogales, Arizona:
2007-2009'' in Appendix D of the Nogales 2012 Plan.
    \44\ See Figures 7 and 14 in ``Analysis of Ambient
PM10 Levels, Topography, and Meteorological Data in
Nogales, Arizona: 2007-2009'' in Appendix D of the Nogales 2012
Plan.
---------------------------------------------------------------------------

     From the Nogales NA and Nogales Municipality, Mexico
emission inventories, the State estimated pollution loads may differ by
a ratio of 1.8 (low estimate)--4.6 (high estimate) to one on a south-
to-north basis in relation to the international border.
     The largest sources of PM10 emissions in the
Ambos Nogales area are reentrained dust from unpaved and paved roads.
     Overall, elevations drop approximately 709 feet across the
entire south-to-north local transect, from the southernmost edge of the
Nogales, Mexico urban boundary to the Nogales NA northern boundary
line.
     Of the 29 exceedance days in 2007-2009, 26 of those days
showed a similar pattern of ambient PM10 concentrations,
wind speeds, wind direction, and temperature variation over a 24-hour
period; the three exceptions were January 1, 2007, May 22, 2008, and
January 1, 2009.
     On exceedance days, the largest proportions, 71-92
percent, of hourly values exceeding 150 [mu]g/m\3\ and almost all of
the highest observed PM10 concentrations of observations
above 450 [mu]g/m\3\, 92 percent, are associated with a southerly wind
direction quadrant.\45\
---------------------------------------------------------------------------

    \45\ See Table 11 above. For a visual representation of this
data, see the pollution roses in Figures 11 and 12, ``Analysis of
Ambient PM10 Levels, Topography, and Meteorological Data
in Nogales, Arizona: 2007-2009'' in Appendix D of the Nogales 2012
Plan.
---------------------------------------------------------------------------

     The ambient PM10 concentration attributed to
the southerly wind quadrant exceeds 150 [mu]g/m\3\ on all 29 exceedance
days. In contrast, two exceedance days from the ``all other wind
direction'' quadrants show a value greater than 150 [mu]g/m\3\: January
1, 2007, and January 26, 2008.
     Only one of 29 exceedance days shows the concentration
attributed to the ``all other wind direction'' quadrants greater than
that of the concentration attributed to the southerly wind quadrant:
January 1, 2007.
     On exceedance days, the average ratio of the southerly
wind quadrant share of 24-hour ambient PM10 values to all
other wind quadrants share of ambient values is 3.83 to one. This ratio
is relatively consistent with the estimated pollution loads ratio of
1.8-4.6 to one, from south-to-north across the international border.
This comparison of the hourly ambient PM10 value/wind
direction ratio and the pollution load ratios suggests that the
pollution load ratios and the low and high emissions inventory
estimates are both conservatively low and high estimates of ambient
conditions.

Upon review of the ambient PM10 data, meteorology, and the
State's analyses, we concur with the State's findings listed above.
e. Arizona's Demonstration of Attainment for the Nogales Nonattainment
Area but for International Sources of PM10 Emissions
(i) Daily Analysis to Demonstrate Attainment but for International
Sources of PM10 Emissions
    As described above, 26 of the 29 2007-2009 exceedances showed a
similar pattern of ambient PM10 concentrations, wind speeds,
wind direction, and temperature variation over a 24-hour period; the
exceptions were January 1, 2007, May 22, 2008, and January 1, 2009. Two
of these days, January 1, 2007, and January 1, 2009, with higher early
morning PM10 concentrations, only vary from the diurnal
profile of PM10 concentrations observed for the other
exceedances, but have similar meteorological and concentration patterns
throughout the rest of the day. Two of the 29 exceedance days, January
1, 2007, and January 26, 2008, had high average ambient concentrations
during hours when the wind was out of directions other than the south.
Thus, there are 25 exceedance days that are equivalent and can be
considered as a group, setting aside the dissimilar exceedance days
listed above, January 1, 2007, January 26, 2008, May 22, 2008, and
January 1, 2009.
    A Conceptual Model of 2007-2009 Exceedance Days. Considering these
25 similar exceedance days, the State explained how the elements of
pollution loads and sources, temperature changes, and wind direction
may contribute to producing the majority of observed ambient
PM10 values exceeding the NAAQS in Nogales, Arizona.\46\ The
data concerning January 1, 2007, January 26, 2008, May 22, 2008, and
January 1, 2009 are reviewed later in more detail in this daily
analysis.
---------------------------------------------------------------------------

    \46\ For a graphical depiction of the interplay between ambient
PM10 concentrations, wind speed, and temperatures
described by the conceptual model, see Figure 3 in ``Clean Air Act,
Section 179B Attainment Determination for the Nogales
PM10 Nonattainment Area'' in Appendix A of the Nogales
2012 Plan. As explained in the footnote to Figure 3, although the
diurnal emissions pattern of the January 26, 2008 exceedance day is
very similar to the 25 exceedance days summarized by the conceptual
model other parts of the discussion may not be consistent with the
observed data from January 26, 2008.
---------------------------------------------------------------------------

    Within the cited Figure 3, the State shows the average
PM10 concentration, wind speed, and temperature across 26
similar exceedance days and including 25 of those days in the
conceptual model. The 24-hour pattern of these variables on these 25
days is similar. Beginning at midnight, the data indicate that there is
a strong pattern of decreasing PM10 concentrations from the
previous day's high values into the early morning hours. Then, there is
a pronounced PM10 increase and drop-off between 6:00 a.m.
and 9:00 a.m., suggesting a regularly occurring direct PM10
source, such as reentrained road dust from the morning commute. As
morning temperatures rise, so does wind speed as wind direction changes
from south to north dispersing the spike in morning PM10
concentrations. The PM10 concentrations continue to fall
through the afternoon and reach their lowest points between 10:00 a.m.
and 4:00 p.m. The morning and afternoon increases in ambient
temperature and wind speed can be attributed to the heating portion of
a diurnal heating and cooling cycle where heated air flows from lower
elevations in the north to the higher elevations in the south.
    On the 25 days, the meteorological and ambient concentration data
also provide an explanation for regularly occurring increases in
PM10 concentrations during the evening hours. As sunset
approaches and night falls, the diurnal cooling cycle begins. Ambient
temperatures drop and lower elevation air masses no longer rise with
convection, causing wind speed to drop and wind direction to be
variable. As temperatures continue to drop after sunset, wind speeds
drop and cold air masses flow downslope from higher elevations, causing
wind direction to shift from a variable/northerly direction to a
southerly direction. A pronounced spike in PM10
concentration is then

observed beginning between 4:00 p.m. and 6:00 p.m.; roughly
corresponding with the evening commute hours. Concentrations remain
high for several hours into the evening and gradually begin to decrease
as midnight approaches. The highest concentrations of PM10
occur in these evening hours when reentrained dust from unpaved and
paved roads may be captured by cold air flows moving south to north
from higher to lower elevations (later in the discussion this
phenomenon is referred to as ``downslope air flows''). Also, home
heating combustion may add a component to the evening PM10
load and also be captured in the evening southerly and downslope air
flows from Nogales, Mexico into Nogales, Arizona.
    This pattern of exceedances is usually observed during times when
the general weather pattern allows for stagnation and a relatively
still air mass subject to movement by the diurnal cooling and heating
cycle. At other times of the year, frontal systems move through often
enough and with enough energy to prevent a stagnant air mass in the
Ambos Nogales region and this diurnal heating and cooling cycle exerts
less influence on the local meteorology.
    The conceptual model the State has presented to explain the
exceedances in the Nogales NA is consistent with the study by Arizona
State University, ``Atmospheric, Hydroclimatic, and Anthropogenic
Causes of Fugitive Dust in the Nogales, Arizona-Nogales, Sonora
Airshed.'' \47\ In this study--based on a regression analysis of 815
daily PM10 observations at Nogales, Arizona, and 457 daily
PM10 observations at Nogales, Mexico, and other
information--the authors conclude that stagnant atmospheric conditions
over a large scale (i.e., a stagnant synoptic atmosphere) is the most
important factor in predicting high daily PM10
concentrations.
---------------------------------------------------------------------------

    \47\ Completed in 2002 by A.W. Ellis, the final report is
available through The Southwest Center for Environmental Research
and Policy at http://scerpfiles.org/cont_mgt/doc_files/A-02-2.pdf.
---------------------------------------------------------------------------

    For the 25 similar days examined by ADEQ, the ambient
PM10 concentration attributed to the southerly wind
direction quadrant always exceeds the 150 [mu]g/m\3\ level, in most
cases markedly.\48\ Conversely, the ambient concentration attributed to
the ``all other wind direction'' quadrants never exceeds the 150 [mu]g/
m\3\ level. Across all 25 days, the average of the hourly monitored
PM10 concentration values for the hours with a southerly
wind direction ranges from 163 to 369 [mu]g/m\3\ for each of the days,
with an average value across the 25 days of 264 [mu]g/m\3\. In
comparison, the average of the hourly concentration values for all
other wind direction quadrants ranges from 38 to 148 [mu]g/m\3\ for
each of the days, with an average value across the 25 days of 80 [mu]g/
m\3\. This suggests that emissions sources to the south in Mexico are
contributing significantly to those hourly ambient concentrations and
the resulting 24-hour average concentrations.
---------------------------------------------------------------------------

    \48\ For the estimated values providing the basis for the
conceptual model's 25 exceedance day values discussed in this
paragraph, see Table 12 in ``Clean Air Act, Section 179B Attainment
Determination for the Nogales PM10 Nonattainment Area''
in Appendix A of the Nogales 2012 Plan.
---------------------------------------------------------------------------

    In sum, for 25 of the 29 exceedance days, the State provided a
conceptual model explaining how exceedances of the PM10
NAAQS occur in the Nogales NA. Moreover, for all of these 25 days, the
origin and contribution of PM10 to exceedances of the
standard at the Nogales, Arizona Post Office monitor has a very large
southerly component. Given the wind direction, the proximity of the
monitor to the border, and the comparison of the magnitude of emissions
on either side of the border, the majority of the emissions that result
in these 25 exceedances most likely originate from the Nogales, Mexico
side of the international border.
    Analysis of Four Days Differing From Conceptual Model: January 1,
2007; January 26, 2008; May 22, 2008; and, January 1, 2009. The
conceptual model of Mexican influence on Nogales NA PM10
concentrations described above fits the observations on 25 of the 29
exceedance days in 2007-2009. The State identified four specific
exceedance days that differ in one or more ways from the 25-day
conceptual model of PM10 exceedances in the Nogales NA:
January 1, 2007, May 22, 2008, January 26, 2008, and January 1, 2009.
See Table 8 for more information.

Table 8--24-hour PM10 Concentration ([mu]g/m\3\) and Hourly Concentration Averages ([mu]g/m\3\) Disaggregated by
              Southerly Wind Direction Quadrant for Exceedance Days Differing From Conceptual Model
----------------------------------------------------------------------------------------------------------------
                                                             Southerly wind quadrant    All other wind direction
                  Date                        24-hour         (135 to 224 degrees)        (225 to 134 degrees)
                                           concentration      average concentration      average concentration
----------------------------------------------------------------------------------------------------------------
January 1, 2007........................               210  199 (15 of 24 values).....  231 (9 of 24 values).
January 26, 2008.......................               204  257 (7 of 24 values)......  182 (17 of 24 values).
May 22, 2008...........................               217  217 (24 of 24 values).....  No Observed Values.
January 1, 2009........................               238  323 (14 of 24 values).....  119 (10 of 24 values).
----------------------------------------------------------------------------------------------------------------
Data Source: Air Quality System database; and, Table 4.2 in Nogales 2012 Plan.

    The State examined each of these days in further detail to evaluate
the influences on the high ambient PM10 values that occurred
on those days and to determine whether the four remaining exceedance
days--January 1, 2007, January 26, 2008, May 22, 2008, and January 1,
2009--should be assigned to the category of exceedance days having a
significant contribution from emission sources originating from the
Nogales, Mexico side of the international border. The State's analysis
is summarized below.
    January 1, 2007 Exceedance Day Review. Considering the January 1,
2007 exceedance day, it differs from the conceptual model average
exceedance day in the timing and distribution of observed ambient
PM10 values and high PM2.5 component most likely
caused by a combustion source.\49\ The PM10:
PM2.5 ratio for January 1, 2007 is the lowest in the 29-day
sample (1.49 to 1). What differs in the case of the January 1, 2007
exceedance is that the 270 degree wind direction quadrants contain
enough high values to contribute disproportionately to the overall 24-
hour average concentration. Although more detailed and different field
studies might prove otherwise, with the information available, the
State's analysis is inconclusive as to whether this exceedance is
attributable to a disproportionate international

contribution and the Nogales NA would not have exceeded the 24-hour
PM10 standard but for Mexican emissions.\50\
---------------------------------------------------------------------------

    \49\ For the complete discussion of coarse versus fine
particulate matter on all exceedance days, see Section 4.4 and Table
8 in ``Analysis of Ambient PM10 Levels, Topography, and
Meteorological Data in Nogales, Arizona: 2007-2009'' in Appendix D
of the Nogales 2012 Plan.
    \50\ For a detailed review of the January 1, 2007 exceedance
day, see Section 4.2.1 of ``Clean Air Act, Section 179B Attainment
Determination for the Nogales PM10 Nonattainment Area''
in Appendix A of the Nogales 2012 Plan.
---------------------------------------------------------------------------

    January 26, 2008 Exceedance Day Review. The State's review of the
January 26, 2008 exceedance day suggests that this day is most like the
conceptual model average exceedance day in the timing and distribution
of observed ambient PM10 values. While the southerly wind
direction quadrant contains enough high values to contribute
disproportionately to the overall 24-hour average concentration, there
are enough remaining high values in the 17 of 24 hourly observations
from the 270 degree wind direction quadrants to be above the 150 [mu]g/
m\3\ level. Again, while specifically designed field studies might help
clarify the relative contributions to this exceedance, with the
information available, the State's analysis is inconclusive as to
whether this exceedance is attributable to a disproportionate
international contribution and the Nogales NA would not have exceeded
the 24-hour PM10 standard but for Mexican emissions.\51\
---------------------------------------------------------------------------

    \51\ For a detailed review of the January 26, 2008 exceedance
day, see Section 4.2.2 of ``Clean Air Act, Section 179B Attainment
Determination for the Nogales PM10 Nonattainment Area''
in Appendix A of the Nogales 2012 Plan.
---------------------------------------------------------------------------

    May 22, 2008 Exceedance Day Review. The May 22, 2008 exceedance day
is wholly different from the State's conceptual model exceedance day
given the relative high wind speeds, a 17 mph high observation, and
higher than usual coarse PM component likely from disturbed
surfaces.\52\ The PM10:PM2.5 ratio for May 22,
2008 is the highest in the 29-day sample (10.96 to 1), well beyond the
sample average of 6.24 to 1. As with total PM10 emissions,
emissions of coarse PM (e.g., unpaved roads) are higher from Nogales,
Mexico, than they are from the Nogales NA. The wind direction is from a
southerly quadrant in all hourly observations. See Table 8. Given this
information, we concur that the day should be placed with the 25 other
exceedance days in the conceptual model, because it is likely that the
sources of PM10 causing the exceedance originated from the
Nogales, Mexico side of the international border.\53\
---------------------------------------------------------------------------

    \52\ See Figure 6 in ``Clean Air Act, Section 179B Attainment
Determination for the Nogales PM10 Nonattainment Area''
in Appendix A of the Nogales 2012 Plan.
    \53\ For a detailed review of the May 22, 2008 exceedance day,
see Section 4.2.3 of ``Clean Air Act, Section 179B Attainment
Determination for the Nogales PM10 Nonattainment Area''
in Appendix A of the Nogales 2012 Plan.
---------------------------------------------------------------------------

    January 1, 2009 Exceedance Day Review. Like the January 1, 2007
exceedance, the January 1, 2009 exceedance day is different from the
conceptual model exceedance day in the timing and distribution of
observed ambient PM10 values and high PM2.5
component most likely caused by a combustion source. As with total
PM10 emissions, emissions of fine PM (e.g., combustion
sources) are higher from Nogales, Mexico, than they are from the
Nogales NA. For example, a comparison of the 2008 Nogales Municipality,
Mexico and Nogales NA emissions inventories for the residential
woodburning source category shows 176 tpy compared to 24 tpy,
respectively (see Tables 2 and 6, above). The key factor for assigning
this day is the contribution of high hourly ambient concentrations with
a southerly wind direction quadrant compared to the remaining 270
degree wind direction quadrants. See Table 8. Consequently, we concur
that the day should be placed with the 25 other exceedance days in the
conceptual model, because it is likely that the sources of
PM10 causing the exceedance originated from the Nogales,
Mexico side of the international border.\54\
---------------------------------------------------------------------------

    \54\ For a detailed review of the January 1, 2009 exceedance
day, see Section 4.2.4 of ``Clean Air Act, Section 179B Attainment
Determination for the Nogales PM10 Nonattainment Area''
in Appendix A of the Nogales 2012 Plan.
---------------------------------------------------------------------------

    To summarize, the State concludes that two exceedance days, May 22,
2008 and January 1, 2009, should be categorized with the 25 exceedance
days where the State found that there was a high likelihood of a large
contribution of PM10 from sources on the Nogales, Mexico
side of the international border such that the Nogales NA would likely
have attained the PM10 standard but for emissions from
Mexico. The two remaining exceedance days, January 1, 2007 and January
26, 2008, have contributions from PM10 sources on the
Nogales NA side of the international border such that it cannot be
determined that there is a similarly high likelihood that the Nogales
NA would not have exceeded the PM10 standard but for
PM10 emissions originating from the Mexican side of the
international border. Therefore, according to this daily analysis, the
State found that at least 27 of 29 exceedances of the PM10
NAAQS observed in the Nogales NA during 2007-2009 can be attributed
primarily to sources of PM10 from across the international
border. Based on these two exceedances and on data completeness and
every day sampling for the 2007-2009 timeframe, the State calculated a
maximum expected annual exceedance rate of 0.7 exceedances per year.
    (ii) Hourly Analysis to Demonstrate Attainment But For
International Sources of PM10 Emissions
    In a second analysis, the State classified each hourly
PM10 concentration value from the 29 exceedance days based
on the likely influence of emissions from Mexico and then recalculated
the 24-hour average concentration that would have occurred but for
international transport of PM10 emissions from Nogales,
Mexico. An hourly concentration was classified as influenced by
international transport if it met one of four criteria, or decision
rules, related to hourly observations of wind direction, wind speed,
and temperature change:
    (1) Hours with sustained (more than one hour consecutively)
southerly winds greater than 4.5 mph (2 meters/second (m/s)),
suggesting the primary influence of wind-blown PM10 from
across the international border;
    (2) hours with southerly winds or air flow and decreasing or stable
temperatures preceded by or followed by hours with similar conditions,
suggesting sustained downslope air flows from higher elevations south
of the international border;
    (3) any hour preceded by and followed by hours with southerly wind
or air flow and decreasing or stable temperatures, suggesting continued
influence of downslope air flow from higher elevations south of the
international border; and,
    (4) surface wind speed less than or equal to 1.1 mph (0.5 m/s),
preceded by or followed by hours with similar conditions, suggesting
sustained air mass stagnation where PM10 emissions suspended
in previous hours remain suspended in the stagnant air mass.

The first decision rule identifies periods consistent with sustained
high winds from the south carrying wind-blown PM10, as
discussed earlier concerning the May 22, 2008 exceedance day. The
second and third decision rules identify periods influenced by
downslope wind flow conditions described in the conceptual model as
usually occurring in the late afternoon and evening and transporting
PM10 from higher elevations in Nogales, Mexico to lower
elevations in the Nogales NA. The fourth decision rule identifies
periods of sustained air mass stagnation usually found in the late
night and early morning hours after the early evening downslope wind or
air flow has ebbed and before sunrise, after which wind speeds begin to
increase from their overnight low values.

    Using the low estimate of total Nogales Municipality, Mexico
PM10 emissions, the analysis of emissions inventories
discussed earlier showed that U.S. sources are responsible for a
maximum of 36 percent of PM10 emissions in the Ambos Nogales
region; see Table 9. Conversely, using the high estimate of total
Nogales Municipality, Mexico emissions, U.S. sources are responsible
for a minimum of 17 to 18 percent of PM10 emissions in the
Ambos Nogales region in 2008 and 2011, respectively.

   Table 9--2008 and 2011 Total PM10 Emission Inventories: Nogales NA, Arizona and Nogales Municipality, Mexico
                                          [Low estimate, tons per year]
----------------------------------------------------------------------------------------------------------------
                                                                  2008              2011             Percent
----------------------------------------------------------------------------------------------------------------
Nogales NA, Arizona.......................................             1,524             1,521                36
Nogales Municipality, Mexico..............................             2,713             2,757                64
                                                           -----------------------------------------------------
    Total Ambos Nogales Region............................             4,237             4,278               100
----------------------------------------------------------------------------------------------------------------
Source: Tables 6-7 from ``Clean Air Act, Section 179B Attainment Determination for the Nogales, Arizona PM10
  Nonattainment Area'' in Appendix A of the Nogales 2012 Plan.

    Therefore, for each hour that meets one of the four criteria listed
above, instead of assuming that the concentration is entirely due to
Mexican sources, a more conservative assumption is that up to 36
percent of the hourly concentrations may be due to contributions from
U.S. emission sources. Therefore, in this next step, the observed
hourly concentrations were weighted by 0.36 for each hour that meets
any one of the four criteria listed above and used this weighted
concentration to estimate the 24-hour average concentration that would
have occurred in the Nogales NA but for international transport from
Mexico.
    To show the effects of each decision rule, an estimated 24-hour
concentration was calculated after the application of Rule 1, Rules 2
and 3, Rules 1-3, and Rules 1-4. The results are summarized below.\55\
---------------------------------------------------------------------------

    \55\ The observed concentrations and meteorological data for
each hour of each exceedance day, the classification based on the
criteria listed above, and the re-calculation of the estimated 24-
hour average concentrations but for international transport are
provided in Section 3.7 of ``Analysis of Ambient PM10
Levels, Topography, and Meteorological Data in Nogales, Arizona:
2007-2009'' in Appendix D of the Nogales 2012 Plan.
---------------------------------------------------------------------------

     The application of Rule 1 only removes one day, May 22,
2008, leaving 28 days showing a concentration value greater than 150
[mu]g/m\3\.
     The application of Rules 2 and 3 removes 27 days, leaving
January 1, 2007 and January 26, 2008 showing a concentration value
greater than 150 [mu]g/m\3\; 196 [mu]g/m\3\ and 244 [mu]g/m\3\,
respectively.
     The application of Rules 1, 2, and 3 again removes 27
days, leaving January 1, 2007 and January 26, 2008 showing a
concentration value greater than 150 [mu]g/m\3\; 196 [mu]g/m\3\ and 244
[mu]g/m\3\, respectively.
     The application of Rules 1, 2, 3, and 4 removes 29 days,
leaving no estimated days with a value greater than 150 [mu]g/m\3\. The
highest 24-hour average concentration estimated was 107 [mu]g/m\3\.

In sum, based on this analysis apportioning hourly concentration data
using the four criteria to produce an estimated 24-hour average
concentration but for international emissions, no exceedance days would
have been expected to occur in the Nogales NA, but for transport from
Mexico.
    Considering the relatively large differences in emissions
inventories between the Nogales NA and Nogales Municipality, Mexico and
the meteorology described by the conceptual model, it is likely that
observed pollution during southerly downslope wind flows originating
from Nogales, Mexico also contributed to observed pollution during
following hours of sustained stagnation. With the wind direction
varying under low wind speeds and stable temperatures, it remains
possible, however, that a greater proportion of PM10
pollution during hours of sustained stagnation may be coming from U.S.
sources. Therefore, a slightly more conservative approach would be to
relax the decision rules by not considering sustained stagnation (Rule
4) and assign PM10 levels during these hours entirely to the
Nogales NA. Consequently, when considering Mexican influence to only
occur under conditions of relative high wind speeds (Rule 1) and
sustained downslope wind flows from the south (Rules 2 and 3), two
exceedance days would have been expected to occur but for international
transport: January 1, 2007 and January 26, 2008. Given the finding that
no more than two exceedance days would have occurred applying criteria
one through three, as determined by this hourly analysis of
concentration data, the maximum expected number of annual exceedances
is 0.7.
3. Proposed Action on the Nogales Nonattainment Area Section 179B
Analysis and Demonstration of Attainment but for International Sources
of PM10 Emissions
    We propose to approve Arizona's section 179B analysis and
demonstration of attainment but for international sources of
PM10 emissions. After meeting the PM10 NAAQS from
1994-1997, an increasing number of exceedances occurred in the Nogales
NA. While population in the Nogales NA has grown slightly since 1995,
the Nogales Municipality population has increased 65 percent, such that
in 2010, 90 percent of the Ambos Nogales regional population is the
Nogales Municipality, Mexico area. This difference in relative
population and population growth over time supports the inference that
a much larger proportion of PM10 in the Nogales NA comes
from emissions sources on the Nogales, Mexico side of the international
border.
    A comparison of 2008 and 2011 emission inventories between the
Nogales Municipality and the Nogales NA shows that pollution loads may
differ by a ratio of 1.8-4.6 to one on a south-to-north basis relative
to the international border. The Nogales NA contributes 17 to 36
percent of PM10 emissions in the Ambos Nogales region,
depending on the emissions inventory estimate chosen for the Nogales
Municipality, Mexico. Conversely, the Nogales Municipality, Mexico
contributes 83 to 64 percent of PM10 emissions in the Ambos
Nogales region.
    In its review of the ambient PM10 data, meteorological
data, and through its analyses, Arizona found that the Ambos Nogales
area's meteorology and topography influence the observed exceedances of
PM10 NAAQS and there is a definite south-to-north
directional component to the ambient air quality

data underlying the exceedances of the PM10 NAAQS. Finally,
daily and hourly analyses of the most recent three years of quality
assured and State certified ambient PM10 and meteorological
data from 2007-2009 show that no more than two, and likely none, of the
29 exceedances would have occurred in the Nogales NA, but for
PM10 emissions from Mexico.
    Based on these two exceedances, data completeness, and every day
sampling for the 2007-2009 timeframe, the calculated maximum expected
annual exceedance rate is 0.7 exceedances per year. The standard we use
to demonstrate attainment of the PM10 NAAQS, ``but for''
international emissions, is that the expected number of days per
calendar year with a 24-hour average concentration above 150 [mu]g/m\3\
must be equal to or less than one. To conclude, we propose to determine
that Arizona has met this standard and to approve their section 179B
Analysis and demonstration of attainment but for international
emissions for the Nogales NA.