Document ID: EPA-HQ-OAR-2007-0562-0435
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2009-11-13T05:00Z

Attachment 1

ARIZONA

Area Designations for the PM2.5

24-Hour Fine Particle National Ambient Air Quality Standards

The table below identifies the county in Arizona that EPA has designated
as not attaining the 2006 24-hour fine particle (PM2.5) standard.  A
county (or part thereof) is designated as nonattainment if it has an air
quality monitor that is violating the standard or if the county is
determined to be contributing to the violation of the standard.

 

Area	Arizona Recommended Nonattainment Counties	EPA’s Final Designated
Nonattainment Counties

Nogales	Santa Cruz County (partial) 	Santa Cruz County (partial) 

EPA has designated the remaining counties in the state as
“unclassifiable/attainment.” EPA designated a county as
“unclassifiable” when:  one or more of its monitors recorded a
violation in 2004-2006; all monitors in the county with complete
2005-2007 data showed attainment; and one or more other monitors in the
county had 2005-2007 monitoring data that was not complete and could not
be used for determining compliance with the standard.

EPA Technical Analysis for Nogales

Pursuant to section 107(d) of the Clean Air Act, EPA must designate as
nonattainment those areas that violate the NAAQS and those nearby areas
that contribute to violations.  This technical analysis for the Nogales
area identifies the county with monitors that violate the 24-hour PM2.5
standard and evaluates the counties that potentially contribute to fine
particle concentrations in the area.  EPA has evaluated these counties
based on the weight of evidence of the following nine factors
recommended in EPA guidance and any other relevant information:

- pollutant emissions

- air quality data

- population density and degree of urbanization

- traffic and commuting patterns

- growth

- meteorology

- geography and topography

- jurisdictional boundaries

- level of control of emissions sources

Figure 1 is a map of Santa Cruz County which encompasses the
nonattainment area. The map includes other relevant information such as
the locations and design values of air quality monitors, the
metropolitan area boundary, and other nearby counties. 

 

                                                             Figure 1

In a letter dated December 19, 2007, Arizona recommended that a portion
of Santa Cruz County be designated as “nonattainment” for the 2006
24-hour PM2.5 standard based on air quality data from 2004 - 2006. 
These data are from Federal Reference Method (FRM) monitors located in
the state. The Nogales area did not previously violate the 1997 PM2.5
NAAQS, but now has one monitor that violates the 2006 24-hour PM2.5
NAAQS.

The Arizona Department of Environmental Quality (ADEQ) noted that the
monitor located at the Nogales Post Office in Nogales, Arizona has
recorded two exceedances of the 24-hour PM2.5 NAAQS in 2006, and ADEQ
recommended that the area be designated as nonattainment based on the
most recent three years of air quality data.  The term
“nonattainment” means an area is violating the PM2.5 standards or
contributing to violations of the standards. A “not classifiable”
designation is appropriate for areas where monitors have insufficient
data, but where available data support attainment of standards. A State
submittal that only addressed whether monitored violations are occurring
in an area does not suffice as the sole justification for designating
the boundaries of a nonattainment area.

The Nogales Arizona Area is already designated as a PM10 nonattainment
area and ADEQ has recommended that the boundaries of the 2006 24-hour
PM2.5 nonattainment area coincide with the existing PM10 nonattainment
area boundaries. The Nogales Arizona area was not previously designated
nonattainment for the 1997 PM2.5 standards.

Based on EPA's technical analysis described below and currently
available information, EPA has designated a portion of Santa Cruz
County, the Nogales area, as nonattainment for the 24-hour PM2.5
air-quality standard. This county is listed in the table below.

Nogales Area	State-Recommended Nonattainment Counties	EPA’s Designated
Nonattainment Counties

Arizona	Santa Cruz County (partial) 	Santa Cruz County (partial) 

The following is the technical analysis for the Nogales, Arizona 24-hour
PM2.5 nonattainment area.

Factor 1:  Emissions data

For this factor, EPA evaluated county level emission data for the
following PM2.5 components and precursor pollutants:  “PM2.5 emissions
total,” “PM2.5 emissions carbon,” “PM2.5 emissions other,”
“SO2,” “NOx, “VOCs,” and “NH3.”  “PM2.5 emissions
total” represents direct emissions of PM2.5 and includes:   “PM2.5
emissions carbon,” “PM2.5 emissions other”, primary sulfate (SO4),
and primary nitrate.  (Although primary sulfate and primary nitrate,
which are emitted directly from stacks rather than forming in
atmospheric reactions with SO2 and NOx, are part of “PM2.5 emissions
total,” they are not shown on the template or data spreadsheet as
separate items).  “PM2.5 emissions carbon” represents the sum of
organic carbon (OC) and elemental carbon (EC) emissions, and “PM2.5
emissions other” represents other inorganic particles (crustal). 
Emissions of SO2 and NOx, which are precursors of the secondary PM2.5
components sulfate and nitrate, are also considered.  VOCs (volatile
organic compounds) and NH3 (ammonia) are also potential PM2.5 precursors
and are included for consideration. 

Emissions data were derived from the 2005 National Emissions Inventory
(NEI), version 1.  See
http://www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html.

EPA also considered the Contributing Emissions Score (CES) for each
county.  The CES is a metric that takes into consideration emissions
data, meteorological data, and air quality monitoring information to
provide a relative ranking of counties in and near an area.  Note that
this metric is not the exclusive manner for consideration of data for
these factors.  A summary of the CES is included in attachment 2, and a
more detailed description can be found at
http://www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html#C.

Table 1 shows emissions of PM2.5 and precursor pollutant components
(given in tons per year) and the CES for violating and potentially
contributing counties in the Nogales area.  Potentially contributing
counties are listed in descending order by CES.

Table 1.  PM2.5 Related Emissions and Contributing Emissions Score

County	State Recom-mended Non-attain

ment	CES	PM2.5

emissions 

total

(tpy)	PM2.5

emissions 

carbon

(tpy)	PM2.5

emissions 

other

(tpy)	SO2

(tpy)	NOx

(tpy)	VOCs

(tpy)	NH3

(tpy)

Pima 	No	66	4,220	1,678	2,542	6,133	30,326	32,966	1,947

Santa Cruz 	Yes, P	18	631	287	344	116	1,520	3.156	466

Cochise 	No	7	1,691	496	1,195	3,712	16,151	7,123	3,404

Based on emission levels and CES values only, Pima, and Cochise Counties
in Arizona are candidates for a 24-hour PM2.5 nonattainment designation.
Santa Cruz County has a violating monitor so it is a candidate for a
nonattainment designation despite the low emission levels and low CES
value.

It is interesting to note that Santa Cruz County has much lower PM2.5
and precursor emissions than either Pima County or Cochise County where
there are no PM2.5 violations.  ADEQ provided summary information from a
winter 2005 study by Arizona State University (ASU), which indicated
that the majority of PM2.5 concentrations in Nogales, Arizona are a
result of transport from Nogales, Sonora, Mexico, via nighttime drainage
flows. The ASU study identified wood burning, food cooking, open
burning, dust emissions, and tailpipe emissions from on-road and
off-road vehicles as the primary sources.  The Sonora, Mexico side has
94% of the total emissions, i.e. its emissions are some 15 times higher
than the U.S. side, according to PM10 and PM2.5 emission inventories
developed by ADEQ for the 1993 Nogales PM10 SIP.  Since the CES tool
does not include these Mexican emissions, the CES county results above
are not conclusive for purposes of identifying relative contributions to
total PM2.5 levels on days with high PM2.5 levels.

With respect to the CES values, Pima County has the highest CES score of
66, followed distantly by the violating county, Santa Cruz, and Cochise
County.  Tucson, Arizona, in Pima County, is located approximately 60
miles north of Nogales and does not appear to contribute to the PM2.5
air quality problems in Nogales, due to topography and meteorologic
conditions during wintertime exceedances of the 24-hour PM2.5 NAAQS, and
this is discussed under factors 6 and 7 below.  In addition, there is no
commuting relationship between Tucson and Nogales, and there are no
violating monitors in Pima County, or in Green Valley to the south of
Tucson.  Based on these factors, EPA  eliminated Pima County from
consideration as part of the 24-hour PM2.5 nonattainment area.  As
indicated by the very low CES, Cochise County does not impact Nogales at
all and is, in fact, separated from Nogales by the Patagonia Mountains. 
Considering several factors, including low emissions, low CES score, and
the topographical barriers, EPA also eliminated Cochise County from
consideration in the nonattainment area.  

Although it has low emissions and a low CES value, Santa Cruz County has
a violating monitor.  Emissions sources in the part of Santa Cruz County
where the violating monitor is located are concentrated along Interstate
19, in the valley formed by the Nogales Wash (see population factor
below).  

Based on emissions information, including consideration of the
information provided by the State of Arizona documenting substantial
emissions from Nogales, Sonora, Mexico, EPA’s nonattainment area
designation of a part of Santa Cruz County includes the population (and
thus, the population-associated emissions), as well as major nearby
highways and stationary sources, taking into account the topographical
separation of the Nogales area from part of the county.  

Factor 2:  Air quality data 

This factor considers the 24-hour PM2.5 design values (in µg/m3) for
air quality monitors in counties in the Nogales area based on data for
the 2005-2007 period.  A monitor’s design value indicates whether that
monitor attains a specified air quality standard. The 24-hour PM2.5
standards are met when the 3-year average of a monitor’s 98th
percentile values are 35 µg/m3 or less.  A design value is only valid
if minimum data completeness criteria are met. 

The 24-hour PM2.5 design values for counties in the Nogales area are
shown in Table 2.

Table 2.  Air Quality Data 

County	State 

Recommended

 Nonattainment	24-hr PM2.5 Design 

Values, 2004-2006

(µg/m3)	24-hr PM2.5 Design 

Values, 2005-2007

(µg/m3)

Santa Cruz County, AZ	Yes, P	38	39

City of Nogales	Yes	38	39

P = partial

The City of Nogales in Santa Cruz County, Arizona shows a violation of
the 24-hour PM2.5 standard, and so, Santa Cruz County is included in the
nonattainment area.  There are no monitors with violating values
elsewhere in Santa Cruz County or in Pima County to the north, therefore
it is appropriate to include Santa Cruz County in the nonattainment
area.   The absence of a violating monitor alone is not sufficient to
eliminate a portion of Santa Cruz County as a candidate for
nonattainment status.  EPA considered eight other factors (plus other
relevant factors or circumstances) when determining the boundaries of
the Nogales nonattainment area and, as further explained below and based
on the weight of evidence, concurs with the State’s recommendation.  

Air quality monitoring data on the composition of fine particle mass are
available from the EPA Chemical Speciation Network and the IMPROVE
monitoring network.  Analysis of these data indicates that the days with
the highest fine particle concentrations occur in cool seasons, and the
average chemical composition of the highest days is typically
characterized by high levels of organic carbon. 

High PM2.5 values in Nogales generally occur under stagnant, light wind
conditions during winter, and are associated with southerly flow (i.e.,
from the direction of Mexico).  Measurements of PM2.5 composition during
an Arizona State University study during the winter of 2005-2006 (quoted
in ADEQ recommendation), showed that the main emission sources are wood
burning, food cooking, open burning, dust emissions, and tailpipe
emissions from on-road and off-road vehicles. Carbonaceous emissions
with about 15% EC are consistent with wood burning or food cooking, and
not consistent with diesel emissions, although diesel emissions are
important at the U.S.-Mexico border crossing and elsewhere in the
Nogales area.

Note:  Eligible monitors for providing design value data generally
include State and Local Air Monitoring Stations (SLAMS) at
population-oriented locations with an FRM monitor.  All data from
Special Purpose Monitors (SPM) using an FRM is eligible for comparison
to the relevant NAAQS, subject to the requirements given in the October
17, 2006 Revision to Ambient Air Monitoring Regulations (71 FR 61236). 
All monitors used to provide data must meet the monitor siting and
eligibility requirements given in 71 FR 61236 to 61328 in order to be
acceptable for comparison to the 24-hr PM2.5 NAAQS for designation
purposes.

Factor 3: Population density and degree of urbanization (including
commercial development)

Table 4 shows the 2005 population for each county in the area being
evaluated, as well as the population density for each county in that
area.  Population data gives an indication of whether it is likely that
population-based emissions might contribute to violations of the 24-hour
PM2.5 standards. 

Table 4.  Population

County	State Recommended Nonattainment	2005 Population	2005 Population
Density (pop/sq mi)

Santa Cruz County, AZ	Yes, P 	46,907	38

City of Nogales	Yes	21,746	1,046

Pima County	No	925,000	101

Cochise County 	No	126,153	20

These data show that the City of Nogales is densely populated, but that
Santa Cruz County is sparsely populated.  In the entire County, there
are only 47,000 people, or 38 people per square mile.  Based solely on
this factor, Santa Cruz County would not be considered as high-ranking
for designation as nonattainment. The figures supplied by ADEQ indicate
that the population density in Nogales, Sonora in Mexico is 17,411
people per square mile. This supports the contention that the major
portion of PM2.5 emissions in the City of Nogales, Arizona comes from
Nogales, Sonora, Mexico, which has a large metropolitan population of
approximately 200,000 people. 

Based on this factor, the Nogales area is a candidate for a
nonattainment status for PM2.5 and the rest of Santa Cruz County is not.
Pima and Cochise Counties are included for comparison.  Their population
densities are very low, and these counties were previously eliminated
from inclusion in the nonattainment area.

Factor 4: Traffic and commuting patterns 

This factor considers the number of commuters in each county who drive
to another county within the Nogales area, the percent of total
commuters in each county who commute to other counties within the
Nogales area, as well as the total Vehicle Miles Traveled (VMT) for each
county in millions of miles (see Table 5). A county with numerous
commuters is generally an integral part of an urban area and is likely
contributing to fine particle concentrations in the area.  

Table 5.  Traffic and Commuting Patterns

County / City	State Recommended Nonattainment	2005 VMT

(millions of mi)	Number Commuting to any violating counties

	Percent

Commuting to any violating counties 

	Number Commuting to and Within Statistical  Area	Percent Commuting to
and Within Statistical Area

Santa Cruz 	Yes, P	464	10,960	87%	10,960	87%

City of Nogales 	Yes

	Pima County	No	8,759	1,420	0	1,420	0%

Cochise County	No	1,906	130	0	130	0%

Major highways in the Nogales, Arizona area are Arizona State Route 82,
which connects Nogales, Arizona with Patagonia, Arizona (19 miles) and
Sonoita (31 miles) to the northeast, and US Interstate 19 which connects
Tucson, Arizona to Nogales, Arizona and continues south into Mexico. The
numbers above indicate that most traffic is local and stays within Santa
Cruz County. There is no commuting pattern between the City of Nogales
and the City of Tucson, in Pima County to the north, as indicated by a
value of 0% for people commuting to any violating counties from Pima
County.  Rather, most of the traffic is personal or commercial coming to
and from the ports of entry. Since truck traffic is an integral part of
the Nogales Area, this information has been included as well.

Figure 2

Two ports of entry connect the Nogales, Arizona area with Sonora,
Mexico.  These ports of entry are the Mariposa Port of entry, which
handles primarily commercial truck traffic, and the DeConcini Port of
Entry, which primarily handles passenger cars, buses and non-commercial
trucks.  In 2006, approximately 3.2 million personal vehicles, 289,000
trucks, and 11,500 buses used the Nogales ports of entry.  This traffic
is the major commuting pattern in the Nogales Area and in Santa Cruz
County.  The average daily truck traffic is  approximately 2001-5000 
trucks.

(See map (Figure 2) “Nogales, Arizona, Emission Sources and Population
Density” which shows the major highways in the Nogales area.)

There are a significant number of vehicles traveling from within Santa
Cruz County to the City of Nogales.  In addition, there is significant
traffic from the Mexican border to beyond the city limits of the City of
Nogales. The Figure 2 map indicates that the heaviest truck and daily
traffic is on Interstate 19 that winds from the Mexican border to the
City of Nogales and through Santa Cruz County to Tucson. ADEQ indicates
the possibility of high levels of mobile source PM2.5 emissions due to
the high level of car and truck traffic within the Nogales area.

Because the truck traffic goes beyond the city limits of the City of
Nogales, and with the analysis of geography and topography described in
factor 7 below, we agree with the recommendation of Arizona that the
northern border of the PM2.5 nonattainment area should be drawn
coincident with the the current northern border of the PM10
nonattainment area in order to capture the contributing
transportation-related emissions from the Interstate 19 corridor. This
area extends beyond the City of Nogales, north into Santa Cruz County,
as shown in Figure 3.  

The 2005 VMT data used for tables 5 and 6 of the technical analysis have
been derived using methodology such as that described in "Documentation
for the  2005 Mobile National Emissions Inventory, Version 2," December
2008, prepared for the Emission Inventory Group, U.S. EPA.  This
document may be found at:   HYPERLINK
"ftp://ftp.epa.gov/EmisInventory/2005_nei/mobile_sector/documentation/20
05_mobile_nei_version_2_report.pdf" 
ftp://ftp.epa.gov/EmisInventory/2005_nei/mobile_sector/documentation/200
5_mobile_nei_version_2_report.pdf 

Factor 5:  Growth rates and patterns  

This factor considers population growth for 2000-2005 and growth in
vehicle miles traveled for 2000-2005 for counties in Nogales area, as
well as patterns of population and VMT growth.  A county with rapid
population or VMT growth is generally an integral part of an urban area
and likely to be contributing to fine particle concentrations in the
area.  Table 6 below shows population, population growth, VMT and VMT
growth for Santa Cruz, Pima, and Cochise Counties, as well as the 2005
population for the City of Nogales.  

Table 6.  Population and VMT Values and Percent Change.

County	Population (2005)	Population % change (2000 - 2005)	2005 VMT

(million)	VMT

Growth (% 2000 to 2005)

Santa Cruz County	42,012	9 %	464	1%

City of Nogales 	21,746

	Pima County	925,000	9%	8,759	52%

Cochise County	126,153	7%	1,906	29%

Figure 3

map source: ADEQ Technical Support Document for Nogales Recommendation



ADEQ estimates that population growth for Nogales, Arizona and the
Nogales PM10 nonattainment area will be small, and the population growth
for Santa Cruz County is projected to be moderate.  Between 2007 and
2015, the population of Santa Cruz County is projected to increase 20%
to 56,144 residents, compared to a significantly slower growth rate of
10% or 23,858 residents for the City of Nogales.  However, Nogales,
Arizona will still represent a plurality of the population of Santa Cruz
County.  

With regard to VMT, Santa Cruz County shows a 1% decrease in VMT from
2000 to 2005.  According to ADEQ, there will be moderate population
growth in Santa Cruz County from 2007 and 2015 and the number of
registered vehicles in Santa Cruz County has grown from 41,527 in
January 2004 to 47,460 in July 2007.  There are no statistics for what
proportion of the vehicles are owned by residents of the City of
Nogales, but since 50% of the total county population resides in the
City of Nogales, a large proportion of VMT is within the City of
Nogales.

While there is population growth, it is contained within the boundaries
of the area proposed by ADEQ which coincides with the existing PM10
nonattainment area.  There is no evidence of significant population
growth or high VMT anywhere else in Santa Cruz County.

Factor 6:  Meteorology (weather/transport patterns)

For this factor, EPA considered data from National Weather Service
instruments in the area.  Wind direction and wind speed data for
2004-2006 were analyzed, with an emphasis on “high PM2.5 days” for
each of two seasons (an October-April “cold” season and a
May-September “warm” season).  These high days are defined as days
where any FRM or FEM air quality monitors had 24-hour PM2.5
concentrations above 95% on a frequency distribution curve of PM2.5
24-hour values. 

EPA developed a “pollution rose” to understand the prevailing wind
direction and wind speed on the days with highest fine particle
concentrations. The pollution rose for Nogales, Arizona, site 040230004,
shown in Figure 4, indicates that elevated levels of particulate matter
occur during the cool season during time periods when prevailing winds
are light, consistent with the analysis submitted by Arizona.

Note:  the meteorology factor is also considered in each county’s
Contributing Emissions Score because the method for deriving this metric
included an analysis of trajectories of air masses for high PM2.5 days.

  

                                                                Figure 4

                                                                        
                         

EPA also directly examined exceedance day backward wind trajectories
from the HYSPLIT model, based on Eta Data Assimilation System (EDAS)
gridded meteorological data.  These trajectories were not conclusive. 
However, that is not surprising given that EDAS has a 40 km grid
resolution, whereas the valley containing Nogales is at most 20 km wide:
the data were not resolved enough to portray the south-north valley
drainage flows that are a key feature of local Nogales meteorology.

The Arizona Department of Environmental Quality maintains a
meteorological site located at the Nogales Post Office.  Review of four
years of hourly wind data, from the meteorological site at the Nogales
Post Office, indicates that 47% of the time winds were out of the south
(between 145 and 225 degrees).  Winds out of the north (315 to 45
degrees) occurred 18% of the time, east (45 through 145 degrees) 20% of
the time, and west (225 through 315 degrees) occurred 15% of the time. 
These data show a definite south to north wind flow along the Nogales
Wash, likely aided by the decrease in elevation from the south to the
north.  

The 2005 study by Arizona State University (ASU) mentioned above
confirmed that PM2.5 crosses the border from Mexico to Arizona,
primarily via southerly drainage flow during the evening hours. This
drainage flow is strongest during the winter months when there are
strong temperature inversions and light to no winds.  Air quality
modeling performed in 1999 found that PM10 concentrations drop to low
levels by the time they reach the northern boundary of the PM10
nonattainment area.  A similar pattern would hold for PM2.5, though
concentrations would be expected to drop off more slowly due to its
smaller diameter and lower deposition rate. The information provided by
ADEQ indicates that there is transport from Mexico to the Nogales area,
and that concentrations decline with distance from the border.

Factor 7:  Geography/topography (mountain ranges or other air basin
boundaries)

The geography/topography analysis looks at physical features of the land
that might have an effect on the air shed and, therefore, on the
distribution of PM2.5 over the Nogales area.

The Nogales area is situated between three mountain ranges located east,
west, and north of the city.  (See terrain shown in Figures 2 and 3.) 
The City of Nogales, Arizona encompasses 21 square miles and lies 3,865
feet above sea level.  With the Pajarito, Atascosa, and Tumacacori
Mountains about 7 miles to the west and the Patagonia Mountains roughly
13 miles east, Nogales rests between the two mountain ranges in the
Nogales Wash.  While the crests of these ranges are roughly 20 miles
apart, the intervening high terrain makes for a narrow north-south
valley.  This valley contains the Nogales Wash, Interstate 19, and the
anthropogenic sources in the southern portion of the county.  

To the north and northeast are the Santa Rita mountains, including Mount
Wrightson at 9,432 feet.  .This range forms a partial barrier to
northward flow; after angling to the west, the main north-south valley
continues northward.  This partial barrier is a reasonable northern edge
to the PM2.5 area; the range separates the Nogales area, on the north,
from the rest of the county.

The elevation decreases from south of the US/Mexico border heading north
toward Tucson, Arizona.  Thus, nighttime drainage flow is typically from
south to north along the Nogales Wash.  According to ADEQ, the narrow
valley topography can trap air pollutants, especially during the evening
hours when the diurnal flow is from the south, out of Nogales, Sonora. 

While it is possible that some PM2.5 may travel beyond the northern
edge, the PM2.5 nonattainment area includes the portions with high
concentrations, and includes nearly all of the U.S. sources that
contribute to PM2.5 levels on days with exceedances of the standard (see
factors 3-5).  Considering these factors together, EPA believes that the
State’s recommended PM2.5 nonattainment boundary, which coincides with
the existing PM10 nonattainment area, is justified.

Factor 8:  Jurisdictional boundaries (e.g., existing PM and ozone areas)

The analysis of jurisdictional boundaries considered the planning and
organizational structure of the Nogales Area to determine if the
implementation of controls in a potential nonattainment area can be
carried out in a cohesive manner.  Santa Cruz County has no air quality
planning agency.  Consequently, the Arizona Department of Environmental
Quality is responsible for developing any state implementation plans
that are required for the Nogales Area.

Santa Cruz County is an existing nonattainment area for the 24-hour PM10
standard and is in attainment for the 8-hour ozone standard.  ADEQ is
the air quality agency responsible for preparing the PM10 Plan, and is
also responsible for preparing any PM2.5 plan that is required.  EPA
believes that the boundaries established for implementing the existing 
PM10 standards are appropriate for implementing the new 24-hour PM2.5
standard. Establishment of the same boundaries for nonattainment of both
PM standards facilitates overall air quality planning for attaining both
standards. 

Factor 9:  Level of control of emission sources 

This factor considers emission controls currently implemented for major
sources in the Nogales PM2.5 nonattainment area.  In considering
county-level emissions, EPA used data from the 2005 National Emissions
Inventory version 1, the most current version of the national inventory
available at the beginning of the designations process in late 2007. 
However, EPA recognized that for certain counties, emissions may have
changed significantly since 2005.  For example, certain power plants or
large sources of emissions in or near an area may have installed
emission controls or otherwise significantly reduced emissions since
2005.  

For the Nogales nonattainment area, there are no large contributing
sources for which this factor would apply.  

Conclusion

EPA agrees with the State’s nonattainment area boundary recommendation
and is, therefore, designating part of Santa Cruz County, the same part
that comprises the existing PM10 nonattainment area, as nonattainment
for the 2006 24-hour PM2.5 standard.  This is the Nogales area within
Santa Cruz County.  EPA believes that the emissions data, air quality
data, population data, growth patterns, traffic and commuting patterns,
meteorology, and topography all support establishing a smaller
nonattainment area than the whole county.  Taking into account the
topography in this area, the nonattainment area boundary includes nearly
all of the U.S. sources that contribute to PM2.5 levels on days with
exceedances of the standard.     

Additional information regarding responses to specific State comments
can be found in EPA's Response to Comments document at   HYPERLINK
"http://www.epa.gov/pmdesignations/2006standards/tech.htm" 
http://www.epa.gov/pmdesignations/2006standards/tech.htm 

Attachment 2

Description of the Contributing Emissions Score

The CES is a metric that takes into consideration emissions data,
meteorological data, and air quality monitoring information to provide a
relative ranking of counties in and near an area.  Using this
methodology, scores were developed for each county in and around the
relevant metro area.  The county with the highest contribution potential
was assigned a score of 100, and other county scores were adjusted in
relation to the highest county.  The CES represents the relative maximum
influence that emissions in that county have on a violating county.  The
CES, which reflects consideration of multiple factors, should be
considered in evaluating the weight of evidence supporting designation
decisions
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	Major PM2.5 components:  total carbon (organic carbon (OC) and
elemental carbon (EC)), SO2, NOx, and inorganic particles (crustal).

	PM2.5 emissions for the highest (generally top 5%) PM2.5 emission
days (herein called “high days”) for each of two seasons, cold
(Oct-Apr) and warm (May-Sept)

	Meteorology on high days using the NOAA HYSPLIT model for
determining trajectories of air masses for specified days

	The “urban increment” of a violating monitor, which is the urban
PM2.5 concentration that is in addition to a regional background PM2.5
concentration, determined for each PM2.5 component

	Distance from each potentially contributing county to a violating
county or counties

[A more detailed description of the CES can be found at
http://www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html#C.]

 EPA designated nonattainment areas for the 1997 fine particle standards
in 2005.  In 2006, the primary and secondary 24-hour PM2.5 standards
were revised from 65 micrograms per cubic meter (average of 98th
percentile values for 3 consecutive years) to 35 micrograms per cubic
meter; the level of the primary and secondary annual PM2.5 standards 
remained unchanged at 15 micrograms per cubic meter (average of annual
averages for 3 consecutive years).   

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PAGE   

 PAGE   15 

Concentration:

  > 40 µg/m3

  35 - 40 µg/m3

  30 - 35 µg/m3

  < 30 µg/m3

Season:

     cool (Oct-Apr)

     warm (May-Sep)

2007

2006

2005

TUCSON_INTERNATIONAL_AP (ID=23160)

Meteorological data from 55.0 miles away

Nogales, AZ

(due to missing or variable wind data)            

2 exceedance(s) not plotted                       

   39-NA

Value

Design

.

4

1

# days > 35

28.2

56.2

33.0

98th %-ile

2006

2005

2004

Year

Pollution Rose, 2005-2007

Nogales, AZ [Santa Cruz County, AZ]

Site 040230004

Wind Speed (mph)

12+

10

8

6

4

2

E

W

S