Appendix C – Maintenance Receptors 

1

Technical Support Document for EPA’s Proposed Action on the 

Montana’s 2010 Infrastructure State Implementation Plan (Transport Portion) for the 

2006 24-hour Fine Particulate (PM2.5) National Ambient Air Quality Standard 

Appendix C – Maintenance Receptors 

1. EPA Region 9 States – California and Arizona 

1. California/Mexico Border – Imperial County 

2. Northern California/San Francisco Bay Area – Alameda, Santa Clara, and Solano Counties 

3. Northern California – Sacramento County 

4. Southern California – Orange, Los Angeles, Riverside, and San Bernardino Counties 

5. Arizona – Pinal County (Cowtown) 

6. Arizona – Santa Cruz County (Nogales) 

2. EPA Region 8 States – Utah 

1. Utah – Utah County (Provo-Orem) 

2. Utah – Salt Lake and Weber Counties (Greater Salt Lake Area) 

Note: See Appendix E for further technical data for Greater Salt Lake Area, Utah. 

 
Appendix C – Maintenance Receptors 

2

1.1. California/Mexico Border – Imperial County 

Levels of PM2.5 in Imperial County that exceed the 2006 24-hour PM2.5 NAAQS are recorded at the 

Calexico – Ethel Street monitoring site, which is located near the Mexican Border (see Figure 1.1.A, 

below). The elevated levels of PM2.5 occur primarily in the wintertime, and fifteen of the eighteen 

values above the level of 35 μg/m3 in the years 2006-2010 occur in December or January (See Table 

1.1.A, below). 

The levels of PM2.5 measured at the Brawley and El Centro monitoring sites are always lower – usually 

much lower – than those measured at the Calexico Ethel Street monitor on days when the PM2.5 levels 

at the Calexico Ethel Street monitor exceed 35 μg/m3

 (see Figure 1.1.B, below). Because these nearby 

monitors do not show similar increases in monitored values, it appears that the cause of elevated PM2.5 

appears to be from a local source near the California/Mexico Border. This consideration, combined with

the significant distance between Montana and this receptor, make it reasonable to conclude that 

emissions from Montana will not interfere with the maintenance of the 24-hour PM2.5 standard at this 

receptor. 

Figure 1.1.A: PM2.5 Monitoring Sites in Imperial County, California 
Appendix C 

Table 1.1.A: Calexicofor the Years 2006 through 2010

Year 

2006 January 8, 14, 17, 29 and December 22, 25

2007 November 20, and December 5, 17

2008 January 4, and December 22, 25

2009 September 4, and December 12, 21, 24

2010 June 28, and December 4

Figure 1.1.B: PM2.5 levels 

from 2006 to 2010

0

10

20

30

40

50

60

70

Appendix C – Maintenance Receptors 

3

-Ethel Street: Dates of PM2.5 Values Above 35μg/m

for the Years 2006 through 2010

Date 

January 8, 14, 17, 29 and December 22, 25

November 20, and December 5, 17

January 4, and December 22, 25

September 4, and December 12, 21, 24

28, and December 4

: PM2.5 levels (μg/m3) at Imperial County PM2.5 Monitoring Sites

from 2006 to 2010, for Days of Elevated PM2.5 

bove 35μg/m3

PM2.5 Monitoring Sites

Calexico -Ethel

El Centro

Brawley
Appendix C – Maintenance Receptors 

4

1.2. Northern California/San Francisco Bay Area – Alameda, Santa Clara, and Solano Counties 

Table 1.2.A: San Francisco Bay Area Maintenance Receptor Design Values 

County ID 

Design Value 

(μg/m3) 

2006-2008 

Design Value 

(μg/m3) 

2007-2009 

Design 

Value(μg/m3) 

2008-2010 

Alameda 060010007 36 34 30

Santa Clara 060850005 36 34 30

Solano 060950004 36 36 31

The maintenance receptors in the San Francisco Bay Area, shown in Table 1.2.A and Figure 1.2.A, were 

only slightly above the standard (1 μg/m3

 above) for the years 2006-2008 and 2007-2009. There is a 

downward trend in the 24-hour PM2.5 design value at each of the receptors. 

The monitored PM2.5 values above 35μg/m3 likely reflect localized sources because they occur during 

wintertime temperature inversions with low winds that persist for several days, which allow directly 

emitted and secondary PM to build up over a few days.

1

 Figures 1.2.B and 1.2.C illustrate the wind 

speed and direction for various levels of PM2.5, and show that elevated levels of PM2.5 occur during 

wintertime conditions with light wind speed. The Bay Area Air Quality Management District analyses 

show that during the winter peak season wood burning can account for 33 to over 80 percent of the 

PM2.5 concentrations. Ammonium nitrate and elemental carbon from diesel sources can also contribute 

to elevated PM2.5 concentrations. 

During summer, the Pacific high pressure system (Pacific High) typically strengthens and resides in the 

northeastern Pacific Ocean, pushing the jet stream and its attendant storm systems well to the north of 

California. In winter, the Pacific High weakens and shifts southward, which allows storm systems to 

come across central California. As a result, almost all of the Bay Area’s annual precipitation takes place 

in the November through April period. During the winter rainy periods, inversions are weak or 

nonexistent, winds are often moderate and air pollution potential is very low. However, during winter

periods when high pressure systems become established over the Bay Area, inversions become strong, 

winds are light and pollution potential is high. These periods are characterized by winds that flow out of 

the Central Valley into the Bay Area and often include tule fog. 

The San Francisco Air Basin encompasses approximately 5,430 square miles and consists of all of 

Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo and Santa Clara counties, the southern 

half of Sonoma County and the southwestern portion of Solano County. The region is characterized by 

 

1

 BAAQMD Reports: Sources of Bay Area Fine Particulates (2008); Fine Particulate Matter Data Analysis and Modeling in 

the Bay Area (2009); Evaluation of the Effectiveness and Benefits of the Wood Burning Rule in the San Francisco Bay Area 

for winter 2009-2010 (2010); Trends in Bay Area Ambient Particulates (2011). 
Appendix C – Maintenance Receptors 

5

complex terrain, consisting of coastal mountain ranges, rugged hillsides, and inland valleys and bays. 

Elevations range from sea level to 1,500 feet. 

EPA believes that, based on the local nature of the PM2.5 problem and the distance of the area from 

Montana, it is reasonable to conclude that emissions from Montana will not interfere with the 

maintenance of the 24-hour PM2.5 standard at these receptors. 
Appendix C – Maintenance Receptors 

6

Figure 1.2.A: San Francisco Bay Area 
Appendix C – Maintenance Receptors 

7

Figure 1.2.B: Alameda County Pollution Rose for 2005-2007 

S

W E

2 4 6 8 10 12+

Wind Speed (mph)

Site 060011001

San Francisco Bay Area, CA [Alameda County, CA]

Pollution Rose, 2005-2007

Year

2004

2005

2006

98th %-ile

27.6

30.4

33.3

# days > 35

.

2

2

Design

Value 30-A

2 exceedance(s) not plotted 

(due to missing or variable wind data) San Francisco Bay Area, CA

Meteorological data from 11.8 miles away

SAN_JOSE_INTL_AP (ID=23293)

2005

2006

2007

Concentration:

 > 40 μg/m3

 35 - 40 μg/m3

 30 - 35 μg/m3

 < 30 μg/m3

Season:

cool (Oct-Apr)

warm (May-Sep)
Appendix C – Maintenance Receptors 

8

Figure 1.2.C: Santa Clara County Pollution Rose for 2005-2007 

 
Appendix C – Maintenance Receptors 

9

1.3 Northern California – Sacramento County 

County Site Name Description 

Sacramento Site ID: 

060670010 

This receptor is in the Sacramento Metro nonattainment area. 

The monitored exceedances likely reflect localized sources occurring 

during wintertime temperature inversions with low winds that persist 

for several days. The speciation data on days during cold seasons in 

2004-2006 with the highest 5 percentile of PM2.5 concentrations show 

that the mass is 47-60% total carbon mass (TCM), which indicates 

residential wood burning as a significantly contributing source of 

emissions. 

(http://www.epa.gov/pmdesignations/2006standards/final/TSD/tsd_F_li

mited_2004-06_speciation_data_report.pdf). 

The largest single source of Sacramento's wintertime PM2.5 emissions 

is wood and other solid fuel burning in fireplaces, wood inserts, wood 

stoves and pellet stoves. 

There is a strong downward trend in this area in recent years 

(http://www.epa.gov/region9/air/trends/pm25.html). 

The Northern Sacramento Valley Air Basin is bounded on the north and west by the Coastal Mountain 

Range, and on the east by the southern portion of the Cascade Mountain Range and the northern portion 

of the Sierra Nevada Mountains. These mountain ranges reach heights in excess of 6,000 feet with peaks

rising much higher. This provides a substantial physical barrier to locally created pollution. 

The maintenance receptor identified above likely reflects localized sources occurring during wintertime 

temperature inversions with low winds that persist for several days in an area that trap emissions 

together with complex topography. The speciation data from high concentration days indicates that the 

PM2.5 has a high carbon mass, which also indicates that residential wood burning emissions are the 

dominant source.2

 Also, as noted above, the largest single source of Sacramento's wintertime PM2.5 

emissions is wood and other solid fuel burning in fireplaces, wood inserts, wood stoves and pellet 

stoves. 

For the area under consideration, high PM2.5 concentrations mostly occur during stagnant conditions 

during winter, with radiation inversions. The cooling of the ground, as heat is radiated away, creates an 

inversion, since air near the ground is cooler than that above. This inhibits mixing and confines 

pollutants to a relatively shallow layer near the ground. A typical value for maximum mixing height 

during high PM2.5 conditions is 300 meters (984 feet) AGL (above ground level). EPA recognizes that 

an inversion height is not a rigid boundary extending through a fixed elevation. In reality the inversion 

would be partly terrain-following, and the degree of stagnation would be subject to additional influences 

 

2

 ARB Almanac website (2006). http://www.arb.ca.gov/ei/maps/statemap/cntymap.htm
Appendix C – Maintenance Receptors 

10

at the foothill edges, such as strong diurnal slope flows. In any case, mixing heights vary by site and 

date, so any single height can provide only a scale for comparison, not a definitive value. Nevertheless, 

the inversion height provides an indicator of the area over which inversions may be enhancing pollution 

concentrations, and of the extent of the area that may interfere with maintenance of the NAAQS. 

Because these areas in Northern California have topographical features higher than the typical daytime

height of the inversion layer, EPA considered the inversion height to estimate the size of the area likely 

to have similar pollution conditions and to interfere with maintenance of the NAAQS. 

In summary, topography is an important factor for this area, given that the inversion layer, which can

lead to winter PM2.5 exceedances in the Sacramento Valley, is blocked by the Sierra foothills. 

The steep terrain and speciated PM2.5 data at this Northern California receptor supports the fact that the 

PM2.5 is largely of local origin. This consideration, combined with the distance between Montana and 

this receptor, make it reasonable to conclude that transport from Montana does not interfere with 

maintenance of the 2006 PM2.5 standard at this receptor. 

1.4. Southern California – Orange, Los Angeles, Riverside, and San Bernardino Counties 

Table 1.4.A: South Coast Air Basin Maintenance Receptor Design Values 

County Site Name Description 

Orange Anaheim The 2008-2010 design value for this site is 30 μg/m3. The 

previous design values were 38 μg/m3 in 2006-2008, and 37 

μg/m3 in 2007-2009. 

The current design value for the Anaheim site is 30 μg/m3, well below the 2006 24-hour PM2.5 

standard. The Anaheim site has shown a consistent downward trend over the last ten years. (see Figure 

1.4.A: 24-Hour Average PM2.5 Design Concentrations 1999-2010). The South Coast Air Quality 

DRAFT 2012 Air Quality Management Plan air quality projections show that the emission control 

strategy outlined in the plan will result in air quality improvement at this site. (see Figure 1.4.B: 

Maximum 24-Hour Average PM2.5 Design Concentrations: 2008 Baseline, 2014 and 2014 Controlled). 

These considerations, combined with the distance from Montana to this receptor, make it reasonable to 

conclude that emissions from Montana will not interfere with the maintenance of the 24-hour PM2.5 

standard at this receptor. 

A more complete discussion of air quality in the South Coast Air Basin is included in Appendix B - 

Non-attainment Receptors at B.1.3. 
Appendix C – Maintenance Receptors 

11

Figure 1.4.A : 24-Hour Average PM2.5 Design Concentrations 1999-2010.3

Figure 1.4.B: Maximum 24-Hour Average PM2.5 Design Concentrations: 2008 Baseline, 2014 and 

2014 Controlled4

 

3

 Joe Cassmassi, Item #2, Scientific, Technical & Modeling Peer Review (STMPR) Advisory Group Meeting #1 for 2012 Air 

Quality Management Plan (AQMP), September 22, 2011.

http://aqmd.gov/gb_comit/stmpradvgrp/2012AQMP/meetings/2011/sept22/stmpr01.pdf 
Appendix C – Maintenance Receptors 

12

1.5 Arizona – Pinal County (Cowtown) 

This monitoring site is a middle scale site and is only comparable to the 24-hour NAAQS due to its 

close proximity to nearby sources. 

Emission inventory data, combined with speciation and source apportionment data, point to agricultural

activities and cattle feedlots, as well as other nearby sources of PM2.5, as primary sources contributing 

to PM2.5 levels at the Cowtown monitor on days with exceedances of the 24-hour PM2.5 NAAQS.5

The Cowtown monitor has a consistent PM2.5/PM10 ratio that is lower than other nearby sites, 

suggesting that the causes of Cowtown PM2.5 concentrations are different than for other locations. The 

correlation of the PM2.5/PM10 ratio with PM2.5 concentrations at the Cowtown monitor also suggests 

that PM2.5 and PM10 concentrations at the Cowtown monitoring site are generated by the same sources. 

Also, the Cowtown monitor is the only site in the area with a pronounced diurnal pattern, with high PM

in the morning and evening hours, further suggesting the influence of local sources. 

EPA believes that, based on the local nature of the PM2.5 problem and the distance of the area from 

Montana, it is reasonable to conclude that emissions from Montana do not interfere with the 

maintenance of the 24-hour PM2.5 standard at this receptor. 

 

4

http://www.aqmd.gov/aqmp/2012aqmp/draft/Chapters/Ch5.pdf

5

 Arizona Area Designations for the PM2.5 24-Hour Fine Particle National Ambient Air Quality Standards, 2011 (76 FR 

6056). 
Appendix C – Maintenance Receptors 

13

Figure 1.5.A: Emission Sources and Design Values within Fifty Miles of the Cowtown Monitor 
Appendix C – Maintenance Receptors 

14

Figure 1.5.B: Cowtown Wind Roses 6

(Note: The different colors in Figure 1.5.B correspond to various levels of PM2.5: 

Blue is less than or equal to 30 μg/m3; Yellow is 30-35 μg/m3; Red is 35-40 μg/m3; and Black greater 

than or equal to 40 μg/m3. Also, values measured between October-April are shown as triangles, and 

values measured between May-September are shown as circles.) 

1.6 Arizona – Santa Cruz County (Nogales) 

The PM2.5 monitoring site in Santa Cruz County is located near the United States border with Mexico. 

(See Figure 1.6.A: Nogales PM2.5 Nonattainment Area). 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). Nighttime drainage flow is typically from south to north along the Nogales Wash 

and 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.

7

 Measurements of PM2.5 composition show that 

the main emission sources are wood burning, food cooking, open burning, dust emissions, and tailpipe 

emissions from on-road and off-road vehicles. 

EPA believes that, based on the local nature of the PM2.5 problem, the direction of flow during periods 

of elevated PM2.5, and the distance of the area from Montana, it is reasonable to conclude that 

emissions from Montana do not interfere with the maintenance of the 24-hour PM2.5 standard at this 

receptor. 

 

6

 Technical Support Document for Determination that the Cowtown Monitor is Ineligible for Comparison with 

the Annual PM2.5 NAAQS, U.S. EPA, Region 9, April 26, 2010. 

7

 Arizona Area Designations for the PM2.5 24-Hour Fine Particle National Ambient Air Quality Standards, 2009 (74 FR 

58688). 
Appendix C – Maintenance Receptors 

15

Figure 1.6.A: Nogales PM2.5 Nonattainment Area 
Appendix C – Maintenance Receptors 

16

2.1 Utah – Utah County 

(Site ID: 490495008) 

A large portion of Utah County is designated as the Provo 2006 PM2.5 nonattainment area (NAA). 

The Provo NAA is situated in the Utah Valley. Like the other low-lying valleys just west of the Wasatch 

Front, the topography of the Utah Valley makes it prone to temperature inversions. During inversions, 

the Provo NAA features stagnant air masses and calm to light winds. As the airshed is essentially 

capped, PM2.5 is formed in-situ (and trapped) via fresh locally derived emission sources and secondary

chemistry. Transport of air masses between the Salt Lake City and Provo NAAs can occur during 

inversions, as there is a 4.75-mile gap in the mountains separating these airsheds which is below the 

approximate average inversion height for these areas. 

Speciation data from exceedance days from 2004-2006 showed the average composition of PM2.5 in 

Provo is 71% ammonium nitrate, with 21% carbon and organic compounds, 8% ammonium sulfate, and 

2% crustal. 

Several factors, including significant mobile, point and area sources contribute to primary and 

secondarily formed PM2.5 that violate the standard during stable weather events associated with strong

inversions. Because of the local nature of these events, it is reasonable to conclude that emissions from 

Montana do not interfere with the maintenance of the 24-hour PM2.5 standard at this receptor. 

2.2 Utah – Salt Lake and Weber Counties (Greater Salt Lake Area) 

(Site IDs: 490353007 (Salt Lake County) and 490570007 (Weber County)) 

 

All of Salt Lake County and a portion of Weber County are designated as part of the Salt Lake City 

(SLC) 2006 PM2.5 nonattainment area (NAA). 

The SLC NAA is situated in a valley bordered on the west by the Stansbury Mountains, Promontory 

Mountains and the Great Salt Lake, and on the east by the Wasatch Front. During inversions, the NAA 

becomes a closed airshed (save for potential transport between the Salt Lake City and Provo NAAs) 

featuring stagnant air masses and light winds which typically oscillate back and forth within the airshed. 

As the airshed is essentially capped during inversions, PM2.5 is formed in-situ (and trapped) via fresh 

locally derived emission sources and secondary chemistry. 

Speciation data from exceedance days from 2004-2006 showed the average composition of PM2.5 in 

Salt Lake City is 58% ammonium nitrate, 31% carbon and organic compounds, 8% ammonium sulfate, 

and 2% crustal. 

Several factors, including significant mobile, point and area sources contribute to primary and 

secondarily formed PM2.5 that violate the standard during stable weather events associated with strong

inversions. Because of the local nature of these events, it is reasonable to conclude that emissions from 

Montana do not interfere with the maintenance of the 24-hour PM2.5 standard at these receptors.