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
Cowtown and the Annual PM2.5 NAAQS 

EPA has concluded that data from the PM2.5 monitor at the Cowtown monitoring station 

(AQS ID: 04-021-3013) in Pinal County should not be compared to the annual PM2.5 

NAAQS. 

40 CFR 58.30(a) states that “(1) PM2.5 data that are representative, not of areawide but 

rather, of relatively unique population-oriented microscale, or localized hot spot, or 

unique population-oriented middle-scale impact sites are only eligible for comparison to 

the 24-hour PM2.5 NAAQS” and “(2) There are cases where certain population- oriented 

microscale or middle scale PM2.5 monitoring sites are determined by the Regional 

Administrator to collectively identify a larger region of localized high ambient PM2.5 

concentrations. In those cases, data from these population-oriented sites would be 

eligible for comparison to the annual PM2.5 NAAQS.” 

For example, if the PM2.5 monitoring site is adjacent to a unique dominating local PM2.5

source or can be shown to have unique annual average concentrations representative 

of a smaller than neighborhood spatial scale, then data from a monitor at the site would 

only be eligible for comparison to the 24-hour PM2.5 NAAQS.1

 However, if there are 

potentially many such small scale impacts throughout the monitoring region, then the 

established monitoring site may be judged to be representative of community-wide 

exposure and eligible for comparison to the annual standard. 

EPA has evaluated the comparability of data from the Cowtown site to the annual PM2.5 

standard on four criteria: the population oriented monitoring, the spatial scale, localized 

hot spot conditions, and the uniqueness of the site, 

Population-oriented monitoring:

40 CFR 58.1 states that “population-oriented monitoring (or sites) means 

residential areas, commercial areas, recreational areas, industrial areas where 

workers from more than one company are located, and other areas where a 

substantial number of people spend a significant fraction of their day”. 

There are four different feedlot facilities in the area: Sawyer Cattle Company, OK 

Cattle Co., Maricopa Feedyard LLC, and Pinal Feeding Co.2,3 Also, there is an 

ethanol/ grain processing facility and a commercial composting facility located 

nearby. The Cowtown site is representative of the conditions that workers from 

these various facilities may be exposed to and thus can be considered 

“population-oriented.”The Pinal County 2009 annual network plan states that the 

site’s type is “population/source impact”. This is an appropriate description of the 

 

1

40 CFR 58.30 (a) (1) 

2

 Dun & Bradstreet Agricultural Data 

3

 See Figure 1 
situation at hand and is consistent with EPA guidance which states that “a site 

may be population-oriented and at the same time be source oriented or reflective 

of maximum concentration”.4

Spatial Scale:

The preamble to the 2006 monitoring regulations states: “In practice, the majority 

of PM2.5 monitors are deployed at neighborhood scale and larger, meaning that 

they are located far enough from large emission sources that they represent the 

fairly uniform air quality across an area with dimensions of at least a few 

kilometers and thus can be considered community oriented”.5

 In addition, the 

CFR states, specifically for PM2.5, that a neighborhood scale site would represent 

“conditions throughout some reasonably homogenous urban sub-region with 

dimensions of a few kilometers”.6

 Based on available evidence, EPA believes 

that the area of reasonably homogenous concentrations around this monitor is 

considerable smaller than a few kilometers in diameter or width. Moreover, the 

area around the monitor, even extending out several kilometers, is definitely not 

associated with community-wide exposure. 

The Cowtown monitor is surrounded by agricultural fields and cattle feedlots, and 

does not represent the conditions expected in urban sub-regions or communitywide population PM2.5 exposure. Furthermore, the CFR gives an example of a 

neighborhood scale site: “such a site would likely be located in a residential or 

commercial area having a high overall PM2.5 emission density but not in the 

vicinity of any single dominant source”.7

 The preamble also states “The EPA is 

presently aware of fewer than ten PM2.5 monitors that are sited in relatively 

unique population-oriented microscale areas, localized hot spots, or unique 

population-oriented middle-scale areas. Such sites may have higher 

concentrations than neighborhood scale sites on at least some days because 

they may be close to and downwind of large emission sources, but the number of 

people exposed to such concentrations is not large relative to the surrounding 

communities”.4 It is very clear that the Cowtown monitor is located in the vicinity 

of a dominant source and does not meet this definition of neighborhood scale 

and community-wide exposure. 

When determining the appropriate spatial scale for a particular site, the 

homogeneity of the particulate matter concentrations, land use, and land surface 

characteristics must also be considered. The Cowtown site is located near a 

 

4 Guidance for Network Design and Optimum Site Exposure for PM2.5 and PM10: EPA-454/R-99-022 

5

61264 Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 

6

40 CFR App. D 4.7.1 (c) (3) 

7

 40 CFR App. D 1.2 (d) 
number of cattle feedlots that have been shown, through chemical speciation8

, to 

contribute to elevated PM2.5 concentrations at the site. Analysis of wind and 

pollution roses during exceedance days also shows that the Cowtown monitor is 

significantly influenced by the cattle feedlots located upwind. The majority of the 

elevated concentrations occur during periods when there are light winds (≤ 8 

mph) from the South and Southeast.9

 

Due to the close proximity of a known source, the PM2.5 concentrations measured 

at the Cowtown site are higher in magnitude than surrounding areas and do not 

represent homogenous particulate concentrations in an area of a few 

kilometers10. In fact, modeling studies show that there are very sharp gradients 

within several km of an emission sources.11 EPA guidance further explains that 

“neighborhood scale monitors do not show significant differences in particulate 

concentrations with spacing of a few kilometers”.12 This suggests that a similar 

monitor located a few kilometers away should experience the similar levels of 

particulate matter. Again, due to the close proximity of a known source, it is 

reasonable to assume that PM2.5 concentrations a few kilometers away may be 

significantly different than those measured at the Cowtown site. Based on the 

available information, the Cowtown monitor can be considered to be 

representative of an area smaller than a neighborhood spatial scale and does not 

represent community-wide exposure. As stated in the Pinal County 2009 annual 

network plan, the Cowtown monitor is representative of a microscale 

environment. 

Localized Hot Spot:

The monitoring regulations have limited discussion about “hot-spots”. The term is 

generally associated with areas of highest concentrations whether it be from 

mobile or multiple stationary sources. Microscale and middlescale monitors are 

mentioned.13 For the purpose of this discussion, we will treat hot-spot as 

synonymous with monitoring locations representing either a micro or middlescale 

environment.

Uniqueness: 

PM2.5 concentrations recorded at this site are consistently higher in magnitude 

than all surrounding sites in the county. Also, preliminary analysis shows that 

 

8

 Pinal County Air Quality Control District Source Apportionment Study: July 29, 2005 

9

 See Figure 2 and Figure 3 

10 See Figure 1 and 4 

11 Brode, Review of TRC Comments on PM NAAQS (2006). 

12 EPA-454/R-99-022

13 Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations
PM2.5 measurements at the Cowtown site are not correlated to any other sites in 

the area. 

There are seven14 feedlots in Pinal County, and most are in the vicinity of 

Cowtown. However, the Cowtown monitor may be representative of a relatively 

unique source of emissions. The monitor is immediately downwind of the 

“Cowtown Complex” which consists of four different cattle feedlot facilities: 

Sawyer Cattle Company, OK Cattle Co., Maricopa Feedyard LLC, and Pinal 

Feeding Co15. This accounts for a total of approximately 424 acres of feedlot 

facilities. Also, there is an ethanol/grain processing facility and a commercial 

composting facility located nearby. This source mix represents a relatively unique 

situation in Pinal County. 

The remaining feedlots are located in the areas to both the East and West 

Stanfield and South of Picacho Peak near the border of Pima County.16 The only 

comparable situation to that of Cowtown is the “Red River Complex” west of 

Stanfield, which has approximately 278 acres of feedlot facilities that are in close 

proximity to a number of dairies. This location may be considered different than 

Cowtown: the feedlot sources are smaller and the overall source mix is 

considerably different. Dairies have been shown to have lower PM10 emissions 

than those of cattle feedlots.17 Also, the data show that the Stanfield PM10

monitor located approximately 4 km from the nearest feedlot, records much lower 

PM10 concentrations: the 2006-2008 average PM10 expected exceedances at the 

Cowtown station is approximately 10 times higher than the Stanfield monitor.18 It 

is reasonable to assume that this location also experiences lower PM2.5 

concentrations than the Cowtown monitor. This is reinforced by the Pinal County 

Source Apportionment Study that showed there was little contribution from the 

cattle feedlots at the Stanfield monitoring station for both PM10 and PM2.5.

19

 

The areas to the East of Stanfield and to the South of Picacho Peak contain 

feedlots of much lesser size. The total areas of these locations, the “Benedict 

Spread” and the “Red Rock Feedlot”, are approximately 186 and 70 arces, 

respectively. While there are other feedlots within Pinal County, it appears that 

the Cowtown monitor represents a relatively unique monitoring location due to 

the size and composition of the nearby sources. 

 

14 Pinal County Advanced Notice of Proposed Rule Making: April 9, 2009 

15 Dun & Bradstreet Agricultural Data 

16 See Figure 5 

17 http://www.arb.ca.gov/ei/areasrc/arbmiscprocresfarmop.htm

18 2009 Pinal County Annual Network Plan 

19 See Figure 6 
Conclusion

40 CFR 58.30 – states that a “relatively unique population-oriented microscale, or 

localized hot spot, or unique population-oriented middle-scale impact sites” is only 

eligible for comparison to the 24-hour PM2.5 NAAQS, not the annual PM2.5 NAAQS. 

Based on the analysis above, the Cowtown site should be considered a relatively 

unique, population-oriented, microscale site Furthermore, the monitoring site is located 

in close proximity to a “unique dominating local PM2.5 source” and is likely to have 

“concentrations representative of a smaller than neighborhood spatial scale.” 

Therefore, the PM2.5 data from the Cowtown site in Pinal County should not be 

compared to the annual PM2.5 NAAQS. 
Figure 1: PM2.5 Design Values 

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50-Mile Buffe

Mesa24-Hr DV: 17.6

Annual DV: 9.3

Orang24-hr 

Annua

South Phoenix24-hr DV: 26.9

Annual DV: 12

West Phoenix24-hr DV: 26.8

Annual DV: 11.6

JLG Supersite24-hr DV: 22

Annual DV: 9.5

Casa Grande24-Hr DV: 19.9

Annual DV: 9.5

Apache Junction

24-Hr DV: 13.1

Annual DV: 6.6

Scottsdale24-Hr DV: 15.1

Annual DV: 7.7

Cowtown

24-Hr DV: 47.8

Annual DV: 21.6

296

37

487

163

67

429

230

226

201 118

9

54

47 45

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482 485

461

440

426 418

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407

374 341 311 333 292

285

Maricopa County

Pinal County

Pima County

G

La Paz County

§ ̈¦10

§ ̈¦17

§ ̈¦19

Emission Sources Within Fifty Miles of the Cowtown PM-2.5 Monitor

AIR0901679_

Sources: NEI (2005), AQS (2009), TANA (2006), USGS (2006), ESRI (2006), Du

!C(

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Feedlots Near Cowtown Monitor

Cowtown

24-Hr DV: 47.8

Annual DV: 21.6

!C( PM-2.5 Monitor Violating 2006 24-Hour Std!C PM-2.5 Monitor Attaining 2006 24-Hour Std

#* Additional Cattle Operations (MFlagg)#* Feedlots#* Dairies

Facilities Emitting > 1 TPY PM-2.5

1 - 10 Tons Per Year10.1 - 25 TPY25.1 - 50 TPY50.1 - 100 TPY100.1 - 125 TPY

City boundariesRes: 1 : 284550.498High : 2786Low : 113

Numbers refer to corresponding OBJECTID record in Emitting_Facilites.xls

0 

5 
Figure 2: Cowtown Pollution Rose20

Figure 3: Cowtown Wind Roses 20 The different colors correspond to various levels of PM2.5: Blue ≤ 30 μg/m3, Yellow 30-35 μg/m3, Red 35-40 μg/m3, and Black ≥ 40 μg/m3. Als

October-April are shown as triangles, and values measured between May-September are shown as circles. 
Figure 4: Spatial Scale 
Spatial Scale Surrounding Cowtown Monitor

AIR0901679_

Sources: GlobeXplorer (April 1, 2008), Dunn and Bradstreet (2008), Arizona State Land De

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Feedlots NearCowtown Monitor

Cowtown

24-Hr DV: 47.8

Annual DV: 21.6

4000 meters

2000 meters

500 meters

100 meters

Maricopa 

City Limits

Figure 5: Main Feedlot Locations 
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50-Mile Buffer

Mesa24-Hr DV: 17.6

Orange 24-Hr DV

Ch24-

South Phoenix24-Hr DV: 26.9

West Phoenix24-Hr DV: 26.8

JLG Supersite24-Hr DV: 22

Casa Grande24-Hr DV: 19.9

Apache Junction

24-Hr DV: 13.1

Scottsdale24-Hr DV: 15.1

Cowtown

24-Hr DV: 47.8

Maricopa County

Pinal County

Pima County

Gil

La Paz County

Tohono O'Odham

Gila River

Salt River

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Cluster 3

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Cluster 2

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Cattle Operations Near Cowtown

AIR0901679_5 

Sources: NEI (2005), AQS (2009), TANA (2006), USGS (2006), ESRI (2006), Dunn 

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Feedlots Near Cowtown Monitor

Cowtown

24-Hr DV: 47.8

Cattle operations cluster boundaries

!C( Violating PM-2.5 Monitor!C Active PM-2.5 Monitor

#* Additional Cattle Operations#* Feedlots#* Dairies

Tribal LandsCity boundaries12,000+ feet~100 feet

0 10 20 5 Miles  ̅

Figure 6: 2003 Speciation Results