Exhibit 1 

Assessment of Compliance with the PM2.5 National Ambient Air Quality 

Standard (NAAQS) for the State of Arizona 

(Exceedances and violations are highlighted) 

Table 1: 2004 to 2006 Annual Average PM2.5 Compliance (in μg/m3) 

City or Site 

Federal Reference Monitors 2004 2005 2006 

ThreeYear 

Average 

Cochise County 

Douglas Red Cross 7.11 7.33 6.78 7 

Coconino County 

Flagstaff Middle School 6.77 6.01 6.61 6 

Gila County 

Payson Well Site 9.54 8.38a 9.04 N/A 

Maricopa County 

JLG Supersite 9.73 9.72 10.22 10 

Mesa (Opened 4/28/2005) N/A 8.35a 9.66 N/A 

South Phoenix (Opened 1/1/2005) N/A 12.84 12.69 N/A 

Tempe Community Center 

(Closed 7/26/2004) 

7.30a N/A N/A N/A 

West Phoenix 11.60 11.87 13.52 12 

Pima County 

Children’s Park 5.57 5.91 5.79 6 

Orange Grove 5.79 6.32 5.80 6 

Pinal County 

Apache Junction Fire Station 5.5a 5.52 5.31 N/A 

Casa Grande Downtown 7.13 7.33 7.55 7 

Santa Cruz County 

Nogales Post Office 10.83 13.1 16.19 13 

a Indicates the data do not satisfy EPA’s summary criteria, usually meaning less than 75 percent valid data 

recovery available in one or more calendar quarters. 
Table 2. 2004 to 2006 24-Hour Average PM2.5 Compliance (in μg/m3

)

98th Percentile 

Samples City or Site b

Federal Reference Monitors 

2004 2005 2006 

ThreeYear 

Average 

Cochise County 

Douglas Red Crossd 22.5 16.0 14.0 18 

Coconino County 

Flagstaff Middle Schoole 20.7 12.7 13.7 16 

Gila County 

Paysond 19.3 22.9 23.4 22 

Maricopa County 

JLG Supersitee 27.6 28.2 24.6 27 

Mesa (Opened 4/28/2005)e N/A 17.5 20.1 N/A 

South Phoenixe

(opened 1/1/2005) 

N/A 36.4f 28.8 N/A 

Tempe Community Center (Closed 

7/26/2004) 

14.8 N/A N/A N/A 

West Phoenixe 29.9 40.5f 28.8 33 

Pima County 

Children’s Parke 10.3 10.7 12.1 11 

Orange Grovee 13.3 13.7 16.3 14 

Pinal County 

Apache Junction Fire Statione 10.3 10.6 9.3 10 

Casa Grande Downtownd 13.7 16.9 15.4 15 

Santa Cruz County 

Nogales – Post Officed 25.1 33.0 56.2f 38g 

b The 98th percentile value will be the second highest value for sites on an every 6th day sample schedule. 

The 98th percentile value will be the 3rd highest value for sites on an every 3rd day sample schedule. 

c

 Samples collected every day – 365 sample days in 2006. 

d Samples collected every sixth day - 61 sample days in 2006. 

e Samples collected every third day - 122 sample days in 2006. 

f Exceedance of the 24-hour average NAAQS of 35 μg/m3 

g Violation of the 24-hour average NAAQS of 35 μg/m3
Technical Support Document 

for Recommendation that 

 the Nogales, Arizona Area 

Be Designated as a PM2.5 Nonattainment Area 

Air Quality Division 

Arizona Department of Environmental Quality 

Janet Napolitano, Governor 

Stephen A. Owens, Director 

December 3, 2007 
i

TABLE OF CONTENTS 

INTRODUCTION.......................................................................................................................................1 

PM2.5 MONITORING.................................................................................................................................1 

Analysis of Monitoring Data ....................................................................................................................2 

GEOGRAPHY AND TOPOGRAPHY .....................................................................................................2 

Analysis of Geography and Topography..................................................................................................3 

DEMOGRAPHICS.....................................................................................................................................3 

Santa Cruz County, Arizona and Nogales, Arizona .................................................................................3 

Nogales, Sonora, Mexico..........................................................................................................................7 

Summary of Population Data ...................................................................................................................8 

Population Density .................................................................................................................................10 

Analysis of Demographics......................................................................................................................10 

JURISDICTIONAL BOUNDARIES ......................................................................................................11 

Analysis of Jurisdictional Boundaries ....................................................................................................11 

PLANNED DEVELOPMENTS...............................................................................................................12 

Nogales, Arizona ....................................................................................................................................12 

Nogales, Sonora, Mexico........................................................................................................................12 

Analysis of Planned Developments........................................................................................................12 

METEOROLOGY....................................................................................................................................15 

Temperatures ..........................................................................................................................................15 

Geography and Drainage........................................................................................................................15 

Variations in Localized Winds ...............................................................................................................15 

ADEQ Wind Monitoring in Nogales, Arizona .......................................................................................15 

Synoptic Weather Patterns in the Southwest US....................................................................................16 

Analysis of Meteorology ........................................................................................................................17 

TRANSPORT OF EMISSIONS – MEXICO TO U.S............................................................................17 

Analysis of Transport of Emissions........................................................................................................18 

EMISSION SOURCES.............................................................................................................................18

Industrial Point Sources.......................................................................................................................18 

Nogales, Arizona................................................................................................................................ 18 

Nogales, Sonora, Mexico ................................................................................................................... 18 

Growth of Point Sources.................................................................................................................... 18 

Area Sources .........................................................................................................................................21 

Nogales, Arizona................................................................................................................................ 21 

Nogales, Sonora, Mexico ................................................................................................................... 21 

Growth of Area Sources..................................................................................................................... 21 
ii

Onroad Mobile Sources........................................................................................................................21 

Growth of Onroad Mobile Sources.................................................................................................... 22 

Nonroad Mobile Sources......................................................................................................................23 

Growth of Nonroad Mobile Sources.................................................................................................. 23 

Analysis of Emission Sources...............................................................................................................23 

COMMUTING PATTERNS....................................................................................................................24 

Analysis of Commuting Patterns............................................................................................................24 

CONTROL MEASURES .........................................................................................................................25 

Analysis of Control Measures ................................................................................................................25 

SUMMARY OF ANALYSES ..................................................................................................................25 

CONCLUSIONS .......................................................................................................................................27 

REFERENCES..........................................................................................................................................28 

APPENDIX A – ASU’s Nogales Winter Study.......................................................................................30

LIST OF TABLES

Table 1 - Nogales, Arizona PM2.5 Monitoring Data for Years 1999 - 2006.................................................1 

Table 2 - Land Ownership in Santa Cruz County ........................................................................................3 

Table 3 - Population Data for Ambos Nogales ............................................................................................8 

Table 4 - Population Projections for Ambos Nogales.................................................................................8 

Table 5 - Population Projections for Rio Rico CDPs..................................................................................9 

Table 6 – Population Projections for Nogales PM10 Nonattainment Area ...................................................9 

Table7 - Persons Per Square Mile for Ambos Nogales..............................................................................10 

Table 8 - Wind Climatology for Nogales, Arizona ....................................................................................15 

Table 9 - PM2.5 and PM10 Emissions from Industrial Point Sources – Santa Cruz County ........................20 

Table 10 - Registered Vehicles in Santa Cruz County and Nogales, Sonora, Mexico...............................21 

Table 11 - Santa Cruz County July 2007 Vehicle Registration Report.....................................................22 

Table 12 -Nogales, Arizona Ports of Entry – Vehicle and Train Traffic ...................................................24

LIST OF FIGURES 

Map 1 Arizona & Sonora Highway System................................................................................................4 

Map 2 Topography of Ambos Nogales .......................................................................................................5 

Map 3 Land Ownership in Santa Cruz County ...........................................................................................6 

Map 4 Jurisdictional Boundaries of Ambos Nogales................................................................................13 

Map 5 Planned Developments in Nogales, Arizona..................................................................................14 

Map 6 Drainage from Nogales, Sonora to Nogales, Arizona....................................................................16 

Map 7 Locations of Emission Sources......................................................................................................19
1

INTRODUCTION 

EPA has requested Governors to recommend designation of nonattainment areas in their jurisdictions 

under the revised PM2.5 National Ambient Air Quality Standard (NAAQS) and submit these 

recommendations to EPA by December 18, 2007. The PM2.5 monitor in Nogales, Arizona, operated by 

ADEQ, has recorded two exceedances of the 24-hour average PM2.5 NAAQS of 35 μg/m3

 and one 

exceedance of the annual average PM2.5 NAAQS of 15 μg/m3 during eight years of PM2.5 monitoring at 

that site. As a result of these exceedances, ADEQ is recommending that the Nogales, Arizona area be 

designated as a PM2.5 Nonattainment Area. To select boundaries for the proposed Nogales PM2.5

Nonattainment Area, ADEQ examined the following ten criteria: (1) PM2.5 monitoring data, (2) 

geography and topography, (3) demographics, (4) jurisdictional boundaries, (5) planned developments, 

(6) meteorology, (7) transport of PM2.5 emissions, (8) emission sources, (9) commuting patterns, and (10) 

control measures (EPA 2007). This document is organized into ten sections corresponding to the 

preceding ten criteria. 

PM2.5 MONITORING 

The following table summarizes the 24-hour PM2.5 measurements recorded at the Nogales Post Office 

monitor in Nogales, Arizona. 

Table 1 - Nogales, Arizona PM2.5 Monitoring Data (EPA AQS Site ID: 04-023-0004) 

 24-Hour and Annual Averages for Years 1999 - 2006

Year 1st 24-Hour 

Maximum 

2nd 24-Hour 

Maximum 

3rd 24-Hour 

Maximum 

4th 24-Hour 

Maximum 

Annual Average 

1999 45.9 39.1 27.9 21.4 12.52

2000 36.0 34.4 31.5 27.6 12.80

2001 35.2 25.7 20.2 17.6 10.71

2002 29.7 25.4 25.3 24.0 12.18

2003 37.0 35.0 25.1 23.4 11.30

2004 28.0 25.1 22.0 18.2 10.83

2005 49.7 33.0 31.8 28.9 13.11

2006 79.8 56.2 48.5 47.0 16.19

On October 17, 2006, EPA lowered the 24-hour PM2.5 standard from 65 μg/m3 to 35 μg/m3.

 The effective date 

of the new standard was December 18, 2006. The annual PM2.5 standard remained the same at 15 μg/m3

. 

 24-hour PM2.5 Exceedances are highlighted and bolded: 

• 79.8 μg/m3 on December 31, 2006 

• 56.2 μg/m3

 on December 25, 2006 

Note: A 48.5 μg/m3 PM2.5 measurement occurred on June 16, 2006, and a 47.0 μg/m3 PM2.5 measurement occurred on 

January 11, 2006. These were not considered 24-hour PM2.5 exceedances because these dates were before the December 

18, 2006 effective date for the lower 24-hour PM2.5 standard of 35 μg/m3

. 

Annual PM2.5 Exceedance is highlighted and bolded: 16.19 μg/m3 in Year 2006 

Data Source: USEPA AQS website (www.epa.gov/ttn/airs/airsaqs/aqsweb/) 
2

Analysis of Monitoring Data 

Review of the 24-hour PM2.5 monitoring data in Table 1 shows that only two years of the 1999 – 2006 

monitoring record had 24-hour maximums less than the current EPA PM2.5 standard of 35 μg/m3

. Time 

lapse photography of the pollutant plume from Nogales, Sonora and emission analyses studies conducted 

in the Nogales PM10 Nonattainment Area (Arizona Department of Environmental Quality, 1993; ENSR, 

1999) indicated that the current location of the Nogales PM10 Monitor at the Nogales, Arizona Post Office 

is adequate for measuring PM10 levels in the nonattainment area. Based on previous air quality studies 

that show the current location of the Nogales, Arizona PM10 monitor is adequate for measuring PM10

levels in the Nogales PM10 Nonattainment Area, ADEQ concluded that the current location of the PM2.5

monitor at the Nogales, Arizona Post office is also adequate for measuring PM2.5 levels in the proposed 

Nogales PM2.5 Nonattainment Area. 

GEOGRAPHY AND TOPOGRAPHY 

Nogales, Arizona is located 60 miles south of Tucson, Arizona in the mid-southernmost portion of Santa 

Cruz County. Nogales, Sonora, México lies directly south of Nogales, Arizona across the international 

border. Nogales, Sonora is 140 miles north of Hermosillo, Sonora. 

Referred to as Ambos Nogales, the communities of Nogales, Arizona and Nogales, Sonora, México 

comprise the largest international border community in Arizona, with a combined population of 177,732 

inhabitants as of the 2000 U.S. and Mexican censuses (U.S. Census Bureau 2000, Instituto Nacional de 

Estadistica Geografia e Informatica 2000). Ambos Nogales is located in the Sonoran Desert; this desert 

covers 120,000 square miles and has a minimum elevation of 2,500 feet (McGinnies 1976). The Sonoran 

Desert is in the Basin and Range topographic province, characterized by north-south elongated valleys 

surrounded by mountain ranges. Ambos Nogales is located in such a valley created by the Nogales Wash, 

a tributary of the Santa Cruz River. The narrow valley topography can trap air pollutants, especially 

during the evening hours when diurnal wind flow is out of the south from Nogales, Sonora, México. The 

mean elevation in Nogales, Arizona is 3,865 feet, while that in Nogales, Sonora is 4,265 feet (Instituto 

Nacional de Estadistica Geografia e Informatica 2005). 

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 U.S. Interstate 19 

which connects Tucson, Arizona to Nogales, Arizona and continues south into México, where it becomes 

Federal Highway 15. See Map 1 for highway system in Arizona and Sonora. The Alvaro Obregón 

Boulevard, Luis Donaldo Colosio Boulevard, and the Corredor Fiscal toll road are the main transportation 

routes in Nogales, Sonora. The Alvaro Obregón Boulevard runs the length of the narrow Nogales Wash 

valley, and is a highly congested route used by local and some cross-border commercial traffic. The 

Corredor Fiscal is a toll road for cross-border commercial traffic, and its on- and off-ramp access is 

limited to the border junction at Mariposa Road, and in the south outside of the city limits of Nogales, 

Sonora. This road absorbs the majority of cross-border commercial traffic. 

Mountain ranges near Ambos Nogales include the Patagonia Mountains to the east and the Tumacacori, 

Atascosa, and Pajarito mountains to the west. Approximately 25 miles to the north are the Santa Rita 

Mountains and Madera Canyon in the Coronado National Forest, where Mount Wrightson rises to an 

elevation of 9,432 feet. Northwest of Interstate 19 are the Cerro Colorado, Las Guijas, and Sierrita 

Mountain Ranges. In Nogales, Sonora the highest elevation areas (5,380 feet) are in the Cerro de los 

Nogales (Nogales’ Hill), west of where the Obregón and Colosio routes meet, near the southern end of the 

city. See Map 2 for the topography of the Ambos Nogales Area; city boundaries of Nogales, Arizona and 

Nogales, Sonora; boundaries of the Nogales PM10 Nonattainment Area, and location of the PM2.5 monitor 

in Nogales, Arizona. 
3

Analysis of Geography and Topography 

From the preceding discussion and review of Map 2, it can be seen that the current Nogales PM10

Nonattainment Area encompasses both the mountain valley where Nogales, Arizona is located and a 

buffer area of mountains outside of this valley. Thus, the Nogales PM10 Nonattainment Area contains 

topographic features that form an airshed that traps emissions generated in the nonattainment area. Based 

on the topography of the area, ADEQ concluded that the Nogales PM2.5 Nonattainment Area boundaries 

should coincide with the current boundaries of the Nogales PM10 Nonattainment Area. 

 

DEMOGRAPHICS 

This section presents the demographics of the Ambos Nogales area, including degree of urbanization, 

population growth (past, present, and future), and population density. The Mexican population has 

consistently exceeded the U.S. population in Ambos Nogales by an order of magnitude, and this is 

expected to continue for the foreseeable future. 

Santa Cruz County, Arizona and Nogales, Arizona 

The 2000 U.S. Census classified the population of Santa Cruz County as 68 percent urban and the 

population of Nogales, Arizona as 94 percent urban (U.S. Census Bureau 2000). The urbanized 

population is expected to continue to grow. Between 1980 and 2007, Santa Cruz County grew from a 

population of 20,459 to 46,907 persons, representing a growth rate of 129 percent. 

Santa Cruz County contains a total of 791,632 acres. Land ownership in Santa Cruz County consists of a 

mix of private land, US Forest Service, Bureau of Land Management, and State of Arizona Trust Land. 

Table 2 lists the number of acres and percent of Santa Cruz County by land ownership

Table 2 - Land Ownership in Santa Cruz County 

Land Ownership Acres Percent 

Coronado National Forest 417,738 52.769 

Private Land 297,816 37.621 

State Trust Land 61,706 7.795 

Bureau of Land Management. 13,550 1.712 

Parks and Recreation 600 0.076 

Other 212 0.027 

Tumacacori National Monument 10 0.001 

Total Acres 791,632 100 

Data Source: Arizona State Land Department, Arizona Land 

Resources Information System. 1994 

See Map 3 for land ownership in Santa Cruz County, Nogales PM10 Nonattainment Area boundaries, 

township and range, and location of Longitude 111° West in relation to the nonattainment area. 
Map 1 Arizona & Sonora Highway System 

4
Map 2 Topography of Ambos Nogales 

5
Map 3 Land Ownership in Santa Cruz County 

6
7

The mid-year 2007 population estimate for Nogales, Arizona, is 21,746, compared to a population of 

19,489 in 1990 and 15,683 in 1980. Compared to the County’s growth rate, this represents a relatively 

slow growth of about 39 percent over the 27-year time frame, or one-third the growth rate of Santa Cruz 

County. The County is estimated to have grown by 22 percent compared to only 4 percent growth for 

Nogales, Arizona from 2000 to 2007 (Table 3). This growth trend is expected to continue (Arizona 

Department of Economic Security 2007). Due to the greater growth in Santa Cruz County, the proportion 

of the county population residing in Nogales, Arizona, has steadily declined from about 77 percent in 

1980 to 46 percent in 2007. 

Between 2007 and 2015, the population of Santa Cruz County is projected to increase 20 percent to 

56,144 residents, compared to a significantly slower growth rate of 10 percent, or 23,858 residents for 

Nogales, Arizona (Table 4). Regardless, Nogales, Arizona will still represent a plurality of the population 

of Santa Cruz County. 

About 82 percent of the resident population of Santa Cruz County is classified as “Minority” (other than 

White). Persons of “Hispanic or Latino” ethnicity comprise more than 98 percent of the minority 

population. In comparison, more than 94 percent of the resident population of Nogales, Arizona, is 

classified as “Minority” with 99 percent comprising the category of “Hispanic or Latino” ethnicity (U.S. 

Census Bureau, Census 2000 Redistricting Summary File). 

In 2000, the population of the Nogales PM10 Nonattainment area was estimated at 22,500 persons, 

comprising 59 percent of Santa Cruz County’s population of 38,381. The nonattainment area includes the 

City of Nogales, Arizona, the southern part of Rio Rico, and unincorporated portions of Santa Cruz 

County. The population residing in the nonattainment area was estimated using a Geographic Information 

System (GIS) to determine the census blocks and block groups contained in the nonattainment area and 

summing the population counts. 

Nogales, Sonora, México 

The residents of Nogales, Sonora, México reside on the slopes of the narrow Nogales Wash valley that 

extends from south to north, as well as in the southern portion of the city. The entirety of the population 

of Nogales, Sonora is classified as urban by the Mexican Census, although many neighborhoods — and 

specially those in the urban fringe — lack basic urban services, such as paved roads, potable water, and 

adequate housing stock (Declet 2006). The Mexican Census of 2000 reported a population of 156,854 for 

Nogales, Sonora (INEGI 2005). This represents a growth rate of approximately 50 percent since 1990. 

The intradecadal household population survey of 2005 reported that the Nogales, Sonora population had 

increased to 193,517 (INEGI 2005). Due to the potential for census undercounts on both sides of the 

border, the resident populations of Ambos Nogales could be higher than documented. 

Nogales, Sonora experienced a growth rate of more than 23.4 percent during the five years between 2000 

and 2005, which averages an annual growth rate of 4.7 percent. If that same growth rate continues, by 

2015, the population of the city could reach more than 300,000, although the official projections are 

slightly more conservative and predict 242,335 residents (Table 4). If growth for Nogales, Sonora stays 

within these values the city would have a population that would be 10 to 11 times the size of Nogales, 

Arizona. Such a large disparity in population size between Nogales, Arizona, and Nogales, Sonora, 

México will mean a higher potential for transport of air pollutants, including PM2.5, from Nogales, Sonora 

into Nogales, Arizona. 

Rio Rico, Arizona 

Río Rico is an unincorporated community in Santa Cruz County that is located 12 miles north of the U.S.-

México border and 57 miles south of Tucson. It is made up of 39,000 acres on the foothills of the Santa 
8

Rita Mountains and has an elevation of 3,418 feet above sea level. Río Rico was part of the Baca Float, a 

100,000-acre U.S. Congress grant to the descendants of Luis María Baca honoring an earlier Spanish 

government grant (Arizona Department of Commerce 2007). The community began in 1969. 

 

According to the U.S. Census, Río Rico is listed as four Census Designated Places (CDPs): Rio Rico 

Northeast, Rio Rico Northwest, Rio Rico Southeast, and Rio Rico Southwest. As of 2000, the total 

number of inhabitants in the four Rio Rico CDPs was listed as 10,413 inhabitants (U.S. Census 2000). 

Economic activities include light manufacturing, and produce staging and distribution in the industrial 

facilities of the Río Rico South Industrial Park. Most of Rio Rico Southeast CDP and a portion of Rio 

Rico Southwest CDP are in the Nogales PM10 Nonattainment Area. 

Summary of Population Data 

Table 3 summarizes historical and current population data for the Ambos Nogales area. 

Table 3 - Population Data for Ambos Nogales 

1980 1990 2000 

Nogales, AZ 15,683 19,489 20,878 

Santa Cruz County 20,459 29,676 38,381 

Nogales, AZ / Santa Cruz County 77% 66% 54% 

Nogales, Sonora 65,603 105,873 156,854 

Sonora Municipio 1,513,700 1,823,600 2,217,000 

Nogales, Sonora / Sonora Municipio 4% 6% 7% 

Table 4 summarizes population projections for the Ambos Nogales area. It is not known how much of 

Santa Cruz County’s population increase will occur in the Nogales PM10 Nonattainment Area, but it is 

likely that the nonattainment area’s population growth will be minimal because the population projections 

for Nogales, Arizona are relatively small. 

The population of Sonora is expected to increase by 7 percent to 2,631,985 persons, while Nogales, 

Sonora’s population is projected to increase from 203,719 to 242,335 residents, a jump of 19 percent. 

Table 4 - Population Projections for Ambos Nogales 

2007 2008 2009 2010 2011 2012 2013 2014 2015 

Nogales, AZ 21,746 22,454 22,659 22,863 23,065 23,265 23,465 23,662 23,858 

Santa Cruz County 46,907 47,777 48,998 50,210 51,418 52,607 53,800 54,973 56,144 

Nogales, AZ / 

Santa Cruz County 

48% 47% 46% 46% 45% 44% 44% 43% 42% 

Nogales, Sonora 203,719 208,901 213,976 218,948 223,820 228,591 233,266 237,845 242,335 

Sonora Municipio 2,463,707 2,487,608 2,510,562 2,532,639 2,553,919 2,574,433 2,594,255 2,613,415 2,631,985 

Nogales, Sonora 

 / Sonora 

Municipio

8% 8% 9% 9% 9% 9% 9% 9% 9% 

Data Sources: 

(1) Arizona Department of Economic Security, July 2, 2007 Population Estimates for Arizona’s Counties, Incorporated Places, and 

Balance of County. 

(2) Nogales, Arizona population projections submitted to POPTAC by SEACOG. 

(3) CONAPO (Mexican Population Projections, 2005-2050).
9

Table 5 lists the population projections for the unincorporated community of Rio Rico. The majority of 

the Rio Rico SE CDP (89%) is in the Nogales PM10 Nonattainment Area and a portion of the Rio Rico 

SW CDP (19%) is in the Nogales PM10 Nonattainment Area. Moderate population growth is projected 

for the Rio Rico CDPs from 2007 to 2015.

Table 5 - Population Projections for Rio Rico CDPs

2007 2008 2009 2010 2011 2012 2013 2014 2015 

Rio Rico 

NE CDP 4,376 4,559 4,741 4,921 5,100 5,277 5,454 5,628 5,802 

Rio Rico 

NW CDP 4,625 4,888 5,149 5,408 5,666 5,920 6,175 6,425 6,675 

Rio Rico 

SE CDP 2,866 3,059 3,249 3,439 3,628 3,814 4,000 4,183 4,366 

Rio Rico 

SW CDP 4,469 4,724 4,977 5,228 5,479 5,725 5,972 6,215 6,458 

Total 16,336 17,230 18,116 18,996 19,873 20,736 21,601 22,451 23,301

Data Source: “Arizona Subcounty Population Projections July 1, 2006 to July 1, 2055 by County, Census 

County Division, Place, and Reservation,” Arizona Department of Economic Security, Research 

Administration, Population Statistics Unit, 12/01/06

Table 6 lists the population projections for the Nogales PM10 Nonattainment Area and the percent of the 

Nogales PM10 Nonattainment Area population in Santa Cruz County. The relative percentage of the 

Nogales PM10 Nonattainment Area’s population to Santa Cruz County’s population remains fairly 

constant between years 2007 to 2015. 

Table 6 – Population Projections for Nogales PM10 Nonattainment Area 

City % Area in 

Nonattainment 

Area 

2007 2008 2009 2010 2011 2012 2013 2014 2015 

Nogales, Arizona 100.0% 21,746 22,454 22,659 22,863 23,065 23,265 23,465 23,662 23,858 

Rio Rico SE CDP 88.7% 2,542 2,713 2,882 3,050 3,218 3,383 3,548 3,710 3,873 

Rio Rico SW CDP 19.1% 854 902 951 999 1,046 1,093 1,141 1,187 1,233 

Total Population 

in Nonattainment 

Area 

-- 

25,142 26,070 26,491 26,912 27,330 27,741 28,154 28,559 28,964 

Nonattainment 

Area Population / 

Santa Cruz County 

Population 

-- 

54% 55% 54% 54% 53% 53% 52% 52% 52%

Data Sources: 

(1) Percent of surface area of Rio Rico SE CDP and Rio Rico SW CDP in the Nogales PM10 Nonattainment Area were based 

on surface area data calculated using GIS. 

(2) Population projections derived from Tables 4 and 5. 
10

Population Density 

Past and projected population densities for Ambos Nogales, and projected population densities for Rio 

Rico and the Nogales PM10 Nonattainment Area are illustrated in Table 7. The population density of Rio 

Rico is less than that of Santa Cruz County. This illustrates that the population of Rio Rico has a rural 

rather than urban population density. 

 

Table 7 – Population Density for Ambos Nogales and Rio Rico 

(persons / square mile)

1990 2000 2007 2010 2015 

Santa Cruz County 24 31 38 41 45 

Nogales, Arizona 937 1,004 1,046 1,099 1,147 

Nogales, Sonora 11,764* 13,392 17,411 18,713 20,712 

Rio Rico -- -- 26 30 37 

PM10 Nonattainment Area 320 342 369 

Data Sources: 

(1) Population density data for Ambos Nogales derived from Tables 3 and 4 using 1,236 square miles for 

Santa Cruz County, 20.8 square miles for Nogales, Arizona and 11.7 square miles for Nogales, Sonora. 

(2) Population estimates for Nogales, Sonora are 203,719 for 2007, 218,948 for 2010, and 242,335 for 

2015 (CONAPO 2007), and assuming densification of the existing built up land base will occur. 

* Note: Nogales, Sonora’s population in 1990 was 105,873 and the urban area was 9 square miles 

 (INEGI 2005). 

(3) Population density data for Rio Rico derived from Table 5 and using 39,000 acres (62.4 square miles) 

for Rio Rico. 

(4) Population density data for Nogales PM10 Nonattainment Area derived from Table 6 and using 78.6 

square miles in the nonattainment area (area calculated using GIS). 

Analysis of Demographics 

Population growth for Nogales, Arizona and the Nogales PM10 Nonattainment Area is projected to be 

small, while population growth for Santa Cruz County is projected to be moderate. The unincorporated 

community of Rio Rico is also projected to have moderate growth. The ratio of the population of the 

Nogales PM10 Nonattainment Area versus the population of Santa Cruz County is relatively constant for 

years 2007 – 2015 (Table 6). The population density of Santa Cruz County increases 7 persons per square 

mile, while the population density of the Nogales PM10 Nonattainment Area increases by 49 persons per 

square mile between years 2007 and 2015. The population growth of Nogales, Sonora is projected to be 

large resulting in a population that will be eleven times larger than that of Nogales, Arizona. 

ADEQ concluded that the Nogales PM2.5 Nonattainment Area boundaries should coincide with the current 

Nogales PM10 Nonattainment Area boundaries, because: (1) Ratio of future population growth between 

the Nogales PM10 Nonattainment Area and Santa Cruz County remains relatively constant, and (2) 

Changes in future population density of Santa Cruz County is relatively constant, while the future 

population density of the Nogales PM10 Nonattainment Area shows a moderate increase. 
11

JURISDICTIONAL BOUNDARIES

Santa Cruz County, comprising 1,236 square miles of land, was created by the 20th Territorial Assembly 

in 1899, thirty-six years after Arizona became a separate territory. The county is named after the Santa 

Cruz River that flows into México and then back across the border to the U.S. 

 

Commercial and cultural ties between Nogales, Arizona and Nogales, Sonora, México are strong. 

Nogales, Arizona was founded in 1882 along a north-south rail line to promote trade between the U.S. 

and México. Nogales, Sonora officially became a municipality on July 11, 1884 by Public Law No. 29, 

which was signed the prior day by Governor of Sonora, Luis Emeterio Torres. Nogales, Arizona was 

declared a city on January 1, 1920. Ambos Nogales represents the first railroad connection between the 

U.S. and México. 

Fast population and economic growth in Nogales, Sonora began with tariff exemptions and the 

stimulation of the manufacturing sector through México’s Border Industrialization Program (BIP) of 

1965. In this year, the first maquiladora in México was established in Nogales, Sonora. Maquiladoras 

import raw materials primarily from the United States, assemble them into value-added products, and ship 

them across the border for distribution to markets outside of México (Lorey 1999). Since this time, the 

maquiladora sector has thrived in México, receiving a big boost after the Mexican peso devaluation of 

1982—making Mexican labor cheaper—and more recently due to the signature of the North American 

Free Trade Agreement (NAFTA) of 1994. The NAFTA deregulated many of the existing trade barriers 

between Canada, México and the U.S., and has been a driver of dramatic increases in urban and industrial 

growth in Nogales, Sonora and other border communities. Today, Ambos Nogales is one of the most 

important ports of entry for NAFTA-related trade. 

The U.S. Congress National Highway Systems Designation Act of 1995 designated Nogales, Arizona as 

part of the Canada-México (CANAMEX) trade corridor, implemented by Public Law 104-59. As the 

CANAMEX project develops, it is expected that Ambos Nogales will continue in its role as a vital hub as 

a transshipment node in U.S.-México trade (Lara et al. forthcoming). 

Nogales, Arizona, contains 20.8 square miles, See Map 4 for city boundaries of Nogales, Arizona and 

Nogales, Sonora; boundaries of the current Nogales PM10 Nonattainment Area; and locations of ports of 

entry and Nogales, Arizona PM2.5 monitor. 

Analysis of Jurisdictional Boundaries 

The Nogales PM10 Nonattainment Area is located in Santa Cruz County. The southern boundary of the 

nonattainment area is the U.S./ México border. Nogales, Arizona is the largest city in the nonattainment 

area. Portions of the Coronado National Forest are near the east and west boundaries of the Nogales PM10

Nonattainment Area. Because the current boundaries of the Nogales PM10 Nonattainment Area 

encompass all major population centers, most of the private land, and existing and planned PM2.5

emission sources in Santa Cruz County, ADEQ concluded that the Nogales PM2.5 Nonattainment Area 

boundaries should coincide with the Nogales PM10 Nonattainment Area boundaries. 

The City of Nogales, Sonora is located directly south of Nogales, Arizona across the international border. 

Although the City of Nogales, Sonora is not in the Nogales PM10 Nonattainment Area, its emissions have 

a large impact on the air quality of the Nogales PM10 Nonattainment Area. 
12

PLANNED DEVELOPMENTS 

The following section describes planned developments in Nogales, Arizona and Nogales, Sonora, 

México. 

Nogales, Arizona 

The following development projects are planned for Nogales, Arizona 

(Nogales Community Development 2007): 

• Mariposa Port of Entry Expansion 

• Construction of a hospital between Nogales and Exit 12 on Interstate 19 

• Morley Avenue Entrepreneurial Development Area 

• Northern Meadow Hill Industrial Park 

• Construction of Candlewood Suites (100 – 200 rooms) next to Holiday Inn Express 

• Commercial and industrial development in various residential, commercial, and industrial areas in 

western Nogales, Arizona 

See Map 5 for locations of the planned development projects listed above in relation to the Nogales, 

Arizona city boundaries and Nogales PM10 Nonattainment Area, and township range sections. 

The Mariposa Port, constructed in 1973 on a 43-acre site, sits on a plateau at an elevation of about 50 feet 

higher than the surrounding ground. It is located on Arizona State Route 189, Mariposa Road, and 

Mexican Federal Highway 15. State Route 189 connects to Interstate 19 four miles to the north. The port 

is the main commercial crossing between the U.S. and México. The Mariposa Port of Entry Expansion is 

a U.S. General Services Administration project for improving truck throughput at the Nogales Mariposa 

U.S. Port of Entry by speeding the inspection of commercial trucks that have been certified by the U.S. 

Customs and Border Protection for hauling freight from México to the U.S (AZTEC & GSA 2007). 

Nogales, Sonora, México 

The following development projects are planned for Nogales, Sonora, México 

(Nogales Community Development 2007): 

• Industrial park expansion 

• Residential development (12,000 residential units by 2009) 

• Expansion of maquiladora program (numerous maquiladora operations have received ISO 9002 

certification) 

Analysis of Planned Developments

The preceding discussion and review of Map 5 shows that the major planned developments in Santa Cruz 

County will occur within the Nogales PM10 Nonattainment Area. Because these planned developments 

are within the current Nogales PM10 Nonattainment Area, ADEQ concluded that the Nogales PM2.5

Nonattainment Area boundaries should coincide with the current boundaries of the Nogales PM10

Nonattainment Area. Future planned developments in Nogales, Sonora may impact air quality in the 

Nogales PM10 Nonattainment Area. 
Map 4 Jurisdictional Boundaries of Ambos Nogales

13
Map 5 Planned Developments in Nogales, Arizona 

14
15

METEOROLOGY 

The following section describes the meteorological conditions observed in the Ambos Nogales area. 

Temperatures 

The average high temperature in the winter time is about 65°F with the average low winter-time 

temperature in the upper 20s. The coldest temperature recorded at the Nogales International Airport was 

-4°F in December of 1978. During the summer, the average high temperature is around 93°F with the 

average low in the mid to upper 70s. The hottest temperature recorded there was 112°F in June of 1990. 

There is not much difference in elevation between the Nogales International Airport and Nogales, 

Arizona, thus the temperatures at the airport are representative of those in Nogales, Arizona. 

Geography and Drainage 

The City of Nogales, Arizona encompasses 21 square miles (54 square kilometers) and lies 3,865 feet 

above sea level. With the Pajarito and Atascosa Mountains about 7 miles west and the Patagonia 

Mountains roughly 13 miles east, Nogales, Arizona rests between the two mountain ranges in the Nogales 

Wash. The elevation decreases from south of the U.S./México border heading north towards Tucson and 

ultimately Phoenix, Arizona. Thus, under calm wind conditions, the nighttime drainage is typically from 

south to north along the Nogales Wash. See Map 6 for depiction of elevation change from Nogales, 

Sonora to Nogales, Arizona. 

Variations in Localized Winds 

Due to the complex terrain in the Ambos Nogales area, wind patterns can be extremely localized (i.e., 

there may be very different winds from one wash to another). The Nogales International Airport is 

located about 8 miles to the northeast of the Nogales, Arizona’s city center. The airport is in a separate 

wash that flows northeast to southwest and is different from the conditions in the Nogales Wash. 

Therefore, it is not appropriate to use the airport winds to represent typical local winds in Nogales, 

Arizona. Upper levels winds are generally uniform across the region, thus Tucson’s upper-level wind 

data will be representative for the Ambos Nogales area. 

ADEQ Wind Monitoring in Nogales, Arizona 

The Arizona Department of Environmental Quality maintains a meteorological site located at the Nogales 

Post Office. As previously mentioned, it is more appropriate to use surface weather data within the city 

of Nogales, Arizona to represent the winds along the Nogales Wash than to use wind data from the 

Nogales International Airport. Review of four years of hourly wind data, from the meteorological site at 

the Nogales Post Office (June 2003 – November 2007), indicates that 47% of the winds were out of the 

south (between 145° and 225°). Winds out of the north (315° through 45°) occurred 18% of the time, east 

(45° through 145°) 20%, and west (225° through 315°) 15% of the time. These data show a definitive 

south to north wind flow along the Nogales Wash, likely aided by the decrease in elevation from the south 

to the north (see Table 8). 

Table 8 - Wind Climatology for Nogales, Arizona (June 2003-November 2007) 

North 

(315°-45°)

South 

(145°-225°)

East 

(45°-145°)

West 

(225°-315°)

Total 

Number of 

Observations 6,956 17,755 7,675 5,639 38,146 

% of 

Occurrence 18% 47% 20% 15% 100% 
Map 6 Drainage from Nogales, Sonora to Nogales, Arizona

Synoptic Weather Patterns in the Southwest U.S. 

During the spring months (March-May) the southwestern U.S. experiences trough passages with 

associated cold fronts. As these systems approach the region, winds are typically out of the south and 

southwest. Once the cold front moves through, the winds shift out of the west and decrease. Cooler, drier 

air replaces warmer, moist air. High pressure usually returns for a short time before the next system 

moves through. Winds are generally lighter under high pressure, allowing localized winds patterns to 

take effect. 

During the summer months (June-August), temperatures are the warmest of the year and mixing heights 

are at their highest levels, promoting afternoon mixing of the air. By July, high pressure sets up near the 

Four Corners region (Utah/Arizona/New Mexico/Colorado). This shifts the dominant wind flow out of 

the southeast and east. Moisture from México is drawn into the region producing the potential for 

afternoon thunderstorms nearly every day. These Monsoon thunderstorms can generate extremely 

localized wind gusts and periods of blowing dust, many times followed by rain. 

The weather pattern shifts once again during the fall months (September-November) with high pressure 

retreating to the south. This allows trough passages to again push through the region from west to east. 

16
17

Like the troughs during the spring, winds come out of the south and southwest ahead of the cold front, 

turning out of the west and northwest once the front pushes through. Lighter winds return under high 

pressure allowing local terrain to influence the wind direction. 

Finally, during the winter months (December-February), high pressure tends to be the dominant weather 

feature. Centered off the coast of southern California, it directs most disturbances north through 

Washington and Oregon, then southeast through Wyoming, Nevada and Utah, and occasionally into 

Arizona. If the high pressure system strengthens and moves over the region, these disturbances are 

generally deflected away from Arizona, and lighter winds prevail. It is during the winter months that 

mixing heights are at their lowest levels during the year with inversions that trap accumulated pollution. 

With the established south-to-north drainage flow from México into the United States along the Nogales 

Wash, emissions from Nogales, Sonora, México are frequently transported north into Nogales, Arizona. 

Analysis of Meteorology 

The preceding discussion and Map 6 show that the typical meteorological conditions associated with high 

PM10 and PM2.5 levels in the Nogales PM10 Nonattainment Area are nighttime drainage of emissions from 

Nogales, Sonora along the Nogales Wash into Nogales, Arizona. The nighttime drainage is strongest 

during the winter months when there are strong temperature inversions and light to no winds. 

Because the nighttime drainage from Nogales, Sonora into Nogales, Arizona follows the mountain 

valley(s) within the Nogales PM10 Nonattainment Area, ADEQ concluded that the Nogales PM2.5

Nonattainment Area boundaries should coincide with the current boundaries of the Nogales PM10

Nonattainment Area. 

TRANSPORT OF EMISSIONS – MÉXICO TO U.S. 

PM2.5 emissions are generated from various sources, which include open burning, residential heating – 

both wood burning and natural gas, and tailpipe emissions from on-road and off-road vehicles. The 1993 

Nogales PM10 State Implementation Plan (ADEQ 1993) indicated that 94 percent of the PM10 emissions 

in Nogales, Arizona originated from Nogales, Sonora, México. This finding was based on PM10

monitoring data, emissions inventory data, terrain and meteorological analyses, and a time lapse camera 

study that showed a plume of air pollutants being transported from Nogales, Sonora, México into 

Nogales, Arizona primarily during the evening hours (ADEQ 1993). 

In addition, intensive monitoring studies of ambient PM10 levels along a south-north transect from 

Nogales, Sonora into Nogales, Arizona (ADEQ 1993; ENSR 1999) and air quality modeling of the 

transport of Mexican emissions (ENSR 1999) indicate that PM10 levels decrease with distance to low 

levels as these emissions move north. Similarly, it is reasonable to expect that PM2.5 levels would also 

decrease with distance to low levels at the northern boundary of the Nogales PM10 Nonattainment Area. 

Thus, areas outside of the Nogales PM10 Nonattainment Area, such as Rio Rico, should not be impacted 

by the northward transport of PM2.5 emissions from Nogales, Sonora. 

A recent air quality study in the Nogales, Arizona area by Arizona State University (ASU) researchers 

also indicated that the majority of PM2.5 emissions in Nogales, Arizona are a result of transport of these 

emissions from Nogales, Sonora, México into Nogales, Arizona (See Appendix A for this report by Dr. 

Allen, Dr. Anderson, and Dr. Herckes that describes their preliminary findings from the Nogales Winter 

2005-2006 Study). The ASU study suggests that the main sources of PM2.5 emissions are wood burning, 

food cooking and dust emissions. Because the Nogales, Sonora population exceeds the Nogales, Arizona 

population by approximately an order of magnitude, the vast majority of these emissions originate on the 

Mexican side of the border. 
18

Analysis of Transport of Emissions 

Based on two air quality studies of the Ambos Nogales area, ADEQ concluded that the Nogales, Sonora 

area is the largest contributor to PM2.5 levels in the proposed Nogales PM2.5 Nonattainment Area. This 

may lead to a Section 179B demonstration that, but for international transport of emissions, the Nogales, 

Arizona area would attain the PM2.5 NAAQS. 

EMISSION SOURCES 

The following section describes the major emission sources in the Ambos Nogales area according to their 

locations – Nogales, Arizona and Nogales, Sonora, México. See Map 7 for locations of industrial point 

sources; highways; railroad lines; ports of entry; PM2.5 monitor in Nogales, Arizona; Nogales PM10 

Nonattainment Area boundaries; US/ México boundary; and city boundaries of Nogales, Arizona and 

Nogales, Sonora. Major PM2.5 emission sources are within the Nogales PM10 Nonattainment Area, except 

for those sources in Nogales, Sonora, México. 

Industrial Point Sources 

Nogales, Arizona

There are no major PM2.5 point sources in the Nogales PM10 Nonattainment Area. Table 9 lists the 

fourteen ADEQ permitted industrial sources in Santa Cruz County. Of these fourteen sources, only three 

sources - portable rock crusher / screener plants - appear to be operating outside of the Nogales PM10

Nonattainment Area. EPA’s 2005 National Emissions Inventory lists the following annual PM2.5

emissions for Santa Cruz County: 1 ton of PM2.5 from electrical generating units and 295 tons of PM2.5

from other stationary sources (www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html#F-2). 

Nogales, Sonora, México

The “Development of the Hazardous Air Pollutant Emissions Inventory for Ambos Nogales” (ENSR 

1997) lists 49 point sources for the Nogales, Sonora, México. Currently, there may be fewer than 49 

point sources in Nogales, Sonora, México due to sources closing or moving to other locations. 

Growth of Point Sources

Communications with ADEQ Southern Regional Office staff based in Tucson, Arizona indicate little 

potential for future increases in the number of point sources in Nogales, Arizona. However, there is a 

potential for an increase in the number of point sources in Nogales, Sonora, México based on that 

numerous maquiladora operations have received ISO 9002 certification (Nogales Community 

Development 2007). 
Map 7 Locations of Emission Sources 

19
20

Table 9 - PM2.5 and PM10 Emissions from Industrial Point Sources – Santa Cruz County

Year 2005 

Facility Name Address PM2.5 Emissions 

(tons/year)

PM10 Emissions 

(tons/year)

C.S. McCrossan Construction, Inc. 

• Asphalt Batch Plant 

• Rock Crusher/Screener 

91 Old Ruby Road 

Nogales, AZ 85621 Not Reported 4.12 

Coronado Group – Todd Pit 

(Coronado Group LLC) 

• Portable Rock Crusher / 

Screener Plant 

Santa Cruz County 

Note: Source may be located 

outside of nonattainment area 

0.07 0.14 

Granite Construction Company 

• Portable Asphalt Batch Plant 

Rio Rico Not Reported 0.02 

GRG Construction Rock 

(Rio Rico Aggregates & Asphalt) 

• Portable Rock Crusher / 

Screener Plant 

Rio Rico 

0.31 1.65 

Kino Crusher Plant 

(Huachuca Concrete, Inc.) 

• Portable Rock Crusher / 

Screener Plant 

Santa Cruz County 

Note: Source may be located 

outside of nonattainment area 

0.22 1.72 

Padilla Sand & Gravel Redi-Mix 

(CPC Southwest Materials, Inc.) 

• Portable Rock Crusher / 

Screener Plant 

Rio Rico 

Not Reported 0.34 

Rio Rico Hot Mix Asphalt Plant 

(Rio Rico Aggregates and Asphalt) 

• Portable Asphalt Batch Plant 

Rio Rico Not Reported 0.26 

Santa Cruz County Health Dept. 

• Boiler 

• Diesel Generators - 2 

2150 North Congress Drive 

Nogales, AZ 85621 Not Reported 0.01 

Steris Isomedix Services 

• Boilers 

• Oxidation Systems 

• Hot Cells 

1370 Industrial Park Drive 

Nogales, AZ 85621 Not Reported 0.59 

Superstition Crushing Plant #23 

(Superstition Crushing, LLC) 

• Portable Rock Crusher / 

Screener Plant 

Santa Cruz County 

• Note: Source may be 

located outside of 

nonattainment area 

0.27 1.80 

United Metro Materials (Rinker) 

• Portable concrete batch plant 

Nogales, Arizona Not Reported 2.5 

United Metro Materials (Rinker) 

• Concrete Batch Plant 

91 Old Ruby Road 

Nogales, AZ 85621 

Not Reported 2.5 

Valencia Power Plant 

 (Unisource, Inc.) 

1741 North Grand Avenue 

Nogales, AZ 85621 

Not Reported 0.11 

Wash 'n Clean World 

• Dry Cleaner / Laundry 

1989 North Grand Avenue 

Nogales, AZ 85621 

Not Reported Not Reported 

Data Source: Annual Emissions Inventory Questionnaires submitted to ADEQ AQD Compliance Section 
21

Area Sources 

Nogales, Arizona

The 1993 Nogales PM10 SIP (ADEQ 1993) and the 1997 Development of the Hazardous Air Pollutant 

Emissions Inventory for Ambos Nogales, ENSR report (ENSR 1997) listed the following PM2.5 area 

sources for the Nogales, Arizona area: (1) Residential Fuel Combustion (e.g., natural gas, kerosene), (2) 

Residential Wood Combustion, and (3) Structural fires. EPA’s 2005 National Emissions Inventory lists 

287 tons of annual PM2.5 emissions from fires in Santa Cruz County. 

(www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html#F-2). 

Nogales, Sonora, México

The 1993 Nogales PM10 SIP (ADEQ 1993) and the 1997 Development of the Hazardous Air Pollutant 

Emissions Inventory for Ambos Nogales, ENSR report (ENSR 1997) listed the following PM2.5 area 

sources for the Nogales, Sonora, México area: (1) Residential Fuel Combustion (e.g., natural gas, 

propane, kerosene), (2) Residential Wood Combustion, (3) Structural Fires, (4) Charbroiling, (5) Wire 

Reclamation, (6) Open Burning, and (7) Residential Biomass / Waste Combustion. 

Growth of Area Sources

From review of census data and other population data for Nogales, Arizona, it appears that the Nogales, 

Arizona’s population will not increase substantially over the next 10 to 15 years. Thus, ADEQ assumes 

that the corresponding growth of area sources in the Nogales, Arizona will be minimal. ADEQ SRO staff 

concur that the growth of area sources in Nogales, Arizona will be minimal. 

 

The Nogales Community Development report (Nogales Community Development 2007) and 

communications with ADEQ SRO staff indicate that the population of Nogales, Sonora, México is 

expected to increase in the next ten to fifteen years with a concurrent increase in residential wood 

combustion and residential fuel combustion (natural gas, propane, kerosene), and the potential for 

increases in structural fires. By Year 2015, the population of Nogales, Sonora, México is projected to be 

11 times larger than the population of Nogales, Arizona. 

Onroad Mobile Sources 

The “Plan of Action for Improving Air Quality in Ambos Nogales” report (Border 2012 Ambos Nogales 

Air Quality Task Force and the Border Liaison Mechanism Economic and Social Development Subgroup 

2005) listed the number of registered vehicles in Santa Cruz County and Nogales, Sonora, México as 

shown in Table 10. However, the report did not list which year the vehicle data refers to and did not 

provide gasoline and diesel fuel classifications. 

Table 10 - Registered Vehicles in Santa Cruz County and Nogales, Sonora, México 

Region Vehicle type Location Number of Vehicles 

Passenger Vehicles Santa Cruz County 39,882 

Buses Santa Cruz County 160 

U.S. 

Commercial Trucks Santa Cruz County 3,091 

Passenger Vehicles Nogales, Sonora 41,073 + 

15,000 unregistered 

Buses + Light Trucks Nogales, Sonora 16,015 

México 

Heavy Commercial Trucks Nogales, Sonora 1,072 

Data Source: Plan of Action for Improving Air Quality in Ambos Nogales, 2005 
22

Every six months, the Arizona Motor Vehicle Division (MVD) generates vehicle registration reports for 

Arizona vehicles. Table 11 lists the vehicles registered in Santa Cruz County, as of July 2007, by vehicle 

year and vehicle type (e.g., 4 light-duty diesel cars of vehicle year 2007 and 23 light duty diesel cars of 

vehicle year 2006 were registered in Santa Cruz County according to MVD’s July 2007 records). 

Table 11 - Santa Cruz County July 2007 Vehicle Registration Report

Light-Duty 

Cars Light-Duty Trucks 

Heavy-Duty 

Vehicles Buses Other Vehicles 

Year of 

Vehicle Total Gas Diesel 

Gas 

TK1 

Gas 

TK2 Diesel Gas Diesel Gas Diesel 

Electric 

Alt 

Fuel 

Motor

-cycle 

2007 1,723 1,129 4 402 12 43 5 55 0 14 0 0 59

2006 2,550 1,670 23 588 17 78 11 50 4 9 1 1 98

2005 2,552 1,784 12 541 16 45 11 53 3 8 2 0 77

2004 2,534 1,704 12 642 18 67 9 17 7 3 0 0 55

2003 2,356 1,636 11 517 27 53 4 31 3 10 1 0 63

2002 2,515 1,789 13 541 28 47 8 19 2 9 0 0 59

2001 2,528 1,730 16 611 15 50 9 46 5 4 0 3 39

2000 2,878 2,014 14 653 23 49 15 49 5 1 9 7 39

1999 2,701 1,944 8 506 32 67 19 79 6 3 0 3 34

1998 2,515 1,808 13 551 25 17 15 54 2 1 2 3 24

1997 2,442 1,607 11 611 34 52 22 81 6 0 0 0 18

1996 1,943 1,315 5 468 31 29 5 70 3 0 0 1 16

1995 2,143 1,466 4 490 38 31 15 69 3 3 1 1 22

1994 1,879 1,251 8 483 31 21 9 60 1 2 1 2 10

1993 1,616 1,109 4 341 35 29 7 74 2 1 0 0 14

1992 1,231 841 3 273 25 13 10 50 1 2 0 0 13

1991 1,204 854 1 264 26 14 6 32 0 0 0 1 6

1990 1,190 841 2 256 24 11 7 39 2 1 1 0 6

1989 1,169 768 7 290 34 5 9 42 1 2 0 1 10

1988 952 608 2 247 24 1 13 47 4 1 0 0 5

1987 818 543 3 201 15 7 11 30 1 2 0 0 5

1986 837 458 2 278 19 9 15 35 4 0 0 0 17

1985 736 398 10 235 24 9 14 27 2 1 0 0 16

1984 552 298 9 173 24 17 2 22 0 0 0 0 7

1983 322 155 12 110 15 10 6 5 1 0 0 0 8

1982 3,574 1,650 35 1,415 179 43 88 56 17 2 1 1 87

Totals 47,460 31,370 244 11,687 791 817 345 1,192 85 79 19 24 807

Data Source: Arizona Motor Vehicle Division. 2007 Vehicle Registration Report 

Growth of Onroad Mobile Sources

The number of registered vehicles in Santa Cruz County has grown from 41,527 in January 2004 to 

47,460 in July 2007 (Arizona Motor Vehicles Division 2007). This increase in the number of vehicles in 

Santa Cruz County may not directly correspond to growth in the number of vehicles in the Nogales PM10

Nonattainment Area because the population of Nogales, Arizona has not grown as fast as the surrounding 

Santa Cruz County. Although it is not known what proportion of the vehicles listed in Table 11 are owned 

by residents in the Nogales PM10 Nonattainment Area, in Year 2000, 80 percent of Santa Cruz County’s 

population resided in Nogales, Arizona and Rio Rico, Arizona (Nogales, Arizona, the majority of Rio 
23

Rico Southeast CDP, and a portion of Rio Rico Southwest CDP are in the Nogales PM10 Nonattainment 

Area). 

According to the “Plan of Action for Improving Air Quality in Ambos Nogales,” (Border 2012 Ambos 

Nogales Air Quality Task Force and the Border Liaison Mechanism Economic and Social Development 

Subgroup 2005), traffic congestion, resulting in increased tailpipe emissions, is a result of the following 

conditions: (1) Volume of the port of entry traffic and local traffic, (2) Design of local streets and limited 

available parking, and (3) Route of the Union Pacific/Grupo México railroad. 

 

Thus, the mix and volume of vehicles on both sides of the border result in excess traffic lines and 

congestion. For example, produce trucks that enter the U.S. at the Mariposa Port of Entry travel on local 

streets destined for warehouses (e.g., Mariposa Avenue and northern Grand Avenue). Local passenger 

traffic also occurs along these streets since most of the commercial development in Nogales, Arizona is 

located along these streets. 

EPA’s 2005 National Emissions Inventory lists 23 tons of annual PM2.5 emissions from onroad mobile 

sources in Santa Cruz County (www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html#F-2). 

Nonroad Mobile Sources 

Union Pacific Railroad operates railroad lines in Santa Cruz County. Table 12 lists the annual number of 

trains that travel between Nogales, Arizona and Nogales, Sonora, México. 

According to the “Plan of Action for Improving Air Quality in Ambos Nogales,” (Border 2012 Ambos 

Nogales Air Quality Task Force and the Border Liaison Mechanism Economic and Social Development 

Subgroup 2005), the Union Pacific railroad tracks split the Ambos Nogales area into a western half and an 

eastern half in both Nogales, Arizona, and Nogales, Sonora, México. Train activity along this route often 

results in several blocked intersections when trains pass through Ambos Nogales increasing idling time 

for motor vehicles. In Nogales, Arizona, there is only one vehicle-bridge crossing over the railroad 

tracks. The first vehicle-bridge crossing over the railroad tracks in Nogales, Sonora was constructed only 

as recently as 2005. 

 

EPA’s 2005 National Emissions Inventory lists 25 tons of annual PM2.5 emissions from nonroad mobile 

sources in Santa Cruz County (www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html#F-2). 

Growth of Nonroad Mobile Sources

Union Pacific Railroad staff do not expect growth in train traffic in Santa Cruz County over the next ten 

years (communication with Union Pacific Railroad 2007). 

Analysis of Emission Sources 

All of the major industrial point sources and area sources, and most of the onroad mobile and nonroad 

mobile sources in Santa Cruz County are within the Nogales PM10 Nonattainment Area. Because 

minimal PM2.5 emissions in Santa Cruz County originate outside of the Nogales PM10 Nonattainment 

Area, ADEQ concluded that the Nogales PM2.5 Nonattainment Area boundaries should coincide with the 

current boundaries of the Nogales PM10 Nonattainment Area. 

It is important to note that the largest contributor to PM2.5 emissions in the Nogales PM10 Nonattainment 

Area are those emission sources located in Nogales, Sonora. 
24

COMMUTING PATTERNS 

Two ports of entry connect the Nogales, Arizona area with Sonora, México. These ports of entry are the 

Mariposa Port of Entry, which primarily handles commercial truck traffic, and the DeConcini Port of 

Entry, which primarily handles passenger cars, buses, and non-commercial trucks. Table 12 lists the 

annual number of vehicles and trains that used these ports of entry during years 2000 through 2006. 

According to the “Draft Environmental Assessment, Nogales Mariposa U.S. Port of Entry, 2007” 

(AZTEC & GSA September 2007), the number of commercial trucks passing through the Mariposa Port 

of Entry has increased steadily since 2002. In 2006, 289,590 trucks passed through this port of entry. By 

comparing this number with the figure in Table 11, it can be inferred that in 2006, commercial trucks 

exclusively used the Mariposa Port of Entry and not the DeConcini Port of Entry. The report also 

mentions that bus traffic at the Mariposa Port of Entry has doubled since 2002 to 8,920 in 2006. 

Comparing this number with the Year 2006 bus traffic number in Table 11, it can be inferred that 77% of 

the buses entered the U.S. through the Mariposa Port of Entry and 23% entered through the DeConcini 

Port of Entry. 

Table 12 – Nogales, Arizona Ports of Entry – Vehicle and Train Traffic

Year Personal 

Vehicles 

Trucks Buses Total 

Vehicles 

Trains 

2000 4,681,567 254,694 8,899 4,693,240 774 

2001 4,590,933 249,327 7,238 4,600,807 635 

2002 3,978,640 242,237 6,796 3,988,045 607 

2003 3,836,372 243,365 6,705 3,845,537 457 

2004 3,571,230 247,553 6,842 3,580,520 444 

2005 3,445,984 266,233 8,988 3,457,762 785 

2006 3,282,781 289,590 11,521 3,296,961 653 

Data Source: U.S Bureau of Transportation Statistics, 

http://www.transtats.bts.gov

The “Development of the Hazardous Air Pollutant Emissions Inventory for Ambos Nogales, July 1997” 

report by ENSR (ENSR 1997) states that typically the wait times / idling times for vehicles crossing from 

Nogales, Arizona to Nogales, Sonora, México were minimal. However, wait times/idling times for 

vehicles crossing from Sonora, México to the Nogales, Arizona area can be substantial. ADEQ SRO staff 

report that at the DeConcini Port of Entry, wait times range between 25 minutes to 1 hour during 

weekdays and more than one hour on weekends. The “Economic Benefits of Expanding the BorderCrossing for Commercial Vehicles at the Mariposa Crossing in Nogales, Arizona, Final Report, June 

2007” by RTI International (RTI 2007) states that the wait times at the Mariposa Port of Entry range from 

22 minutes to over four hours, with a midpoint wait time of 2.35 hours. The longest wait times occur 

during the months of November through January. 

Analysis of Commuting Patterns 

The preceding discussion notes that the major commuting pattern in Nogales, Arizona area is the vehicle 

traffic that crosses the U.S./ México border at two ports of entry: (1) Mariposa Port of Entry, which 

primarily handles commercial truck traffic, and (2) DeConcini Port of Entry, which primarily handles 

passenger cars, buses, and non-commercial trucks. Because there are no other major commuting patterns 

within the Nogales PM10 Nonattainment Area or within Santa Cruz County, ADEQ concluded that the 

Nogales PM2.5 Nonattainment Area boundaries should coincide with the current boundaries of the 

Nogales PM10 Nonattainment Area. 
25

CONTROL MEASURES 

ADEQ permitted sources in the Nogales PM10 Nonattainment Area are in full compliance with the control 

measures listed in their permits (ADEQ AQD Compliance Section). 

Nogales, Arizona paved 5 miles of unpaved roads in 2006 and plans to pave similar amounts of unpaved 

roads in the next five years (Internet search of Nogales, Arizona news stories by ADEQ SRO staff). 

Nogales, Sonora, México also plans to pave 37 kilometers (23 miles) of unpaved roads in the next 3 years 

(Nogales, Sonora, México city officials provided these data to ADEQ SRO). 

The Mariposa Port of Entry is close to completion of construction of additional “Free and Secure Trade 

Program” (FAST) lanes to speed the inspections of commercial trucks that have been certified by the US 

Customs and Border Protection for hauling freight from México to the U.S. (RTI International 2007). 

These FAST lanes are expected to reduce the wait times associated with inspecting commercial trucks 

from México. 

Analysis of Control Measures 

The main PM2.5 control measure found for the proposed Nogales PM2.5 Nonattainment Area was the 

expansion of the Mariposa Port of Entry to increase the number of lanes for inspecting commercial trucks 

that have been certified by the U.S. Customs and Border Protection for hauling freight from México to the 

U.S. This control measure will decrease the idling times of commercial truck and the resulting PM2.5

emissions from truck exhaust. 

 

SUMMARY OF ANALYSES 

ADEQ examined the following ten criteria in order to recommend boundaries for the proposed Nogales 

PM2.5 Nonattainment Area: 

• PM2.5 Monitoring Data 

• Geography and Topography 

• Demographics 

• Jurisdictional Boundaries 

• Planned Developments 

• Meteorology 

• Transport of PM2.5 Emissions 

• Emission Sources 

• Commuting Patterns 

• Control Measures 

ADEQ also examined air quality and transportation studies conducted for the Ambos Nogales area and 

communicated with ADEQ SRO staff who participated in many of these studies and have a working 

knowledge of the area, its emission sources, and meteorology. 

The following list summarizes ADEQ’s analyses of the above criteria: 

1. PM2.5 Monitoring Data - Previous air quality studies by ADEQ and ENSR indicate that the 

current location of the Nogales, Arizona PM10 monitor is adequate for measuring PM2.5 levels in 

the Nogales PM2.5 Nonattainment Area. 
26

2. Geography and Topography - The current Nogales PM10 Nonattainment Area encompass both the 

mountain valley, where Nogales, Arizona is located, and a buffer area of mountains outside of 

this valley. Based on the geography and topography data, ADEQ concluded that the Nogales 

PM2.5 Nonattainment Area boundaries do not need to be extended beyond the current boundaries 

of the Nogales PM10 Nonattainment Area. 

3. Demographics - Based on projected population growth in and near the Nogales PM10

Nonattainment Area, ADEQ concluded that the Nogales PM2.5 Nonattainment Area boundaries 

do not need to be extended beyond the current boundaries of the Nogales PM10 Nonattainment 

Area. 

4. Jurisdictional Boundaries - The current boundaries of the Nogales PM10 Nonattainment Area 

encompasses all major cities and most of the private land in Santa Cruz County. Based on these 

jurisdictional boundary data, ADEQ concluded that the Nogales PM2.5 Nonattainment Area 

boundaries do not need to be extended beyond the current boundaries of the Nogales PM10

Nonattainment Area. 

5. Planned Developments – Data on planned developments were found only for Nogales, Arizona 

(and Nogales, Sonora). Based on these planned development data, ADEQ concluded that the 

Nogales PM2.5 Nonattainment Area boundaries do not need to be extended beyond the current 

boundaries of the Nogales PM10 Nonattainment Area. 

6. Meteorology - Based on that the nighttime drainage from Nogales, Sonora into Nogales, Arizona 

follows the mountain valley(s) in the Nogales PM10 Nonattainment Area, ADEQ concluded that 

the Nogales PM2.5 Nonattainment Area boundaries do not need to be extended beyond the current 

boundaries of the Nogales PM10 Nonattainment Area. 

7. Transport of Emissions – México to U.S. - Based on two air quality studies of the Ambos 

Nogales area, ADEQ concluded that the Nogales, Sonora area is the largest contributor, 94%, to 

PM2.5 levels in the proposed Nogales PM2.5 Nonattainment Area. 

8. Emission Sources - Based on that minimal PM2.5 emissions in Santa Cruz County originate 

outside of the Nogales PM10 Nonattainment Area, ADEQ concluded that the Nogales PM2.5

Nonattainment Area boundaries do not need to be extended beyond the current boundaries of the 

Nogales PM10 Nonattainment Area. 

9. Commuting Patterns - The major commuting pattern in the Nogales PM10 Nonattainment Area is 

the traffic in Nogales, Arizona area that crosses the U.S. / México border at two ports of entry: (1) 

Mariposa Port of Entry, which primarily handles commercial truck traffic, and (2) DeConcini 

Port of Entry, which primarily handles passenger cars, buses, and non-commercial trucks. Based 

on that there are no other major commuting patterns in the Nogales PM10 Nonattainment Area, or 

in Santa Cruz County, ADEQ concluded that the Nogales PM2.5 Nonattainment Area boundaries 

do not need to be extended beyond the current boundaries of the Nogales PM10 Nonattainment 

Area. 

10. Control Measures - The expansion of the Mariposa Port of Entry will increase the number of 

lanes for inspecting commercial trucks that have been certified by the U.S. Customs and Border 

Protection for hauling freight from México to the U.S. This control measure will decrease the 

idling times of commercial trucks and the resulting PM2.5 emissions from truck exhaust. 
27

ADEQ did consider including all of the unincorporated community of Rio Rico (39,000 acres) in the 

proposed Nogales PM2.5 Nonattainment Area, but did not for the following reasons: 

• Low Population Density: The Year 2007 population density of Rio Rico is estimated at 26 

persons per square mile, while the Year 2007 population density of Santa Cruz County is 38 

persons per square mile (see Table 7 in the preceding Demographics section of this report). 

Thus, Rio Rico’s population density is more rural than urban in nature. 

 

• No Major Permitted Industrial Sources – ADEQ’s records show no major permitted 

industrial sources in Rio Rico. There may be portable asphalt batch plants and portable rock 

crusher / screener plants that move in and out of Rio Rico as needed for construction projects. 

• Small Number of Businesses – Rio Rico has three businesses: Rio Rico Market, Ryan's Sports 

Pub, and the Las Trankas Mexican Restaurant. All three businesses are located at 1131 West 

Frontage Road; Rio Rico, AZ 85648 (frontage west of Interstate 19). 

• Minimal PM2.5 Emissions – The main source of PM2.5 emissions in Rio Rico is vehicle traffic. 

Due to the low population density, these emissions will be dispersed over a large area. Also, 

because Rio Rico is north of Nogales, Arizona, Rio Rico will not contribute to the elevated 

PM2.5 levels recorded at the Nogales PM2.5 monitor that occur when there is nighttime drainage 

of emissions from Nogales, Sonora into Nogales, Arizona (i.e., emissions are being transported 

from south to north along the Nogales Wash). 

Most of the Rio Rico Southeast CDP (89%) and a portion of the Rio Rico Southwest CDP (19%) are 

within the Nogales PM10 Nonattainment Area. 

CONCLUSIONS 

Based on the above analyses, data, studies, and communications, ADEQ recommends that the boundaries 

of the proposed Nogales PM2.5 Nonattainment Area coincide with the boundaries of the current Nogales 

PM10 Nonattainment Area as listed in the Code of Federal Regulations, 40, Parts 81 to 85, Revised as of 

July 1, 2006, page 82: 

The portions of the following Townships which are within the State of Arizona and lie east of 111 degrees 

longitude: 

 T23S, R13E 

 T23S, R14E 

 T24S, R13E 

 T24S, R14E 

As previously discussed, the large majority of PM2.5 emissions in the proposed Nogales PM2.5

Nonattainment Area are due to transport of PM2.5 emissions from Nogales, Sonora, México and it is these 

emissions that are the major cause of the PM2.5 exceedances in Nogales, Arizona. 
28

REFERENCES 

Arizona Department of Commerce. 2006. Nogales: Community Profile. 

http://www.azcommerce.com/SiteSel/Profiles/Community+Profile+Index.htm. 

Arizona Department of Commerce. 2007. Río Rico: Community Profile. 

http://www.azcommerce.com/doclib/COMMUNE/rio%20rico.pdf. 

Arizona Department of Commerce. 2006. Santa Cruz: County Profile. 

http://www.azcommerce.com/SiteSel/Profiles/County+Profiles.htm

Arizona Department of Economic Security, Population Statistics Unit. 2007. http://www.workforce.gov. 

Arizona Department of Environmental Quality. 1993. Final State Implementation Plan for the Nogales 

PM10 Nonattainment Area. 

Arizona Department of Transportation. 2006. HPMS Database. 

Arizona Motor Vehicle Division 2007. Vehicle Registration Report. 

AZTEC & GSA. September 2007. Draft Environmental Assessment, Nogales Mariposa US Port of 

Entry, Nogales, Arizona.

Border 2012 Ambos Nogales Air Quality Task Force and the Border Liaison Mechanism Economic and 

Social Development Subgroup. 2005. Plan of Action for Improving Air Quality in Ambos Nogales. 

Colegio de Sonora. 2007. Boletín Mensual de la Unidad de Información Regional. (Regional Information 

Unit Monthly Newsletter). Año 1, número 8, Septiembre 2007. 

CONAPO. Proyecciones de la Población de México, 2005-2050. (Population Projections in México, 

2005-2050). http://www.conapo.gob.mx/00cifras/5.htm. Last Accessed on November 15, 2007. 

Declet Barreto, J. 2006. Vulnerability to Technological Hazards in México-United States Border Cities: 

the Case of Ambos Nogales. Masters thesis, Arizona State University School of Geographical 

Sciences. 

Desert USA. Sonoran Desert. http://www.desertusa.com/du_sonoran.html. 

Last Accessed November 14, 2007. 

Engineering Science. 1987. PM10 Emissions Inventory Data for the Nogales Planning Area. 

ENSR. July 1997. Development of the Hazardous Air Pollutant Emissions Inventory for Ambos Nogales. 

Prepared for Arizona Department of Environmental Quality. 

ENSR. June 1999. Ambos Nogales Hazardous Air Pollution and Particulate Matter Study, Executive 

Summary. 

EPA 2007. Area Designations for the Revised 24-Hour Fine Particle National Ambient Air Quality 

Standard. Memo by Robert J. Meyers, Acting Assistant Administrator. 

http://www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html. 
29

EPA. 2005. National Emissions Inventory. http://www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html. 

INEGI (Instituto Nacional de Estadistica Geografia e Informatica). 2005. Cuaderno Estadístico Municipal 

de Nogales, Sonora, edición 2005. (Statistical Municipal Workbook for Nogales, Sonora, 2005 

edition). http://www.inegi.gob.mx/est/contenidos/espanol/sistemas/cem05/info/son/m043/mapas.pdf

 Last Accessed on November 15, 2007. 

———. 2005. Conteo de Población y Vivienda 2005. (Population and Household Count 2005). 

http://www.inegi.gob.mx. Last Accessed on November 14 2007. 

Lara Valencia, F., J. Declet Barreto and E. Keys. Spatial Equity and Transportation Hazard along CrossBorder Trade Corridors: The Case of Ambos Nogales. (forthcoming). 

Lorey, D. E. 1999. The U.S.-Mexican Border in the Twentieth Century. Scholarly Resources. 

McGinnies, W. G. 1976. An Overview of the Sonoran Desert. Second Annual Conference of the 

Consortium of Arid Lands Institutions. 

http://alic.arid.arizona.edu/Sonoran/documents/mcginnies/McGinnies_overview.html. 

Last Accessed November 16, 2007. 

Nogales Community Development. November 8, 2007. Communication with Yvonne Delgadillo, 

Executive Director, Nogales Community Development, 124 B. North Terrace Avenue, Nogales, 

Arizona. 

RTI International. 2007. The Economic Benefits of Expanding the Border-Crossing for Commercial 

Vehicles at the Mariposa Crossing in Nogales, Arizona. Final Report. Prepared for Gary Becker, 

U.S. Department of Homeland Security, Private Sector Office, Washington, D.C. 20528 

U.S Census Bureau. 2000. Population Estimates Program. Race / Ethnicity. 

http://www.census.gov/popest/race.html. 

U.S. Census Bureau. 2006. Population Estimates Program. 

http://www.census.gov/popest/estimates.php. 

U.S. Census Bureau. 2000. Río Rico Northeast CDP, Arizona; Río Rico Northwest CDP, Arizona; Río 

Rico Southeast CDP, Arizona; Río Rico Southwest CDP, Arizona. http://www.census.gov. Last 

accessed on November 28, 2007. 

U.S. Customs and Border Protection. 2006. CBP Border Wait Times. http://apps.cbp.gov/bwt/. 

U.S. Department of Transportation, Bureau of Transportation Statistics. 2006. Border Crossing Data. 

http://www.transtats.bts.gov/bordercrossing.aspx. 
30

APPENDIX A – ARIZONA STATE UNIVERSITY’S NOGALES WINTER STUDY 

Preliminary Summary of Findings from Nogales 

Winter 2005-2006 Study 

J. O. Allen, J. R. Anderson, and P. Herckes 

17 Oct 2007 

Between 29 December 2005 and 13 January 2006 our research team 

measured particulate matter (PM) using on-line instruments and 

collected PM samples for off-line analyses. The sampling site was the 

Nogales Post Office air quality monitoring site which is located 

approximately 0.5 km north of the U.S.-México border. The sampling 

site is near the main road through Nogales, AZ, and downslope from 

Nogales, Sonora. Due to downslope flow from higher ground to the 

south, winds were generally from the south from approximately an 

hour after sunset until the breakup of the nocturnal boundary layer at 

approximately 1000 the next morning. 

Data from all the on-line aerosol instruments (BAMs, DusTrak, 

SMPS, AMS) reflect high PM2.5 concentrations between ~1900 and 

~1000. PM2.5 concentrations during these periods routinely exceed 35 

μg/m3 and for short periods exceed 100 μg/m3. Overnight PM2.5

concentrations were the main contributors to the daily average PM2.5

concentrations which were in the range 10-38 μg/m3 during this study. 

Samples collected during the study have been analyzed for bulk 

composition by ion chromatography (sulfate, nitrate), colorimetry 

(ammonium), carbon evolution (organic carbon and elemental 

carbon), and scanning electron microscopy (mineral dust). Inorganic 

ions, sulfate, nitrate and ammonium, were present at very low 

concentrations indicating little contribution to PM from gas-phase 

SOx, NOx, and NH3 emissions. Carbonaceous PM contributions were 

17-37 μg/m3 with approximately 85% of this organic carbon, and the 

remainder elemental carbon. A substantial fraction of the 25,000 

particles analyzed by SEM were mineral dust particles; these results 

suggest PM2.5 mineral dust concentrations of ~10 μg/m3. 

Based on the correlation of high PM2.5 measurements with light 

southerly winds, and the small number of emission sources between 

the sampling site and the U.S.-México border, the predominant 

sources of PM2.5 at the sampling site were in México. This is 

consistent with emission inventory results that indicate 94% of PM2.5

at the sampling site is from Nogales, Sonora. Based on the bulk 

composition data, the main sources of PM2.5 are open burning, food 

cooking, and dust emissions. Carbonaceous emissions with ~15% EC 

are consistent with wood burning or food cooking, and not consistent 
31

with an aerosol dominated by diesel emissions for which EC makes 

up ~60% of the carbonaceous emissions. 

Please note that these conclusions apply to the Post Office monitoring 

site during winter high PM events. Diesel engine emissions, for 

example, may be important at the main truck border crossing located 

~3 km west of the sampling site. Because the main truck border 

crossing is located in an adjacent valley, movement of air from this 

crossing to the sampling site may be limited, especially on winter 

nights. 

We have analyzed the Nogales samples for organic tracer species by 

GC/MS and for mineral dust composition by SEM. Using these data 

in addition to the bulk PM2.5 composition data we will provide a more 

accurate and precise PM2.5 source apportionment.

J:\AQD\AQD\ASSESS\Special Application Unit\Nogales PM2.5 Designation - Proposed\DRAFT FINAL TSD\Supporting Info For Governor's 

PM2.5 Letter - Dec. 3, 2007.pdf