1

ATMOSPHERIC, HYDROCLIMATIC, AND ANTHROPOGENIC

CAUSES OF FUGITIVE DUST IN THE NOGALES, ARIZONANOGALES, SONORA, AIRSHED

PROJECT NUMBER: A-02-02

ANDREW WALTON ELLIS, ARIZONA STATE UNIVERSITY

NARRATIVE SUMMARY

The research reported here intended to identify the relative importance of

different factors that presumably contribute to high concentrations of fugitive dust

in the Nogales airshed (anthropogenic signals, atmospheric characteristics,

surface conditions). The basic theoretical questions addressed are:

• What are the physical and anthropogenic conditions that evolve to a point in

time when PM10 concentrations in Nogales are high?

• What are the relative contributions of atmospheric support and surface

conditions in the daily concentration of PM10 on either side of the border at

Nogales?

• Does soil condition play a larger role in PM10 concentrations on the Sonora

side of Nogales due to the supposed difference in infrastructure (paved

roadways)?

Each of these questions was addressed in supporting the ultimate goals of the

production of daily forecasts of PM10 concentrations and determining an answer

to the question of whether or not mitigation is realistic.

The findings of the work indicate that atmospheric support (stagnant synoptic 

atmosphere) is most important in controlling daily PM10 concentrations in

Nogales; the condition of the surface (soil moisture) does have value, but of a

secondary nature. An artificial representation of the anthropogenic variable day 

of the week (created based on historical variation in PM10 by day of the week)

showed no strong relationship with daily PM10 concentrations on either side of

the border at Nogales. Furthermore, it was found that when PM10 concentrations 

are anomalously high (greater than the mean) for Nogales (Arizona and Sonora

combined), atmospheric controllers have little bearing on the concentration of

PM10 relative to normal on one side of the border versus the other. However,

during fall and winter, dry soil moisture conditions are associated with relatively 

higher concentrations on the Arizona side of Nogales than on the Sonora side

relative to normal at each location. This indicates that the condition of the surface

is a greater influence on moving high PM10 concentrations even higher on the

Arizona side of the border than on the Sonora side. This idea fits with intuition,

given the supposed difference in infrastructure (paved roadways) between the
2

Arizona and Sonora sides of Nogales and with the fact that concentrations on the

Sonora side are almost always higher than on the Arizona side.

The findings that PM10 concentrations on the Sonora side of Nogales tend to be

considerably higher than on the Arizona side and that concentrations at both

locations tend to be highest in fall and winter and lowest in summer are not

surprising and not of great additional benefit. However, of interest to the scientific 

community and local decision-makers is the seasonality of the concentrations 

despite the fact that the highest amounts of soil moisture occur climatologically in

fall and winter. This immediately suggests the importance of the large-scale

atmosphere over the condition of the surface and indicates that the Nogales

airshed, from a PM10 perspective, is largely at the mercy of an atmosphere

conducive to high concentrations of dust. To the general community of Nogales,

intra-annual periods of high risk are rather apparent.
3

ATMOSPHERIC, HYDROCLIMATIC, AND ANTHROPOGENIC

CAUSES OF FUGITIVE DUST IN THE NOGALES, ARIZONANOGALES, SONORA, AIRSHED

PROJECT NUMBER: A-02-02

ANDREW WALTON ELLIS, ARIZONA STATE UNIVERSITY

INTRODUCTION

Small solid or liquid airborne particles of varying physical and chemical

characteristics, collectively referred to as atmospheric particulate matter (PM),

are introduced into the atmosphere by both natural (pollination, wind erosion,

wildfire) and anthropogenic (combustion, ground traffic) processes. PM size is a

critical determinant of the associated effects of PM on human health. Coarse PM,

that of a diameter ranging from 2.5 microns (μm) to 10 μm, or PM10, is capable of

being inhaled and is typically deposited in the upper portions of the respiratory 

system. Inhalation of PM10 has been associated with excess mortality, bronchial

aggravation, and alteration of pulmonary functions within children (ADEQ 2000).

In 1995, the Arizona Comparative Environmental Risk Project examined

statewide human exposure to all environmental risks and ranked air quality,

specifically PM, as the highest threat to the collective health of the state’s 

inhabitants (ADEQ 2000). Within the study it was estimated that PM10

concentrations are responsible for greater than 900 premature deaths annually;

in recent years across Arizona there have been increases in hospital admissions 

for respiratory disease, asthma episodes, lower respiratory symptoms, and

coughs (ADEQ 2000).

PM10 is predominantly dust-introduced to the lower atmosphere by ground traffic 

on paved and unpaved roadways, construction efforts, and wind of a magnitude

capable of producing soil erosion (generally 7.72 meters per second [m/s]-1 (15

miles per hour [mph]) or greater; Pewe 1981; ADEQ 2000). Unlike smaller-sized

PM, PM10 concentrations are not evenly distributed spatially, as the amount of

dust within the lower atmosphere at a specific location is largely a function of

proximity to source areas. PM concentrations along the Arizona-Sonora

international border (Figure 1) have been a concern of the Arizona Department of

Environmental Quality (ADEQ) and the U.S. Environmental Protection Agency 

(EPA) for years, prompting several special border studies. In studying PM10

within Douglas, Ariz., ADEQ scientists found concentrations to be 21% to 117%

higher at the border than at a location one mile north (ADEQ 1999). In a later

study, ADEQ collected data for fine PM (PM2.5) on either side of the border at

Nogales for 20 months. Concentrations in Arizona were comfortably within the

federal standard, while just south of the border concentrations were found to be
4

three times the standard (ADEQ 1999). Along the Arizona-Sonora border,

suspended PM is a common occurrence due to the generally weak atmospheric 

dynamics and the confining topography (Berman, et al. 1995).

In the most recent (2000) annual report from ADEQ, the PM monitoring site in

Nogales, Arizona recorded an annual mean PM10 concentration of 52.5

micrograms per cubic meter (μg/m3) during 1999, which was the eighth highest

among the 86 monitoring sites across the state (ADEQ 2000). The highest oneday concentration of 169 μg/m3 at Nogales was the fifth highest across the state

of Arizona, while the second highest one-day concentration (164 μg/m3) at the

Nogales site was the third highest within the state. Across the border at Nogales,

Son., the annual mean concentration of PM10 during 1999 (59.8 μg/m3) was 

higher than across the border in Arizona, as were the first and second highest

one-day concentrations (180 μg/m3 and 141 μg/m3 respectively; ADEQ 2000).

These findings suggest that lower atmospheric concentrations of PM10 in

Nogales continue to be rather high and are most likely higher on the Sonoran

side of the border.

High PM10 concentrations in Nogales may be at least partially related to the fact

that the Nogales port is the busiest in the state of Arizona. The greater than 4.5

million pedestrians that move through the port annually represent nearly 60% of

the pedestrians that cross the Arizona-Sonora border each year. The greater

than 251,000 trucks crossing the border at Nogales each year represents nearly 

75% of the total along the Arizona-Sonora border. Finally, nearly 4 million

passenger cars pass through the Nogales port each year, accounting for 41% of

that form of traffic crossing the Arizona-Sonora border each year.

RESEARCH OBJECTIVES

The research reported here intended to identify the relative importance of

different factors that presumably contribute to high concentrations of fugitive dust

in the Nogales airshed (anthropogenic signals, atmospheric characteristics,

surface conditions). The basic theoretical questions addressed are:

• What are the physical and anthropogenic conditions that evolve to a point in

time when PM10 concentrations in Nogales are high?

• What are the relative contributions of atmospheric support and surface

conditions in the daily concentration of PM10 on either side of the border at

Nogales?

• Does soil condition play a larger role in PM10 concentrations on the Sonora

side of Nogales due to the supposed difference in infrastructure (paved

roadways)?

Each of these questions was addressed in supporting the ultimate goals of the

production of daily forecasts of PM10 concentrations and determining an answer

to the question of whether or not mitigation is realistic.
5

RESEARCH METHODOLOGY/APPROACHES

In order to pursue the research objectives, a rather large daily dataset was 

created for Nogales. First, daily PM10 data for Nogales, Ariz. (1985–2001) and

Nogales, Son. (1993–2001) were collected from ADEQ. Data underwent

extensive quality assurance procedures at ADEQ prior to database assemblage,

and therefore the data were taken as accurate and as complete as possible. As 

with all standard practice in measuring PM, data measurements were taken on a

six-day interval with daily measurements at the two sites coinciding beginning in

December 1993. The result was 815 daily observations at Nogales, Ariz., 457

daily observations at Nogales, Son., and 266 daily observations that were

coincident between the two.

PM data were subjected to descriptive statistical analyses in order to

characterize the concentration of PM10 on both sides of Nogales as well as one

side relative to the other. PM10 data were then stratified by month and day of the

week for the full data record from each side of Nogales, and again subjected to

descriptive statistical analyses for each of the stratified groups (i.e., by month

and by day of the week). Based upon climatological analysis of Nogales 

(temperature, precipitation) and the monthly characterizations of PM

concentrations, the annual calendar was divided into seasons. The result was a

distribution of seasons defined in a standard way: December through February 

was deemed the winter season; March through May was deemed spring; June

through August was deemed the summer season; and September through

November was deemed fall.

In an attempt to reveal an anthropogenic signal in PM data in association with

ground traffic activity, the U.S. Customs Service was contacted for daily border

crossing data for the port at Nogales. It was learned that daily data is translated

to monthly totals at the end of each month and the original daily data is 

discarded. This reduced the capacity for detection of an anthropogenic signal

and resulted in the creation of an artificial indicator of an anthropogenic signal

based on the stratification by day of the week. Given the distribution of PM

concentrations by day of the week, an artificial representation of the day of the

week, from one to seven, was created based on association with the highest PM

concentrations (Friday = 1, Sunday = 2, 3 = Monday, 4 = Thursday) and lowest

PM concentrations (5 = Saturday, 6 = Wednesday, 7 = Tuesday).

As might be expected, PM data for both sides of Nogales were found to be

positively skewed with positive kurtosis (skewed toward smaller values and

clustered about a central value). The data were transformed with a logarithmic 

function to produce two normally distributed data sets—one for each of the two

sites in Nogales. Once fitted to a normal distribution, high daily values of PM10

concentrations were deemed to be those in the upper quartile, low daily values 

were those in the lower quartile, and all values between were considered nonextreme. The result was a distribution of 144 high PM days (≥ 78.0 μg/m3

) and

146 low PM days (≤ 31.3 μg/m3) at Nogales, Ariz., and 89 high PM days (≥ 95.0
6

μg/m3) and 91 low PM days (≤ 39.0 μg/m3) at Nogales, Son. The normalized

values were then able to be stratified once again by month and day of the week.

For the seasons characterized by the highest PM10 concentrations, which is to

say fall, winter, and spring, days upon which the PM concentration was high on

both sides of Nogales were extracted and used to create synoptic composite

maps of 500 hectopascals (hPa) height and 850 hPa wind. Data for composites

were extracted from the gridded database of the National Center for

Environmental Prediction (NCEP)/National Center for Atmospheric Research

(NCAR) reanalysis database (Kalnay, et al. 1996). Composites were based on 18

days within the fall season and 20 days from the winter season, while no

composites were constructed for spring since there was not a single occurrence

of coincidental high PM days at the Nogales stations. Height maps were

constructed to illustrate the climatological pattern of height (ridge/trough pattern)

along with the height anomalies on days of high PM concentration in order to

characterize the relative stagnation (upper atmospheric high pressure) on such

days. The same comparison was made for lower atmospheric wind

(climatological wind vectors v. composite wind vectors from high PM days) in

order to again illustrate the stagnation of the synoptic atmosphere underneath

which Nogales is situated.

Daily lower atmospheric wind data (850 hPa u and v vector components) and

500 hPa height data were extracted from the North American radiosonde

database of the National Climatic Data Center (NCDC) and the Forecast

Systems Laboratory (FSL). Data were extracted for Tucson, which represents the

nearest radiosonde station to Nogales (Figure 1). High resolution surface wind

data for Nogales is desired, but no permanent station (i.e. long record) exists in

Nogales. Wind data for Tucson at the 850 hPa level were taken in hopes of

representing the general flow in the lower atmosphere across the region.

However, the general stagnation of the lower atmosphere influences the degree

to which the wind data at Tucson are representative of the region and not just the

local thermally driven circulation. Data representing 500 hPa height at Tucson

are used to capture the general intensity of the typical middle-atmospheric high

pressure across the region.

Daily maximum and minimum temperature data for Nogales were extracted from

NCDC’s Summary of the Day database, along with climatological (1971–2000)

daily mean values of maximum and minimum temperature provided by the

Western Regional Climate Center. The two daily data sets were combined to

calculate daily maximum and minimum temperature anomalies for Nogales.

These data were used to characterize the daily climatic condition at Nogales,

again as a representation of atmospheric stagnation whereby higher than normal

maximum temperatures and lower than normal minimum temperatures (large

daily temperature range) generally indicate a stagnant, dry atmosphere.
7

Also extracted from the NCDC database was daily precipitation at Nogales, from

which several variables were calculated in order to characterize the dryness of

the soil. First, quite simply, the number of days since the last precipitation event

was calculated for each day of the PM concentration record (1985–2001).

Second, daily precipitation and daily mean temperature (calculated from

maximum and minimum temperatures described above) were employed in a

climatic water budget model in order to simulate the daily soil moisture at

Nogales. The Thornthwaite-Mather water budget model (Thornthwaite and

Mather 1955) is a mass conservation technique that balances estimated inputs of

water (precipitation, P) to the soil with water loss (evaporation plus plant

transpiration or evapotranspiration). The climatic water budget employs empirical

techniques to estimate the potential evapotranspiration (PE), or atmospheric 

demand given ample soil moisture, and the actual evapotranspiration (AE). PE is 

calculated based on mean temperature, while the lesser amount of AE is 

calculated from a function based on PE and the level of soil moisture relative to

field capacity. Monitoring the input (P) to the soil against the loss (AE) allows for

estimation of the soil moisture on daily to monthly time scales. The ThornthwaiteMather method of PE calculation is generally accepted as the simplest yet most

accurate method available (Leathers, et al. 2000). Using the climatic water

budget on a daily time step, daily values of P-PE (input minus atmospheric 

demand; daily drying) and soil moisture were produced for Nogales. To some

extent, the accuracy of the soil moisture amount, although historically accurate

when using this method, is irrelevant to this research, as the greatest interest is 

in relative amounts of moisture associated with episodes of high and low PM

concentrations rather than specific levels of soil moisture (i.e. thresholds).

Lastly, as another indicator of the dryness of the soil, monthly values of the

Palmer Drought Severity Index (PDSI) for the climatic region encompassing

Nogales were extracted from the PDIS database of the Climate Diagnostics 

Center of the National Oceanic and Atmospheric Administration. The PDSI

(Palmer 1965; Alley 1984) is the most widely used index for monitoring soil

moisture conditions across the United States. The method is similar to that

associated with the climatic water budget, whereby inputs (P) are compared to

water outputs (PE and AE). Monthly PDSI values are produced by the Centers 

for Disease Control (CDC) for U.S. climate divisions, which are climatically 

homogeneous regions largely defined by clusters of counties. Positive values 

indicate moist soil conditions while negative values indicate unusually dry soil

moisture conditions. Here, the gross monthly values were matched with the daily 

PM concentrations for Nogales.

Once the daily historical PM concentrations for both sides of Nogales and the two

taken together as a mean were characterized, and atmospheric and surface data

representing the condition of each were brought alongside the PM data,

statistical analyses relating the data were employed. The overarching goal was to

assess the relative contributions of atmospheric support and surface condition in

the daily concentration of PM10 on either side of the border at Nogales.
8

Initially, all data for each of the two PM stations in Nogales were stratified into

groups of days of high PM concentrations and days of low PM concentrations as 

determined from the normalized PM data at each station. The stratification was 

done by season to produce groups of high and low PM concentrations during fall,

winter, and spring, with the summer season omitted due to relatively infrequent

occurrences of high PM. A two-sample t-test was used to perform a hypothesis 

test and compute a confidence interval of the difference between the means of

the populations for high and low PM days. A confidence level of at least 0.10 was 

considered to be significant. Simply, this test illustrated the differences in the

variables (day of the week, atmospheric/surface conditions) associated with high

and low concentrations of PM10. In order to further illustrate the relationships 

between the independent variables and PM concentrations on each side of

Nogales, simple linear correlation analysis was performed to assess the nature

and significance of the relationships across the full spectrum of observations.

In an attempt to highlight the independent variables that explained the greatest

variability in PM concentration and to assess its predictability, stepwise multiple

regression analysis was employed. Only added to the regression model were

variables that were normally distributed, had regression coefficients that were

statistically significant (using the 0.10 level of confidence), and did not possess a

significant collinear relationship with previously added variables. The result was a

combination of independent variables for each season and for each side of

Nogales that explained a specific amount of the variance in PM concentration

through either a direct or inverse relationship.

One of the goals of the research was to establish the relative importance of the

condition of the atmosphere to the condition of the surface (soil) in the production

of high PM concentrations on either side of Nogales. In order to generalize this,

composites of both the atmospheric variables and soil moisture variables were

constructed from standardized values of the appropriate individual variables. The

sign of each variable used in the composites was determined based on the sign

of the correlation coefficient of each as it related to PM concentration. For each

season, the atmospheric composite was created from the four representations of

the vector components of the 850 hPa wind at Tucson (positive relationships),

along with maximum (positive) and minimum (negative) temperature, and 500

hPa height (positive). The only exception was with minimum temperature, where

in spring the relationship with PM concentration on the Sonora side of Nogales 

was positive. Therefore, minimum temperature was eliminated as an input to the

atmospheric composite in spring. The soil moisture component was constructed

from the number of days since the last precipitation event (positive), soil moisture

(negative), P-PE (negative), and the PDSI (negative in winter, but positive for the

Arizona side in fall and spring). The conflicting nature of the relationship between

PDSI and PM concentration on the Arizona side of Nogales in fall and in spring

eliminate it from calculation of the surface composite during those two seasons.

The construction of the composites was performed through the summation of
9

standardized z-scores for the variables. Z-scores are simply a subtraction of the

raw data point from the mean of the population, which is then divided by the

standard deviation of the population. Once standardized, the variables used for

each composite were either added (positive relationships with PM concentration)

or subtracted (negative relationships), to produce a composite variable that

possesses a positive relationship with PM concentration.

Using the atmospheric and surface composites and standardized PM

concentration values, linear correlation analysis was performed to assess the

strength of the relationships of the two variables with PM concentration on each

side of Nogales and for fall, winter, and spring. Subsequently, the data for each

season were stratified into relatively high (positive z-scores) and low (negative zscores) PM concentration, and the correlation analysis was re-run on the high

and low PM populations separately. This was done to discern the relative

importance of the atmospheric condition to that of the surface in moving PM

concentrations from low to moderate levels, and from moderate to high levels.

Lastly, in an attempt to differentiate the relative importance of the conditions of

the atmosphere and the surface to PM concentrations on one side of Nogales 

relative to the other, differences between PM concentrations at the two stations 

were matched with the composite representations for the atmosphere and

surface. Since the greatest interest is obviously in the conditions that move PM

concentrations from moderate to high, only relatively high PM concentration days

as determined from z-scores calculated from the mean daily concentration taken

from both sides of Nogales were considered for each season. Once these days 

were extracted, the difference in the z-score for PM concentration at each site in

Nogales was calculated as that on the Sonora side minus that on the Arizona

side. Taking the difference in raw PM concentrations from each side of Nogales 

would simply result in an inordinate number of values that illustrate the fact that

concentrations on the Sonora side are most often greater than on the Arizona

side. However, z-scores indicate the magnitude of the PM concentrations at each

station relative to the mean and data spread at that station, and therefore

differencing these gets at the difference between the daily anomaly in PM

concentration at each station. Once this difference was taken for the group of

days of relatively high PM concentration across Nogales it was matched with the

composites of the atmospheric and surface conditions for linear regression

analysis. The number of days upon which this analysis was run varied from fall

(n=45) to winter (n=37) to spring (n=36).

PROBLEMS/ISSUES ENCOUNTERED

Two major problems restricted the work as it was initially proposed. First, in

hoping to ascertain the anthropogenic contribution to high daily PM10

concentrations in Nogales via the high amount of border crossing activity, daily 

traffic at the Nogales port was scheduled for analysis and introduction as a

potential independent variable. However, upon communicating with the U.S.

Customs Office and officials at the U.S. port in Nogales, it was found that daily 

data are not maintained once translated to monthly values. This problem left
10

unanswered the question of the apparent anthropogenic signal in PM10

concentration when stratified by day of the week. Are the peaks on Sunday and

Friday on the Arizona side and on Sunday and Thursday on the Sonora side a

function of border crossing activity? This cannot be discerned without daily data.

Second, at the time during which the research idea was being formulated

scientists from ADEQ indicated there was a surface weather station in Nogales 

that included anemometry for wind measurements. This information was found to

be in error, as the only anemometry within Nogales occurred only on a casestudy basis for a specific ADEQ study. Therefore, this project was left without a

climatological representation of wind speed and direction within Nogales, making

identification of specific source areas of PM10 impossible. One of the original

objectives of the project was to match PM10 concentration data with wind

direction information and landuse/landcover data to trace back to the source of

fugitive dust. Given the lack of a historical wind record for Nogales, this is 

impossible from a climatologial perspective and would now require case-study 

fieldwork for short-term research work.

RESEARCH FINDINGS

The results of the research are headlined by nine major findings:

1. PM10 concentrations on the Sonora side of Nogales tend to be considerably 

higher than on the Arizona side. Daily concentrations on both sides of

Nogales significantly covary (correlation coefficient (r) = 0.68), however

values on the Sonora side are considerably higher no matter the measure of

the distribution (minimum, maximum, mean, median; Figure 2a). As expected,

the spread of the data (standard deviation) is greater on the Sonora side than

on the Arizona side. Both the median and the mean PM10 concentrations at

Sonora are greater than the annual U.S. federal standard of 50 μg/m3. On the

Arizona side, the overall mean concentration is greater than the annual

standard while the median is less. However, the value of the 95th percentile

on both the Arizona and Sonora sides of Nogales are below the 24-hour

standard of 150 μg/m3. Examining the full record during which measurements 

from the two sides of Nogales coincided, it was found that 86% of the days 

were characterized by higher concentrations of PM10 on the Sonora side than

on the Arizona side.

2. On both the Sonora and Arizona sides of Nogales, concentrations tend to be

highest in fall and winter and lowest in summer, despite the fact that the

highest amounts of soil moisture occur climatologically in fall and winter. The

arid climate of Nogales is marked by rather high temperatures (Figure 3a) and

two wet periods annually (Figure 3b). The North American monsoon season

(approximately July through September) is evident in the pattern of

precipitation, while the secondary wet season of December through March is 

evident, albeit less dramatic than the monsoon season. However, due to

lower temperatures during the winter season, the amount of precipitation (P)

relative to the amount of potential evapotranspiration (PE), or the supply of

water relative to the demand for water (P-PE) is much higher in winter than
11

during the monsoon season (Figure 3c). This translates to a considerably 

moister soil at Nogales in winter than during the summer months, including

during the monsoon season (Figure 3d). Still, with a soil moisture capacity 

(field capacity) of nearly 178 millimeters (mm), the most moist portion of the

year at Nogales (February and March; Figure 3d) is still well below field

capacity, which is illustrative of the dry soil conditions on average.

Intuitively, the greatest PM10 concentrations at Nogales would occur during

the timing of the driest soil conditions. However, this is not the case, as 

concentrations on both sides of Nogales are lowest during June, July, and

August when the soil is driest (Figures 2b and 2c). Conversely,

concentrations are highest during November and January, when soil moisture

is relatively high. The weak relationship between the hydroclimatological

conditions and PM10 concentration historically is even more apparent when

examining PM10 concentrations for the seasons associated with the higher

concentrations—fall (September through November), winter (December

through February), and spring (March through May). Values in fall and winter

are comparable on both the Arizona and Sonora sides of Nogales (Figure 2d)

despite the very different soil moisture conditions (Figure 3d). Historically,

PM10 concentrations in spring are less than in fall and winter despite the soil

being very dry at the end of spring. Given the climatological soil moisture

conditions throughout the year at Nogales, it appears that although the dry

soil is conducive to high concentrations of PM10, the relative dryness 

throughout the year has less bearing on PM10 levels than might be thought.

However, this climatological representation says nothing about the annual or

even daily variability in soil moisture and its relationship to PM10

concentrations within each season.

3. PM10 concentrations show somewhat of a pattern by day of the week,

whereby highest values tend to occur on the shoulders of weekends—Friday 

and Sunday on the Arizona side, and Thursday and Sunday on the Sonora

side. In what appears to be an anthropogenic signal in the concentration of

PM10 in Nogales, the highest concentrations (from measures of the median to

measures of the 95th percentile) are higher on the shoulders of weekends 

when examined by day of the week (Figures 2e and 2f). Peaks occur on

Sunday on both the Arizona and Sonora sides of Nogales, values decrease

through Tuesday, and increase again to peaks on Friday on the Arizona side

and Thursday on the Sonora side. This may correlate with ground traffic 

activity in Nogales, possibly even with border crossing traffic, but the lack of

ground traffic data in general and the lack of daily border traffic data make

this an assumption. However, the somewhat systematic pattern in PM10

concentration when stratified by an unnatural variable suggests the likelihood

of an anthropogenic effect.

4. Explaining the inconsistency in the mean relationship between soil moisture

and PM10 concentration in fall and winter at Nogales, the prevalence of a
12

stagnant synoptic atmosphere during those two seasons lends great

atmospheric support for higher concentrations of PM10. A stagnant synoptic 

atmosphere is clearly an important ingredient for high PM10 concentrations in

Nogales (Figure 4). Days of high concentrations in fall and winter (timing of

the greatest frequency) are marked by large 500 hPa height increases over

the southwestern U.S./northwestern Mexico region (Figures 4a and 4b). This 

is indicative of higher pressure aloft in the middle portion of the atmosphere

(approximately 5,200 meters [m] to 5,800 m above sea level), which is 

typically associated with relatively clear, calm atmospheric conditions. The

height increases are in the vicinity of a ridging in the typical pattern of 500

hPa over the western United States, and are accompanied by large height

decreases in the eastern Pacific Ocean in the vicinity of the eastern portion of

typical troughing in the height pattern. This amplification of the system

indicates the strength of the overall anomaly associated with the stagnation of

the atmosphere over the southwestern U.S./northwestern Mexico region.

Further evidence of a stagnant atmosphere is the pattern of winds at 850 hPa

(approximately 1,400 m to 1,500 m above sea level on average). However,

winds during episodes of high PM10 concentrations are not very different from

the climatological mean across the region of study during fall (Figure 4c) or

winter (Figure 4d). With neither wind speed nor direction do large differences 

exist, indicating little importance of lower atmospheric synoptic wind on a

gross scale. This may well not be the case for measured winds local to the

area of Nogales.

5. During fall, winter, and spring, episodes of high (low) PM10 concentrations on

both the Sonora and Arizona sides of Nogales are associated with northerly 

(southerly) early morning winds, greater (lesser) diurnal temperature ranges,

higher (lower) middle-atmospheric pressure across the region, drier (moister)

soil conditions, greater (lesser) daily surface drying, and a relatively long

(short) period of time since the last precipitation event.

Stratifying the PM10 concentration data independently for the Arizona and

Sonora sides of Nogales for fall, winter, and spring into episodes of high (top

quartile) and low (bottom quartile) concentrations allows for a simple

comparison of the physical differences associated with the two situations.

Testing for the significance of the difference between the two populations in

terms of an artificial day of the week variable and both atmospheric and

surface condition variables simplistically points to the contributors to episodes 

of high PM10 concentrations (Table 1). Correlation coefficients that represent

the significance of the linear relationship between PM10 concentrations and

the individual variables indicate the strength of the relationships across the

full range of the data (Figure 5).

The artificial representation of the day of the week (created based on

historical variation in PM10 by day of the week) was not characterized by a
13

significant difference between episodes of high and low concentrations for

either the Arizona or Sonora sides of Nogales for any season (Table 1; Figure

5). The common finding in the area of lower atmospheric synoptic winds (at

Tucson) was the clear tendency toward northerly winds during the early 

morning hours (0500 LST) during episodes of high concentrations contrasted

with southerly winds associated with low PM10 concentrations. This was 

common for both the Arizona and Sonora sides of Nogales and for all

seasons (Table 1), but most notably in winter (Figure 5b). Northerly winds 

during the early morning hours are likely indicative of a diurnal flow pattern in

the area of the wind measurements, Tucson, whereby anabatic (downsloping)

flow from higher terrain (north-northeast of Tucson) occurs. This is typical

within a stagnant synoptic flow pattern, which allows the smaller-scale

thermodynamic wind to operate, and therefore likely indicates the presence of

the type of synoptic pattern conducive to high PM10 concentrations (Figure 4).

During winter and spring, early morning winds are significantly more easterly 

(less westerly) during episodes of high concentrations on the Arizona side of

Nogales than during low concentration episodes (Table 1 and Figure 5). This 

is again indicative of an early morning anabatic wind pattern at Tucson,

whereby northeasterly flow downward from the high terrain to the northeast

invades Tucson. The same pattern of a tendency toward a northeasterly wind

in late afternoon (1700 LST) is evidenced by linear correlation coefficients.

The caveat with the lower atmospheric wind vectors as they are represented

by a correlation coefficient is that a positive value for W-E wind does not

necessarily mean more of an easterly wind, but could mean a westerly wind

(negative) of a lesser magnitude. This is more sensible in either the case of

early morning or late morning winds—calmer synoptic scale winds would be

more conducive to the suspension of PM10.

For all seasons on both the Arizona and Sonora sides of Nogales, episodes 

of high PM10 concentrations are marked by positive anomalies in maximum

temperature, but negative anomalies during episodes of low concentrations 

(Table 1; Figure 5). Negative (positive) anomalies in minimum temperature

are associated with episodes of high (low) concentrations during all seasons 

on the Arizona side of Nogales and only during winter on the Sonora side

(Table 1; Figure 5), but are of a lesser significance than with maximum

temperature. Higher maximum temperatures and lower minimum

temperatures are both indicative of a dry, stagnant synoptic atmosphere

whereby significant surface heating through a clear, calm atmosphere during

daytime hours is countered by efficient radiational cooling at night under the

same conditions.

The indication of a stagnant synoptic atmosphere during episodes of high

PM10 concentrations compared to episodes of low concentrations as revealed

by both lower-atmospheric wind and daily temperature data is further

supported through examination of 500 hPa heights as measured at Tucson.

The heights indicate the presence of upper-atmospheric high pressure, or a
14

stagnant synoptic atmosphere. Episodes of high concentrations were marked

by significantly higher 500 hPa heights than during episodes of low

concentrations during all seasons on the Sonora side of Nogales, and during

winter and spring on the Arizona side (Table 1; Figure 5).

The Palmer Drought Severity Index (PDSI) indicates the relative amount of

moisture in the soil on a monthly basis across the region as a calculated

index. During winter and spring on the Sonora side, the PDSI is more

negative (drier) during episodes of high PM10 concentrations than during

episodes of low concentrations (Table 1), although only significantly from a

linear correlation standpoint during spring (Figure 5). With relatively low soil

moisture climatologically in Nogales during fall (after the dry soil moisture

conditions associated with summer), it is not surprising that the PDSI is not

significantly different during high concentration episodes. The higher soil

moisture of winter and spring provide the variability necessary for the

distinction between episodes of high and low PM10 concentrations. On the

Arizona side of Nogales, only one of the relationships between PDSI values 

during episodes of high and low PM10 concentrations was significant. This 

was during the spring season, and was actually positive, suggesting that

general (monthly, regional) soil moisture conditions only play a role on the

Sonora side of Nogales.

Soil moisture at Nogales does appear to be significantly different during

episodes of high PM10 concentration compared to low episodes. This is 

especially so during fall and spring on the Sonora side and during fall and

winter on the Arizona side; soil moisture is significantly lower in association

with high PM10 concentrations than with low concentrations (Table 1; Figure

5). The rate of the daily drying of the soil—the difference between

precipitation and potential evapotranspiration—is significantly different

between episodes of high and low concentrations, especially during the fall

and winter seasons on the Sonora side of Nogales, but only during winter on

the Arizona side (Table 1; Figure 5). In each case, the rate of daily drying is 

greater (P-PE values smaller) during episodes of high concentrations of PM10.

Finally, during all seasons on the Arizona side of the border and during fall

and winter on the Sonora side, episodes of high PM10 concentration were

marked by generally more than twice the length of time since the last

precipitation event than episodes of low concentration (Table 1; Figure 5).

In summary, the day of the week does not seem to be significantly different

during episodes of high PM10 concentration than during episodes of low

concentration at Nogales. However, the synoptic atmosphere does seem to

be more stagnant, and the soil moisture conditions do appear to be drier

during high concentration episodes. Although this is important knowledge,

these findings are rather intuitive.
15

6. Daily prediction of PM10 concentrations on either side of the border at

Nogales appears to be an elusive goal, as one-to-four predictors only 

explains between approximately 12% and 41% of the variation in daily PM10

concentration, depending on the season.

Stepwise multiple regression analysis for diagnostic and prognostic purposes 

indicates the variables most associated with explaining PM10 concentrations 

for each season on both the Arizona and Sonora sides of the border while

characterizing the viability of daily predictions. Daily predictions of winter PM10

concentrations seem most possible, as three variables explain 33.9% and

40.8% of the variance in concentrations on the Arizona and Sonora sides,

respectively (Table 2). Analysis for the fall season reveals that four and three

variables explain 16.8% and 29.5% of the variance in concentrations on the

Arizona and Nogales sides, respectively. The values drop to one and two

variables to explain 11.6% and 21.9% of the variance on the Arizona and

Nogales sides (Table 2). The low values indicate that daily prediction of PM10

concentration in Nogales is an elusive goal. This is likely a product of two

things: (1) anthropogenic contributions to PM10 are capable of varying

dramatically from one day to the next, making it difficult to forecast

concentrations very accurately even if the atmospheric controllers are

forecasted well, and (2) environmental conditions across the region are

naturally conducive to high PM10 concentrations in the lower atmosphere on a

consistent basis, with relatively little variability, which makes daily prediction

more difficult.

In each case, the condition of the atmosphere (maximum temperature, lower

atmospheric wind, 500 hPa height) are the primary indicators.

Representations of surface condition are included during winter on the

Arizona side (soil moisture) and on the Sonora side (P-PE). Interestingly, day 

of the week is introduced into the suite of variables on the Arizona side of

Nogales in fall (Table 2). The results seem to indicate the importance of the

atmosphere over that of the condition of the surface. This is sensible in that

the atmosphere has to be supportive of high concentrations of PM10 before

the availability of dust from a dry surface even matters.

7. An artificial representation of the day of the week (created based on historical

variation in PM10 by day of the week) showed no strong relationship with daily 

PM10 concentrations on either side of the border at Nogales, and failed to

show any value as a predictive tool.

Based upon both the test of the differences in the populations associated with

high and low PM10 concentration episodes and the multiple regression model

building analyses, it is rather clear that the generic anthropogenic indicator

day of the week is not useful as an indicator of PM10 concentration. Daily 

ground traffic data may indeed reverse this finding, but this data does not

exist beyond small case study sampling.
16

8. Composite representations of atmospheric and surface conditions indicate

that atmospheric support (stagnant synoptic atmosphere) is most important in

controlling PM10 concentrations while the condition of the surface (soil

moisture) does have value, but of a secondary nature.

Composite representations (averaged from standardized scores, or z-scores,

of individual variables) of atmospheric conditions and surface conditions used

to create correlation coefficients that express the linear relationship with PM10

concentrations show the importance of the atmosphere relative to the surface

in producing episodes of high PM10. The representation of the atmospheric 

component correlated significantly with PM10 concentration when using the

full spectrum of values for all seasons on both the Arizona and Sonora sides 

of Nogales (Table 3). The relationships were also significant when examining

only the high (top half) and low (bottom half) concentrations at each location

and for each season except for the high concentrations on the Arizona side in

fall and on the Sonora side in spring. Overall, this indicates that daily PM10

concentrations, across the spectrum of values (full record, top and bottom

halves), are closely linked to the condition of the atmosphere. This is a bit in

contrast to the importance of the condition of the surface (Table 3). The

condition of the surface significantly correlates with PM10 concentration on

both the Arizona and Sonora sides of Nogales when considering the full

spectrum of values for all seasons except for during winter on the Sonora

side. However, when the data are stratified into halves based on relatively 

high and low PM10 concentrations the only significant correlation occurs with

low values in winter on the Sonora side and in spring on the Arizona side.

This weakness seems to indicate the importance of the atmosphere as a

controller of PM10 concentrations over the value of the surface condition. As 

stated before, this is sensible in that the atmosphere must be supportive of

suspending particulate matter before the condition of the source—the

surface—is an issue.

9. When PM10 concentrations are anomalously high (greater than the mean) for

Nogales (Arizona and Sonora combined), atmospheric controllers have very 

little bearing on the concentration of PM10 on one side of the border versus

the other relative to their individual means. However, during fall and winter,

dry soil moisture conditions are associated with higher concentrations relative

to the mean on the Arizona side of Nogales than on the Sonora side. This 

indicates that the condition of the surface is a greater influence on moving

high PM10 concentrations even higher on the Arizona side of the border than

on the Sonoran side, which is in agreement with intuition, given the supposed

difference in infrastructure (paved roadways) between the two sides of

Nogales. Under moister soil conditions, the concentration of PM10 on the

Sonora side of Nogales is higher relative to normal than on the Arizona side,

meaning that under the given atmospheric condition for the region,

concentrations are locally higher on the Sonora side. However, when the soil
17

is anomalously dry, PM10 concentrations at Nogales, Ariz., become higher

relative to normal than on the Sonora side (relative to normal there as well),

which is indicative of an unusual source of dust for the Arizona side. The

unusual source of dust is likely the Sonora side of Nogales, given that

concentrations there are almost always higher than on the Arizona side.

In taking the daily average of the PM10 concentrations from the Sonora and

Arizona sides of Nogales and standardizing to z-scores, historical data were

stratified into relatively high (positive scores) and relatively low (negative

scores) values. In examining only the relatively high values, the daily 

difference between z-scores (created from the full record at each location) at

the two locations was taken as the value on the Sonora side minus that on

the Arizona side. This provides a comparison of the two stations, one relative

to the other, based on the standardized score at each station. This is to say 

that concentrations on the Sonora side are most often higher than on the

Arizona side, but this is nullified when considering the z-scores calculated for

each station individually. Therefore, there are many days when the

concentration on the Arizona side is much higher relative to its own mean

than the concentration on the Sonora side relative to its own mean. The result

of the difference between the z-scores from both sides of Nogales is a

distribution of daily values during relatively high overall episodes (Nogales as 

a whole) within which many days on the Sonora side of Nogales are

higher/lower than on the Arizona side relative to normal at each location. The

differences are then matched with the composite representations of the

atmospheric and surface conditions to examine the linear relationships. The

atmospheric representation is constructed such that positive atmospheric 

values indicate greater synoptic stagnation and positive surface values 

indicate drier atmospheric conditions.

No significant relationship exists between the atmosphere as a whole and the

difference in the relative PM10 concentration on either side of Nogales (Table

4). However, a significant inverse relationship exists between the surface

condition and the difference during fall and winter (Table 4). This indicates 

that when the soil is drier, the difference in relative PM10 concentrations 

(Sonora minus Arizona) is smaller (or more negative), suggesting that the

condition of the surface is a greater influence on moving high PM10

concentrations even higher on the Arizona side of the border than on the

Sonoran side. This is indicative of an unusual source of dust for the Arizona

side, which is likely the Sonora side of Nogales, given that concentrations 

there are almost always higher than on the Arizona side.

CONCLUSIONS

The findings of the work indicate that atmospheric support (stagnant synoptic 

atmosphere) is most important in controlling daily PM10 concentrations in

Nogales; the condition of the surface (soil moisture) does have value, but of a

secondary nature. An artificial representation of the anthropogenic variable day 
18

of the week (created based on historical variation in PM10 by day of the week)

showed no strong relationship with daily PM10 concentrations on either side of

the border at Nogales. Furthermore, it was found that when PM10 concentrations 

are anomalously high (greater than the mean) for Nogales (Arizona and Sonora

combined), atmospheric controllers have little bearing on the concentration of

PM10 relative to normal on one side of the border versus the other. However,

during fall and winter, dry soil moisture conditions are associated with relatively 

higher concentrations on the Arizona side of Nogales than on the Sonora side

relative to normal at each location. This indicates that the condition of the surface

is a greater influence on moving high PM10 concentrations even higher on the

Arizona side of the border than on the Sonora side. This idea fits with intuition,

given the supposed difference in infrastructure (paved roadways) between the

Arizona and Sonora sides of Nogales and with the fact that concentrations on the

Sonora side are almost always higher than on the Arizona side.

RECOMMENDATIONS FOR FURTHER RESEARCH

Beyond the findings discussed above, this work has shown the need for intensive

case-study analysis that involves instrumentation deployment within the Nogales

airshed. The spirit of this work was to determine the broader controllers of PM10

concentrations, particularly the importance of the atmospheric condition versus 

the condition of the surface. These are natural physical controllers that provide

the ingredients for high concentrations of dust. In order to mitigate the problem of

dust concentration in the airshed, the specific sources that generate the dust

during the ideal physical conditions must be identified. This can only be done

through an intensive fieldwork campaign. The suggestion for further research is 

the establishment of a temporary network of instrumentation for measuring PM10

concentration and human surface transportation activity alongside wind direction

and speed across Nogales, Arizona and Sonora, during a targeted period

between October and January. During periods for which the large-scale

controllers of PM10 concentration (atmospheric and surface) are conducive to

episodes of high PM10, a fairly dense network of instrumentation should be

capable of indicating source areas. Once identified, suspected source areas can

be stratified by type (e.g., unpaved road, construction, agriculture, natural desert)

for an assessment of just how realistic thoughts of mitigation strategies are. In

other words, the environmental conditions, as identified in this study, are

conducive to high PM10 concentrations naturally (stagnant atmosphere, dry soil),

so is the idea of mitigation a battle that will be nearly impossible to win?

Determining the relative importance of the anthropogenic contribution to the PM10

problem is an absolute necessity, and there is virtually no historic data to help

with that effort. The result is a need for intense field study and the assumption

that what takes place during the field campaign is the rule rather than the

exception such that the information can be incorporated into mitigation efforts.

RESEARCH BENEFITS

The research discussed here is most beneficial to the scientific community and

policy- and decision-makers in the short term, but ultimately to the community of

Nogales in the long term. Furthermore, a key association between PM10
19

concentrations in Arizona and Sonora uncovered by the work has implications for

the international relationship within Nogales.

The findings that PM10 concentrations on the Sonora side of Nogales tend to be

considerably higher than on the Arizona side and that concentrations at both

locations tend to be highest in fall and winter and lowest in summer are not

surprising and not of great additional benefit. However, of interest to the scientific 

community and local decision-makers is the seasonality of the concentrations 

despite the fact that the highest amounts of soil moisture occur climatologically in

fall and winter. This immediately suggests the importance of the large-scale

atmosphere over the condition of the surface and indicates that the Nogales

airshed, from a PM10 perspective, is largely at the mercy of an atmosphere

conducive to high concentrations of dust. To the general community of Nogales,

intra-annual periods of high risk are rather apparent.

Also of interest to policy and decision-makers and the community of Nogales is 

the fact that PM10 concentrations show somewhat of a pattern by day of the

week, whereby the highest values tend to occur on the shoulders of weekends:

Friday and Sunday on the Arizona side, and Thursday and Sunday on the

Sonora side. Again, when combined with the idea of the seasonality of the

concentrations, high risk periods are apparent. The seemingly anthropogenic 

signal should be of interest to policy- and decision-makers, as it appears to

suggest the influence of ground traffic. However, an artificial representation of the

day of the week (created based on historical variation in PM10 by day of the

week) showed no strong relationship with daily PM10 concentrations on either

side of the border at Nogales. Overall, the suggestion is that an anthropogenic 

control is likely present in Nogales, but its resolution requires more intense study 

(see the sections Recommendations for Further Research and Problems/Issues 

Encountered).

Beneficial to scientists attempting to gain a better understanding of the PM10

problem in Nogales are the atmospheric and surface characteristics associated

with episodes of high (low) concentrations—northerly (southerly) early morning

winds, greater (lesser) diurnal temperature ranges, higher (lower) middleatmospheric pressure across the region, drier (moister) soil conditions, greater

(lesser) daily surface drying, and a relatively long (short) period of time since the

last precipitation event. All of this suggests the requirement of a stagnant

synoptic atmosphere and a dry surface to produce high PM10 concentrations.

However, of greater importance is that fact that composite representations of

atmospheric and surface conditions indicate that atmospheric support (stagnant

synoptic atmosphere) is most important in controlling PM10 concentrations, while

the condition of the surface (soil moisture) is of secondary importance. This 

seems logical, in that dry soil conditions are rather meaningless if the

atmosphere is not conducive to high PM10 concentrations. Again, the emphasis 

on the atmosphere is of importance to policymakers charged with discerning

surface types or areas that are potential contributors to the PM10 problem in
20

Nogales. In other words, the environmental conditions, as identified in this study,

are conducive to high PM10 concentrations naturally (stagnant atmosphere), so is 

the idea of mitigation a battle that will be nearly impossible to win given the lesser

role of the surface condition?

Of importance to the community of Nogales, policy- and decision-makers for the

area, and the international debate over PM10 on either side of the border at

Nogales is the finding that when PM10 concentrations are anomalously high

(greater than the mean) for Nogales (Arizona and Sonora combined),

atmospheric controllers have little bearing on the concentration of PM10 on one

side of the border versus the other. However, during fall and winter, dry soil

moisture conditions are associated with relatively higher concentrations on the

Arizona side of Nogales than on the Sonora side (relative to normal at each

location). This indicates that the condition of the surface is a greater influence on

moving high PM10 concentrations even higher on the Arizona side than the

Sonoran side, which is in agreement with intuition, given the supposed difference

in infrastructure (paved roadways) between the two sides of Nogales. This 

seemingly has been a point of contention—that a lesser level of infrastructure on

the Sonora side of Nogales directly leads to high concentrations of PM10 from

ground transportation, which in turn could be transported by the atmospheric flow

across the border into Nogales, Arizona. This is likely the explanation of the

finding here, whereby under more moist soil conditions the concentration of PM10

on the Sonora side of Nogales is higher relative to normal than on the Arizona

side, meaning that under the given atmospheric condition for the region,

concentrations are locally higher on the Sonora side. However, when the soil is 

anomalously dry, PM10 concentrations at Nogales, Ariz., become higher relative

to normal than on the Nogales, Son., side (relative to normal there as well),

which is indicative of an unusual source of dust for the Arizona side. The unusual

source of dust is likely the Sonora side of Nogales, given that concentrations 

there are almost always higher than on the Arizona side.

Finally, of interest to policy- and decision-makers, the community of Nogales, and

scientists in the discipline is the fact that daily prediction of PM10 concentrations 

on either side of the border at Nogales appears to be an elusive goal. This is 

likely a product of two things: (1) anthropogenic contributions to PM10 are

capable of varying dramatically from one day to the next, making it difficult to

forecast concentrations very accurately even if the atmospheric controllers are

forecasted well, and (2) environmental conditions across the region are naturally 

conducive to high PM10 concentrations in the lower atmosphere on a consistent

basis, with relatively little variability, which makes daily prediction more difficult.

Overall, the benefits of the work lie in the finding that high PM10 concentrations 

on the Arizona side of Nogales appear to be enhanced by dust from the Sonora

side during episodes of dry soil conditions and in the work’s diagnosis of the

atmospheric and surface conditions conducive to high concentrations throughout

Nogales. These findings should aid the community through a better
21

understanding of the problem, policy- and decision-makers through the

information on the Sonora-Arizona discrepancy, and the community of scientists 

through a first-ever inclusion of surface conditions (soil moisture) in the study of

PM10.

From the academic perspective, this research helped David A. Brommer obtain

an Master’s degree in Geography from the Department of Geography at Arizona

State University. The results of this research are being prepared for publication

as a refereed research article.

ACKNOWLEDGMENTS

PM10 concentration data and case study description were provided by Sandra

Wardwell of the Arizona Department of Environmental Quality. Climatological

data for Nogales, Ariz., were provided by the National Climatic Data Center.

REFERENCES

Arizona Department of Environmental Quality. 1999. Arizona Department of

Environmental Quality Douglas/Agua Prieta Air Pollution Study Protocol.

Memorandum 222-8-17-99.

Arizona Department of Environmental Quality. 2000. “Arizona Department of

Environmental Quality 2000 Annual Report, Air Quality Report.” ARS 49-424.10.

http://www.adeq.state.az.us/comm/pubs/download/ar.pdf.

Alley, W. 1984. “The Palmer Drought Severity Index: limitations and

assumptions.” Journal of Climate and Applied Meteorology 23: 1100–1109.

Berman, N. S., D. L. Boyer, A. J. Brazel, S. W. Brazel, R. R. Chen, H. J. S.

Fernando, and M. J. Fitch. 1995. “Synoptic Classification and Physical Model

Experiments to Guide Field Studies in Complex Terrain.” Journal of Applied

Meteorology 34: 719–730.

Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M.

Iredell, S. Saha, G. White, J. Woollen, Y. Zhu, A. Leetmaa, and R. Reynolds.

1996. “The NCEP/NCAR Reanalysis 40-year Project.” Bulletin of the American

Meteorological Society: 77: 437–471.

Leathers, D. J., A. J. Grundstein, and A. W. Ellis. 2000. “Growing Season

Moisture Deficits Across the Northeastern United States.” Climate Research 14:

43–55.

Palmer, W. C. 1965. “Meteorological Drought.” Weather Bureau Research Paper

No. 45. Washington, D.C.: U.S. Department of Commerce.

Pewe, T. L. 1981. “Desert Dust: An Overview,” In Desert Dust: Origins,

Characteristics, and Effect on Man, T. L. Pewe, ed. Geological Society of

America Special Paper 186: 1-10.
22

Thornthwaite, C. W., and J. R. Mather. 1955. “The Water Balance.” Publications 

in Climatology 8: 1–104.
Figure 1. The greater region of the southwestern U.S. and northwestern Mexico,

including Nogales, Arizona, and Noagles, Sonora.
Figure 2. Box plots of PM10 concentrations for Nogales, Arizona, and Nogales,

Sonora, including annual statistics (a), by month at Arizona (b) and Sonora (c),

by season (d), and by day of the week at Arizona (e) and Sonora (f). Black 

horizontal lines within each box indicate the median, white horizontal lines 

indicate the mean, the top and bottom of each box indicate the 75th and 25th

percentiles, the whiskers indicate the 90th and 10th percentiles, and the dots 

indicate the 95th and 5th percentiles. Solid horizontal lines indicate the 24-hour

(150 μg m-3) and annual (50 μg m-3) U.S federal standards.
Figure 3. Mean monthly air temperature (a), precipitation (b), difference between

precipitation and potential evapotranspiration (c), and soil moisture (d) for

Nogales, Arizona, 1971-2000.

Figure 4. Climatological (1971–2000) means and anomalies for days of high

PM10 concentration in Nogales for 500 hPa height (gpm) in fall (a) and winter (b),

and 850 hPa wind (vectors) in fall (c) and winter (d). Mean 500 hPa lines are

solid and in bold, while height anomalies are dashed (high PM10 days minus 

climatology) and shaded (light = positive; dark = negative). Mean wind vectors 

are black while wind vectors for days of high PM10 concentration are white.
Figure 5. Correlation coefficients for the relationship between daily PM10

concentrations and day of the week (artificial), daily vector wind components (WE and N-S), daily maximum and minimum temperature anomalies, daily 500 hPa

height, monthly Palmer Drought Severity Index (PDSI), daily soil moisture, daily 

P-PE value, and number of days since last precipitation for fall (n= 210 for

Arizona and 117 for Sonora; a), winter (n=180 and 131; b), and spring (n=205

and 110; c). Asterisks indicate statistically significant relationships with a p-value

less than or equal to 0.10.
Table 1. Mean values for variables with statistically significant (p-value ≤ 0.10)

differences between days of high (upper quartile) and low (lower quartile) PM10

concentrations at Nogales, Arizona (top) and Nogales, Sonora (bottom).

Arizona Fall Winter Spring

Variable High Low High Low High Low

(n=210) (n=180) (n=205)

Day of week (artificial) -- -- -- -- -- --

Atmospheric

0500 u wind (ms-1) -- -- +6.0 -2.5 -- --

0500 v wind +1.0 -1.9 +2.4 -1.0 +3.2 -2.1

1700 u wind -- -- -- -- -- --

1700 v wind -- -- -- -- +0.9 -3.8

max temperature anomaly (

o

C) +1.8 -1.1 +3.4 -2.7 +1.6 -1.9

min temperature anomaly -1.1 +1.6 -0.9 +0.7 -0.8 +1.4

500 hPa height (gpm) -- -- 5759.6 5642.2 5766.9 5703.4

Surface

PDSI (dimensionless) -- -- -- -- +2.3 +0.7

soil moisture (mm) 18.2 37.5 59.1 81.9 -- --

P-PE -- -- -0.7 +12.9 -- --

last precipitation (days) 14.1 5.9 25.2 11.7 21.4 10.1

Sonora (n=117) (n=131) (n=110)

Day of week (artificial) -- -- 3.3 4.1 -- --

Atmospheric

0500 u wind (ms

-1

) -- -- -- -- -- --

0500 v wind +1.2 -3.0 +2.0 -2.2 +3.2 -2.1

1700 u wind -- -- -- -- -- --

1700 v wind -- -- -- -- -- --

max temperature anomaly (

o

C) +2.5 -1.8 +4.0 -2.2 +2.0 -2.2

min temperature anomaly -- -- -1.6 +0.1 -- --

500 hPa height (gpm) 5829.5 5741.9 5758.9 5640.4 5806.7 5683.6

Surface

PDSI (dimensionless) -- -- -0.8 +0.4 -2.8 +0.3

soil moisture (mm) 20.7 47.1 62.5 89.3 15.3 70.2

P-PE -38.5 -12.1 -7.1 +11.7 -61.5 -38.1

last precipitation (days) 14.1 7.0 31.3 13.4 -- --
Table 2. Results of stepwise multiple regression analysis (p-value ≤ 0.10 to

enter) by season, including the variables chosen and the sign and coefficient of

determination (R2) associated with each variable

Variable Sign R2 Variable Sign R2

Arizona Fall (n=210) Sonora Fall (n=117)

max temperature anomaly + 11.3 max temperature anomaly + 21.5

1700 u wind + 14.8 1700 u wind + 26.9

day of week - 15.8 0500 v wind + 29.5

1700 v wind + 16.8

Arizona Winter (n=180) Sonora Winter (n=131)

500 hPa height + 30.7 500 hPa height + 28.7

0500 v wind + 32.6 0500 v wind + 37.3

soil moisture - 33.9 P-PE - 40.8

Arizona Spring (n=205) Sonora Spring (n=110)

max temperature anomaly + 11.6 max temperature anomaly + 20.2

0500 u wind + 21.9
Table 3. Linear correlation coefficients for relationships between PM10

concentrations and mean atmospheric and surface characteristics by season for

the full record (all) and for only high PM10 (greater than mean) and low PM10 (less 

than mean) episodes. Significant relationships (p-value ≤ 0.10) are in bold.

Arizona Sonora

all high low all high low

Fall (n=210) (n=117)

atmosphere 0.37 0.02 0.29 0.48 0.36 0.45

p-value 0.00 0.88 0.00 0.00 0.01 0.00

surface 0.26 0.03 0.07 0.24 -0.12 0.19

p-value 0.00 0.79 0.42 0.01 0.41 0.13

Winter (n=180) (n=131)

atmosphere 0.45 0.24 0.38 0.61 0.27 0.55

p-value 0.00 0.08 0.00 0.00 0.08 0.00

surface 0.19 0.15 0.03 0.11 -0.13 0.23

p-value 0.01 0.24 0.77 0.20 0.34 0.05

Spring (n=205) (n=110)

atmosphere 0.41 0.22 0.48 0.37 -0.11 0.36

p-value 0.00 0.05 0.00 0.00 0.53 0.01

surface 0.18 0.00 0.29 0.22 0.18 0.14

p-value 0.01 0.99 0.00 0.02 0.22 0.27

Table 4. Linear correlation coefficients for relationships between daily

differences in relative PM10 concentrations (Sonora minus Arizona)

and mean atmospheric and surface characteristics by season for

episodes of high PM10 concentrations (greater than mean).

Significant relationships (p-value ≤ 0.10) are in bold.

Fall Winter Spring

(n=45) (n=37) (n=36)

atmosphere -0.16 -0.18 -0.10

p-value 0.33 0.36 0.59

surface -0.34 -0.31 0.09

p-value 0.02 0.06 0.59