The Science of the Total Environment, 105 (1991 ) 233-258 Elsevier Science Publishers B.V., Amsterdam

233

The changing environment of a desert boomtown P.A. Walsh and T.E. Hoffer Desert Research Institute, Energy and Environmental Engineering Center, P.O. Box 60220, Reno, NV 89506-0220, USA (Received April 26th 1990; accepted August 22nd, 1990) ABSTRACT World population growth has prompted the exploration and habitation of geographical regions previously considered undesirable or unsuitable for human comfort. The impact of humans and their civilization on desert regions, where water and vegetation are scarce, is not well understood. The high plains desert of the southwestern United States is the fastest growing region of the nation. Historically, the desert atmosphere was distinguished by extraoldinary visibility and negligible particle and chemical pollution. Unfortunately, visitors and residents of the region have pelceived a decline in the air quality during a 15-20-year period which corresponds to a rapid influx of population and the development of isolated urban areas. This study atlempts to assess the relative impacts of demography, meteorology and air chemistry on the air quality of a rapidly growing, small city located in the Mohave Valley on the Nevada/Arizona border. Statistically significant trends were identified in the local meteorology and air quality over a 10-year period. Temperature and relative humidity values were observed to increase at the urban site. Increases were also noted in the concentrations of total suspendea particulates (TSP) and the oxides of nitrogen. Observations at the urban site were compared with similar measurements at nearby non-urban sites and with the results of studies at two larger cities in the desert southwest, Phoenix and Tucson, AZ. Conclusions based on the combined analyses indicate that the desert environment has been strongly influenced in the immediate urban area and that the changes are due primarily ~o demographic influences. Changes in urban air quality observed in the Mohave Valley were more pronounced and were apparem over a shorter period of time than air quality changes observed elsewhere in the southwest. INTRODUCTION

The impact of man and his civilization on the global environment i~,:an important question, both scientifically and politically, in the world today. Three factors shape the magnitude and intensity of these impacts: rapid increase in population during the twentieth century, the tendency of people to concentrate in urban areas (urbanization), and advances in technology and industrialization (Beaumont, 1989). Arid, or desert, regions of the world cover 36% of the land surface. Desert environments are harsh, yet fragile. A delicate balance exists between water, soil, geological and vegetative processes (Hills, 1966). In the latter half of the 0048-9697/$03.50

© 1991- -Elsevier Science Publishers B.V.

234

P.A. WALSH AND T.E. HOFFER

twentieth century, desert regions of the world have become important to the global community. The desert southwest of the United States is the fastest growing region of the world's most industrialized nation, while the countries of the Middle East control much of the world's oil supply and the Third World countries of Africa and Asia, with their burgeoning populations, add complexity ,to the future. It is important to assess the impact of the three factors described above on arid environments. Desertification (the advance of desert landscapes into non-desert regions), depletion of groundwater tables, accelerated erosion, desolation of native vegetation, decreases in visibility and climatic change are potential manifestations of unbalanced systems (Glantz, 1977; Sheridan, 1981; E1-Baz, 1984; Miller et al., 1990). Atmospheric impacts are especially hard to quantify. Sparse population and the perception of desert regions as unimportant to human survival has limited the historical information available to researchers. In addition, changes are confined to a short time span and to limited areas within desert regions. The most accurate data available has been collected in the southwestern United States. This paper describes the relative impacts of demography and meteorology on the air quality of a rapidly growing, small city in the southwest US desert. Air chemistry and meteorological data analyzed for the study were collected by the Desert Research Institute (DRI) as part of a long-term monitoring program (Hoffer et al., 1981). The program was designed to assess possible effects of emissions from the 1580 MW Mohave Power Project (MPP) on the ambient air quality of the Mohave Valley, a portion of th,~ Colorado River valley which extends along the Nevada/Arizona border south of Boulder Dam to Needles, CA. The MPP is located in the Mohave Valley at Laughlin, NV, on the west bank of the Co!t~rado Rive:. (Fig. 1). The area around Laughlin, NV/Bullhead City, AZ, was, until very recently, a sparsely populated region. Bullhead City, AZ, originated in 1945 as a construction camp for Davis Dam, a reclamation facility located 4.8 km north of town. Two small casinos began operation on the Nevada side of the river in the 1950s. Major growth began around 1968 when Southern California Edison Company began construction of the Mohave Power Project south of Laughlin's casino row on the west bank of the river. The power plant began operation in April 1971. The casino industry experienced significant growth during the mid-to-late-1979s. Tour,sm (gaming and recreation) forms the basis of the local economy and is the primary source of employment for the area. Bullhead City, AZ, incorporated in 1984, maintains the majority of housing and other public services for the permanent community. Laughlin, NV, remains an unincorporated township under the Clark County, NV, government.

235

CHANGING ENVIRONMEN/OF A DESER-IBOOMI-OWN

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Fig. I. Map of the study area showing sites mentioned in the text. Inset shows location of the study area with respect to state and county boundaries.

236

r,.A. WALSH A N D T.E. HOFF-ER

The Laughlin, NV/Bullhead City, AZ, area is surrounded by unspoiled desert terrain. Mountain ranges parallel the north/south orientation of the river. Elevations of the surrounding mountains are typically about 900 m msl. Spirit Mountain, the highest local peak (1719 mmsl), is 19.3 km northwest of MPP. Las Vegas, NV, 152.9 km north-northwest in Clark County, NV, is the closest major metropolitan area. To the west, the Mojave Desert makes up the bulk of San Bernardino County in southern California Bordering the westernmost edge of the county is the greater Los Angeles metropolitan area, a heavily populated and highly industrialized area. Mohave County, AZ, to the east, is almost uninhabited. Bullhead Cky is the largest population center of Mohave County, AZ. The Phoenix, AZ. metropolitan area is approximately 325 km southeast of the study area. Residents of Laughlin, NV/Bullhead City, AZ, have perceived a decline in local air quality and have brought the problem to the attention of the City Council, the local planning commission and the local newspaper. Three air pollution sources have the potential for significantly affecting the air quality of the Laughlin/Bullhead City area: (i) local generation associated with traffic and construction; (ii) effluent from the coal-fired Mohave Power Project located approximately 4.8 km south of the Laughlin casino row; (iii) long-range transport from the greater Los Angeles, CA, and Phoenix, AZ, urban areas. Synoptic-scale flow is typically from west to east. Air pc llutants from the Los Angeles metropolitan area may be transported through the dry atmosphere of the Mojave Desert to the Laughlin/Bulhead City area and beyond, into the National Parks located in southern Utah and northern Arizona (Miller et al., 1990; White et al., 1990). During the winter months, synoptic and mesoscale circulation oround low pressure areas combine to transport pollution from the Phoenix area. The influence of demographics, emissions and local meteorology on the air quality within the river valley is described and summarized in the following sections. DEMOGRAPHICS

The desert southwest ot" the continental United States is experiencing the fastest population growth in the nation. Dry desert climate and low land prices have made the area attractive to retirees and businesses. A 1986 study released by the US Department of Commerce Bureau of the Census reported that Nevada and Arizona led the rest of the nation in population growth from 1976 to 1986. For the purposes of this study, research focused on three counties surrounding the l, aughlin/Bullhead City area: San Bernardino

237

CHANGING ENVIRONMENTOF A DESERTBOOMTOWN

County, CA, to the west, Clark County, NV, to the north, and Mohave County, AZ, to the east. All three counties reported sustained growth of resident populations over the 18-year period• San Bernardino County population increased from 681 600 in 1970 to 1 167200 persons in 1987, a 171.2% gain• The 1987 San Bernardino County population is approximately twice the combined populations of Clark and Mohave Counties. Clark County experienced a 231.2% increase from a 1970 base population of 273288, while Mohave County, heavily weighted by Bullhead City, grew at the fastest rate of 310.8% from a base of 26300 in 1970. Bullhead City, AZ, experienced explosive growth over the same period, increasing the population from a base of 3759 in 1970 to 20 315 in 1987, a Bullhead

City,

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San Bernardino County, CA, 681 600.

238

P.A. WALSH AND T.E, HOFFER

540.4% increase. Peak growth rates were experienced in 1977 and in 1985 in association with major casino expansion periods. Figure 2 compares the rate of growth of Bullhead City, AZ, with comparable figures for the three counties San Bernardino, Clark and Mohave. For each site the value plotted is the ratio of the indicated year to the 1970 census value for that site. Discontinuous lines are due to missing population reports. Rapid growth of the permanent population in the Laugnim/'~tiuii,ieata"' City area is overwhelmed by the increase in part-time residents and tourists patronizing the various recreational and gaming facilities. The Laughlin Chamber of Commerce estimated that over two million tourists visited the area in ~987 Beginning in 1980, gaming revenues reported to the State of Nevada increased from approximately $30million to $300 million in 1988. (Demographics data courtesy of the Bullhead City, AZ, and Laughlin, NV, .Chambers of Commerce.) Average Daily Traffic C o u n t s Ap#roach Routes to Laughlin/Bullhead

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Fig. 3. Annual daily traffic counts for four primary approaches to the Laughlin, NV/Bullhead City, AZ, area, Refer to text for a description of approaches, Data compiled by the Nt.vada and Arizona Departments of Transportation.

CHANGING ENVIRONMENT OF A DESERT BOOMTOWN

239

Figure 3 displays the average daily traffic counts collected by the Nevada Department of Transportation for four approach routes to Laughlin, NV. The source areas of the four routes are broadly defined. NV South - - Los Angeles, San Diego and Southern California; NV Northwest - - Las Vegas and points northwest; NV East - - Davis Dam bridge from Bullhead City (Arizona and points northeast); and AZ S o u t h - Phoenix and other Arizona - . - . . . . . . . . ,, ...... ~,..... a . . . . . . . . . . . . . . ~ . . . . . . . . . s . . . . . . . . . . . . . . . prlmary ,,,,..,,. arteries through Laughlin and Bullhead City. During peak travel times, traffic jams occur on both sides of the river, significantly increasing the amount of combustion products released into the atmosphere. In 1986 a new bridge across the Colorado River connecting Bullhead City to Laughlin's casino row was added to the traffic configuration. The new route is shorter than the Davis Dam bridge and caused a significant drop in traffic along that route. Since traffic counts for the new bridge approach were unavailable for study, data for both bridge approaches were omitted from the figure. The rapid increase in population and the related economic and physical growth of the Laughlin/Bullhead City area impacts air quality in three ways. First, construction of housing, casinos and businesses disrupts the stable land surface of the desert and introduces particulate matter (dust) into the atmosphere. Second, the operation of businesses in extreme heat and arid conditions increases the demands on power and water utilities and increases the emissions of halocarbons and other pollutants. And third, a substantial increase in automobile traffic within a nan-ow river valley results in an increase in the emission of nitrogen oxides and other by-products of combustion engines. CLIMATOLOGY

Measurements of temperature, relative humidity and wind have been collected in the Laughlin/Bullhead City area since 1980. Data were collected at two sites: the Bullhead City Airport, 3.2 km northeast of the MPP stack at an elevation of 169 m msl, and on the MPP site, 0.9 km southeast of the MPP stack at an elevation 210m msl. For the purpose of the monitoring study, the sites were considered to be comparable. The data sets have been merged and cover the period from October 1980 through May 1987. The Bullhead City Airport site was installed in 1980. Continuous mcasurements were made until 30 May 1986 when the site was moved due to airport expansion to a site near the MPP. The short-term record for the MPP s;te was consistent with the longer term Bullhead City Airport data set. At both sites wind measurements were collected at a height of 10m above the ground and temperature ;~nd relative humidity data were collected at 8 m. Precipitation measurements were not made at either site.

240

P.A WALSH AND T.E. HOFFER

Meteorological measurements were also made at Cottonwood Cove, a small marina located 39.4 km north of the Mohave Power Project on the west shore of the Colorado River (Fig. 1). Data collected at Cottonwood Cove, site elevation 274 m msl, were representative of a non-urban background in the river valley.

Temperature Monthly mean, minimum and maximum temperatures were calculated from the hourly data base for the Bullhead City and Cottonwood Cove sites. A summary of temperature data is presented in Table 1. Temperatures at Cottonwood Cove displayed the same yearly pattern as those observed at TABLE I Temperature by month for Mohave Valley sites, including means and extremes (°C) Month

Mean temperat~:tre

Mean maximum temperature

Extreme maximum temperature

Mean minimum temperature

13.6 14.6 ! 7. ! 2 !.6 27.3 3 i.9 34,9 33.7 29,7 22.3 16.4 ! 3.3

23. ! 27.0 29.9 35.7 40.5 43,8 45.9 45.1 42.2 35. I 28.8 22.3

24.6 32.7 34.7 37. ! 44,3 47,2 47,5 40,6 44,2 38,9 30.6 25,3

3.6 3.0 6.2 8.0 12.7 18.6 21.9 21.8 16.6 10.8 4.2 3. I

1.9 0.3 3.5 5.0 10.0 15.8 19.8 21.2 13.6 8.9 2.5 1.2

Cottonwood Cove January 12, I February 14.1 March 16.7 April 21.5 May 27.3 June 32.9 July 34.7 August 34,0 September 29,1 October 23,3 November 16,6 December ! 2,3

16,3 18,8 22,2 27,2 32,9 38,8 40,3 39,6 34.8 28,6 20.9 16. !

24.0 31.0 33.5 37.0 43.1 46,7 47.4 47.3 44.1 41.8 34,1 27,0

8.5 9.9 11.5 i 5.6 21.1 26.4 28.7 28.2 23.4 18.4 12.6 8.9

i.3 - 1.2 2.4 6.0 I0.0 i 7.7 20.3 21.9 12.5 10.5 2.2 - 0.6

Bullhead City January February March April May June July August September October November December

Extreme minimum temperature

CHANGING ENVIRONMENT OF A DESERT BOOMTOWN

241

Bullhead City. The monthly mean temperature and extreme minimum and extreme maximum temperatures were approximately equal to values observed at Bullhead City. Over the period of record. July had the ~.i~hest mean temperature at both sites. December was the coldest month J~: ~ullhead City while January had the coldest mean in Cottonwood Cove. Systematic differences were apparent between the monthly mean maximum and mean minimum temperatures. Bullhead City's monthly mean maximum temperature was consistently warmer than Cottonwood Cove by an average of 6.9°C. In contrast, the monthly mean minimum temperature at Bullhead City was systematically 69°C cooler than the Cottonwood Cove observations. Physical characteristics of the two sites may account for observed differences in the monthly mean maximum and mean minimum values. The Cottonwood Cove site is located on a slope adjacent to Lake Mohave, a large, stable heat sink which moderates the temperature extremes. Near the Bullhead City Airport/Mohave Power Project sites, the Colorado River valley widens and reaches a lower mean elevation. The flat valley floor emphasizes the effects of daily heating, increasing the monthly mean maximum temperature and enhancing the nightly radiative cooling tb.ereby depressing the monthly mean minimum temperature. Pooling of dense cool air at lower elevations would also tend to decrease the monthly mean minimum temperature. Dry climate, infrequent cloud cover and lack of vegetation has a distinct effect on the diurnal temperature trends. Intense solar heating during the daytime hours is followed by strong radiative cooling at night. For Bullhead City, this cycle is most pronounced during the summer months of June, July and August when the daily temperature reaches a maximum i~J th~ lute afternoon followed by a steady drop in temperature until sunrise the following morning. In contrast, nighttime temperatures during the winter months (December, January and February) cool rapidly to a minimum value shortly after midnight and remain constant until sunrise the following morning. The stabilization of the minimum temperature implies that a balance is reached between the incoming solar heating during the daylight hours and the outgoing long-wave radiation during the night. Conditions favorable to the development of a nocturnal inversion are present year-round in Bullhead City, but sufficient solar insolation is available to break the inversion every day.

Relative humidity A time series of the monthly mean, minimum and maximum relative humidity observations at Bullhead City is shown in Fig. 4. The highest observed values (mean and extreme) of relative humidity are associated with the winter months. June consistently experienced the minimum average humidity level and the smallest range in humidities.

242

P.A. WALSH AND T.E. HOFFER

Relative Humidity

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Fig. 4. Time series of relative humidity measure~nents at the Bullhead City Airport/Mohave Power Project site. Values shown are monthly means of average minimum and maximum relative humidity.

The normal inverse relationship was observed between hourly values of temperature and relative humidity on a diurnal basis. Maximum relative humidity levels were observed at sunrise during both winter and summer seasons. Minimum values coincide with the daily peak temperatures. Over the period of record, Bullhead City experienced 72 days during the winter months when the maximum relative humidity exceeded 90% and 46 days during the spring (March, April and May). During the summer and fal~ (September, October and Novembe,') seasons, the maximum relative humidity exceeds 90% on 19 and 35 days, respectively. Relative humidity is a function of observed ambient temperature and is an important parameter in assessing the perception of local weather on human beings. A one-to-one correspondence does not exist between relative humidity and absolute humidity. Higher values associated with cold temperatures do not necessarily indicate elevated absolute humidity. The data c~!!ccted at the Bullhead City Airport/Mohave Power Project sites would imply that winter is perceived to be more humid (and therefore more uncomfortable) than other seasons of the year, but no information exists on the absolute levels of water vapor in the environment. No camparable measurements were available at Cottonwood Cove. Winds

Wind measurements were collected at both Bullhead City Airport/Mohave Power Project and Cottonwood Cove sites. Synoptically, weather systems in

243

CHANGING ENVIRONMENT OF A DESERT BOOMTOWN

Day

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Speed

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Bullhead Airport / Mohave Power Project 1980- 1986 Fig. 5. The frequency of wind direction and speed at the Bullhead Airport/Mohave Power Project site over the period of record. The vertical axis for each plot is frequency of occurrence. Wind direction measurements are divided into 16 bins of 22.5 ° each. Bin I is the 22.5* bin centered on North, Bin 9 is the bin centered on South. Wind speed measurements are divided into six bins. Wind speeds are categorized as follows: (I) 0.0-2.0, (2) 2.0--4.0, (3) 4.0-6.0, (4) 6.0-8.0, (5) 8.0-10.0, (6) > 10.0ms -t .

the western US move from west to east and active systems are associated with westerly winds. Winds observed at the valley floor are constrained by topography to two primary directions, north and south, and governed by locally generated circulation patterns. Seasonal histograms of wind direction and speed at the Bullhead Airport/Mohave Power Project site are displayed in Fig. 5. Mean daily wind speeds were of similar magnitude, 3.0-4.0 m s -~ , at both sites. Little variation was found on a seasonal basis between the mean daily wind speeds, although there is a slight indication that summer wind speeds are slower than those observed during the winter months. Sunlight and the daily cycle of nighttime stability and afternoon instability has a marked effect on the hourly wind speeds and direction at the Bullhead City Airport/Mohave Power Project sites. As a result, wind parameters correlated well with the daily temperatures. Peak daily wind speeds were observed during the late afternoon hours and the lowest wind speeds were observed during the non-daylight hours. Strong seasonal trends in mean wind direction were more pronounced when wind observations were limited to the daylight hours. Wind direction at the Bullhead City Airport/Mohave Power Project sites tended to remain aligned with the valley axis (SSW to NNE), suggesting that air in the Bullhead City area stagnates during the nighttime hours. At Cottonwood Cove, a drainage flow oriented along a small valley north of Spirit Mountain causes the nighttime winds to shift to the southwest. The

244

P.A. WALSH AND T.E. HOFFER

'~=~~

Day

Night

Direction

N

E

S W

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N

E

1988

S W

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Fig, 6, The frequency of wind direction and speed at the Cottonwood Cove site over the period of record. See Fig, 5 tbr a description of categories.

seasonal wind direction and speed plots for the Cottonwood Cove site are displayed in Fig. 6. Discussion

Siting and exposure of meteorological instrumentation are important factors in the assessment of urbanization on climate (Chandler, 1967; Bailing and Brazel, 1987b). The loca-fion of instruments may emphasize or minimize the subtle effects of urbanization, heat islands and air pollution problems. Bullhead City instrumentation, located on the valley floor in close proximity to the urban area, emphasizes the urban effect. Measurements taken at the Cottonwood Cove site reflect the non-urban background within the river valley. Statistical analysis of daily mean temperatures showed an increase in ambient temperatures at Bullhead City over the 6.5-yea.r data record. A linear regression of 1714 daily mean temperatures to the year of record produced a positive slope of 0.278°Cyear -t with a standard deviation (tr) of 0.146°C year -~. The corresponding two-tailed significance level (~) based on the t-statistic is 0.056. Linear regression of the detiiy maximum temperature (slope = 0.499°Cyear -t, tr = 0.160°C year -I, ~ = 0.002)also produced a positive slope. Table 2 summarizes the linear regression results of daily temperature data to the year of record for both Bullhead City and Cottonwood Cove. In a similar manner, linear regressions of daily relative humidity observations to the year of record at Bullhead City were calculated. The daily mean

CHANGING ENVIRONMENT OF A DESERT BOOMTOWN

245

TABLE 2 Temperature linear regression coefficients Partition

Parameter

N u m b e r of observations

Bullhead A i r o o r t / M o h a v e Power Project Full year Minimum i740 (1981-1985) Mean Maximum

Slope (°C y e a r - i )

Standard deviation

Significance level

0.082 0.278 0.499

0.129 0. ! 46 0.1.60

0.528 0.056 0.002

Spring

Minimum Mean Maximum

517

0.106 0.389 0.639

0.188 0.136 0. ! 57

0.369 0.004 0.090

Summer

Minimum Mean Maximum

442

- 0.260 - 0.086 0.186

0.108 0.101 0.121

0.016 0.391 0.126

Fall

Minimum Mean Maximum

5| 6

0.033 0.074 0.207

0.158 0.170 0.192

0.834 0.665 0.282

Winter

Minimum Mean Maximum

480

0.098 0.120 0.140

0.091 0.084 0.099

0.277 0. ! 54 0.158

28 ! 9

- 0.256 - 0.228 - 0.2 ! 2

0.059 0.065 ~,¢~71

0.000 0.000 0.003

Cottonwood Cove Full year Minimum Mean Maximum Spring

Minimum Mean Maximum

665

0.030 0.071 - 0.00 i

0.083 0.09 ! 0.099

0.722 0.852 0.99 i

Summer

Minimum Mean Maximum

734

- 0.157 - 0. I ! 5 - 0.098

0.046 0.044 0.048

0.001 0.009 0.043

Fall

Minimum Mean Maximum

763

- 0.203 - 0. ! 37 - 0.068

0.082 0.091 0.103

0.013 0.133 0.509

Winter

Minimum Mean Maximum

657

- 0.446 - 0.459 - 0,490

0.051 0.054 0.064

0.000 0.000 0.000

relative humidity disolayed a positive slope of 3.789% year -j . The daily minimum relative hm,'fidity also reflected the strong positive trend. Both regressions were statistically significant at a probability of < 0.020. i able 3 summarizes the relative humidity regression results for Bullhead City.

240

P,A. WALSrl AND T.E. HOFFER

TABLE 3

Relative humidity linear regression coefficients. Bullhead Airport/Mohave Power Project Partition

Parameter

Number of observations

=,,'-: . . . . .

M;,,;,,,--,,m Mean

!7!a

n oTq 0.789 0.342

517



I.J.ll

ff~,Li.lt

itltltiltt...a.lt



Standard deviation

Significance level

0 21a 0.331 0,475

0.000 0.017 0.471

- 0.196 - 0.572 - 0.973

0,220 0.430 0.674

0,375 0.184 0. i 50

1.632 2.342 3.376

0.386 0.577 0,827

0.000 0.000 0.000

515

1.790 2,661 3.194

0.280 0,429 0,606

0.000 0.000 0.000

480

1.657 ! 633 !. 166

0.394 0.565 0.767

0.000 0.004 0.129



Maximum Spring

Minimum Mean

Maximum Summer

Minimum

442

Mean

Maximum Fall

M inimum Mean

Maximum Winter

Minimum Mean

Maximun

Slope (% year-t ) -,.,-+z

: .

.

.

.

.

.

Temperature trends at the non-urban Cottonwood Cove site were quite different from those at Bullhead City. A linear regression of daily mean temperatures to the year of record for the period 1980-1988 indicated a negative trend with a slope of -0.228°Cyear-~ (a = 0.065°Cyear : ) . Regressions of the daily minimum and daily maximum temperatures also exhibited negative trends. All three regressions were ,,tatistically significant with 0t ~< 0.005, In an effort to identify the relative influences of meteorological and demographical factors, the meteorological data were partitioned to look at the trends by season. Temperature increases at Bullhead City were most distinct during the spring months. Late winter/early spring is the peak tourist season in the Bullhead City/Laughlin area. Warming during those months was indicated by a positive trend in the mean and maximum daily temperatures. The slope of the linear regressions exceeded those of the full year data regressions. A weak, negative trend was detected during the summer months. Results of linear regressions on fall and winter temperature data were statistically insignificant. The overall decrease in temperature at Cottonwood Cove was dominated by the very strong negative trends (daily minimum, mean and maximum) associated with summer and winter seasons. A negative trend was also discer-

CHANGING ENVIRONMENT OF A DESERT BOOMI"OWN

247

nible during the fall season, but regression results were not statistically significant (refer to Table 2). The seasonal partitioning of Bullhead City relative humidity data were inversely related to the temperature results. Relative humidity levels showed statistically significant, strong positive trends during the summer and fall seasons. Rapid warming observed during the spring months was accompanied by equally strong, but less statistically significant, reductions in relative humidity values. Yearly regressions show that Bullhead City has experienced a general warming and an increase in relative humidity since 1981. Seasonal analyses highlight an inverse relationship between seasons experiencing significant warming trends and those showing strongly positive relative hurrfidity trends. Warming trends in Bullhead City are associated with the peak tourist season. The impact of tourist traffic at Cottonwood Cove is negligible because the resort is primarily a residential community and does not have a shopping/ entertainment area. A very weak positive trend during the spring season reflects the 'peak" tourist influence on the area. Based on the available d:~ta it is not possible to determine the causes of the observed negative trends in temperature over the period of record at Cottonwood Cove. Results of the Bullhead City analysis have been compared with similar studies on long-term records for two rapidly growing cities of the desert southwest, Phoenix and Tucson, AZ. Phoenix has experienced rapid growth over the past decade and has emerged as North America's fastest growing metropolitan area with at least one million residents (Robey, 1985). Using data from the National Weather Service for the years 1948-1984, investigators linktd rapid urbanization with substantial increases in maximum and miaimum temperatures throughout the year. Dew point levels have decreased, and when combined with the large increases in temperatures, relative humidity values have dropped sharply (Cayan and Douglas, 1984; Brazel and Bailing, 1986; Bailing and Brazel, 1987a, c). Local land-use changes and increased pollution levels associated with urban exp,lnsion were cited as the primary causes of the observed climatic changes. Studies in the Tucson area found little change in temperature, dew point, or relative humidity over the 37-year period from 1945 to 1995, but detected signiticant increases in summer afternoon temperatures (assumed daily maximum) in an Ig-year subperiod from 1968 to 1985. Differences between the Phoenix and Tucson results were attributed to differences in the magnitude of absolute population growth and the topographic settings of the two cities which influences the transport and dispersion of poilutants, the positioning of the meteorological instrumentation relative to the urban center's and the nature of the landscape being replaced by the expanding cities (Bailing and Brazel, 1987b).

248

P.A. WALSH AND T.E. HOFFER

The present study s,ggests that the climate of Bullhead City has experienced a greater degree of change than either Phoenix or Tucson and that the change was apparent over a shorter per,_'edof time. All three sites experienced positive temperature trends, while only Bullhead City showed statistically significant increases i~l relative humidity over the period of record. The published results indicate that Phoenix and Tucson are influenced by the classic "urban heat island" effect (Landsberg, 1981). The observed increase in relative humidity at Bullhead City does not agree with the classic pattern. The rate of population growth, the areal size of the city, the degree of alteration of surface conditions, the amount of heat production and the topography increase the ambiguity of a heat island description for the Bullhead City urban area. AIR CHEMISTRY Criteria pollutant gases: sulfur dioxide, the oxides of nitrogen, and ozone, have been measured at many desert sites over the last two decades. In the Mohave Valley, sulfur dioxide (SO2) levels are generall~ at or below the detectable limits of instrumentation. Observed ambient concentrations have never exceeded the 3-h, 24-h or annual standards. Federal and State standards for the criteria pollutants discussed in this paper are presented in Table 4. Values of the oxides of nitrogen (NO/NO2/NOx) in the Mohave Valley are also low compared with the published Criteria Pollutant Standards. In the southwest desert the concentration of ozone sometimes exceeds the Federal l-h standard. Emissions from the Mohave Power Project include SOs and oxides of nitrogen. The mass of particulate matter suspended in the atmosphere (total suspended particulates, TSP) is also subject to EPA standards. The TSP standard is rarely exceeded in the undisturbed desert. In urban areas, where the desert soil has been disturbed, TSP values are elevated. Recently, in response to medical studies, ambient standards for suspended particulates have been changed to limit the measurement to the particle size range with aerodynamic diameters < 10gin. This standard, known as PMI0, is also described in Table 4. Air quality of tk: Mohave Valley can be described in terms of the pollutants listed above. Air chemistry data were collected using the Mohave Valley Ambient Air Quality Monitoring Network, which has been a dynamic system since its ineo)tion. Stations have been added and deleted in response to ongoing analysis. Measurements have also been added or deleted at existing sites. Instruments have been redesigned and upgraded in response to new technology, and sampling and calibration techniques have been refined. For all these reasons, the data described in the following sections will have

CHANGING ENVIRONMENT OF A DESERT BOOMTOWN

249

TABLE 4 Ambient air quality standards (~lg m-3) (standards in parentheses in ppb) Parameter

Period

Nevada '~

Arizona b

California c

National

SO2

Ih 3 hd 24h d Anntml arithmetic mean

1300(500) 365(140) 80(31)

1300(500) 365(140) 80(31)

655(252) 131(50) -

365(140) 80(3!)

TSP

24 h d Annual geometric mean

!5 75

150~ 75

-

-

PM to

24 h Annual arithmetic mean Annual geometric mean

_r t

_~ _g

50 _

! 50 50

_t

_g

30

Ozone

NO,

Ih !h Annual average

-

235(120)

235(I 20) h

180(90)

235(120)

100(50)

100(50)

470(235) -

100(50)

~'The ambient air quality standards for the state of Nevada appear in the Nevada Air Pollution Control Law, Chapter 445, Nevada Air Quality Regulations, Article 12 - - Ambient Air Quality Standards "The ambient air qnality standards for the State of Arizona appear under the Arizona Rules and Regulations tC,.L~ Air Pollution Control, Arb:ona Code, Title 18, Chapter 2. The ambient air quality standards for the State of California appear in the California Code of Regulations, Title 17, Sections 70100-70201 a ,';tandards, other than annual arithmetic or geometric means, are not to be exceeded more than ortce per year ~A recommendation has been made to keep the Arizona 24-h TSP standard, in addition to ar, ticipated PMto standards (footnote g). fAs yet, no standards have been proposed for Nevada. Nevada standards will be at least as strict as National standards. It is anticipated that Arizona PMt0 standards will be proposed in late 1989 or early 1990 and that they will be the same as National standards. h Reduced standard level, 195 # g m - 3 defined for the Lake Tahoe Basin. -, no standard ,"or particular time interval.

different beginning and end periods and all parameters are not available for all sites. Data collected early in a station's history will be considered to have a lower level of validity. For this article, data were processed and quality control checked by data processing methods developed for the project. In addition, external quality assurance was provided by Environmental Research and Technology, lnc. (ERT) through 1985.

250

P.A, WALSH AND T.E. HOFFER

Sulfur dioxide (S02) Emissions from the Mohave Power Project (MPP) are a primary source of SO2 in the Mohave Valley area. Atmospheric sulfur dioxide was detected by the flame-photometric technique using a Meloy Model SA185-2 instrument until 1986 (minimum detection limit 5-10ppb) and then upgraded to a Meloy SA285E (minimum detection limit 0 ppb). Both flame-photometric instruments were equipped with H2S scrubbers to remove all gases except SO2 and Teflon filters to remove particulate sulfur. Instantaneous SO2 values are averaged over l-h intervals and recorded for post-analysis. Observed values of SO., were typically at or below the threshold level of the instrument. No distinct diurnal trend was apparent in the data. Weekly time series showed no significant distinction between weekday and weekend concentrations. A time series of the monthly mean SO, echoed the minimum detectable level of the instrumentation over the entire period of record. Background values were exceeded more often during the summer months and higher values were associated with fumigation episodes and higher midafternoon wind speeds. The MPP is operational 24 h a day and is the primary source of SO2 in the area. The absence of discernible temporal trends in SO., measurements was an expected result. Due to a major system failure, the Mchave Power Project did not operate fiom 9 June to 5 December 1985. During that period, SO, was not detected at the Mohave Valley monitoring sites except for a few hours (< 10) ,it the Bullhead City site.

Oxides of nitr,,~gen ( NO/NO2/NO~ ) Data were collected by instruments sited at the Desert Research Institute olfice on Arizona State Route 95, the main highway in Bullhead City. Concentrations of the oxides of nitrogen (NO, NO, and NO,.) were derived from measurements made with a Columbia Scientific Industries Model i600 oxides of nitrogen analyzer. The sum of NO and NO, is designated NO,.. Approximately one-half of the hourly measurements were at or below the minimum detectable limits of the instrumentation (10 ppb). For observations greater than the minimum instrument detection level, mean NO and NO, levels average 3 ppb higher during the winter months than summer values. On a monthly basis, the highest mean NO and NO,. levels were reported in Noveml:er and July, and August average0 the It,west mean concentrations. Monthly average maximum concentrations emphasized the same pattern. Winter average ma:dmum values were 15 ppb higher than summer observations and coincided with the winter tourist season and an influx of

CHANGING ENVIRONMENT OF A DESERT BOOMTOWN

25 I

recreation vehicle and automobiles. Automobile traffic was also reflected on a weekly time scale. NOx concentrations were lowest on Monday and Tuesday and peaked on Friday and Saturday. The magnitude of the weekly pattern was not great ( < 5 ppb), but was a discernible feature in the data record. Diurnal trends for both NO and NOx compounds were related to the solar insolation cycle. Minimum daily values were observed during the daylight hours. After sur~set, higher concentrations were recorded with a peak occurring at dawn. Pigher nighttime concentrations correlated with long winter nights and the formation and dissolution of the nocturnal inversion. A distinct upward trend was detected in mean and maximum values of NO, over the period 1978 through 1987. A linear regression analysis of daily mean NO, (2901 points) to the year of record produced a slope of 1.80ppb year-' (a = 0.089ppbye~r -~, 0~ ~< 0.0005). Daily maximum values experienced Monthly Mean and Maximum NO x Bullhead C i t y , A Z I

I

I

1

I

[

I

I

I

t

I

I

I

250

200

150 CL

0 z

X

,°°I .,4,f J ;

l

01/77 01/76

01/78

~ .. : 01/79 01/80

i

[

01/81

1

L_

01/83

01/82

I

--1 .............J

01185 01/84

01/86

!

01/87

01/88

D~te Fig. 7. Time series of monthly mean and monthly maximum NO, for Bullhead City, AZ. Data collected at the Desert Research Institute office on Arizona S~ate Route 95.

252

P.A. WALSH AND T.E. HOFFER

a more precipitous rise (slope = 4.99 ppb year -~, a = 0.204 ppb year- ~, 0t - 0.0005). A comparison of the monthly mean and maximum values of NOx over the period of record is shown in Fig. 7. Oxides of nitrogen measurements at Bullhead City, AZ, were compared with observations collected at Fort Mohave, AZ, a non-urban site surrounded by disturbed desert land (agricultural), 11.3km to the south/southwest. Monthly mean and maximum NO.~ values measured at Bullhead City averaged 1.6 and 2.8 times comparable measurements obtained at the Fort Mohave site. A weekl.¢ pattern was not obvious. Linear regression analysis of daily mean NO.~ (836 points) to the year of record showed a slight positive trend with a slope of 0.192 ppb year -~ (tr = 0.048 ppb year -I, ~ = 0.0005). Daily maximum concentrations increased at a slightly greater rate, slope = 1.754ppbyear -~ (tr = 0.135 ppb year -~, ~ = 0.0005). The observed patterns of NO and NOx described above and the comparison of urban versus non-urban measurements suggest that local generation of the oxides of nitrogen related to tourist tra~c is responsible for some fraction of the nitrogen oxides in the Bullhead City area, and nocturnal inversions, especially during the winter months, serve to concentrate those emissions to measureable levels. The Bullhead City area has two major sources of NO/NO.,/NO.,., the Mohave Power Project and automobile traffic. The power station is a major source of SO2 and oxides of nitrogen. With the exception of the period from 9 June to 5 December 1985, no significant change in SO: was observed over the period of record and Southern California Edison has confirmed, through private communication, that the sulfur content of coal used at MPP has not changed over the same period. The MPP outage was not detectable in the NO/NO:/NO,. concentrations. This v~ould imply that observed increases in NO/NO,/NO,. are probably attributable to the increased automobile traffic associated with tourists and rapid growth of population in the Bullhead City urban area.

Total suspended particulates ( TSP) Total suspended particulate matter is the general term for particles found in the atmosphere. TSP were measured every sixth day by the high-volume sampling method (hivol), which is the EPA reference method (EPA, 40CFR50). A medium volume PM~0 sampler was collocated at the Bullhead City site. The region around Bullhead City is covered by 'desert pavement', a stable surface layer that minimizes the impact of wind and water erosion (Beaumont, 1989). Disruptinn of the native desert surface by construction and agriculture has a significant impact on the airborne particles. Fugitive dust has been

C H A N G I N G E N V I R O N M E N T OF A DESERT BOOMTOWN

253

regarded as the primary source of ambient particulate matter in the Bullhead City area. A recent study (Coulombe, 1990) confirmed that hypothesis. In a two-phase study, chemical analyses were performed on medvol PMI0 filters collected for a l-year period (September 1988 through August 1989). The Chemical Mass Balance (CMB) receptor model (Watson et al., 1990a, b) was applied to the data. Results showed that soil contributions constituted 75% of the average PM~0. Local motor vehicles, long-range transport and the MPP contributed 17, 4 and 0.1% respectively to the average PM~0 particulate loading. Three sites within the Mohave Valley monitoring network have been collecting particulate data continuously since 1976: Cottonwood Cove (rural, surrounded by undisturbed terrain), Bullhead City (urban) and Fort Mohave

(TSP)

Total Suspended Particulates

ThreeSites (5-pt weighted smoothing) 2501 ;

i

I

t

i

-I

I

I

l

I

i

i "--T--i

--- Bullhead

ii

- - Fort Mohave

.... Co~:~onwood

200 f

100'

"

"

A^ !Alr 01177 01179 01/81 01/83 01185 01187 01/89 01/76 01/78 01/80 01/82 01/84 01/86 01/88 Date F i g . 8. Time series of monthly mean TSP concentrations fcr three Mohave Valley sites: Bullhead City, Cottonwood Cove an~J Fort Mohave.

254

P.A. WALSH

AND

T.E. HOFFER

(non-urban, surrounded by disturbed desert). Time series of the monthly mean TSP for each of the three sites demonstrate a standard yearly cycle of higher summer and lower winter concentrations (Fig. 8). A comparison of monthly mean values over the period of record showed Bullhead City concentrations to average twice Fort Mohav¢ levels and to be a factor of 3-4 greater than Cottonwood Cove measurements. Linear regression of the monthly mean concentrations to the year of record produced negative trends for all three sites. Anomolous concentrations (high) were observed at each of the three sites during 1976. The elevated observations would be consistent with a significant increase in airborne dust associated with construction and agriculture during an extremely dry season. Linear regressions calculated without the 1976 values indicate that TSP concentrations decreased over the remaining study period at Cottonwood Cove (slope = -0.302, a = 0.268, 0t = 0.262)and Fort Mohave (slope = - 1.453, t~ = 0.508, ~ = 0.004). In contrast, Bullhead City TSP concentrations increased from 1977 to the present (slope : 1.356, t~ = 0.614, 0t = 0.028). Figure 9 compares the annual geometric mean for the three sites. Bullhead City exceeded the Federal primary standard (75/~gm -3) for > 75% of the years of record and the Federal secondary standard (60/~gm -3) for 100% of the 13-year period. Fort Mohave recorded one violation of the _,:ederal secondary standard in 1976 and has not been in violation since then. Cottonwood Cove did not record any violations. Characterietics of TSP measurements coUected at the three sites since 1976 show a sharp distinction between the urban and non-urban sites. The systematically high concentrations and the observed increase over the period of record suggest that TSP in Bullhead City is closely linked to the urbanizatio,~ of the area, including construction and increa,~ed automobile traffic. ,

Bullheaci

City

,o0 , /(

0 ~'l

, Cottonwood

.......

~"~"../

i

i,

J

,

Cove

Federal Primary

V

I

i

l

Fort Mohave

~

i

~

,

,

I

,

76 77 78 79 80 81 82 83 ~4 85 8b 87 88 Annual Geometric Mean of TSP

Fig. 9. Annual geometric mean of total suspended particulates (TSP) for three Mohave Valley sites: Bullhead City, Cottonwood Cove and F~r~ Mohave, Horizontal lines indicate the US Primary TSP Standard, 75pg m ~ (top), and US Secondary TSP Standard, 60t~g m-3 (bottom).

CHANGING ENVIRONMENT OF A DESERT BOOMTOWN

255

Ozone Ozone measurements have been made at Cottonwood Cove and Spirit Mountain, a solitary peak located 19.3 km northwest of Bullhead City, since 1976. Ozone was not measured at Bullhead City. Instruments at Spirit Mountain are mounted at an elevation of 1719 m msl near the top of the mountain and are assumed to represent the synoptic-scale of motion and the results of long-range transport. Data are collected with a Monitor Labs Model 8410A ozone analyzer or with Dasibi ozone monitors (model numbers 1003AH and 1003AAS). Minimum detectable level is 10ppb. Ozone data trace a strong yearly cycle, peaking in the early summer months (May/June) and reaching minimum values during the winter (December/ Monthly Mean Ozone Spirit M o u n t a i n and Cottonwood Cove 1-..... - r ~

I m--I

.... - - T - - - - - - T - •

I

I

I...............T -

75----

Spirit M t n .

.....+ ..... Cottonwood

h, II

Cove

65

i. i

55

P

0 45

35 ~-

+

+

+

++

+

+ ~

+

"~. +

25

I

I

01/79

I

I

01/8~

0ii80

I

I

01/83

01/82

I

I

01185

01/84

I

I

01/87

01/86

L 01/1~J9

01/88

Date

Fig. i0. Comparison of time series of mean munthly ozone measurements at the Cottonwood Cove and Spirit Molmtain sites. Cottonwood Cove is representative of non-urban conditions at the valley floor. Spirit Mountain site is representative of synoptic-scale conditions.

256

P.A. WALSH AND T.E. HOFFER

January). Time series of monthly mean ozone at the two sites are shown in Fig. 10. A linear regression of the monthly mean ozone concentration at Cottonwood Cove to the year of record produced a negative slope of - 0 . 8 9 8 p p b y e a r -~ (a = 0.479 ppb year -~, ~ = 0.064). A negative trend in ozone concentration was also detected at the Spirit Mountain site (slope = - 0.486 ppb year-~, a = 0.340 ppb year-~ ), but the linear regression results were less statistically significant (~ = 0.204). Monthly mean ozone values for Spirit Mountain are consistently higher than those observed at Cottonwood Cove. The consistently lower levels of ozone and the more pronounced negative treL~d at the valley ~oor is probably due to the elevated (and increasing) levels of NOx and photochemical reactions. CONCLUSIONS

Laughlin/Bullhead City has experienced significant changes in climate and air quality in the recent past. The climate has become warmer and is perceived to be more humid due to the influences of urbanization, topography and demography. The observed changes are more pronounced and progressing at a faster pace than climate changes in the rapidly expanding urban areas of Phoenix and Tucson. Topography of the Mohave Valley, rapid urbanization and a significant increase in automobile traffi~ through the valley have strongly affected the air quality. Time series and linear regression analyses have indicated a strong positive trend in the oxides of nitrogen. A comparable trend was not observed in measurements of SOs, another major component of the Mohave Power Project effluent. This would suggest that the increase in the oxides of nitrogen is not related to the MPP power plant, but may be attributed to a sharp increase in automobile traffic in the Mohave Valley. This conclusion is reinforced when data for the period June through December 1985 are reviewed, During that period of time the Mohave Power Project was offline. Sulfur dioxide concentrations for that period, with the exception of a few hours, did not exceed the instrument detection limits. No significant drop was observed in the oxides of nitrogen. Total suspended particulate levels in the Laughlin/Bullhead City area are consistently higher than surrounding agricultural areas. Trend analysis of the TSP measurements showed a positive trend over the period of reference at the urban site in Bullhead City. In contrast, negative trends were observed at both of the non-urban/rural sites. Ozone measurements at both Cottonwood Cove and Spirit Mountain indicated statistically significant declines over the period of record. Cottonwood Cove mean monthly ozone concentrations are lower than those observed at the top of Spirit Mountain and are declining at a more rapid pace. Photo-

CHANGING ENVIRONMENT OF A DESERT BOOMTOWN

257

chemical reactions and increasing concentrations of NO/NO2/NOx at the valley floor may be responsible for the observed decreases in ozone. The results of this study emphasize the potential effects of limited urbanization on the climate and air quality of a desert site. The introduction of humans and the trappings of modern, industrialized society rapidly destabilizes the precarious balance maintained by a pristine desert environment. The trarisformation in the Laughlin/Bullhead City environment from pristine desert to a polluted urban area has implications for other locales worldwide. In light of the current global increase in population and the pivotal role of arid ~egions of the Middle East and Africa in world matters, this study emphasizes that even small urban centers can significantly impact desert environments in a negative w ay. REFERENCES Bailing, R.C. and S.W. Brazel, 1987a. Recent changes i~ Phoenix, Arizona summertime diurnal precipitation patterns. Theor. Appl. Climatol., 38: 50-54. Bailing, R.C. and S.W. Brazel, 1987b. Temporal variatio~s in Tucson, Arizona summertime atmospheric moisture, temperature and weather stress levels. J. Climate Appl. Meteorol., 26: 995-999. Bailing, R.C. and S.W. Brazel, 1987c. Time and space characteristics of the Phoenix urban heat island. J. Ariz. Acad. Sci., 21: 75-81. Beaumont, P., 1989. Environmental Management and Development in Dry!ands. Routledge, Mackays of Chatham PLC, Chantham, Kentm UK. Brazel, S.W. and R.C. Bailing Jr, 1986. Temporal analysis of long-term atmospheric moisture levels in Phoenix, Arizona. J. Climate Appl. Meteorol., 25:112-117. Ca~ "~ D.R. and A.V. Douglas, 1984. Urban influences on surface temperatures in the sou'hwestern United States during recent decades. J Climate Appl. Meteorol., 23: 1520-1530. Chandler, T.J., 1967. Absolute and relative humidities in towns. Bull. Am. Meteorol. Soc., 48: 394-399. Coulombe, W.B., 1990. 1989 Annual Report. Atmospheric Survey. Mohave Power Project. Appendix B. Prepared tbr Southern California Edison Company, Rosemead, CA, by the Desert Research Institute, Reno, NV. EI-Baz, F. (Ed.), 1984. Deserts and Arid Lands. Martinus Nijoff Publishers, Kluwer Academic Publishers Group, The Hague, Netherlands. Glantz, M.H. (Ed.), 1977. Desertification: Environmental Degradation in and around Arid Lands. Westview Press, Boulder, CO, USA. Hills, E S. (Ed.), 1966. Arid Lands: A Geographical Appraisal. Methuen & Co. Ltd, Butler & Tanner Ltd, Frome and London, UK. Hoffcr, T.E., D.F. Miller and R.J. Farber, 1981. A case study of visibility as related to regional transport. Atmos. Environ., 15: 1935-1942. Landsberg, H.E., 1981. The Urban Climate. Academic Press, New York, NY, USA. Miller, D.F., D.E. Schorran, T.E. Hoffer, D.P. Rogers and W.H. White, 1990. An analysis of regional haze using tracers of opportunity. J. Air Waste Manage. Assoc., 40: 757-761. Robey, B., 1985. The American People. Trm,mn Talley Books, E.P. Dutton, New york, NY, USA.

258

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Sheridan, D., 1981. Desertification of the United States. Council of Envir~,nmental Quality 198 !. US Government Printing Office, Washington, DC. Watson, J.G., J.C. Chow, J.E. Core, D.A. Dubose, P.L. Hanraham, R.C. Henry, T.G. Pace, N.F. Robinson, H.J. Wiliamson and L. Wijnberg, 1990a. Receptor Model Technical Series, Vol. III, CMB User's Manual, CMB 7.0. US Environmental Protection Agency, Research Triangle Park, NC. Watson, .LG., N.F. Robinson, J.C. Chow, R.C. Henry, B.M. Kim, T.G. Pace, E.L. Meyer and Q. Ngyen, 1990b. The USEPA/DRI Ck~mical Mass Balance Receptor Model, CMB 7.0. Environ. Software, 5: 38-49. White, W.H., E.S. Macias, D.F. Miller, D.E. Schorran, T.E. Hoffer and D.P. Rogers, 1990. Regional transport of the urban workweek: methylchloroform cycles in the Ne,,ada-Arizona desert. Geophys. Res. Lett., 17: 1081-1084.

The changing environment of a desert boomtown.

World population growth has prompted the exploration and habitation of geographical regions previously considered undesirable or unsuitable for human ...
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