Air Pollution

Diem, J.E., 2013. The 1970 Clean Air Act and termination of rainfall suppression in a U.S. urban area. Atmospheric Environment 75:141–146.

The purpose of this paper is to determine the impact of reduced atmospheric particulate resulting from the Clean Air Act of 1970 on changes in summer rainfall in the Atlanta, Georgia USA region. In order to determine if rainfall at nine candidate stations in the metropolitan area was influenced by changes in particulate concentrations within the 1948-2009 period, predicted rainfall characteristics were derived from rainfall frequencies at nine reference stations located more than 80 km from downtown Atlanta. Both parametric and non-parametric tests were used to test for significant differences between observed values and predicted values within 34 overlapping 30-year periods. For the country as a whole, emissions of PM10 (i.e. particulates with a diameter less than or equal to 10 mm) decreased by approximately 40% from 1970 to 1975. The reduction in emissions caused a rapid rebound in summer rainfall in the Atlanta region. There was suppression of rainfall over and downwind of the Atlanta urbanized area during 30-yr periods that comprise all or portions of the decades of the 1950s, 1960s, and 1970s. This suppression occurred even while urban-related factors that promote rainfall enhancement were present. During the 1948e1977 suppression period, there was a decrease in rainfall of at least 40 mm at affected locales, which is substantial given that the mean seasonal rainfall was approximately 300 mm. The rainfall suppression involved a decrease of heavy-rainfall days. Atlanta is most likely not a unique case; therefore, particulate-induced rainfall suppression might have occurred over and downwind of other U.S. urban areas prior to the late 1970s.


Diem, J.E., M.A. Hursey, I.R. Morris, A.C. Murray, and R.A. Rodriguez. 2010. Upper-level atmospheric circulation patterns and ground-level ozone in the Atlanta metropolitan area. Journal of Applied Meteorology and Climatology 49:2185–2196.

The purpose of this paper is to identify middle-troposphere circulation patterns associated with high ozone concentrations during June–August of 2000–07 in the Atlanta, Georgia, metropolitan statistical area (MSA), which is located in the southeastern United States. The methods involved classifying daily 500-hPa geopotential height fields into synoptic types, determining the mean atmospheric conditions (i.e., daytime temperature, daytime relative humidity, daytime cloud cover, morning mixing height, and afternoon mixing height) for the types, determining the mean daily maximum 8-h average ozone concentrations for the types, and performing back-trajectory analyses for high-ozone types. There were a total of 12 synoptic types, and significantly high ozone concentrations across the MSA coincided with the three following types: Atlanta under a continental anticyclone, Atlanta to the east of a continental anticyclone and west of a trough, and Atlanta under the western side of a trough. The continental-anticyclone type was much more prevalent than the other two types. When the MSA was under or just to the east of a continental anticyclone, atmospheric conditions were conducive to increased in situ ozone production and pollutant carryover from the previous day. Between 45% and 60% of the days with those circulation patterns had ozone concentrations exceeding the federal standard. When the Atlanta MSA was under the western side of a trough, not only did the potential for in situ ozone production and pollutant carryover contribute to high ozone concentrations, but there also was a high potential for pollutant transport from the Ohio River valley into the Atlanta MSA.


Diem, J.E. 2009. Atmospheric characteristics conducive to high-ozone days in the Atlanta metropolitan area. Atmospheric Environment 43:3902-3909.

The purpose of this paper is to identify the atmospheric conditions associated with elevated ground-level ozone concentrations during June–August of 2000–2007 at 11 ozone-monitoring stations in the Atlanta, GA, USA metropolitan statistical area (MSA). Analyses were confined to high-ozone days (HODs), which had a daily maximum 8-h average ozone concentration in the 95th percentile of all June–August values. Therefore, each station had 36 HODs. The southeastern and far northern portions of the MSA had HODs with the highest and lowest ozone concentrations, respectively. HODs at nearly all Atlanta MSA ozone monitoring stations were enabled by migratory anticyclones. HODs for most stations were hot, dry, and calm with low morning mixing heights and high afternoon mixing heights. All sets of HODs had daily mean relative humidities and afternoon mixing heights that, respectively, were significantly less than and significantly greater than mean values for the remaining days. Urbanized Atlanta typically was upwind of an ozone-monitoring station on its HODs; therefore, wind direction on HODs varied considerably among the stations. HODs may have been caused partially by NOx emissions from electric utility power plants: HODs in the southern portion of the MSA were linked to air-parcel trajectories intersecting a power plant slightly northwest of Atlanta and plants in the Ohio River Valley, while HODs in the northern portion of the MSA were linked to air-parcel trajectories intersecting two large power plants slightly southeast of the Atlanta MSA. Results from this study suggest that future research in the Atlanta MSA should focus on power-plant contributions to ground-level ozone concentrations as well as the identification of non-monitored locations with potentially high ozone concentrations.


Diem, J.E., and D.M. Styers. 2008. Ozone exposure and potential for vegetation injury within the Atlanta, Georgia metropolitan area. Southeastern Geographer 48:172–183.

Ozone-induced vegetation injury may be prevalent within metropolitan areas in the southeastern United States. The purpose of this study is to explore the relationship between ozone exposure and potential foliar injury within the Atlanta, Georgia metropolitan statistical area (MSA). The main methods involve calculating seasonal ozone exposure at 11 monitoring stations in the Atlanta MSA for 2000-2005 and measuring ozone-induced visible foliar injury in the eastern portion of the MSA in August 2004. Ozone exposure within the Atlanta MSA probably was high enough to injure only highly sensitive species, and the foliar survey did reveal foliar injury to highly sensitive species.


Diem, J.E., C.R. Ricketts, and J.R. Dean. 2006. Impacts of urbanization on land-atmosphere carbon exchange within a metropolitan area in the USA. Climate Research 30:201–213.

Urbanization can cause changes in carbon fluxes, which, in turn, impacts atmospheric carbon dioxide (CO2) concentrations and possibly global surface temperatures. Using the Atlanta, Georgia, region as a case study, this paper explores the impact of urban expansion from 1973 to 2002 on land–atmosphere carbon exchange. The major objectives were to estimate net ecosystem production (NEP) values for multiple land-cover classes and to link urbanization-induced changes in land-cover to changes in NEP and overall carbon fluxes. The principal data were daily climatic data, year-specific land-cover data, annual net ecosystem exchange (NEE) values, and annual anthropogenic carbon emissions estimates. The principal methods were testing for climatic trends, determining the composition of the land-cover classes, estimating annual NEP values for the land-cover classes, and estimating the overall carbon exchange. The major findings: (1) there were no significant trends for any of the climatic variables; (2) the region was only ~16% urbanized in 1973; however, by 2002, the region was ~38% urbanized; (3) the NEP in 1978–1980 of 443gC m–2 yr–1 may have continued until 1996–1998, despite the substantial loss of forest land; and (4) net carbon emissions increased from ~150 g in 1978–1980 to ~940g C m–2 yr–1 in 1996–1998. Therefore, urban expansion greatly increased the carbon emissions of the Atlanta region; however, it is possible that, through increasing the growing-season length as well as increasing nitrogen and CO2 fertilization, urban expansion may not decrease the region-wide NEP.


Diem, J.E. 2004. Explanations for the spring peak in ground-level ozone in the southwestern United States. Physical Geography 25:105–129.

Many remote locations in the Northern Hemisphere have a spring peak, rather than a summer peak, in ground-level ozone concentrations, and the principal cause is presumed to be stratosphere-troposphere exchange (i.e., stratospheric intrusions). Grand Canyon National Park (GCNP) in northern Arizona also has a spring peak, and the purpose of this study is to explore the impact of stratospheric intrusions and another process synoptic-scale pollutant transport on ground-level ozone levels at GCNP from 1996 to 2000. The primary methods involve the stratification of days to identify stratospheric-intrusion days and the compositing of days to assess the impact of pollutant transport on ground-level ozone concentrations. Results indicate that stratospheric intrusions contributed little to the ozone budget at GCNP. In fact, atmospheric pollution originating in southern California was the likely cause of the May peak in ozone. The transported pollution also appeared to be responsible for high ozone days during all spring months. Tracer-based research (i.e., beryllium-7 and methylchloroform) at multiple locales in the southwestern United States is needed to fully confirm the weak impact of stratospheric intrusions and the strong contribution of ozone and its precursors originating in southern California.


Diem, J.E. 2003. A critical examination of ozone mapping from a spatial-scale perspective. Environmental Pollution 125:369-383.

Following the establishment of point measurements of ground-level ozone concentrations have been attempts by many researchers to develop ozone surfaces. This paper offers a critique of ozone-mapping endeavors, while also empirically exploring the operational scale of ground-level ozone. The following issues are discussed: aspects of spatial scale; the spatial complexity of ground-level ozone concentrations; and the problems of previous attempts at ozone mapping. Most ozone-mapping studies are beset with at least one of the following core problems: spatial-scale violations; an improper evaluation of surfaces; inaccurate surfaces; and the inappropriate use of surfaces in certain analyses. The major recommendations to researchers are to acknowledge spatial scale (especially operational scale), understand the prerequisites of surface-generating techniques, and to evaluate the resultant ozone surface properly.


Diem, J.E. 2003. Potential impact of ozone on coniferous forests of the interior southwestern United States. Annals of the Association of American Geographers 93:265–280.

Despite the well-documented negative impacts of ozone on the health of coniferous forests in southern California and the significant growth experienced by southwestern cities over the past several decades, the ozone/forest dynamic in the interior portion of the southwestern United States has been largely ignored. Primarily through a review of literature pertaining to most aspects of ozone and its impact on forest health, this article provides insights on the ozone/forest dynamic within coniferous forests of the interior Southwest. It is suggested that ozone absorption in southwestern coniferous forests may equal that in southern California, owing to the long-distance transport of atmospheric pollutants into the interior Southwest and the presence of the North American monsoon. Nevertheless, research gaps identified in this article suggest a need for future research on ozone exposure levels and the ozone sensitivities of conifer species and varieties in southwestern coniferous forests.


Diem, J.E. 2002. Remote assessment of forest health in southern Arizona, USA: Evidence for ozone-induced foliar injury. Environmental Management 29:373–384.

This paper examines possible ozone-induced foliar injury to ponderosa pine areas in the Rincon Mountains of southern Arizona from 1972 to 1992. Spatiotemporal differences in a satellite-derived vegetation index (VI) are examined with respect to antecedent moisture conditions, temporal variations in ozone exposure levels, and measured foliar injury values from 1985. Seasonal ozone exposure levels (SUM60 and W126) increased from 1982 to 1998 and were significantly correlated (r = 0.49 and 0.53, alpha = 0.05) with annual population totals in the Tucson area. Extensive masking of satellite images from 1972, 1986, and 1992 resulted in two optimal change detection areas, with one site, TVWMica, exposed mostly to the Tucson air pollution plume, while the other site, EMica, was more protected from Tucson-derived pollution. An overall increase in VI from 1972 to 1992 at both sites appears to have been caused by an increase in moisture availability. Larger foliar injury values in 1985 were associated with a smaller increase in VI (i.e., a smaller increase in green leaf biomass) from 1972 to 1986. From 1972 to 1986 and from 1986 to 1992, VI values at TV/WMica increased at a slower rate compared to those at EMica. The reduced increase in “green-up” may have been caused partially by ozone-induced foliar injury and resulting decreases in green leaf biomass. However, these spatial differences in VI values may have also been caused by a number of other factors. Results nevertheless reveal the strong possibility of distinct, topographically based, spatial variations in ozone-induced foliar injury within the Rincons.


Diem, J.E., and A.C. Comrie. 2002. Predictive mapping of air pollution involving sparse spatial observations. Environmental Pollution 119:99–117.

A limited number of sample points greatly reduces the availability of appropriate spatial interpolation methods. This is a common problem when one attempts to accurately predict air pollution levels across a metropolitan area. Using ground-level ozone concentrations in the Tucson, Arizona, region as an example, this paper discusses the above problem and its solution, which involves the use of linear regression. A large range of temporal variability is used to compensate for sparse spatial observations (i.e. few ozone monitors). Gridded estimates of emissions of ozone precursor chemicals, which are developed, stored, and manipulated within a geographic information system, are the core predictor variables in multiple linear regression models. Cross-validation of the pooled models reveals an overall R2 of 0.90 and approximately 7% error. Composite ozone maps predict that the highest ozone concentrations occur in a monitor-less area on the eastern edge of Tucson. The maps also reveal the need for ozone monitors in industrialized areas and in rural, forested areas.


Diem, J.E., and A.C. Comrie. 2001. Allocating anthropogenic pollutant emissions over space: Application to ozone pollution management. Journal of Environmental Management 63:425–447.

An inventory of volatile organic compound (VOC) and nitrogen oxides (NOx) emissions is an important tool for the management of ground-level ozone pollution. This paper has two broad aims: it illustrates the potential of a geographic information system (GIS) for enhancing an existing spatially-aggregated, anthropogenic emissions inventory (EI) for Tucson, AZ, and it discusses the ozone-specific management implications of the resulting spatially-disaggregated EI. The main GIS-related methods include calculating emissions for specific features, spatially disaggregating region-wide emissions totals for area sources, and adding emissions from various point sources. In addition, temporal allocation factors enable the addition of a multi-temporal component to the inventory. The resulting inventory reveals that on-road motor vehicles account for approximately 50% of VOC and NOx emissions annually. On-road motor vehicles and residential wood combustion are the largest VOC sources in the summer and winter months, respectively. On-road motor vehicles are always the largest NOx sources. The most noticeable weekday vs. weekend VOC emissions differences are triggered by increased residential wood combustion and increased lawn and garden equipment use on weekends. Concerning the EI’s uncertainties and errors, on-road mobile, construction equipment, and lawn and garden equipment are identified as sources in the most need of further investigation. Overall, the EIs spatial component increases its utility as a management tool, which might involve visualization-driven analyses and air quality modeling.


Diem, J.E., and A.C. Comrie. 2001. Air quality, climate, and policy: A case study of ozone pollution in Tucson, Arizona. The Professional Geographer 53:469–491.

This article addresses the need to better understand the complex interactions between climate, human activities, vegetation responses, and surface ozone so that more informed air-quality policy recommendations can be made. The impacts of intraseasonal climate variations on ozone levels in Tucson, Arizona from April through September of 1995 to 1998 are determined by relating variations in ozone levels to variations in atmospheric conditions and emissions of ozone’s precursor chemicals, volatile organic compounds (VOCs) and nitrogen oxides (NOx), and by determining month-specific atmospheric conditions that are conducive to elevated ozone levels. Results show that the transport of ozone and its precursor chemicals within the Tucson area causes the highest ozone levels to be measured at a downwind monitor. The highest ozone levels occur in August, due in part to the presence of the North American monsoon. Atmospheric conditions conducive to elevated ozone concentrations differ substantially between the arid foresummer (May and June) and the core monsoon months ( July and August). Transport of pollution from Phoenix may have a substantial impact on elevated ozone concentrations during April, May, and June, while El Paso/Ciudad Juarez –derived pollution may contribute significantly to elevated ozone concentrations in August and September. Two broad policy implications derive from this work. Regional pollutant transport, both within the U.S. and between the U.S. and Mexico, is a potential issue that needs to be examined more intensively in future studies. In addition, spatiotemporal variations in sensitivities of ozone production require the adoption of both NOx and VOC control measures to reduce ozone levels in the Tucson area.


Diem, J.E., and A.C. Comrie. 2000. Integrating remote sensing and local vegetation information for a high-resolution biogenic emissions inventory – application to an urbanized, semi-arid region. Journal of the Air and Waste Management Association 50:1968–1979.

This paper presents a methodology for the development of a high-resolution (30-m), standardized biogenic volatile organic compound (BVOC) emissions inventory and a subsequent application of the methodology to Tucson, AZ. The region’s heterogeneous vegetation cover cannot be modeled accurately with low-resolution (e.g., 1-km) land cover and vegetation information. Instead, local vegetation data are used in conjunction with multispectral satellite data to generate a detailed vegetation-based land-cover database of the region. A high-resolution emissions inventory is assembled by associating the vegetation data with appropriate emissions factors. The inventory reveals a substantial variation in BVOC emissions across the region, resulting from the region’s diversity of both native and exotic vegetation. The importance of BVOC emissions from forest lands, desert lands, and the urban forest changes according to regional, metropolitan, and urban scales. Within the entire Tucson region, the average isoprene, monoterpene, and OVOC fluxes observed were 454, 248, and 91 micrograms/m2/hr, respectively, with forest and desert lands emitting nearly all of the BVOCs. Within the metropolitan area, which does not include the forest lands, the average fluxes were 323, 181, and 70 micrograms/m2/hr, respectively. Within the urban area, the average fluxes were 801, 100, and 100 micrograms/m2/hr, respectively, with exotic trees such as eucalyptus, pine, and palm emitting most of the urban BVOCs. The methods presented in this paper can be modified to create detailed, standardized BVOC emissions inventories for other regions, especially those with spatially complex vegetation patterns.


Diem, J.E. 2000. Comparisons of weekday-weekend ozone: Importance of biogenic volatile organic compound emissions in the semi-arid southwest USA. Atmospheric Environment 34:3445–3451.

This paper examines differences between daily maximum weekday and weekend ambient ozone concentrations in the Tucson, AZ metropolitan area. Temporal variations in the Weekend Effect (i.e. weekend ozone concentrations are larger than weekday concentrations) are not explained entirely by changes in anthropogenic emissions of ozone precursor chemicals (i.e. nitrogen oxides and volatile organic compounds). A dramatic change from the Weekend Effect in June to the Weekday Effect (i.e. weekday ozone concentrations are larger than weekend concentrations) in July is associated with the onset of the North American Monsoon. A transition from a relatively dry atmosphere during the arid foresummer months of May and June to a relatively moist atmosphere during the monsoon months of July and August seems to explain the changes in ozone concentrations. Moist conditions are associated with increases in biogenic volatile organic compound (BVOC) emissions in the urban forest and surrounding desert areas. BVOC emissions appear to be an important source of VOCs, especially during the monsoon months. Therefore, an increase in ambient BVOC concentrations from June to July presumably reverses the sensitivity of ozone production in the Tucson area from VOC- to NO x-sensitive.


Comrie, A.C., and J.E. Diem. 1999. Climatological analysis and forecast modeling of carbon monoxide concentrations in Phoenix, Arizona. Atmospheric Environment 33:5023–5036.

TWe perform a climatology of factors influencing ambient carbon monoxide (CO), in which we examine the relationships between meteorology, traffic patterns, and CO at seasonal, weekly, and diurnal time scales in Phoenix, Arizona. From this analysis we identify a range of potentially important variables for statistical CO modeling. Using stepwise multivariate regression, we create a suite of models for hourly and 8-h ambient CO designed for daily operational forecasting purposes. The resulting models include variables and interaction terms related to anticipated nocturnal atmospheric stability as well as antecedent and climatological CO behavior. The models are evaluated using a range of error statistics and skill measures. The most successful approach employs a two-stage modeling strategy in which an initial prediction is made that may, depending on the forecast value, be followed by a second prediction that improves upon the first. The best models provide accurate daily forecasts of CO, with explained variances approaching 0.9 and errors under 1 ppm.

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