A direct sensitivity analysis technique (DDM-3D) has been integrated into the URM-1 ATM three-dimensional air quality model to efficiently provide an indication of which emission source types from various source areas have the greatest potential to impact pollutant levels. In this study, nine episodes were modeled to represent annual aerosol and wet deposition levels. Direct sensitivity analysis was then used to quantify the source/receptor relationship between emissions from thirteen geographic regions and pollutant levels in the Southern Appalachian Mountains (SAM). In particular, the impact of SO2 emission reductions on aerosol and wet deposition levels at ten Class I areas are discussed. SO2 emission reductions from different geographic regions displayed very different levels of impact on various sites within the SAM region. In general, the receptor sites showed the greatest response to emission reductions in the nearest states and regional sub-domains. Sites in Alabama and Georgia show the greatest response to reductions in emissions from Alabama and Georgia, respectively. The sites in North Carolina and Tennessee show the greatest response to emission reductions in Tennessee. And, the sites in Virginia and West Virginia show the greatest response to emission reductions in Virginia, West Virginia, and the Midwest sub-domain.

During 1992 an intensive study of the process of ozone formation was performed in the Atlanta area. This study included an extensive network of both ground-based and airborne measurements, as well as development of emission inventories and photochemical model scenarios. One of the major issues associated with ozone formation in Atlanta and elsewhere concerns the relation between ozone and its two major anthropogenic precursors, reactive organic gases (ROG) and nitrogen oxides (NO{sub x}). Traditionally, NO{sub x}-ROG sensitivity has been determined from predictions of photochemical models. The Atlanta study pioneered a number of new approaches to this issue, including an attempt to link predictions of NO{sub x}-ROG sensitivity more closely with observations. One of the most promising observation-based approaches involves the species O{sub 3} and total reactive nitrogen (NO{sub y}). During the Atlanta study intensive simultaneous measurements of ozone and NO{sub y} were made during helicopter transects of the urban plume downwind from Atlanta on several days. In this paper the authors will focus on measurements made during August 10--11, 1992, days characterized by warm temperatures, light winds and high ozone. A series of photochemical model scenarios have been developed for the event of August 10--11, 1992 in Atlanta.

This paper presents an analysis of cloud water measurements made during the summers of 1986 and 1987 at Whitetop Mountain, Virginia (36.639° N, 81.605° W). Analysis of cloud water chemistry, cloud type, and air mass origin are made for each cloud event occurring during one 3 to 4 week measurement ‘intensive’ per year. Regional source/receptor relationships are also investigated. Cloud water concentrations of major ions (i.e., H+, SO42−, NO3 −, and NH4+) are consistently higher during orographically formed ‘cap’ cloud events. Differences in cloud liquid water content between cap and frontal cloud events explains most, but not all, of the cloud water ion concentration differences. The remaining difference can be explained by greater rainfall associated with frontal cloud events. Most of the cloud water sulfate measured at Whitetop Mountain is apparently due to nucleation of aerosol sulfate within cloud droplets and not to local in-cloud aqueous phase SO, oxidation. No strong source/ receptor relationships were evident from this analysis. Most 72 hr air trajectories arriving at Whitetop Mountain during the cloud events described in this paper originated in the southeastern United States. Few came from the Ohio River Valley or the northeastern United States.

Recently, a comprehensive air quality modeling system has been developed as part of the Southern Appalachians Mountains Initiative (SAMI). The performance of the model in predicting ozone, size- and composition-resolved aerosols, and acid deposition mass fluxes have been evaluated using measurements during nine episodes between 1991 and 1995. The daily averaged normalized bias and error for ozone are typically within EPA guidance criteria for urban-scale modeling. The mean normalized error was approximately 40% for the sulfate, ammonium, elemental carbon, and organic components that constitute over 75% of the PM2.5 in the region. The error is generally larger for the nitrate and soil components but these components are relatively small. The wet deposition mass fluxes have high spatial variability, but still agree well with observations. The mean normalized errors for sulfate and nitrate wet deposition were approximately 25%. Wet deposition bias was further accentuated by a bias in simulated precipitation. Variations in modeling error with pollutant levels were also examined. Most species showed a systematic overestimation for low levels and an underestimation for high levels.

As part of the Southern Appalachians Mountains Initiative (SAMI), a comprehensive air quality modeling system has been developed to assess the impact of three different emission scenarios on air quality for the years 2010 and 2040. Results have been aggregated over nine characteristic episodes, representing 69 days, to find the expected response of seasonal ozone, annual average PM2.5 and annual average wet and dry deposition. These levels provide the basis for SAMI's regional effects modeling assessment. It was found that ozone can be reduced with nitrogen oxide (NOx) controls. Sulfate aerosols and sulfur deposition decrease significantly in the Class I areas in response to sulfur dioxide (SO2) emission controls. However, an increase in nitrate aerosol levels may result due to an increase in free ammonia becoming available in response to reductions in SO2 and increases in NH3 emissions. Also, changes in total nitrogen deposition were minimal, except when ammonia emissions are controlled.