The resolution of the grid was 12-km over the area shown in Figure 1. Two meteorological periods, one summer (July 12-28, 2001) and one winter (January 1-21, 2002) were simulated to capture the seasonal variation of the responses. Source categories to be analyzed were determined using a tiered approach: any category that showed significant impact in one tier was broken into subcategories for further analysis in the next tier. Responses were calculated for the entire domain and visualized but decisions were made based on responses at national park and wilderness (Class 1) areas of the region as well as its Speciated Trends Network (STN) sites, which are generally located in urban areas.
In the first tier, SO2, NOx, NH3, VOC (both anthropogenic and biogenic) and primary carbon (PC) emissions were reduced uniformly over the entire domain. During the summer period, SO2 emission reduction was the most effective control strategy in reducing fine PM (i.e., PM2.5) levels due to a large decrease in sulfate concentrations. SO2 reduction was also the most beneficial strategy in improving visibility since sulfate is the largest summertime contributor to light extinction in this region. In winter, NH3 reduction was most effective due to large reductions in nitrate and ammonium, followed by NOx and PC reductions. However, since most of the worst visibility days occurred during summer, with the exception of a few coastal Class 1 areas, only SO2 sources were selected as the targets of the next tier. Two more questions were addressed in this tier. First, effects of combined reductions in SO2, NOx, NH3 emissions were compared to the effects of reducing these pollutants individually. It was found that adding individual responses may result in an overestimation of the combined response by about 10 %. Second, several Class 1 areas were paired with the nearest STN site to see if their responses were similar. While some pairs displayed strong correlation others differed, especially in their responses to SO2, NH3.and PC reductions.
In the second tier, SO2 sources were classified as ground-level and elevated. The elevated sources in the VISTAS states (Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, Tennessee, Virginia and West Virginia) were differentiated from elevated sources in the rest of the domain. Elevated VISTAS sources were further broken into three categories: coal-fired power plant (CPP), other power plant, and non-power plant (NPP). Responses to SO2 emission reductions from all these source categories were calculated and analyzed. CPPs of the VISTAS region had, by far, the largest impact on visibility at Class 1 areas: reducing their SO2 emissions decreased sulfate levels as well as light extinction. NPP impacts ranked second, generally by a wide margin except at some Class 1 areas that were not under so much influence of CPPs.
In the third tier, CPPs and NPPs were further broken down by state. Responses to SO2 emission reductions from CPPs and, separately, from NPPs of each individual VISTAS state were calculated. At each Class 1 area visibility was affected at a different level by these reductions. Figure 1 shows the relative reduction ratio of sulfate levels (a ratio smaller than unity indicates a reduction) when SO2 emissions from CPPs in Alabama are reduced by 30%. In this paper, responses to emission reductions described above will be quantified and deviations from general trends will be identified.
Figure 1. Response of sulfate concentrations to a 30% decrease in SO2 emissions from coal-fired power plants in Alabama on July 19, 2001.
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