(BRAVO) Study: Final Report. - Desert Research Institute

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Final Report — September 2004 West 24h tracer 6h tracer no tracer Guadalupe Mtns 6-hour sites Central Texas Wichita Mtns Lake Colorado City Stephenville Monahans Ft McKavett Esperanza McDonald Ft Stockton Marathon Ft Lancaster Sanderson Persimmon Gap Langtry LBJ Presidio Amistad Brackettville Rio Grande Pleasanton Big Bend K-Bar Eagle Pass Near East Laredo Falcon Dam Hagerman Stillhouse Purtis Creek Somerville Everton Ranch Aransas Lake Corpus Christi Padre Island South Texas Laguna Atascosa Eastern Texas Wright Patman Center San Bernard Big Thicket Figure 5-1. Groups of BRAVO sampling sites that showed similar particulate matter behavior. 12000 10000 Sulfate SO2 Sulfate/Total Sulfur 1.20 0.80 Nanograms per cubic meter 8000 6000 4000 0.40 0.00 -0.40 Ratio 2000 -0.80 0 7/23 7/30 8/6 8/13 8/20 8/27 9/3 9/10 9/17 9/24 10/1 10/8 10/15 10/22 10/29 Date -1.20 Figure 5-2. Sulfate and SO 2 time series at K-Bar (left scale) and sulfate as fraction of total sulfur (right scale). Sulfate was closely associated with ammonium, and was present as a mixture of ammonium sulfate and ammonium bisulfate, i.e., ammonium to sulfate ion equivalent ratios varied between 0.5 and 1 (i.e., molar ratios ranged from 1 to 2). Excluding one outlier, the mean ammonium to sulfate ion equivalent ratio was 0.8 ± 0.2, and the aerosol was never 5-2
Final Report — September 2004 fully neutralized. The ratio did not change during sulfate episodes, but it did increase slightly from an average of 0.7 ± 0.1 before mid-September to 0.9 ± 0.1 from mid-September to the end of the study. A ratio of 1.0 indicates fully neutralized sulfate. The nitrate ion concentration was at least a factor of ten lower than that of sulfate, and was not associated with ammonium. During some periods, it was highly correlated with sodium, though. The early part of the study was characterized by several episodes of elevated fine soil concentrations. During these periods, the Fe/Ca ratio was also elevated and 5-day back trajectories frequently indicated rapid transport from the Caribbean, suggesting the possible influence of Saharan dust. (The influence of Saharan dust is covered in more detail in Sections 5.4 and 10.3.) As an example, we will discuss the August 29-September 4 sulfur episode, which had the highest sulfur concentration during the BRAVO Study. A few days earlier (August 27) widespread haze was reported throughout a large area of the eastern United States. High sulfate concentrations first appeared in northeastern Texas and then spread across the state to Big Bend by September 1. Haze was reported on September 1, with visibility of just a few miles at many National Weather Service stations in central and southern Texas (Del Rio, Laredo, San Antonio, Harlingen, and Corpus Christi). Figure 5-3 shows the fine particulate matter composition across Texas during this period. Note the high sulfate and organic mass in eastern Texas, and even at Wichita Mountains, Oklahoma, decreasing across the state to Big Bend. Concentrations in the southern part of the state were lower than elsewhere during this period. nanograms per cubic meter 20000 15000 10000 5000 Sodium Non-soil Potassium Soot mass Soil mass Organic mass (H) Ammonium Sulfate 0 West K-Bar Six-hour sites Near East Central Texas Eastern Texas South Texas Wichita Mtns. Figure 5-3. Composition of PM 2.5 aerosol throughout Texas during the August 29-September 4, 1999, sulfate episode at Big Bend. Organic mass (H) refers to organic matter concentrations inferred from hydrogen atom measurements. All of the particulate sulfur is assumed to be in the form of ammonium sulfate, whose concentration is thus 4.125 times the measured elemental sulfur concentration. 5-3
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