Composition and origin of summertime air pollutants at Deep Creek Lake, Maryland

Abstract

A 1 month intensive summertime field study conducted in rural western Maryland resulted in a comprehensive set of fine particle, gaseous and meteorological data. Sulfur in the assumed form of ammonium sulfate accounted for 67% of the average fine particle mass and had a very high correlation with fine particle mass (r=0.99). Other measured species, includiqg carbon and nitrate, made only minor contributions to the fine mass. Peak sulfate concentrations, averaged over 6-h intervals, exceeded 50 [mu]g m-3. Nitric acid concentrations showed strong daytime maxima and on a 24 h basis were about four times those of fine particle nitrate. Sulfur in the gas phase (SO2) constituted more than half of the total sulfur, indicating that the sampling site was being influenced by local sources. Fine particle selenium was well correlated with fine particle sulfur (r=0.70). The ratio of fine particle sulfur to selenium was 2800, characteristic of a rural site downwind of coal-burning areas. Mixed-layer back trajectories were used to identify possible source regions for the measured parameters. Wind frequency-normalized concentrations of parameters associated with coal-burning (S, Se, SO2 and mass) were highest for back trajectories arriving at Deep Creek Lake from the west-northwest. Use of Rahn and Lowenthal regional signatures showed an overwhelming dominance by the Lower Midwest region, and a surprisingly weak impact by the Upper Midwest region, at the site.