A STUDY OF SO2, SO42- AND TRACE-ELEMENTS IN CLEAR-AIR AND CLOUDS ABOVE THE MIDWESTERN UNITED-STATES

In April and May 1990, as a part of ACID MODES II, a series of aerosol, gas and cloud water samples were collected over OH, WV, KY and IN at an altitude of 1.2-3 km using two aircraft. We report here the concentrations of SO2, SO24- and 12 trace elements measured on 41 filter pack samples collected over 17 flights and pH, conductivity, SO24- and selected trace elements in 17 cloud water samples from four flights. We discuss the composition of the airborne aerosols and cloud water above this area which is a major source region of SO2. Mean mixed layer concentrations of SO24- (mug m-3), and trace elements Zn, As, Se and Sb (ng m-3) were; 7.5+/-3.1, 35+/-19, 1.4+/-0.9, 1.7+/-0.8 and 0.49+/-0.22, respectively. These elements are primarily associated with submicron-sized aerosols and were the only elements measured that were significantly enriched relative to average crustal abundances. While the samples collected in the mixed layer had concentrations comparable to those at nearby surface sites, a 10-fold decrease in concentrations of all species was observed in samples collected immediately above the mixed layer. On average, 68% of the S was present as SO2 (average concentration in mixed layer air was 3.5 ppbv). A strong relationship was observed between total S and aerosol Se and to a lesser degree between As and Sb. The SO24-/Se tracer technique was used for the first time from an aircraft platform to determine in-cloud SO2 oxidation. Although the aircraft collected limited cloud water (generally < 20 ml) and small air volumes (< 10 m3), it is demonstrated that SO24- and Se can be determined with adequate uncertainty to apply the technique. In-cloud oxidation was found in three of the four clouds and accounted for up to 50% of the total SO24-. Examples of SO2 limited, and oxidant limited conditions were also observed.