Inefficient scavenging of aerosol sulfate by non-precipitating clouds in polluted air

We present measurements of the scavenging efficiency (beta) of aerosols bearing Se and As (mass median diameter around 0.5 mu m) by non-precipitating clouds associated with large frontal systems. The measurements were made during July 1995 and September-October 1993 at Whiteface Mountain summit. The site was influenced by highly polluted (based on aerosol loadings) as well as relatively clean air masses. Scavenging efficiency varied from 0.48 to 0.89; however, most values fell between 0.55 and 0.65. The highest values were associated with clouds where the pre-cloud aerosol SO4 concentration was < 5 mu g m(-3), mean beta of 0.78 +/- 0.06 compared to 0.60 +/- 0.05 when SO4 concentration was >5 mu gm(-3). We also demonstrate that beta based on SO4, the most common aerosol used in earlier scavenging studies, can be biased high due to contributions of SO4 in the cloud water from oxidation of dissolved SO2. (C) 1998 Elsevier Science Ltd. All rights reserved.