CLOUD SCAVENGING OF AEROSOL SULFUR: TWO CASE STUDIES

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Abstract

Theoretical estimates of sulfate removal by precipitation have assumed that 40–100% of available clear-air sulfate is scavenged in-cloud. In an experiment performed in an air mass with <1 μgm−3 sulfate, particulate matter of diameter D <3 μm was collected to afford 1-h increment data analysis for sulfur and soil (here, a composite of A1, Ca, Fe, Si, K and Ti) by PIXE analysis. Optical particle spectrometer probes were used to determine number and mass distributions of aerosol and cloud droplets in a diameter range 0.09 <D <23.5 μm. Two in-cloud studies, one 9 h and one 6 h long, show removal efficiencies (by mass) of 80–98% of sulfur and of 35–77% of the soil aerosol. During the first (stratus) cloud event, the total accumulation-mode mass was reduced < 90%. Nucleation scavenging, according to classical closed-parcel cloud microphysics theory, appears to be the dominant mechanism by which precipitation sulfate was formed in this background air mass.

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Author to whom correspondence should be addressed. Present address: Center for Environmental Sciences, Dept. of Physics, Univ. of Colorado, Denver, CO 80202, U.S.A.

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