Sulfate Washout Ratios in Winter Storms

Abstract

Washout ratios for sulfate were computed from simultaneous measurements of sulfate in precipitation and in the air below cloud base. Stratification of the data according to predominant mode of precipitation growth (growth primarily by accretion of liquid cloud droplets versus growth primarily by vapor deposition onto ice particles) illustrated that for cases of accretional growth, the washout ratio was 10?50 times larger than for cases of depositional growth. The depositional growth mode appeared to be a baseline value and was determined by the efficiency of below-cloud scavenging. Sulfate washout ratios (mass weighted) for this baseline value ranged between 1 and 800 and typically averaged near 100. Enhancement of this baseline value caused by accretional growth was a measure of the contribution due to nucleation scavenging and/or aqueous phase conversion of SO2 to SO4 in the cloud water. This accretional mode increased the sulfate washout ratio to values averaging near 1000. Values of the washout ratio for sulfate during periods of growth predominantly by accretion appeared to be related inversely to precipitation rate. Two-thirds of the washout ratio observations were within a factor of 2 of the theoretically predicted values when precipitation growth occurred primarily by accretion of cloud droplets.