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Experimental determination of incloud oxidation of SO l has been lacking because of an inability to distinguish SO 4 in cloud water, derived from aerosol scavenging (i.e., produced from homogeneous gas phase oxidation), from that due to in-cloud SO l oxidation, (SO4)in. A tracer technique has been developed in this laboratory that uses trace elements (M) to resolve the two SO 4 components. Extensive data are reported in this paper on the use of Se as a tracer. Preliminary results are also included to test As and Sb as potential tracers. The technique i• based on simultaneous determination of SO 4 and M in cloud water (cw) and in out-of-cloud air (aa), and the knowledge of the relative scavenging efficiencies a and • of SO½ and Mbearing aerosols, respectively' (SO4)i, = [(SO4/M)c w -a/• (SO4/M)aa] (M)cw. Data from two events are presented which yield the scavenging ratio of SO4/Se as 1.04 ñ 0.29 and 1.04 ñ 0.19. The technique has been used to quantitatively determine in situ SO l oxidation in six summer cloud systems during 1987-1989 at Whiteface Mountain, New York. Initially, cloud water samples were collected hourly, but in later sampling campaigns this interval was reduced to 15 min to better delineate the relationship between chemistry and meteorology in clouds. Aerosol collection was also reduced from 12 to 2 hours. Five cloud systems with pH ranging from 2.8 to 4.0 processed air which, based on our understanding of elemental signatures, had passed through the high-pollution emitting areas of the midwest. In individual samples, (SO4)in varied from 0 to 39%. The amount of (SO4)•n did not vary dramatically from one sample to the next. The data appeared to be consistent with a flowthrough reactor at the site through which updrafted air is passing. Where available, SO l data in clear air and within the cloud yielded a mass balance consistent with our experimentally determined in situ SO 4 formation. A sixth cloud event, a storm system which processed air largely from the north-northwest ("clean air"), yielded pH of 4.3 to 5.0 in the cloud water. Our tracer techniques based on both Se and As revealed no in situ oxidation of SO l . This was confirmed by our observation that SO l concentration was low, and values in the cloud and in cloud-free air were equal. When data from all six summer cloud systems were pooled together, (SO4)i, decreased with increasing pH with negligible oxidation 1Wadsworth Center for Laboratories and Research, above pH 4. Also, (SO4)i, decreased smoothly with increasing aerosol SO4/Se ratio. Coal combustion is the primary source of S and Se in rural atmosphere in the northeast. Relative proportions of SO t and SO 4 are dependent on atmospheric oxidation. Enhancement in oxidation yields higher SO4/Se ratio and at the same time leaves less SO l in the air for oxidation. A simple model is developed that enables an estimation of SO l oxidation in clouds. port. Thus H202 may control the regional sourcereceptor relationship between SO 2 emissions and Paper number 91JD01943. SO 4 deposition. The lim...

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