A study of heterogeneous oxidation of SO2in summer clouds

Abstract

Experimental determination of in‐cloud oxidation of SO2has been lacking because of an inability to distinguish SO4in cloud water, derived from aerosol scavenging (i.e., produced from homogeneous gas phase oxidation), from that due to in‐cloud SO2oxidation, (SO4)in. A tracer technique has been developed in this laboratory that uses trace elements (M) to resolve the two SO4components. 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 is based on simultaneous determination of SO4and M in cloud water (cw) and in out‐of‐cloud air (aa), and the knowledge of the relative scavenging efficiencies α and β of SO4and M‐bearing aerosols, respectively: (SO4)in= [(SO4/M)cw‐ α/β (SO4/M)aa] (M)cw. Data from two events are presented which yield the scavenging ratio of SO4/Se as 1.04±0.29 and 1.04±0.19. The technique has been used to quantitatively determine in situ SO2oxidation 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)invaried from 0 to 39%. The amount of (SO4)indid not vary dramatically from one sample to the next. The data appeared to be consistent with a flow‐through reactor at the site through which updrafted air is passing. Where available, SO2data in clear air and within the cloud yielded a mass balance consistent with our experimentally determined in situ SO4formation. 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 SO2. This was confirmed by our observation that SO2concentration 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)indecreased with increasing pH with negligible oxidation above pH 4. Also, (SO4)indecreased 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 SO2and SO4are dependent on atmospheric oxidation. Enhancement in oxidation yields higher SO4/Se ratio and at the same time leaves less SO2in the air for oxidation. A simple model is developed that enables an estimation of SO2oxidation in clouds.