Sulfur dioxide oxidation in winter orographic clouds

Abstract

The production of sulfate in cloud droplets, and the attendant depletion of sulfur dioxide (SO2) oxidants were studied in wintertime orographic clouds over southeastern Wyoming. By periodic (5–20 min) releases of SO2 into the cloud the mixing ratio of SO2 was raised to about double its background value of 0.7 parts per billion by volume (ppbv). Average values of the in‐cloud reaction time, pseudo‐first‐order SO2 reaction rate, and hydrogen peroxide (H2O2) depletion were 400 s, 2.3×10−4 s−1, and 0.04 ppbv, respectively. The measured sulfate yields and H2O2 depletions show that H2O2 is the dominant oxidant for SO2 in this situation; this finding is consistent with model results. Model simulations reveal that the O2/SO2 reaction pathway (catalyzed by Mn(II) and Fe(III)) was competitive with the H2O2/SO2 reaction pathway on one out of the nine observation days. Organohydroperoxides were not depleted and other modeled reaction pathways (O3/SO2 and HCHO/SO2) were inferred to be inhibited due to either the chemical (pH < 4.7) or physical (temperature below −5°C, cloud water content less than 0.3 cm3 m−3) properties of the cloud. The agreement found in this work between observed and predicted properties of the H2O2/SO2/H2O system contrasts with the work of Chandler et al. (1988a, b, 1989) and Gallagher et al. (1990) who observe a factor of 3 or larger discrepancy between laboratory and field measurements of the H2O2/SO2 reaction rate.