Sulfur dioxide in remote oceanic air: Cloud transport of reactive precursors

Abstract

Reactive surface emissions of reduced sulfur gases can produce SO 2 in the middle and upper troposphere at the levels of 80±30 pptv measured high over the remote oceans. We present simulations with a two‐dimensional “Staubsauger” or “vacuum cleaner” model that combines a photochemical model with a description of vertical transport of trace species by convective clouds within larger synoptic circulations. Emissions of 20–60 Tg (S)/yr of (CH 3)2 S, H 2 S, or CS 2, may produce the observed SO 2. Roughly equal production rates of SO 2 and methane sulfonic acid may be expected. The amount and exact vertical distribution of the SO 2 produced remain uncertain: the greatest chemical uncertainties are the reaction yield of SO 2 expectable under clean tropospheric conditions and also the liquid‐phase removal of SO 2, and the oxidation rate. The amount of upper tropospheric SO 2 produced depends substantially on the proximity of strong reduced S sources to regions of active convection. However, the character of the solutions we present is invariably distinctly different from those obtained with one‐ or two‐dimensional models employing the eddy‐diffusion hypothesis. The results of the model point beyond its original conception, and stress the likely importance of the rainy tropical jungles and mid‐latitude industrial regions, since both regions have large sulfur emissions arid frequently active cumulonimbus convection. This process, however, should contribute mainly to upper‐tropospheric SO 2. Other chemical implications are that tropospheric OH may depend critically on HOOH levels as well as the hydrocarbon and nitrogen oxide cycles. Cloud transport may play an important role in these cycles. The hydroxyl radical concentration depends as much on assumptions regarding HOOH reaction and transport as it does on NO levels.