A new soil source of atmospheric sulfur dioxide

More than 2 million km2 of Holocene-age, pyritic sediments have formed naturally from dissolved sulfate in coastal environments around the world. Oxidation of this pyrite forms acid sulfate soils (ASS). The process is greatly accelerated by human activities, such as excavation or land drainage.

Most of the oxidation efflux from these coastal ASS occurs as acidic dissolved sulfate (H2SO4), but elemental balance studies have suggested that efflux of sulfur dioxide (SO2) may be important. We report our first studies on this issue.

Profiles of atmospheric SO2 concentration were measured above a bare ASS used for sugar cane cultivation by a pulsed-fluoresence SO2 detector and passive gas samplers, along with measurements of the surface energy balance and soil evaporation rate. Direct measurements of SO2 efflux were made with closed chambers employing the SO2 analyser and a circulating air stream.

The atmospheric SO2 concentrations were in the range 1 to 4 ppbv, which were considerably larger than expected for clean rural air. Small fluxes of SO2 from soil to atmosphere were detected. They appeared to be linked to the timing and magnitude of surface evaporation. We suggest that SO2 evolution from ASS may be a significant item in the global atmospheric sulfur budget.