Diffusion-coupled oxidation of atmospheric sulphur dioxide

Abstract

INCREASING evidence of the toxicity of atmospheric sulphates, together with the upward trend in fossil fuel combustion, have made the rate of formation of atmospheric sulphates one of the most important topics in current air pollution research. Many investigators¹ have measured sulphate concentrations in plumes and air sheds, and have usually fitted their data to a linear SO₂ oxidation rate expression. The reported values thus obtained for the rate constant vary by up to two orders of magnitude¹. Moreover, the linear rate expression is inconsistent with the findings of numerous field investigators^2–5, whose data suggest that the oxidation occurs almost exclusively near the SO₂ source, rather than uniformly throughout a dispersing atmosphere. The analysis given here deduces that the oxidation may occur almost wholly near the source, and describes the circumstances in which this will happen. It is also shown that in these conditions the oxidation proceeds not to completion but to a fractional asymptotic limit, and that half the final sulphate yield will be obtained in an amount of time whose limits are predictable, constant for any given dispersion pattern, and independent of the magnitude of the rate constants and other chemical parameters.