1. CEGB, Central Electricity Research Laboratories, Leatherhead, Surrey KT22 7SE, UK

Abstract

During the atmospheric transport of combustion-generated emissions of SO₂ and NO from point sources, the kinetics of oxidation processes are of particular importance in determining the pattern of subsequent wet and dry deposition because oxidation products such as sulphate aerosol and nitric acid may have deposition characteristics totally different from those of primary emissions. Modelling studies¹ have indicated the importance of the ambient air composition and of the dispersion rate on chemistry within plumes during the early stages of transport from point sources, and the important role of mixing in determining oxidant concentrations, particularly ozone, has been demonstrated in near field measurement programmes^2–5. It has been generally assumed (see ref. 5) that the significant consequences of dispersion are confined to the early stages of long-range transport and observations to date (see refs 5, 6) indicate that the strong influence of dispersion on plume chemistry is exerted over distances <100 km from source. We report here results of measurements made by an instrumented aircraft of the chemistry within a labelled plume on a trajectory over the North Sea which indicate that oxidation of plume material may be limited by dispersion at distances of at least 650 km.