The oxidation and long-range transport of sulphur dioxide in a remote region

Abstract

The flux of sulphur dioxide has been measured in the plume of a remotely situated smelter (Mount Isa, Australia) at distances of up to 1000 km from the source. These measurements were made with an airborne correlation spectrometer in plumes ranging in age from 1.0 to 42.5 h, the latter corresponding to a photolytic age of almost two periods of daylight. Ground-based experiments were also performed to determine the rate of dry deposition of sulphur dioxide in the area situated about 500 km from the source. The implications of an error in the deposition velocities, reported by Milne J.W., Roberts D.B. and Williams D.J. (Atmos. Envir. 13, 373–380, 1979.), are considered. The average velocity for the Mount Isa region is higher than that previously employed by Roberts D. B and Williams D.J. (Atmos. Envir. 13, 1485–1489, 1979). Along with a reassessment of the diurnal model and the average mixing height encountered by Roberts and Williams, this necessitates a reduction in the estimate of the photochemical oxidation rate of sulphur dioxide from 0.25 to 0.15%h −1, when averaged over 24 h. It is shown in this study that the rate of loss of sulphur dioxide from the plume out to the furthest distances measured can be accounted for by the combined processes of atmospheric oxidation and dry deposition, as determined by Roberts and Williams for much shorter distances (< 260 km). The lifetime of sulphur dioxide in the region under study is shown to be fourteen days, with about half of the loss being due to atmospheric oxidation.