The Conversion of SO₂ to Sulfate in the Atmosphere

Abstract

The atmospheric chemistry responsible for the conversion of SO2(g) to particulate sulfate in areas impacted by anthropogenic emission of SO2 is reviewed. The major reaction mechanism for the homogeneous conversion process in the absence of clouds or fog is the oxidation of SO2(g) by the hydroxyl radical. The rate of this conversion process increases with both increasing temperature and relative humidity. Correlations are described for the effects of these two variables on the conversion process, and equations given which correlate all of the available literature data for the homogeneous conversion process in ambient atmospheres. The conversion of S(IV) to sulfate via aqueous solution chemistry in clouds and fog is more complex and dependent on several variables, including concentrations of the principal oxidants (hydrogen peroxide and ozone), ammonia, droplet size and composition, and meteorology. The gas‐phase homogeneous conversion process can vary from less than 1% SO2(g) converted per hour to a maximum of about 10% converted per hour at high temperature and relative humidity. In contrast, the rate of conversion of S(IV) to sulfate in the aqueous‐phase homogeneous process is controlled by mixing and reactant limitations, rather than kinetic considerations. The process can involve 100% SO2 converted per hour under optimum conditions. Consequences of the various conversion processes on environmental quality are briefly illustrated with a discussion of the impact of sulfate‐containing aerosols on PM10 concentrations and visibility degradation.

SO2 depletion in tropospheric volcanic plumes

“…The effects of these phenomena upon SO 2 depletion in volcanic plumes have yet to be constrained, and may be difficult to quantify using UV spectroscopy for optically thick ash laden plumes or in fog. However, k values in excess of the highest ever reported for a volcanic plume (5 Â 10 À3 s À1 ) have been found for power station plumes, emitted directly into cloud or fog [Eatough et al, 1994].…”

“…This figure also shows a typical trace of relative humidity and temperature versus time for the measurement period. According to Eatough et al [1994], increases in temperature, insolation and relative humidity will all increase rates of conversion of SO 2 to sulphate via homogenous and/or heterogeneous processes. However, in our case it appears as though there is very little change in flux over the measurement period, during which the typical temperature range was 12°C, R.H. ranged from 100% to %40%, and solar elevations were variable.…”

“…The in-plume SO 2 can be oxidised in the gas or aqueous phase and on the surface of solid particles, with reaction rates increasing with temperature, relative humidity (R.H.), insolation, and aerosol density [Eatough et al, 1994]. Whilst these reactions are relatively well understood in the free atmosphere and for anthropogenic plumes, they are poorly characterised for tropospheric volcanic plumes.…”

“…In a detailed study of the effect of meteorological variables upon these processes, Eatough et al [1994] documented that k varies from <1% h À1 to a maximum of around %10% h À1 , with increasing temperature and R.H. (e.g., %3% h À1 at 20°C, and %8% h À1 at 30°C, for 50% R.H.) for gas phase reactions during daylight.…”

“…They also report diurnal variations in SO 2 loss via gas phase oxidation, with maxima at noon (%7% h À1 , vs. 2% h À1 daily mean, for 25°C and 50% R.H.) when OH concentrations are highest. Aqueous phase SO 2 conversion is more complex and loss rates can vary widely (<2% h À1 to >500% h À1 ) depending upon oxidant concentrations, droplet size and composition [Eatough et al, 1994].…”