Reduction of visibility by sulphates in photochemical smog

Abstract

THE relationship between pollutant emissions and the optical haze characteristic of photochemical smog has proved difficult to unravel^1,2. It is clear that material produced by reaction in the atmosphere is responsible for much of the deterioration in the optical environment, since the ambient aerosol scatters much more light at a given mass concentration than do the primary aerosols emitted by known sources^3,4. Unfortunately, measurement of the scattering contributed by an individual product species is complicated by the fact that most of this secondary material is deposited on existing particles and cannot be isolated for direct observation of its in situ optical properties. In this note I present statistical evidence from measurements in Los Angeles that secondary sulphate compounds scatter visible light more efficiently than do other chemical fractions of the ambient aerosol. My results indicate that sulphur emissions can strongly influence atmospheric visibility even in photochemical smog of the Los Angeles variety, where secondary organics and nitrates contribute most of the pollutant aerosol mass.