Evidence of photochemical haze in the atmosphere of Greater London

Abstract

PHOTOCHEMICAL pollution in Greater London, as evidenced by unnaturally high ozone concentrations, has been reported on several occasions^1–5. In Los Angeles, where photochemical activity has been experienced for many years, an obvious feature of this pollution is aerosol formation, which has been attributed to the participation of SO₂ in the photochemical reaction sequence, with subsequent formation of sub-μm sulphate particles⁶. Although SO₂ concentrations are higher in Greater London, there have been no reported measurements of photochemical haze, but Cox and Penkett⁷, by extrapolation of laboratory measurements of air samples from Harwell, have suggested that a conversion rate of 10% h⁻¹ for SO₂ to sulphate aerosol may be possible in Greater London; this would be sufficient to produce large quantities of sulphate aerosol. Identification of such processes is important because evidence suggests that photochemically produced sulphate aerosols can, in certain circumstances, account for a large proportion of total particlate mass in the 0.1–1.0 µm size range⁸. This size range is important in determining the optical scattering coefficient of polluted air⁹, and hence visibility, and potentially as a respiratory tract irritant because of its ability to penetrate deeply, in combination with its chemical nature¹⁰. We report here preliminary observations based on nephelometric measurements of the optical scattering coefficient of air over central London during June to August 1976, and evidence of haze formation in the London area as educed from the Daily Weather Reports (DWR) of the Meteorological Office (MO), and show how these relate to the occurrence of photochemical processes over Greater London.