Particle formation from homogeneous reactions of sulphur dioxide and nitrogen dioxide

Abstract

Some recent laboratory results relating t o the conversion of gaseous sulphur dioxide and nitrogen dioxide into particulate material and the implication of these results on the atmospheric chemistry of these gases is discussed. Homogeneous oxidation of SO, by photochemically initiated reactions may occur at a rate ranging from 0.1 % hr-' in clean air t o a 0-10 % hr-1 in urban air. The oxidation rate of NO, is more raped (up t o 25% hr-l) but may be effectively less due t o the reversible nature of the reactions. Theoretical and experimental results suggest that the rate of formation of H,SO, in urban air (from SO,) is sufficient for heteromolecular nucleation t o form aqueous sulphuric acid aerosols but in background areas condensation on the existing aerosol is the main fate of the SO, oxidation products. Sufficient gaseous HNO, for nucleation is unlikely to be produced from NO, in the atmosphere and condensation on aerosol particles, particularly hygroscopic particles, is an important removal mechanism for HNO,. LL .arge effect of relative humidity on the rate of the nucleation and condensation processes is demonstrated experimentally.

Photochemical particulate formation in sulfur dioxide-air mixtures

“…(1971). and Cox (1974) cannot be compared with the present data. Cox and Penkett did not report the humjdity in their results; Quon et ui.…”

“…It would be of interest now to compare the present data with results of other investigations. Results of direct measurements of the number of nuclei formed in sulfur dioxide-air mixtures were reported by Cox andPenkett (1970) Cox (1974). Kocmond et al (1973), Quon et (11.…”

“…Renzetti and Doyle observed that the amount of light scattered increased with relative humidity. Cox (1974) also reported that the number of nuclei increased with humidity. Katz and Gale (1971) observed an increase in the conversion rate of sulfur dioxide to sulfur trioxide (or sulfuric acid) as the humidity was increased.…”

Chemical composition of the ambient aerosol in the Sudan Gezira

“…Nitrate formation will also occur in the atmosphere by a number of mechanisms. It has been shown for instance that nitrogen dioxide (NO,) will react with ozone in the presence of a hygroscopic aerosol, such as ammonium sulphate, to produce nitrate (Cox, 1974); this mechanism will be intensified greatly in a cloud system where the water droplets have a much larger surface area and this will lead to nitric acid production on cloud droplets. Another mechanism which will lead to nitric acid production directly in the gas phase is the reaction of hydroxyl radicals with NO,.…”