Particle Formation from NH₃-SO₂-H₂O-Air Gas Phase Reactions

Abstract

RECENT articles on the formation of atmospheric aerosols from NH₃–SO₂–H₂O gas phase reactions^1,2 have considered only those products formed between the anhydrous gases to give 2:1 and 1:1 (NH₃: SO₂) compounds. I wish to draw attention to my work³ on the reaction between these gases at partial vapour pressures in air of 50 to 500 N m⁻². Solid products were formed only in the presence of water vapour at 20° C. Analyses showed that the compounds formed were either ammonium sulphite (with excess NH₃) or ammonium pyrosulphite (with excess SO₂). Vapour pressure measurements at 0 to 23° C indicated that the extrapolated vapour pressures of NH₃ and SO₂ above the sulphite in dry air at −70° C would be about 1 × 10⁻⁴ and 5 × 10⁻⁵ N m⁻² respectively. With 50% saturation of the air with water vapour at −70° C, however, these vapour pressures would be depressed to 8 × 10⁻⁶ and 4 × 10⁻⁶ N m⁻² respectively, that is, to concentrations approximately equal to those reported for the 2:1 compound. The partial vapour pressures of the pyrosulphite under similar conditions would be 3.3 × 10⁻⁵ N m⁻² for both gases. At −52° C the vapour pressures of these gases in dry air would be identical for both the 2:1 compound and ammonium sulphite.