Particle formation from homogeneous reactions of sulphur dioxide and nitrogen dioxide

Some recent laboratory results relating to 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 SO₂ by photochemically initiated reactions may occur at a rate ranging from 0.1% hr^–1 in clean air to a 0–10% hr^–1 in urban air. The oxidation rate of NO₂ is more raped (up to 25% hr^–1) but may be effectively less due to the reversible nature of the reactions. Theoretical and experimental results suggest that the rate of formation of H₂SO₄ in urban air (from SO₂) is sufficient for heteromolecular nucleation to form aqueous sulphuric acid aerosols but in background areas condensation on the existing aerosol is the main fate of the SO₂ oxidation products. Sufficient gaseous HNO₃ for nucleation is unlikely to be produced from NO₂ in the atmosphere and condensation on aerosol particles, particularly hygroscopic particles, is an important removal mechanism for HNO₃. A large effect of relative humidity on the rate of the nucleation and condensation processes is demonstrated experimentally.