SO₂ oxidation products other than H₂SO₄ as a trigger of new particle formation. Part 1: Laboratory investigations

Abstract. Mechanistic investigations of atmospheric H₂SO₄ particle formation have been performed in a laboratory study taking either H₂SO₄ from a liquid reservoir or using the gas-phase reaction of OH radicals with SO₂. Applying both approaches for H₂SO₄ generation simultaneously it was found that H₂SO₄ evaporated from the liquid reservoir acts considerably less effective for the process of particle formation and growth than the products originating from the reaction of OH radicals with SO₂. Furthermore, for NO_x concentrations >5×10¹¹ molecule cm⁻³ the formation of new particles from the reaction of OH radicals with SO₂ is inhibited. This suggests that substances other than H₂SO₄ (potentially products from sulphur-containing peroxy radicals) trigger lower tropospheric new particle formation and growth. The currently accepted mechanism for SO₂ gas-phase oxidation does not consider the formation of such substances. The analysis of new particle formation for different reaction conditions in our experiment suggests that a contribution of impurities to the nucleation process is unlikely.