SO₂ oxidation products other than H₂SO₄ as a trigger of new particle formation. Part 2: Comparison of ambient and laboratory measurements, and atmospheric implications

Abstract. Atmospheric new particle formation is generally thought to occur due to homogeneous or ion-induced nucleation of sulphuric acid. We compare ambient nucleation rates with laboratory data from nucleation experiments involving either sulphuric acid or oxidized SO₂. Atmospheric nucleation occurs at H₂SO₄ concentrations 2–4 orders of magnitude lower than binary or ternary nucleation rates of H₂SO₄ produced from a liquid reservoir, and atmospheric H₂SO₄ concentrations are very well replicated in the SO₂ oxidation experiments. We hypothesize these features to be due to the formation of free HSO₅ radicals in pace with H₂SO₄ during the SO₂ oxidation. We suggest that at temperatures above ~250 K these radicals produce nuclei of new aerosols much more efficiently than H₂SO₄. These nuclei are activated to further growth by H₂SO₄ and possibly other trace species. However, at lower temperatures the atmospheric relative acidity is high enough for the H₂SO₄–H₂O nucleation to dominate.