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

Atmospheric new particle formation is generally thought to occur due to homogeneous or ion-induced nucle- ation of sulphuric acid. We compare ambient nucleation rates with laboratory data from nucleation experiments involving either sulphuric acid or oxidized SO2. Atmospheric nucle- ation occurs at H2SO4 concentrations 2-4 orders of magni- tude 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 oxida- tion 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 H 2SO4. These nuclei are acti- vated to further growth by H2SO4 and possibly other trace species. However, at lower temperatures the atmospheric rel- ative acidity is high enough for the H2SO4-H2O nucleation to dominate.