Quantum chemical investigation of nucleation involving SO₂ AND OH

Despite decades of intensive studies, the key mechanisms controlling atmosphericnucleation remain elusive. Recent state-of-art laboratory experiments, in whichnucleation was initiated via the oxidation of SO₂ and OH, may shed light on thisimportant problem. The experimental threshold of the H₂SO₄ concentration needed toachieve a substantial nucleation was estimated to be about the typical H2SO4concentration in the atmosphere. This means that nucleation mechanisms underlying thelaboratory nucleation events may resemble the new particle formation in the Earth'satmosphere. However, none of the existing theories can explain the nucleation eventsobserved in the laboratory conditions. In the present study, possible mechanismsunderlying nucleation observed in the recent laboratory studies have been explored usingthe quantum methods , and the relative stability of clusters corresponding to differentnucleation pathways has been studied. In the case of ionic nucleation, SO₃^-(SO₃)_m(H₂O)_nclusters of the very high stability have been identified. This suggests that the formation ofSO₃^-(SO₃)_m(H₂O)n may be the dominant pathway for the first few nucleation steps . In thecase of the neutral/homogenous heteromolecular nucleation, complexation of the peroxyradical HO₂ and H₂O₂, whose concentrations in the laboratory experiments are quite high,with (H₂SO₄)_m(H₂O)n molecules has been investigated. We found that neither HO₂ norH₂O₂ are likely to be efficient facilitators/catalysts of the binary H₂SO₄-H₂O nucleation.This may be an indication of the involvement of other species, yet to be identified, in theobserved nucleation events.