Atmospheric Sulfuric Acid Dimer Formation in a Polluted Environment

New particle formation (NPF) contributes significantly to atmospheric particle number concentrations and cloud condensation nuclei (CCN). In sulfur-rich environments, field measurements have shown that sulfuric acid dimer formation is likely the critical step in NPF. We investigated the dimer formation process based upon the measured sulfuric acid monomer and dimer concentrations, along with previously reported amine concentrations in a sulfur-rich atmosphere (Atlanta, USA). The average sulfuric acid concentration was in the range of 1.7 × 10(7)–1.4 × 10(8) cm(−3) and the corresponding neutral dimer concentrations were 4.1 × 10(5)–5.0 × 10(6) cm(−3) and 2.6 × 10(5)–2.7 × 10(6) cm(−3) after sub-collision and collision ion-induced clustering (IIC) corrections, respectively. Two previously proposed acid–base mechanisms (namely AA and AB) were employed to respectively estimate the evaporation rates of the dimers and the acid–amine complexes. The results show evaporation rates of 0.1–1.3 s(−1) for the dimers based on the simultaneously measured average concentrations of the total amines, much higher than those (1.2–13.1 s(−1)) for the acid–amine complexes. This indicates that the mechanism for dimer formation is likely AA through the formation of more volatile dimers in the initial step of the cluster formation.