A Monte Carlo approach to study the effect of ions on the nucleation of sulfuric acid-water clusters.

The nucleation of sulfuric acid-water clusters is a significant contribution to the formation of aerosols as precursors of cloud condensation nuclei (CCN). Depending on the temperature, there is an interplay between the clustering of particles and their evaporation controlling the efficiency of cluster growth. For typical temperatures in the atmosphere, the evaporation of H2 SO4 H2 O clusters is more efficient than the clustering of the first, small clusters, and thus their growth is dampened at its early stages. Since the evaporation rates of small clusters containing an HSO 4 - ion are much smaller than for purely neutral sulfuric acid clusters, they can serve as a central body for the further attachment of H2 SO4 H2 O molecules. We here present an innovative Monte Carlo model to study the growth of aqueous sulfuric acid clusters around central ions. Unlike classical thermodynamic nucleation theory or kinetic models, this model allows to trace individual particles and thus to determine properties for each individual particle. As a benchmarking case, we have performed simulations at T = 300 K a relative humidity of 50% with dipole and ion concentrations of c dipole = 5 × 10 8 - 10 9 cm - 3 and c ion = 0 - 10 7 cm - 3 . We discuss the runtime of our simulations and present the velocity distribution of ionic clusters, the size distribution of the clusters as well as the formation rate of clusters with radii R ≥ 0.85 nm . Simulations give reasonable velocity and size distributions and there is a good agreement of the formation rates with previous results, including the relevance of ions for the initial growth of sulfuric acid-water clusters. Conclusively, we present a computational method which allows studying detailed particle properties during the growth of aerosols as a precursor of CCN.

Evidence for the role of ions in aerosol nucleation.

Aerosol nucleation has been studied experimentally in purified, atmospheric air, containing trace amounts of water vapor, ozone, and sulfur dioxide. The results are compared with model calculations. It is found that an increase in ionization by a factor of 10 increases the production rate of stable clusters by a factor of approximately 3, probably due to ion-induced nucleation.