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It has recently been demonstrated that the O2SO3-1ion forms in the atmosphere as a natural consequence of ionizing radiation. Here, we present a density functional theory-based study of the reactions of O2SO3-1 with O3. The most important reactions are (a) oxidation of O2SO3-1 to O2SO3-1 and (b) cluster decomposition into SO3, O2 and O2. The former reaction is highly exothermic and the nascent O2SO3-1 will rapidly decompose into SO2 and O2. If the origin of O2SO3-1 is SO2 oxidation by O2, the latter reaction closes a catalytic cycle wherein SO2 is oxidized to SO3. The relative rates between the two major sinks for O2SO3-1 is assessed, thereby providing a measure of the maximum turnover number of ion catalysed SO2 oxidation, i.e. how many SO2 can be oxidized per free electron. The rate ratio between reactions (a) and (b) is significantly altered by the presence or absence of a single water molecule, but reaction (b) is in general much more probable. Although we are unable to assess the overall importance of this cycle in the real atmosphere due to the unknown influence of CO2 and NOx, we roughly estimate that ion induced catalysis may contribute with several percent of H2SO4 levels in typical CO2 free and low NOx reaction chambers, e.g. the CLOUD chamber at CERN. [ABSTRACT FROM AUTHOR]

Binary heterogeneous nucleation of watersuccinic/ glutaric/malonic/adipic acid on nanometer-sized particles is investigated within the frame of classical heterogeneous nucleation theory. Homogeneous nucleation is also included for comparison. It is found that the nucleation probabilities depend on the contact angle and on the size of the seed particles. New thermodynamical properties, such as saturation vapor pressure, density and surface tension for all the dicarboxylic acid aqueous solutions are included in the calculations. While the new surface tension and density formulations do not bring any significant difference in the computed nucleation rate for homogeneous nucleation for succinic and glutaric acids, the use of the newly derived equations for the vapor pressure decrease the acid concentrations in gas phase by 3 orders of magnitude. According to our calculations, the binary heterogeneous nucleation of succinic acid-water and glutaric acid-water - although it requires a 3-4 orders of magnitude lower vapor concentrations than the homogeneous nucleation - cannot take place under atmospheric conditions. On the other hand binary homogeneous nucleation of adipic acid-water systems might be possible under conditions occuring in upper boundary layer. However, a more detailed characterization of the interaction between the surface and the molecules of the nucleating vapor should be considered in the future. [ABSTRACT FROM AUTHOR]

Highly oxidised organic vapors can effectively stabilize sulphuric acid in heteronuclear clusters and drive new-particle formation. We present quantum chemical calculations of cluster stability, showing that multifunctional species can stabilize sulphuric acid and also present additional polar functional groups for subsequent cluster growth. We also model the multi-generation oxidation of vapors associated with secondary organic aerosol formation using a two-dimensional volatility basis set. The steady-state saturation ratios and absolute concentrations of extremely low volatility products are sufficient to drive new-particle formation with sulphuric acid at atmospherically relevant rates.

Evaporation of small Lennard-Jones argon clusters has been studied using molecular dynamic simulations. An extensive library of clusters with 4, 5, 6, 11, and 21 atoms has been obtained from an earlier study. Analysis of the evaporation properties of the clusters indicate, that the fraction of dimer evaporations of all evaporation events increases with the total energy of the cluster. The fraction of evaporated dimers from clusters with a constant lifetime is independent of the cluster size for short-lived clusters and increases with cluster size for long-lived clusters. Only a few percent of the clusters which are long lived enough to participate in vapor-liquid nucleation decay by emitting dimers. The mean cluster lifetime as a function of total energy shows the same exponentially decreasing trend for monomer and dimer evaporation channels. The fraction of trimer evaporations is found to be vanishingly small.

We have calculated the critical cluster sizes and homogeneous nucleation rates of water at temperatures and vapor densities corresponding to experiments by Wolk and Strey [J. Phys. Chem B 105, 11683 (2001)]. The calculations have been done with an expanded version of a Monte Carlo method originally developed by Vehkamaki and Ford [J. Chem. Phys. 112, 4193 (2000)]. Their method calculates the statistical growth and decay probabilities of molecular clusters. We have derived a connection between these probabilities and kinetic condensation and evaporation rates, and introduce a new way for the calculation of the work of formation of clusters. Three different interaction potential models of water have been used in the simulations. These include the unpolarizable SPC/E [J. Phys. Chem. 91, 6269 (1987)] and TIP4P [J. Chem. Phys. 79, 926 (1983)] models and a polarizable model by Guillot and Guissani [J. Chem. Phys. 114, 6720 (2001)]. We show that TIP4P produces critical cluster sizes and a temperature and vapor density dependence for the nucleation rate that agree well with the experimental data, although the magnitude of nucleation rate is constantly overestimated by a factor of 2 x 10(4). Guissani and Guillot's model is somewhat less successful, but both the TIP4P and Guillot and Guissani models are able to reproduce a much better experimental temperature dependency of the nucleation rate than the classical nucleation theory. Using SPC/E results in dramatically too small critical clusters and high nucleation rates. The water models give different average binding energies for clusters. We show that stronger binding between cluster molecules suppresses the decay probability of a cluster, while the growth probability is not affected. This explains the differences in results from different water models.