| Current models of homogeneous gas phase nucleation lack thermodynamic data essential to understanding particle growth mechanisms and accurately predicting nucleation rates in the atmosphere. Nucleation model predictions are highly sensitive to the thermodynamics of the initial stages of particle growth, where free energies calculated using classical nucleation theory are expected to be the least accurate. There is strong evidence that H2SO 4, H2O, and NH3 are involved in atmospheric nucleation. These species are also principal components of atmospheric ions, suggesting that they may play important roles in ion-induced nucleation mechanisms. The purpose of this work is to measure thermodynamic parameters for cluster ions composed of NH3, H2SO4, and H2O. Equilibrium constants for the reactions of H2O and NH3 with the cluster ions, H+(H2SO4) s(H2O)w, HSO4−(H 2SO4)s(H2O) w, and NH4+(NH3) n(H2SO4)s(H 2O)w, were measured using an ion flow reactor over a range of temperatures, and thermodynamic quantities (Δ H° and ΔS°) were derived from van't Hoff analyses. Ab initio structures were calculated to elucidate trends in ligand bonding. Current experimental results, combined with our previous measurements of H2SO4 bond enthalpies within cluster ions, are used to determine the nucleating ability of these systems for a range of atmospheric conditions. In general, nucleation of the negative cluster ions is predicted to be preferred over nucleation of positive ions in the atmosphere. For temperatures ≤250 K, relative humidities ≥50%, and H2SO4 concentrations ≥107 molecule cm−3 there is no thermodynamic barrier to nucleation of the HSO4−(H2SO4) s(H2O)w system. Ammonia typically stabilizes H2SO4 within positive cluster ions due to formation of fully ionic crystalline-like clusters of the type NH 4+(NH4+HSO4 −)c, c = 1–3. However, nucleation barriers for the NH4+(NH 3)n(H2SO4) s(H2O)w system are strongly dependent on ambient NH3 concentrations, and positive ion nucleation in the atmosphere may actually be inhibited by the presence of low levels of NH3. |
