Atmos. Chem. Phys. Discuss., 9, 4231-4249, 2009
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Ammonia in positively charged pre-nucleation clusters: a quantum-chemical study and atmospheric implications
A. B. Nadykto, F. Yu, and J. Herb
Atmospheric Sciences Research Center, State University of New York at Albany, 251 Fuller Rd., Albany, NY 12203, USA

Abstract. The quantum-chemical treatment of pre-nucleation clusters consisting of atmospheric nucleation precursors is critically important for the understanding of the molecular nature of atmospheric nucleation. In the present study, the influence of ammonia on the thermochemical stability of positively charged pre-nucleation clusters has been studied using the Density Functional Theory (DFT). The formation of binary (NH4+)(H2O)n and ternary (NH4+)(H2OSO4)(H2O)n ionic clusters and the conversion of (H3O+)(H2O)n−1 into (NH4+)(H2O)n and (H3O+)(H2SO4)(H2O)n−1 into (NH4+)(H2SO4)(H2O)n have been investigated. The thermochemical analysis carried out in the present study shows both (H3O+)(H2O)n−1→ (NH4+)(H2O)n and (H2SO4)(H3O+)(H2O)n−1→ (NH4+)(H2SO4)(H2O)n transformations to be favorable thermodynamically and gives us a clear indication of the important role of ammonia in the conversion of positively charged clusters containing hydronium (H3O+) into those containing protonated ammonia. Under typical continental boundary layer condition, a large fraction of positive sulfuric acid monomer ions may contain ammonia. The ratio of NH4+(H2SO4)(H2O)n+1 to (H3O)+(H2SO4)(H2O)n decreases significantly with the hydration number but is still above ~10 when n=5. In contrast, most of neutral and negative hydrated sulfuric acid monomers do not contain ammonia. The atmospheric implications of the obtained results are discussed.

Citation: Nadykto, A. B., Yu, F., and Herb, J.: Ammonia in positively charged pre-nucleation clusters: a quantum-chemical study and atmospheric implications, Atmos. Chem. Phys. Discuss., 9, 4231-4249, doi:10.5194/acpd-9-4231-2009, 2009.
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