Significance of ammonia in growth of atmospheric nanoclusters.

We apply accurate quantum chemistry methods to study the thermochemistry of molecular clusters containing ammonia, water, and sulfuric acid and investigate initial reaction steps in atmospheric nucleation by calculating free energies for the related reactions. The results indicate that ammonia is a key reactant enhancing the growth of small water-sulfuric acid clusters in atmospheric conditions. The role of ammonia becomes significant when the nanoclusters contain more than one or two sulfuric acid molecules. This implies a lower limit of 1:3 for the NH3/H2SO4 mole ratio of atmospheric sulfuric acid-water-ammonia clusters.