Measurement of the thermodynamics of the hydrated dimer and trimer of sulfuric acid.

We report measurements of the equilibrium constants, K(p), for the water mediated clustering of two and three sulfuric acid molecules for atmospheric temperatures and relative humidities. Limits for K(p) for the formation of the sulfuric acid tetramer and higher clusters as well as the kinetics of small sulfuric acid cluster growth are also presented. These results put strong constraints on the rate of nucleation in the atmosphere via the H(2)SO(4)/H(2)O neutral mechanism. We show that the neutral nucleation of H(2)SO(4) and H(2)O is slower than ion-induced nucleation of H(2)SO(4) and H(2)O for most conditions found in the middle and upper troposphere. These laboratory-based upper limits to the nucleation rates are also much lower than the predictions of the liquid drop model/classical nucleation theory.