under a Creative Commons License. Atmospheric Chemistry and Physics Estimating the NH3:H2SO4 ratio of nucleating clusters in

Abstract. We study the ammonia addition reactions of H2SO4·NH3 molecular clusters containing up to four am-monia and two sulfuric acid molecules using the ab initio method RI-MP2 (Resolution of Identity 2nd order Møller-Plesset perturbation theory). Together with results from pre-vious studies, we use the computed values to estimate an up-per limit for the ammonia content of small atmospheric clus-ters, without having to explicitly include water molecules in the quantum chemical simulations. Our results indicate that the NH3:H2SO4 mole ratio of small molecular clusters in typical atmospheric conditions is probably around 1:2. High ammonia concentrations or low temperatures may lead to the formation of ammonium bisulfate (1:1) clusters, but our re-sults rule out the formation of ammonium sulfate clusters (2:1) anywhere in the atmosphere. A sensitivity analysis in-dicates that the qualitative conclusions of this study are not affected even by relatively large errors in the calculation of electronic energies or vibrational frequencies.