Amines are likely to enhance neutral and ion-induced sulfuric acid-water nucleation in the atmosphere more effectively than ammonia

We have studied the structure and formation thermodynamics of dimer clusters containing H2SO4 or HSO4- together with ammonia and seven different amines possibly present in the atmosphere, using the high-level ab initio methods RI-MP2 and RI-CC2. As expected from e. g. proton affinity data, the binding of all studied amine-H2SO4 complexes is significantly stronger than that of NH3 center dot H2SO4, while most amine-HSO4- complexes are only somewhat more strongly bound than NH3 center dot HSO4-. Further calculations on larger cluster structures containing dimethylamine or ammonia together with two H2SO4 molecules or one H2SO4 molecule and one HSO4- ion demonstrate that amines, unlike ammonia, significantly assist the growth of not only neutral but also ionic clusters along the H2SO4 co-ordinate. A sensitivity analysis indicates that the difference in complexation free energies for amine- and ammonia-containing clusters is large enough to overcome the mass-balance effect caused by the fact that the concentration of amines in the atmosphere is probably 2 or 3 orders of magnitude lower than that of ammonia. This implies that amines might be more important than ammonia in enhancing neutral and especially ioninduced sulfuric acid-water nucleation in the atmosphere.