Amines are likely to enhance neutral and ion-induced sulfuric acid-water nucleation in the atmosphere more effectively than ammonia
- Division of Atmospheric Sciences and Geophysics, Dept. of Physics, P.O.Box 64, 00014 University of Helsinki, Finland
Abstract. 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•H2SO4, while most amine-HSO4− complexes are only somewhat more strongly bound than NH3•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 ion-induced sulfuric acid-water nucleation in the atmosphere.