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

Abstract

We have studied the structure and formation thermodynamics of dimer clusters containing H<sub>2</sub>SO<sub>4</sub> or HSO<sub>4</sub><sup>−</sup> 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-H<sub>2</sub>SO<sub>4</sub> complexes is significantly stronger than that of NH<sub>3</sub>•H<sub>2</sub>SO<sub>4</sub>, while most amine-HSO<sub>4</sub><sup>−</sup> complexes are only somewhat more strongly bound than NH<sub>3</sub>•HSO<sub>4</sub><sup>−</sup>. Further calculations on larger cluster structures containing dimethylamine or ammonia together with two H<sub>2</sub>SO<sub>4</sub> molecules or one H<sub>2</sub>SO<sub>4</sub> molecule and one HSO<sub>4</sub><sup>−</sup> ion demonstrate that amines, unlike ammonia, significantly assist the growth of not only neutral but also ionic clusters along the H<sub>2</sub>SO<sub>4</sub> 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