Abstract
Abstract. The substitution of ammonia by dimethylamine in sulfuric acid – ammonia – dimethylamine clusters was studied using a collision and evaporation dynamics model. Quantum chemical formation free energies were computed using B3LYP/CBSB7 for geometries and frequencies and RI-CC2/aug-cc-pV(T+d)Z for electronic energies. We first demonstrate the good performance of our method by a comparison with an experimental study investigating base substitution in positively charged clusters, and then continue by simulating base exchange in neutral clusters, which cannot be measured directly. Collisions of a dimethylamine molecule with an ammonia containing positively charged cluster result in the instantaneous evaporation of an ammonia molecule, while the dimethylamine molecule remains in the cluster. According to our simulations, a similar base exchange can take place in neutral clusters, although the overall process is more complicated. Neutral sulfuric acid – ammonia clusters are significantly less stable than their positively charged counterparts, resulting in a competition between cluster evaporation and base exchange.
Highlights
Atmospheric aerosols are known to affect the climate by absorbing and scattering radiation and acting as cloud condensation nuclei
The cluster names are written in terms of sulfuric acid (SA), ammonia (Am) and DMA molecules, and the extra proton attached to one base molecule and giving the overall positive charge is not included explicitly
The formation free energies are calculated relative to neutral molecules, except for one positively charged (CH3)2NH+2 ion in case of positive clusters, or one positively charged NH+4 ion for charged sulfuric acid – ammonia clusters with no DMA
Summary
Atmospheric aerosols are known to affect the climate by absorbing and scattering radiation and acting as cloud condensation nuclei. Recent results at the CLOUD experiment in CERN suggest that ammonia does not enhance the growth of sulfuric acid clusters enough to account for the nucleation rates observed in the boundary layer (Kirkby et al, 2011). It has been seen in earlier modeling studies that amines stabilize sulfuric acid clusters significantly more than ammonia (Kurten et al, 2008; Loukonen et al, 2010), different quantum-chemistry methods give somewhat different results concerning the magnitude of the stability difference (Nadykto et al, 2011; Kurten, 2011). We extend our discussion to base substitution in electrically neutral clusters and point out some important differences
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