Reactivity of methanesulfonic acid salt clusters relevant to marine air

Aliphatic amines and methanesulfonic acid (MSA) are important emissions in the marine environment. Although some studies of marine aerosols have shown that particles containing MSA can be internally mixed with ammonium, others have indicated a significant amine component. The present work employs Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) to determine the kinetics and thermodynamics of exchange of dimethylamine (DMA) for ammonia in ammonium methanesulfonate clusters. DMA displaces ammonia with near unit reaction efficiency. The Gibbs free energy for exchange (ΔG°) was <-23.1 kJ mol^-1, strongly suggesting that dimethylaminium methanesulfonate salts are preferred over ammonium methanesulfonate salts. These results are consistent with previous studies of amine exchange in ammonium bisulfate and ammonium nitrate clusters. Additionally, cluster growth, characterized by the addition of a base to neutralize remaining acid in the cluster, was examined. Methanesulfonate clusters grew more effectively by addition of a DMA molecule than by addition of an ammonia molecule, indicating that in some environments, particle growth by amines may compete favorably with growth by ammonia at small particle size. These results also suggest that in diverse environments the presence of aminium salts is preferred over the presence of ammonium salts in sub-3 nm diameter clusters.