The effect of water molecules and air humidity on the cluster formation from benzoic acid with sulfuric acid/ammonia/dimethylamine: A molecular-scale investigation

In the atmosphere, benzoic acid (BA) is one of the most abundant aromatic acids and contributes to new particle formation (NPF). Sulfuric acid (SA), ammonia (NH3), and dimethylamine (DMA) have been identified as critical nucleation precursors. Since water (H2O) is abundant in the atmosphere, the hydration effect is a significant concern. However, the hydration effect on the precise mechanisms of cluster formation remains unclear. In this study, theoretical calculation method was used to investigate the interactions between BA and NH3/DMA/SA in the presence of up to six water molecules at the PW91PW91/6-311++G(3df,3pd) using geometrical analysis, topological analysis, and the Gibbs free energies analysis. Hydrated distributions of clusters under different relative humidity (RH) levels and their atmospheric implications were also estimated. This study shows water molecules have mild impacts on the interaction between BA and NH3/DMA, but have relatively strong effect on the interaction between BA and SA. Hydrated distributions for the three types of clusters show that they are insensitive to different RH levels, and a completely unhydrated (BA)(NH3) cluster dominates the hydrated distributions. When the concentrations of BA are particularly high, especially in heavily polluted areas, the concentrations of (BA)(SA) can reach the same order of magnitude or higher than those of (SA)2. Thus, the importance of BA in prompting NPF has been revealed to a certain extent.