Technical note: A theoretical study on the mechanism of citric acid-driven multi-component nucleation of sulfuric acid-base-water clusters 

Abstract. New particle formation (NPF) is one of the important sources of aerosol and an important reason for the rapid increase of PM2.5 mass concentration in polluted areas. It has been shown that citric acid, present in atmosphere, is a potential precursor of new particles and may play a role in the nucleation process of new particles. However, the exact mechanism by which citric acid contributes to nucleation remain unclear. The thermodynamically stable geometry of SA·AM·Wn·CAm, SA·DMA·Wn·CAm, and SA·AM·DMA·Wn·CAm (n = 0–4, m=0–1) clusters were optimized and the Gibbs free energy and hydration distribution were calculated at the M06-2X/6-311+G(2d, p) level in this study. The results demonstrate that three carboxyl groups (-COOH) and one hydroxyl group (-OH) of citric acid can act as both hydrogen donors and acceptors through hydrogen bonding interactions with sulfuric acid-base-water clusters. This interaction lowers the nucleation barriers (△G﹤ 0), indicating an energetically favorable reaction. At three relative humidities (RH), anhydrous and monohydrate forms dominate in SA·AM·Wn·CA clusters, as well as SA·AM·DMA·Wn·CA (n = 0–4) clusters; while anhydrous form dominates in SA·DMA·Wn·CA (n = 0–4) clusters. These findings suggest that citric acid reduces the hydrophilicity of these clusters. In conclusion, the involvement of citric acid in atmospheric processes is conductive to cluster formation, thereby facilitating the multicomponent nucleation of sulfuric acid-base-water clusters. Overall, our study highlights how citric acid participates in and enhances new particle formation processes.