The enhanced role of formic acid on sulfuric acid-ammonia-driven nucleation in forest regions and polluted city areas

Formic acid (FA) is particularly prominent for its ubiquity and structural simplicity among atmospheric organic acids, and exerts a significant influence on atmospheric acidity. However, the potential contribution of FA to the primary stage of new particle formation (NPF) remains unclear. Herein, molecular dynamics (MD), density functional theory (DFT) and the atmospheric cluster dynamics code (ACDC) model have been utilized to evaluate the mechanism of FA participation in atmospheric SA (sulfuric acid)-A (ammonia) clusters. The MD simulations qualitatively suggest that FA can aggregate with SA and A to form larger clusters, and the aggregation time of the largest clusters decreases as the temperature decreases. The DFT and ACDC findings indicate that the ternary SA-A-FA system is thermodynamically more stable at low temperatures (238.15 K). Simultaneously, in regions with low temperatures, high [FA] (10¹¹ molecules/cm³), low [SA] (10⁶ molecules/cm³) and high [A] (10¹¹ molecules/cm³), FA significantly enhances SA-A cluster formation rates. The low-temperature NPF mechanism implies that FA could facilitate the growth of pure SA-A clusters via a “catalytic” mechanism and play an integral role in the genesis of critical clusters as a “participant”. This dual role differs from the “catalytic” role exhibited by malonic and glycolic acids in our previous studies. This discovery could help identify the sources of unexplained NPFs in regions with high FA concentrations, such as densely forested areas with abundant vegetation, regions affected by biomass burning, or periods with elevated vehicle exhaust emissions and the release of volatile organic compounds like isoprene and terpenoids.