Promotion of CO₂ Reactivity by Organic Acid on Aerosol Surfaces

Recently, the atmospheric aerosol surface, which is reported to be quite acidic, is recognized as an important microreactive medium for atmospheric chemistry, profoundly impacting air quality and global climate. Nevertheless, the molecular-level understanding of the effect of surface-bound acids on atmospheric chemical reactions remains limited. Herein, the reactions between CO₂ and NH₃/amines at the air-water interface with organic acids are investigated using combined molecular dynamic simulations and quantum chemical calculations. The results show that the reactions of CO₂-NH₃/amines predominantly occur at the interface of water droplets since CO₂/NH₃/amines show a surface tendency. At the surface with formic acid (HCOOH), the barrier of C-N compound formation from the CO₂-NH₃ reaction catalyzed by HCOOH is calculated to be 6.8 kcal/mol, which can be easily overcome at ambient temperature and is significantly decreased in comparison to both gas phase and surface without organic acid. Furthermore, the HCOOH-mediated mechanism can also promote CO₂ reactions with alkylamines (methylamine (MA) and dimethylamine (DMA)), as the nucleophilicity of the N-site in these amines is significantly stronger than that in NH₃. Overall, the results highlight the significance of organic acid on the aerosol surface in efficient capture of gaseous CO₂ and uncover the catalytic role of interfacial organic acid for assessing the potential contribution of gas-particle partitioning of pollutants to aerosol formation.