Spontaneous Reaction between CO ₂ and Organic Acids in Water Microdroplets: Implications for the Formation of Secondary Organic Aerosols

Secondary organic aerosols (SOAs) are key components of aerosols, but their formation pathways remain incompletely elucidated. This study reveals a new pathway for SOA formation in water microdroplets by employing both experimental and theoretical methods. It is found that inert CO₂ can react rapidly with atmospheric organic acids in water microdroplets, producing low-volatility compounds that contribute to SOA formation. Radical quenching experiments and direct observation of carbocations indicate that the carbocations generated from organic acids are the key active intermediates. Such carbocations are proposed to subsequently react with the counterion HCO₃^-, derived from CO₂, to yield the observed products. Density functional theory calculations confirm that the carbocation mechanism is most favorable, with a reaction energy barrier of 5.24 kcal/mol. Further studies find that various organic acids can undergo similar reactions, and the reaction efficiency is positively correlated with the number and radius of halogen atoms but negatively correlated with carbon chain length. Overall, this study unveils a novel pathway for SOA formation involving ubiquitous CO₂ and organic acids, offering mechanistic insights into the chemistry of atmospheric CO₂ and SOAs.