Size-Dependent Sigmoidal Reaction Kinetics for Pyruvic Acid Condensation in Single Aqueous Microdroplets

Although aqueous microdroplets have been shown to exhibit enhanced chemical reactivity compared to bulk solutions, mechanisms for these enhancements are not completely understood. Pyruvic acid (PA) is an abundant α-keto acid in aerosols, fogs, and clouds in the atmosphere, and its conjugate base, pyruvate, is an important intermediate in several metabolic pathways. Utilizing in situ micro-Raman spectroscopy as a probe, we investigated the chemistry of PA within aqueous microdroplets in a relative humidity (RH)- and temperature-controlled environmental cell. We found that PA undergoes a condensation reaction to yield mostly zymonic acid (ZA). Interestingly, the reaction follows a size-, RH- and temperature-dependent sigmoidal kinetic profile. We developed a diffusion–reaction–partitioning model to simulate the complex kinetics observed in the microdroplets. Combined experimental measurements and kinetic modeling showed that the condensation reaction of PA in microdroplets is driven by coupled surface reactions and gas-phase partitioning. Importantly, the kinetic model best fits the data when an autocatalytic step is included in the mechanism, i.e. a reaction step where the product, ZA, catalyzes the interfacial condensation reaction. Overall, the dynamic nature of aqueous microdroplet chemistry and the coupling of interfacial chemistry with gas-phase partitioning are demonstrated. Furthermore, autocatalysis of small organic molecules at the air–water interface for aqueous microdroplets, shown here for the first time, has implications for several fields including prebiotic chemistry, atmospheric chemistry and chemical synthesis.