Volatility Change During Droplet Evaporation of Pyruvic Acid

Atmospheric water-soluble organic gases such as pyruvic acid are produced in large quantities by photochemical oxidation of biogenic and anthropogenic emissions and undergo water-mediated reactions in aerosols and hydrometeors. These reactions can contribute to aerosol mass by forming less-volatile compounds. Although progress is being made in understanding the relevant aqueous chemistry, little is known about the chemistry that takes place during droplet evaporation. Here, we examine the evaporation of aqueous pyruvic acid droplets using both the vibrating orifice aerosol generator (VOAG) and an electrodynamic balance (EDB). In some cases, pyruvic acid was first oxidized by OH radicals. The evaporation behavior of oxidized mixtures was consistent with expectations based on known volatilities of reaction products. However, independent VOAG and EDB evaporation experiments conducted without oxidation also resulted in stable residual particles; the estimated volume yield was 10–30% of the initial pyruvic acid. Yields varied with temperature and pyruvic acid concentration across cloud-, fog-, and aerosol-relevant concentrations. The formation of low-volatility products, likely cyclic dimers, suggests that pyruvic acid accretion reactions occurring during droplet evaporation could contribute to the gas-to-particle conversion of carbonyls in the atmosphere.