Roles of the Phase State and Water Content in Ozonolysis of Internal Mixtures of Maleic Acid and Ammonium Sulfate Particles

The ozonolysis of maleic acid/ammonium sulfate (MA/AS, 30/70 wt%) internal mixtures was investigated at various relative humidities (RHs) (80%, 55%, 30%, and <5%) and physical states (aqueous and crystalline solid). Complementary laboratory techniques-an electrodynamic balance coupled with in situ Raman spectroscopy and a reaction flow cell coupled with offline ion chromatography (IC)-were employed to probe the mass changes, spectroscopic changes, product identities, and post-reaction hygroscopic changes in ozone-processedMA/AS mixed particles. The rate of ozonolysis was highly dependent on the physical state and the water content of the particles. Crystalline solid particles underwent negligible changes even after exposure to 10-ppm ozone for six days. Aqueous particles reacted much faster and the rate was related to the RH during reaction. It took 23 h to reduce MA to 50% of its initial mass at 80% RH, whereas it took 87 h to do the same at 55% RH or lower. We believe that the salting-out of ozone in concentrated droplets at 55% RH may have contributed to the reduced ozonolysis rate. The molar conversion of MA to glyoxylic acid was about unity in both the 55% RH and the 80% RH experiments. Exposed particles may have formed amorphous solids when dried and their hygroscopicity was different from that of the corresponding parent particles. Early deliquescence of reacted particles was observed, suggesting the strong influence of aging on aerosol hygroscopicity.