Surfactant Effects in Irradiated, Hanging-Droplet, Aqueous-Phase Glyoxal/Ammonium Sulfate Aerosol Mimic Systems

Abstract

Carbonyl-containing volatile organic compounds (CVOCs) are present in a wide range of atmospherically relevant contexts and can contribute significant portions of water-soluble organic carbon in aqueous aerosols, forming a variety of heterocyclic or oligomeric products under dark conditions. However, condensed-phase chemistry under irradiated conditions is much less understood and results in potential shifts to the overall extent of CVOC oligomerization. This study reports in situ surface tension measurements of glyoxal/ammonium sulfate hanging-droplet aqueous aerosol mimics that are directly irradiated with ultraviolet light and subsequently undergo aqueous-phase photochemistry due to this exposure. The surface tension values of aged glyoxal/ammonium sulfate solutions (79.7 ± 0.2 dyn/cm) are slightly lower than those of ammonium sulfate solutions without organics (80.1 ± 0.2 dyn/cm). Direct photochemistry in hanging droplets results in the formation of products capable of slight (−2.9 ± 2.3%) but statistically significant (p = 0.002) surface tension depression. Changes in the relative quantities of heterocyclic and aldol oligomeric products are observed in irradiated, hanging-droplet aerosol mimics, in contrast to comparably irradiated bulk solutions and non-irradiated hanging-droplet samples. Mimic solutions containing additional surfactants [sodium dodecyl sulfate (SDS) and methylsuccinic acid (MSA)] also exhibit compositional changes but do not demonstrate statistically significant differences in surface tension upon being irradiated (p = 0.563 and 0.422 for SDS and MSA, respectively).