Uptake of Ethylene Glycol by Sodium Nitrate Aerosols and Its Degradation in Mixed Particles Mediated via Nitrate Photolysis

Abstract Organic components constitute a substantial fraction of atmospheric particulate matter mostly incorporated via gas–particle partitioning and thereafter undergo extensive chemical processing, such as formation of secondary organic aerosol (SOA) with elevated O/C ratios via oxidation. Ethylene glycol, a semivolatile polyol, inherently having a high O/C ratio, is directly emitted into the atmosphere mostly in urban atmosphere. Nitrate is a major inorganic constituent of urban atmospheric aerosols, particularly under haze conditions. The present study demonstrates accommodation of ethylene glycol (EG) within sodium nitrate (SN) aerosol via heterogeneous nonreactive uptake with an uptake coefficient, γ, of 2.5 (±0.2) × 10–6 at 80% RH (relative humidity). EG uptake is significantly suppressed at lower humidity, with reductions of ∼20% at 70% RH and ∼46% at 60% RH. Degradation of EG via in-particle nitrate-mediated photolysis proceeds with a rate constant of 6.9 × 10–5 s–1 at 80% RH and increased further on lowering of RH. Notably, even at 80% RH, this particle-phase degradation rate is ∼5-fold higher than the reported gas-phase rate. In situ micro-Raman and offline GC/MS analysis showed formation of compounds with higher O/C ratios, such as glycolaldehyde, glyoxal, glycolic acid, oxalic acid, and formate. The formation of light-absorbing aldehydes (glyoxal, glycolaldehyde) may contribute to reduced solar albedo and brown carbon formation, thus enhancing in-particle photochemistry. Moreover, formation of organic acids may induce physicochemical changes via complexation with other metal ions, resulting in formation of organic salts that will modify the acidity, hygroscopicity, and optical properties of aged particles. Collectively, these findings have implications for polluted urban atmospheres, especially humid coastal regions with increasing trends of both NOx and EG emissions which may significantly contribute to SOA formation and represent an important atmospheric sink for EG.