Production of Formate via Oxidation of Glyoxal Promoted by Particulate Nitrate Photolysis

Abstract

Particulate nitrate photolysis can produce oxidants (i.e., OH, NO 2 , and NO 2 − /HNO 2 ) in aqueous droplets and may play a potential role in increased atmospheric oxidative capacity. Our earlier works have reported on the SO 2 oxidation promoted by nitrate photolysis to produce sulfate. Here, we used glyoxal as a model precursor to examine the role of particulate nitrate photolysis in the formation of secondary organic aerosol (SOA) from particle-phase oxidation of glyoxal by OH radicals. Particles containing sodium nitrate and glyoxal were irradiated at 300 nm. Interestingly, typical oxidation products of oxalic acid, glyoxylic acid, and higher-molecular-weight products reported in the literature were not found in the photooxidation process of glyoxal during nitrate photolysis in the particle phase. Instead, formic acid/formate production was found as the main oxidation product. At glyoxal concentration higher than 3 M, we found that the formic acid/formate production rate increases significantly with increasing glyoxal concentration. Such results suggest that oxidation of glyoxal at high concentrations by OH radicals produced from nitrate photolysis in aqueous particles may not contribute significantly to SOA formation since formic acid is a volatile species. Furthermore, recent predictions of formic acid/formate concentration from the most advanced chemical models are lower than ambient observations at both the ground level and high altitude. The present study reveals a new insight into the production of formic acid/formate as well as a sink of glyoxal in the atmosphere, which may partially narrow the gap between model predictions and field measurements in both species.

Particulate nitrate photolysis in the atmosphere

“…175 Our recent work examined the role of particulate nitrate photolysis in the formation of SOA from particle-phase oxidation of glyoxal by OH radicals. 176 Interestingly, we did not observe typical oxidation products such as oxalic acid, glyoxylic acid, and higher-molecular-weight products previously reported in the literature. Instead, formic acid/formate was the main oxidation product.…”

Nitrate Photolysis in Mixed Sucrose–Nitrate–Sulfate Particles at Different Relative Humidities

“…They found that it can explain up to 60% of the missing sulfate between original modeling and field observations . Most recently, production of formate via oxidation of glyoxal promoted by particulate nitrate photolysis was reported . The enhancement of particulate nitrate photolysis can be potentially attributed to several reasons, such as more incomplete solvent cage in particles than in solution bulk due to the high surface-to-volume ratio of the particle, , increased absorption cross section of nitrate ions when adsorbed on particulate surface, and enhanced internal light intensity by morphology-dependent resonances (MDRs) .…”

Competing Photochemical Effects in Aqueous Carbonyl/Ammonium Brown Carbon Systems

“…The solutions in this study were constantly mixed, consistent with other photochemistry work, in order to prevent the accumulation of surfactant CVOC/AS reaction products at the gas–liquid interface, and to ensure uniform bulk-phase photochemistry across the entire mimic solution. That said, studies exploring CVOC reaction systems have demonstrated potentially surface-dependent behavior under photochemical conditions, owing to their surface-active properties. , The relative importance of this surface behavior depends on SOA loading and composition, aerosol size, and ambient conditions, which are not replicable under laboratory conditions. Regardless of mixing or surface effects, it is likely that formic acid and similar, smaller-MW products are arising from photochemistry and subsequently partitioning to the gas phase .…”

“…That said, studies exploring CVOC reaction systems have demonstrated potentially surface-dependent behavior under photochemical conditions, owing to their surface-active properties. 24,45 The relative importance of this surface behavior depends on SOA loading and composition, aerosol size, and ambient conditions, which are not replicable under laboratory conditions. Regardless of mixing or surface effects, it is likely that formic acid and similar, smaller-MW products are arising from photochemistry and subsequently partitioning to the gas phase.…”