Enhanced Nitrite Production from the Aqueous Photolysis of Nitrate in the Presence of Vanillic Acid and Implications for the Roles of Light-Absorbing Organics

Abstract

A prominent source of hydroxyl radicals (•OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, NO2 –) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving numerous organic compounds in atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cation). Upon adding small oxygenates such as ethanol, n-butanol, or formate as •OH scavengers, the average intrinsic quantum yield of nitrite [Φ­(NO2 –)] was 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ­(NO2 –) was strongly pH-dependent, reaching 8.0 ± 2.1% at a pH of 8 and 1.5 ± 0.39% at a more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) may greatly enhance the nitrite production from the aqueous nitrate photolysis through photosensitizing reactions, where the triplet excited state of BrC may generate solvated electrons, which reduce nitrate to NO2 for further conversion to nitrite. This photosensitization process by BrC chromophores during nitrate photolysis under mildly acidic conditions may partly explain the missing HONO in urban environments.

Particulate nitrate photolysis in the atmosphere

“…128 Wang et al demonstrated the importance of solvated electrons produced from photosensitizing reactions in enhanced nitrite production from nitrate photolysis. 158 They suggested that the solvated electrons are mainly scavenged by nitrate, leading to more NO 2 production for further conversion to nitrite.…”

Snowmelt Leads to Seasonal Nitrous Acid Formation Across Northwestern China

“…, which could continue to react with H 2 O to form NO 2 reactions (12 and 13) (Wang, Huang, et al, 2021).…”

“…The large enhancement of P(HONO) (g) /[NO 3 ˉ] was probably because the complex organic compositions in snowmelt were much more efficient in enhancing HONO and O 2 • ˉ formation (Wang, Dalton, et al., 2020). A recent study has shown that brown carbon (BrC) may greatly enhance the NO 2 − production from the aqueous NO 3 − photolysis through photosensitizing reactions (Wang, Huang, et al., 2021). It has been shown that the triplet state of photosensitizer (P*) are sources of O 2 •− (Canonica et al., 1995).…”

“…It has been shown that the triplet state of photosensitizer (P*) are sources of O 2 •− (Canonica et al., 1995). Besides, P* could transfer a solvated electron ( e s − ) to NO 3 − to produce NO 3 2− , which could continue to react with H 2 O to form NO 2 reactions ( and ) (Wang, Huang, et al., 2021). $$$\mathrm{P}\ast +{\mathrm{N}\mathrm{O}}_{3}^{-}\,\to \,{\mathrm{P}}^{+{\bullet}}+{\mathrm{N}\mathrm{O}}_{3}^{2-}$$$ $$${\mathrm{N}\mathrm{O}}_{3}^{2-}+{\mathrm{H}}_{2}\mathrm{O}\,\to \,{\mathrm{N}\mathrm{O}}_{2}+2{\mathrm{O}\mathrm{H}}^{-}$$$ …”

Aqueous SOA formation from photosensitized guaiacol oxidation: Comparison between non-phenolic and phenolic methoxybenzaldehydes as photosensitizers in the absence and presence of ammonium nitrate

“…On a related note, the significance of photosensitization by BrC (via formation of solvated electrons; Wang et al, 2021) and marine dissolved organic matter (via O 2 •formation; Garcia et al, 2021) in enhanced nitrite production from nitrate photolysis have been reported. A recent study from our group has shown that glyoxal photo-oxidation mediated by both nitrate photolysis and photosensitization can significantly enhance the atmospheric sink of glyoxal (Zhang et al, 2022).…”