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The nitrate anion (NO3−) is abundant in environmental aqueous phases, including aerosols, surface waters, and snow, where its photolysis releases nitrogen oxides back into the atmosphere. Nitrate photolysis occurs via two channels: (1) the formation of NO2 and O− and (2) the formation of NO2− and O(3P). The occurrence of two reaction channels with very low quantum yield (∼1%) highlights the critical role of the solvation environment and spin-forbidden electronic transitions, which remain unexplained at the molecular level. We investigate the two photolysis channels in water using quantum chemical calculations and first-principles molecular dynamics simulations with hybrid density functional theory and enhanced sampling. We find that spin-forbidden absorption to the triplet state (T1) is possible but occurs at a rate ∼15 times weaker than the spin-allowed transition to the singlet state (S1). A metastable solvation cage complex requires additional thermal energy to dissociate the N–O bond, allowing for recombination or non-radiative deactivation. Our results explain the temperature dependence of photolysis, linked to hydrogen bond rearrangement in the solvation shell. This work provides new molecular insights into nitrate photolysis and its low quantum yield under environmental conditions.
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Technical Note: A technique to convert NO2 to NO2− with S(IV) and its application to measuring nitrate photolysis
Nitrate photolysis is a potentially significant mechanism for “renoxifying” the atmosphere, i.e., converting nitrate into nitrogen oxides – nitrogen dioxide (NO2) and nitric oxide (NO) – and nitrous acid (HONO). Nitrate photolysis in the environment occurs through two channels which produce (1) NO2 and hydroxyl radical (OH) and (2) nitrite (NO2-) and an oxygen atom (O(3P)). Although the aqueous quantum yields and photolysis rate constants of both channels have been established, field observations suggest that nitrate photolysis is enhanced in the environment. Laboratory studies investigating these enhancements typically only measure one of the two photo-channels, since measuring both channels generally requires separate analytical methods and instrumentation. However, measuring only one channel makes it difficult to assess whether secondary chemistry is enhancing one channel at the expense of the other or if there is an overall enhancement of nitrate photochemistry. Here, we show that the addition of S(IV), i.e., bisulfite and sulfite, can convert NO2 to NO2-, allowing for measurement of both nitrate photolysis channels with the same equipment. By varying the concentration of S(IV) and exploring method parameters, we determine the experimental conditions that quantitatively convert NO2 and accurately quantify the resulting NO2-. We then apply the method to a test case, showing how an OH scavenger in solution prevents the oxidation of NO2- to NO2 but does not enhance the overall photolysis efficiency of nitrate.
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- Award ID(s):
- 2305164
- PAR ID:
- 10510267
- Publisher / Repository:
- Copernicus Publications
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 24
- Issue:
- 7
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 4411 to 4419
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-24-4411-2024
