Photolysis and oxidation by OH radicals of two carbonyl nitrates: 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone
| Autor: | B. Picquet-Varrault, R. Suarez-Bertoa, M. Duncianu, M. Cazaunau, E. Pangui, M. David, J.-F. Doussin |
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| Jazyk: | angličtina |
| Rok vydání: | 2020 |
| Předmět: | |
| Zdroj: | Atmospheric Chemistry and Physics, Vol 20, Pp 487-498 (2020) |
| Druh dokumentu: | article |
| ISSN: | 1680-7316 1680-7324 |
| DOI: | 10.5194/acp-20-487-2020 |
| Popis: | Multifunctional organic nitrates, including carbonyl nitrates, are important species formed in NOx-rich atmospheres by the degradation of volatile organic compounds (VOCs). These compounds have been shown to play a key role in the transport of reactive nitrogen and, consequently, in the ozone budget; they are also known to be important components of the total organic aerosol. However, very little is known about their reactivity in both the gas and condensed phases. Following a previous study that we published on the gas-phase reactivity of α-nitrooxy ketones, the photolysis and reaction with OH radicals of 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone (which are a β-nitrooxy ketone and γ-nitrooxy ketone, respectively) were investigated for the first time in simulation chambers. The photolysis frequencies were directly measured in the CESAM chamber, which is equipped with a very realistic irradiation system. The jnitrate/jNO2 ratios were found to be (5.9±0.9)×10-3 for 4-nitrooxy-2-butanone and (3.2±0.9)×10-3 for 5-nitrooxy-2-pentanone under our experimental conditions. From these results, it was estimated that ambient photolysis frequencies calculated for typical tropospheric irradiation conditions corresponding to the 1 July at noon at 40∘ N (overhead ozone column of 300 and albedo of 0.1) are (6.1±0.9)×10-5 s−1 and (3.3±0.9)×10-5 s−1 for 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone, respectively. These results demonstrate that photolysis is a very efficient sink for these compounds with atmospheric lifetimes of few hours. They also suggest that, similarly to α-nitrooxy ketones, β-nitrooxy ketones have enhanced UV absorption cross sections and quantum yields equal to or close to unity and that γ-nitrooxy ketones have a lower enhancement of cross sections, which can easily be explained by the larger distance between the two chromophore groups. Thanks to a product study, the branching ratio between the two possible photodissociation pathways is also proposed. Rate constants for the reaction with OH radicals were found to be (2.9±1.0)×10-12 and (3.3±0.9)×10-12 cm3 molecule−1 s−1, respectively. These experimental data are in good agreement with rate constants estimated by the structure–activity relationship (SAR) of Kwok and Atkinson (1995) when using the parametrization proposed by Suarez-Bertoa et al. (2012) for carbonyl nitrates. Comparison with photolysis rates suggests that the OH-initiated oxidation of carbonyl nitrates is a less efficient sink than photodissociation but is not negligible in polluted areas. |
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