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https://doi.org/10.5194/acp-2020-684
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-684
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  09 Oct 2020

09 Oct 2020

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This preprint is currently under review for the journal ACP.

On the importance of atmospheric loss of organic nitrates by aqueous-phase ·OH-oxidation

Juan Miguel González-Sánchez1,2, Nicolas Brun1,2, Junteng Wu1, Julien Morin1, Brice Temime-Rousell1, Sylvain Ravier1, Camille Mouchel-Vallon3, Jean-Louis Clément2, and Anne Monod1 Juan Miguel González-Sánchez et al.
  • 1Aix Marseille Univ, CNRS, LCE, Marseille, France
  • 2Aix Marseille Univ, CNRS, ICR, Marseille, France
  • 3Laboratoire d’Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, France

Abstract. Organic nitrates are secondary species in the atmosphere. Their fate is related to the chemical transport of pollutants from polluted areas to more distant zones. While their gas-phase chemistry has been studied, their reactivity in condensed phases is far from being understood. However, these compounds represent an important fraction of organic matter in condensed phases. In particular, their partition to the aqueous-phase may be especially important for oxidized organic nitrates for which water solubility increase with functionalization. This work has studied for the first time the aqueous-phase ·OH-oxidation kinetics of 5 alkyl nitrates (isopropyl nitrate, isobutyl nitrate, 1-pentyl nitrate, isopentyl nitrate and 2-ethylhexyl nitrate) and 3 functionalized organic nitrates (α-nitrooxyacetone, 1-nitrooxy-2-propanol and isosorbide 5-mononitrate) by developing a novel and accurate competition kinetic method. Low reactivity was confirmed, with kOH (at 296 ± 2 K) ranging from 8·107 to 2.5·109 L mol−1 s−1. Using these results, the previously developed aqueous-phase Structure Activity Relationship (SAR) was extended, and the resulting parameters confirmed the extreme deactivating effect of the nitrate group, up to two adjacent carbon atoms. The achieved extended SAR was then used to determine the ·OH-oxidation rate constants of 49 organic nitrates, including hydroxy nitrates, ketonitrates, aldehyde nitrates, nitrooxy carboxylic acids and more functionalized organic nitrates such as isoprene and terpene nitrates. Their multiphase atmospheric lifetimes towards ·OH-oxidation were calculated using these rate constants, and compared to their gas-phase lifetimes. Large differences were observed, especially for polyfunctional organic nitrates: for 50 % of the proposed organic nitrates for which ·OH-reaction occurs mainly in the aqueous-phase (more than 50 % of the overall removal) their ·OH-oxidation lifetimes increased by 20 % to 155 % under cloud/fog conditions (LWC = 0.35 g m−3). In particular, for 83 % of the proposed terpene nitrates, the reactivity towards ·OH occurred mostly (> 98 %) in the aqueous-phase while for 60 % of these terpene nitrates their lifetimes increased by 25 % to 140 % compared to their gas-phase reactivity. We demonstrate that these effects are of importance under cloud/fog conditions, but also under wet aerosol conditions, especially for the terpene nitrates. These results suggest that taking into account aqueous-phase ·OH-oxidation reactivity of biogenic nitrates is necessary to improve the predictions of their atmospheric fate.

Juan Miguel González-Sánchez et al.

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Juan Miguel González-Sánchez et al.

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Short summary
Organic nitrates play a crucial role on air pollution as they are considered as NOx reservoirs. This work lights up the importance of their reactions with OH radicals in the aqueous-phase (cloud/fog, wet aerosol), which is slower than in the gas-phase. For compounds that significantly partition in water such as polyfunctional biogenic nitrates, these aqueous-phase reactions should drive their atmospheric removal, leading to a broader spatial distribution of NOx than previously accounted for.
Organic nitrates play a crucial role on air pollution as they are considered as NOx reservoirs....
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