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 increases with functionalization. This work has studied for the first time the aqueous-phase ⚫OH-oxidation kinetics of four alkyl nitrates (isopropyl nitrate, isobutyl nitrate, 1-pentyl nitrate, and isopentyl nitrate) and three 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 observed, with kOH ranging from 8×107 to 3.1×109 L mol−1 s−1 at 296±2 K. Using these results, a 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 they were compared to their gas-phase lifetimes. Large differences were observed, especially for polyfunctional organic nitrates: for 50 % of the proposed organic nitrates for which the ⚫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 (liquid water content 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 considering aqueous-phase ⚫OH-oxidation reactivity of biogenic nitrates is necessary to improve the predictions of their atmospheric fate.

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.

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Abstract. Organic nitrates (RONO2) are secondary compounds, and their fate is related to the transport and removal of NOx in the atmosphere. While previous research studies have focused on the reactivity of these molecules in the gas phase, their reactivity in condensed phases remains poorly explored despite their ubiquitous presence in submicron aerosols. This work investigated for the first time the aqueous-phase photolysis-rate constants and quantum yields of four RONO2 (isopropyl nitrate, isobutyl nitrate, α-nitrooxyacetone, and 1-nitrooxy-2-propanol). Our results showed much lower photolysis-rate constants for these RONO2 in the aqueous phase than in the gas phase. From alkyl nitrates to polyfunctional RONO2, no significant increase of their aqueous-phase photolysis-rate constants was observed, even for RONO2 with conjugated carbonyl groups, in contrast with the corresponding gas-phase photolysis reactions. Using these new results, extrapolated to other alkyl and polyfunctional RONO2, in combination with estimates for the other atmospheric sinks (hydrolysis, gas-phase photolysis, aqueous- and gas-phase ⚫OH oxidation, dry and wet deposition), multiphase atmospheric lifetimes were calculated for 45 atmospherically relevant RONO2 along with the relative importance of each sink. Their lifetimes range from a few minutes to several hours depending on the RONO2 chemical structure and its water solubility. In general, multiphase atmospheric lifetimes are lengthened when RONO2 partition to the aqueous phase, especially for conjugated carbonyl nitrates for which lifetimes can increase by up to 100 %. Furthermore, our results show that aqueous-phase ⚫OH oxidation is a major sink for water-soluble RONO2 (KH>105 M atm−1) ranging from 50 % to 70 % of their total sink at high liquid water content (LWC) (0.35 g m−3). These results highlight the importance of investigating the aqueous-phase RONO2 reactivity to understand how it affects their ability to transport air pollution.

Abstract. Organic nitrates (RONO2) are secondary compounds whose fate is closely related to the transport and removal of NOx in the atmosphere. Despite their ubiquitous presence in submicron aerosols, the photochemistry of RONO2 has only been investigated in the gas phase, leaving their reactivity in condensed phases poorly explored. This work aims to address this gap by investigating, for the first time, the reaction products, and the mechanisms of aqueous-phase photolysis of four RONO2 (i.e., isopropyl nitrate, isobutyl nitrate, α-nitrooxyacetone, and 1-nitrooxy-2-propanol). The results show that the reactivity of RONO2 in the aqueous phase differs significantly from that in the gas phase. In contrast to the gas phase, where RONO2 releases NOx upon photolysis, the aqueous phase photolysis of RONO2 leads primarily to the direct formation of HONO, which was confirmed by quantum chemistry calculations. Hence, the aqueous-phase photolysis of RONO2 represents both a NOx sink and a source of atmospheric HONO, a significant precursor of ∙OH and ∙NO. These secondary radicals (·OH and ·NO) are efficiently trapped in the aqueous phase, leading to the formation of HNO3 and functionalized RONO2. This reactivity can thus potentially contribute to the aging of Secondary Organic Aerosol (SOA) and serve as an additional source of aqueous-phase SOA.