Review of ACP manuscript "On the importance of multiphase photolysis..." by J. M. González-Sánchez et al.

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Organic nitrates (RONO₂) are secondary compounds, and their fate is related to the transport and removal of NO_x 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 aerosol. This work investigated for the first time the aqueous-phase photolysis rate constants and quantum yields of four RONO₂ (isopropyl nitrate, isobutyl nitrate, &alpha;-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, as well as other atmospheric sinks (hydrolysis, gas phase photolysis, aqueous and gas phase &middot;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 up to 100 %. Furthermore, our results show that aqueous-phase &middot;OH oxidation is a major sink for water-soluble RONO₂ (K_H &gt; 10⁵ M atm^&ndash;1) ranging from 50 to 70 % of their total sink at high LWC (0.35 g m^&ndash;3). These results highlight the importance of investigating the aqueous-phase RONO₂ reactivity to understand how it affects their ability to transport air pollution.