D. James. Donaldson

An increasing number of studies report on the spontaneous production of OH radicals and H₂O₂ at the air/water interface of aqueous droplets. However, none of these studies have investigated OH production in droplets containing nitrate (NO₃^–) or nitrite (NO₂^–) anions. Those two ions, in particular NO₃^–, play a key role in atmospheric chemistry. The goal of this work was to study the spontaneous production of H₂O₂ in water droplets containing these two anions under dark conditions. Compared to ions that we have already studied (i.e., Cl^– and Br^–), H₂O₂ production in droplets containing NO₂^– or NO₃^– can be either enhanced or inhibited depending on the NO₂^– or NO₃^– concentrations. These findings suggest that, below a certain concentration, NO₂^– and NO₃^– enhance the H₂O₂ production due to their ability to disrupt the water structure at the interface, where at larger concentrations, a specific interfacial NO₂^– or NO₃^– chemistry starts to be important, producing gas-phase species such as HONO and NO₂. By monitoring H₂O₂ and several other gas-phase products, we propose a mechanism to explain this specific interfacial chemistry of NO₂^– and NO₃^–. This chemistry may not be significant during day time due the photolysis of these two anions, but it can have an important impact during night time.

Molecular iodine (I₂) significantly influences atmospheric oxidation capacity, driving new particle formation and contributing to ozone depletion in polar regions. However, its formation mechanisms in these cold environments, particularly at night, remain poorly understood. Here, we present experimental evidence that the dark freezing of iodide-containing bulk solutions and droplets activates iodide (I^-) without any added catalysts or oxidants. This I^- activation is markedly enhanced during the freezing of iodide-containing droplets, leading to substantial I₂ production in both the gas and the aqueous phases. The production follows a Langmuir-Hinshelwood formalism, indicating a surface-mediated process at the frozen air-water interface. A detailed mechanistic investigation reveals that the production of I₂ is driven by the hydroxyl radicals (OH·) and hydrogen peroxide (H₂O₂) formed at the frozen interface, which subsequently oxidize I^-. We attribute the formation of OH· and H₂O₂ to the high electric field possibly induced by the Workman-Reynolds freezing potential. This work identifies a previously unrecognized nighttime formation mechanism of I₂ in atmospheric snow and ice, which has implications for atmospheric multiphase chemistry in cold environments.

Carboxylic acids are ubiquitous in the atmosphere in both the gas and particle phase. One of their major degradation pathways is known to be their reaction with OH radicals. To enhance our understanding of carboxylic acid reactivity, we investigated the gas phase reactions of OH radicals with a series of these acids by performing quantum chemistry calculations at the ωB97X-V/def2-TZVPPD, ωB97M-V/def2-TZVPPD, CCSD(T)/6-311+G(2df,2p), and CCSD(T)/aug-cc-pVTZ level. Our calculations are mainly focused on the reactive decarboxylation pathway energetics that occur following the OH-driven abstraction of the carboxylic hydrogen of formic, acetic, malonic, fumaric, maleic, trifluoroacetic, and trichloro-acetic acids. All the reactions are exoergic, and in all cases, we find that a hydrogen-bonded complex with a 6-membered ring structure is formed between the acid group and the OH, followed by reactive decarboxylation with small or nonexistent barriers to both abstraction and decarboxylation steps. Finally, in the case of trifluoroacetic acid decarboxylation, we calculate the energetics of several pathways that may lead to stable oxidation product formation, and we compare our theoretical work to other experimental and theoretical studies.

(Text in French) Spontaneous interfacial oxidant formation as a key driver of aerosol oxidation. Aerosols and clouds are key players in tropospheric chemistry. These tiny particles, with a radius ranging from a few nanometres to tens of microns, impact atmospheric composition, visibility, climate, and human health. Besides the environmental importance, all these effects have enormous societal and economic consequences. Immense efforts have been undertaken over the last decades to understand their sources, nature (i.e., liquid vs. solid) and chemical transformations during their atmospheric lifetimes. Nonetheless, our ability to describe their chemistry is far from complete. Here, the recently discovered spontaneous formation of hydrogen peroxide (H2O2), and possibly HOx radicals, at the air/liquid interface is presented as a key driver for the oxidation of aerosols and cloud droplets. This may have profound impacts on the oxidation capacity of tropospheric aerosols and clouds by enhancing radical production in multiphase systems.

Recent studies have reported spontaneous production of OH radicals and H₂O₂ at the air-water interface of water droplets. However, the mechanism(s) behind this chemistry remain elusive, with the presence of a strong electric field at the interface being considered one reason for this spontaneous chemistry. Here, we provide evidence that in salt-containing aqueous droplets, the amount of oxidant formation is strongly related to the identity and concentration of the ions present in the solution. Anions have a significantly stronger effect on H₂O₂ formation compared to cations. The effect of the anions' identity on H₂O₂ formation follows the Hofmeister series, which describes changes in the solvation properties of a solution due to the presence of ions. We present a quantitative relationship between two Hofmeister parameters and peroxide concentration derived from our experimental results. This link between H₂O₂ formation and the Hofmeister series suggests that anions disrupt the water structure at the interface, reducing the solvation of OH^- anions and promoting their dissociation into OH radicals and free electrons, leading to an increase in H₂O₂ formation. This study shows that spontaneous formation of H₂O₂ is driven by the solvation properties at the interface and not necessarily (or exclusively) by the presence of a strong electric field.

We report evidence that organic aerosols containing carboxylic acids can be spontaneously oxidized in the dark under normal atmospheric conditions due to interfacial hydroxyl radical production. Product formation is negligible under dry conditions and increases with increasing relative humidity. In a dioxygen-free environment, the oxidation efficiency is substantially decreased. Size-resolved measurements show an increase in the reactivity and product formation yields for smaller particles, correlated with their surface-to-volume ratio. Our findings suggest that spontaneous hydroxyl radical production at the air-water interface of organic nanodroplets may be an important pathway in their oxidation, especially during nighttime.

Surfaces exposed to outdoor and even indoor environments, especially in urban settings, develop a coating comprised of chemical compounds deposited from the overlying atmosphere and products of reactions occurring within this coating. Early work recognized the potential of this coating to reflect the local atmospheric concentrations of pollutants. More recently, the potential of such coatings (“urban grime”) to participate in active exchange of important atmospheric oxidants and their precursors has been explored. Here, I will present my perspective on the history of urban grime chemistry studies and where the future may lie in these.

We present experimental evidence for the spontaneous production of hydrogen peroxide (H₂O₂) at the liquid-ice interface during the freezing of dilute salt solutions. Specifically, sample solutions containing NaCl, NaBr, NH₄Cl, and NaI at concentrations between 10^-6 and 10^-1 M were subjected to freezing-melting cycles and then analyzed for H₂O₂ content. The relationship between the production rate of H₂O₂ and the salt concentration follows that of the Workman-Reynolds freezing potential (WRFP) values as a function of the salt concentration. Our results suggest that H₂O₂ is formed at the liquid-ice interface from the self-recombination of hydroxyl radicals (OH·), produced from the oxidation of hydroxide anions due to the high electric field generated at the aqueous-ice interface under the WRFP effect. Furthermore, the involvement of O₂ likely acting as an electron capturer could promote to produce more OH radicals and hydroperoxyl radicals (HO₂·), thus enhancing the production of H₂O₂ at the liquid-ice interface. Overall, this study suggests a novel mechanism of H₂O₂ formation in ice via its spontaneous production at the liquid-ice interface, induced by the Workman-Reynolds effect.