It is now accepted that one of the important pathways of secondary organic aerosol (SOA) formation occurs through aqueous phase chemistry in the atmosphere. However, the chemical mechanisms leading to macromolecules are still not well understood. It was recently shown that oligomer production by OH radical oxidation in the aerosol aqueous phase from α-dicarbonyl precursors, such as methylglyoxal and glyoxal, is irreversible and fast.

Methyl vinyl ketone (MVK) was chosen in the present study as it is an α,β-unsaturated carbonyl that can undergo radical oligomerization in the aerosol aqueous phase. We present here experiments on the aqueous phase OH-oxidation of MVK, performed under various conditions. Using NMR and UV absorption spectroscopy, high and ultra-high resolution mass spectrometry, we show that the fast formation of oligomers up to 1800 Da is due to radical oligomerization of MVK, and 13 series of oligomers (out of a total of 26 series) are identified. The influence of atmospherically relevant parameters such as temperature, initial concentrations of MVK and dissolved oxygen are presented and discussed. In agreement with the experimental observations, we propose a chemical mechanism of OH-oxidation of MVK in the aqueous phase that proceeds via radical oligomerization of MVK on the olefin part of the molecule. This mechanism highlights in our experiments the paradoxical role of dissolved O₂: while it inhibits oligomerization reactions, it contributes to produce oligomerization initiator radicals, which rapidly consume O₂, thus leading to the dominance of oligomerization reactions after several minutes of reaction. These processes, together with the large range of initial concentrations investigated show the fundamental role that radical oligomerization processes likely play in polluted fogs and atmospheric aerosol

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International audienceAqueous phase oxidation reactions in atmospheric particles can yield high molecular weight products and create secondary organic aerosol (SOA) upon droplet evaporation. Oxidation by hydroxyl radicals to create oligomers in solution that form SOA has been previously investigated; however, mixed organic solutions that can initiate radical chemistry have been largely overlooked. In aqueous solution, pyruvic acid (PA), an a-keto acid found in both the gas and aqueous phases in the atmosphere, photolyzes via a radical mechanism. Here, we use this photochemistry of pyruvic acid to trigger oligomerization of methyl vinyl ketone (MVK), an alpha,beta-unsaturated compound generated by the atmospheric oxidation of isoprene. We closely compare the reaction products and mechanism to a recent work in which the radical oligomerization of MVK initiated by hydroxyl radical is studied in depth. Using mass spectrometry, it is shown that the two reactions create oligomers of similar molecular weights, up to m/z 1200 for initial MVK concentrations of 20 mM. In the MVK and PA photolysis, exploring initial reactant concentrations demonstrates that the same oligomer series are produced regardless of the initial reactant or dissolved oxygen concentrations. However, the size of the oligomers formed increases with increasing initial reactant concentrations, and the oligomerization process is slowed when dissolved oxygen is present. Finally, using a Langmuir trough, that measures the surface tension as a function of liquid surface area, it is shown that these oligomer photoproducts are surface active. These results indicate the importance of mixed organic systems to understanding secondary organic aerosol formation and growth. Consequently, this chemistry may affect gasparticle mass transfer of water and semivolatile aerosol components and, therefore, the way that aerosol interacts with its environment

Abstract. It has recently been established that unsaturated water-soluble organic compounds (UWSOCs) might efficiently form oligomers in polluted fogs and wet aerosol particles, even for weakly soluble ones like methyl vinyl ketone (MVK). The atmospheric relevance of these processes is explored by means of multiphase process model studies in a companion paper. In the present study, we investigate the aging of these aqueous-phase MVK oligomers formed via •OH oxidation, as well as their ability to form secondary organic aerosol (SOA) upon water evaporation. The comparison between aqueous-phase composition and aerosol composition after nebulization of the corresponding solutions shows similar trends for oligomer formation and aging. The measurements reveal that oligomer aging leads to the formation of organic diacids. Quantification of the SOA mass formed after nebulization is performed, and the obtained SOA mass yields seem to depend on the spectral irradiance of the light used to initiate the photochemistry. Investigating a large range of initial MVK concentrations (0.2–20 mM), the results show that their •OH oxidation undergoes competition between functionalization and oligomerization that is dependent on the precursor concentration. At high initial MVK concentrations (≥ 2 mM), oligomerization prevails over functionalization, while at lower initial concentrations, oligomerization is not the major process, and functionalization dominates, resulting in small carbonyls, dicarbonyls and monoacids. The atmospheric implications of these processes are discussed.

A statistical analysis of 295 cloud samples collected at the Puy de Dôme station in France (PUY), covering the period 2001–2018, was conducted using principal component analysis (PCA), agglomerative hierarchical clustering (AHC), and partial least squares (PLS) regression. Our model classified the cloud water samples on the basis of their chemical concentrations and of the dynamical history of their air masses estimated with back-trajectory calculations. The statistical analysis split our dataset into two sets, i.e., the first set characterized by westerly air masses and marine characteristics, with high concentrations of sea salts and the second set having air masses originating from the northeastern sector and the “continental” zone, with high concentrations of potentially anthropogenic ions. It appears from our dataset that the influence of cloud microphysics remains minor at PUY as compared with the impact of the air mass history, i.e., physicochemical processes, such as multiphase reactivity.

Abstract. A total of 480 microorganisms collected from 39 clouds sampled at the Puy de Dôme station (alt. 1465 m; 45°46′19′′ N, 2°57′52′′ E; Massif Central, France) were isolated and identified. This unique collection was screened for biosurfactant (surfactants of microbial origin) production by measuring the surface tension (σ) of the crude extracts, comprising the supernatants of the pure cultures, using the pendant drop technique. The results showed that 41 % of the tested strains were active producers (σ < 55 mN m−1), with 7 % being extremely active (σ < 30 mN m−1). The most efficient biosurfactant producers (σ < 45 mN m−1) belong to a few bacterial genera (Pseudomonas and Xanthomonas) from the Υ-Proteobacteria class (78 %) and a yeast genus (Udeniomyces) from the Basidiomycota phylum (11 %). Some Bacillus strains from the Firmicutes phylum were also active but represented a small fraction of the collected population. Strains from the Actinobacteria phylum in the collection examined in the present study showed moderate biosurfactant production (45<σ < 55 mN m−1). Pseudomonas (Υ-Proteobacteria), the most frequently detected genus in clouds, with some species issued from the phyllosphere, was the dominant group for the production of biosurfactants. We observed some correlations between the chemical composition of cloud water and the presence of biosurfactant-producing microorganisms, suggesting the “biogeography” of this production. Moreover, the potential impact of the production of biosurfactants by cloud microorganisms on atmospheric processes is discussed.

Abstract. Laboratory experiments of efficient oligomerization from methyl vinyl ketone (MVK) in the bulk aqueous phase were simulated in a box model. Kinetic data are applied (if known) or fitted to the observed MVK decay and oligomer mass increase. Upon model sensitivity studies, in which unconstrained rate constants were varied over several orders of magnitude, a set of reaction parameters was found that could reproduce laboratory data over a wide range of experimental conditions. This mechanism is the first that comprehensively describes such radical-initiated oligomer formation. This mechanism was implemented into a multiphase box model that simulates secondary organic aerosol (SOA) formation from isoprene, as a precursor of MVK and methacrolein (MACR) in the aqueous and gas phases. While in laboratory experiments oxygen limitation might occur and lead to accelerated oligomer formation, such conditions are likely not met in the atmosphere. The comparison of predicted oligomer formation shows that MVK and MACR likely do negligibly contribute to total SOA as their solubilities are low and even reduced in aerosol water due to ionic strength effects (Setchenov coefficients). Significant contribution by oligomers to total SOA might only occur if a substantial fraction of particulate carbon acts as oligomer precursors and/or if oxygen solubility in aerosol water is strongly reduced due to salting-out effects.

One of the current essential issues to unravel our ability to forecast future climate change and air quality, implies a better understanding of natural processes leading of secondary organic aerosol (SOA) formation, and in particular the formation and fate of oligomers. The difficulty in characterizing macromolecules is to discern between large oxygenated molecules from series of oligomers containing repeated small monomers of diverse structures. In the present study, taking advantage from previously established radical vinyl oligomerization of methyl vinylketone (MVK) in the aqueous phase, where relatively simple oligomers containing up to 14 monomers were observed, we have investigated the same reactivity on several other unsaturated water soluble organic compounds (UWSOCs) and on a few mixtures of these precursor compounds. The technique used to characterize the formed oligomers was a traveling wave ion mobility spectrometry coupled to a hybrid quadrupoletime of flight mass spectrometer (IMS-MS) fitted with an electrospray source and ultra-high performance liquid chromatography (UPLC). The technique allows for an additional separation, especially for large ions, containing long carbon chains. We have shown the efficiency of the IMS-mass spectrometry technique to detect oligomers derived from MVK photooxidation in the aqueous phase. The results were then compared to other oligomers, derived from ten other individual biogenic UWSOCs. The technique allowed distinguishing 2 between different oligomers arising from different precursors. It also clearly showed that compounds bearing a non-conjugated unsaturation did not provide oligomerization. Finally, it was shown that the IMS-mass spectrometry technique, applied to mixtures of unsaturated conjugated precursors, exhibited the ability of these precursors to co-oligomerize, i.e. forming only one complex oligomer system bearing monomers of different structures. The results are discussed in terms of atmospheric implications for the detection of oligomers in complex chamber and/or field samples.