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International audience; The irradiation under simulated sunlight of some phenolic compounds typically emitted in ambient air by biomass burning, namely vanillin and acetosyringone, yielded intermediates with humic-like fluorescence properties that can be assimilated to humic-like substances (HULIS). Evidence was obtained by ultra-high-resolution mass spectrometry of the occurrence of oligomerization processes up to the formation of trimeric species. In contrast, the irradiation of other biomass-burning compounds such as vanillic acid, m-cresol and guaiacol did not yield either HULIS-type fluorescence or oligomers. We suggest that the photolysis of biomass-burning compounds is a potential HULIS source in the atmosphere, if the relevant substrates undergo photoinduced oligomerization reactions.

Water soluble organic carbon (WSOC) can significantly influence the aerosol optical properties and the aqueous phase chemistry in cloudwater, fogwater and aerosol liquid water. Here, we examine how the changing pH (in acidic range) affects the absorption spectra of aqueous extracts from field biomass burning aerosols, under dark conditions and in presence of simulated sunlight illumination. The observation under dark conditions indicates that pH variation from 2 to 5 induces significantly enhanced light absorbance in the wavelength ranges of 235–270 nm and 300–550 nm, whereas the light absorbance decreased in the range of 270–300 nm, which might be partially ascribed to the deprotonation of carboxylic acids and phenols. During the extract photolysis, light absorption exhibits photo-bleaching below 380 nm and photo-enhancement above 380 nm, indicating that at acidic levels (pH = 2–5), the particle extracts could undergo a significant composition evolution leading to a modification of absorptive properties. Meanwhile, after 12 h-photolysis, the acidity ([H+]) normalized by WSOC concentration in aqueous extracts ([WSOCae]) increased with a variation of Δ[H+]/[WSOCae]=(3.7 ± 0.7) × 10−7 mol mgC−1 (mean ± standard deviation), suggesting the formation of new acidic substances. Although these findings were acquired in aqueous solutions more relevant to cloud and fog water, the similar evolution likely occurs in wetted aerosols. This calls more attention to the effect of acidity on the wetted aerosols in order to better estimate the aerosol radiative forcing.

The ionic strength can affect the photochemical reactions occurring in the liquid core of the aerosol deliquescent particles. Yet, the investigations related to ionic strength effect on aqueous-phase reactions are limited. Here we assess the photochemical degradation of a typical lignin-derived compound, vanillin (VL), upon addition of either sodium nitrate (NaNO3) or sodium sulfate (Na2SO4). The UV-VIS absorption spectra of VL exhibits a hyperchromic effect and a bathochromic shift of the n→π* absorption band, in going from λmax = 310 nm in the dilute aqueous phase to λmax = 346 nm at I = 0.3 M adjusted by Na2SO4. A hypsochromic shift of the n→π* absorption band of VL, in going from λmax = 310 nm in the dilute liquid water to λmax = 303 nm at Ieff = 0.124 M is observed when the ionic strength was adjusted with NaNO3. At fixed pH ≈ 4, representing both the cloud droplets and the liquid core of the aerosol particles, the pseudo-first-order rate constant, k1st (Ieff.→0) = (1.5 ± 0.2) x 10−5 s−1, in dilute aqueous solution (Ieff = 0 M, NaNO3), linearly decreases to (k1st = (3.7 ± 0.2) x 10−6 s−1), at Ieff = 0.124 M (NaNO3) with kinetic salting coefficient b = -(4.8 ± 0.2) L mol−1. The very subtle pH change of the absorption spectra of irradiated VL is observed due to the formation of carboxylic acids, hydroxylated compounds and oligomers which presence was confirmed by FT-ICR MS analysis. Most likely aliphatic like CHO compounds are formed upon photodegradation of VL in dilute aqueous phase while the condensed aromatic CHO and CHON compounds are produced in presence of nitrate ions (Ieff = 0.124 M). These results suggest that inorganic ions present in the liquid water of aerosols alters the kinetics and the product formation of photochemical degradation of organic compounds which in turn can affect the light absorption properties of aerosols.

The ionic strength effect can influence the kinetics and products distribution within the liquid water content (LWC) of aerosol deliquescent particles. The heterogeneous reactions of atmospheric oxidants (O3, SO2, and NO2, among others) with liquid core of aerosols can be also influenced by ionic strength, which in turn can impact important atmospheric processes such as haze formation. However, the ionic strength effects on liquid phase reactions and on heterogeneous reactions of atmospheric relevance have been barely studied in the past. Finally, the gas-liquid equilibrium can be affected by ionic strength of the liquid water. Here we highlight the importance of ionic strength effects on chemical processes in the aerosol deliquescent particles. We give the future directions on this topic emphasizing that ionic strength can affect the absorption and/or scattering properties of aerosol particles and cloud droplets which in turn is crucial for accurate estimates of radiative forcing of clouds and aerosols.

International audience; There is an increasing concern about the pesticides spread in the atmosphere. They can be found in the gas-phase but because of their physico-chemical properties they mostly remain adsorbed on the surface of the atmospheric particles. The kinetic data of the adsorbed pesticides towards atmospheric oxidants such as ozone and hydroxyl radicals are very scarce. Here we present the kinetic data set for the heterogeneous reactions of gas-phase ozone and eight commonly used pesticides adsorbed on silica particles. The obtained rate constants are described by both kinetic patterns, i.e., Langmuir-Rideal (L-R) and Langmuir-Hinshelwood (L-H) mechanisms. Concerning the L-R mechanism the rate constants range between 3.4 . 10(-19) cm(3) molecules(-1) s(-1) and 8.8 . 10(-19) cm(3) molecules(-1) s(-1). With respect to the L-H mechanism, the values of air-silica partitioning coefficient (K-(O3)) vary between 21 . 10(-16) cm(3) molecules(-1) and 358 . 10(-16) cm(3) molecules(-1). The maximum rate constants range between 0.4 . 10(-4) and 1.7 . 10(-4) s(-1). The estimated lifetimes of the selected pesticides according to both L-R and L-H mechanism span from 8 to >33 days, demonstrating that these species are very persistent with respect to ozone reactivity in the atmosphere. The results obtained in this study can contribute to better describe the atmospheric fate of pesticides in the particulate phase. (C) 2014 Elsevier Ltd. All rights reserved.

CAPRAM 3.0 is the latest development of the chemical aqueous phase radical mechanism (CAPRAM) series which is incorporating CAPRAM 2.4 (Ervens et al., 2003a, Journal of Geophysical Research—Atmospheres 108) and a new extended reaction mechanism for atmospherically relevant hydrocarbons containing more than two and up to six carbon atoms. The chemistry of organics containing three and four carbon atoms is now described in detail. Almost 400 new reactions are now implemented considering the chemistry of organic compounds containing different functional groups, i.e. alcohols, carbonyl compounds, mono- and dicarboxylic acids, polyfunctional compounds as well as some esters and one heterocyclic compound. The aqueous chemistry has been coupled to the gas phase mechanism RACM (regional atmospheric chemistry modeling) (Stockwell et al., 1997, Journal of Geophysical Research—Atmpspheres 102, 25847–25879), and phase exchange is treated using the resistance model of Schwartz (1986. In: Jaeschke, W. (Ed.), Chemistry of Multiphase Atmospheric Systems, NATO ASI Series, Springer, Berlin, pp. 415–471). The CAPRAM remote scenario which was chosen as the standard scenario showed that the introduction of the higher organic chemistry has a relevant influence on the standard subsystems. The diurnal peak concentration of OH radical in the droplets decreases with about 40% and the reactions of OH with hydrocarbons containing 3 or 4 carbon atoms account for about 10% out of the total sinks of OH in the droplets. A slightly stronger acidification of the aqueous phase in comparison to CAPRAM 2.4 is observed. The simulations for the standard scenario showed that there is an increase of organic mass within the droplets where the organic compounds containing 4 carbon atoms represent the 67.5% of the total mass, whereas in the urban and in the marine scenario the contribution of two carbon atom compounds is dominating. The formation and accumulation of substituted mono- and dicarboxylic acids such as tartaric, mesoxalic and acetic acid in the aqueous phase are also observed.