Reactions of organic peroxy radicals, RO 2 , with substituted and biogenic alkenes at room temperature: unsuspected sinks for some RO 2 in the atmosphere? †

International audienceUntil now the reactions of organic peroxy radicals (RO 2) with alkenes in the gas phase have been essentially studied at high temperature (T $360 K) and in the context of combustion processes, while considered negligible in the Earth's atmosphere. In this work, the reactions of methyl-, 1-pentyl-and acetylperoxy radicals (CH 3 O 2 , C 5 H 11 O 2 , and CH 3 C(O)O 2 , respectively) with 2-methyl-2-butene, 2,3-dimethyl-2butene and for the first time the atmospherically relevant isoprene, a-pinene, and limonene were studied at room temperature (298 AE 5 K). Monitoring directly the radicals with chemical ionization mass spectrometry led to rate coefficients larger than expected from previous combustion studies but following similar trends in terms of alkenes, with (in molecule À1 cm 3 s À1) k II CH3O2 ¼ 10 À18 to 10 À17 Â 2/2 and k II CH3COðOÞO2 ¼ 10 À14 to 10 À13 Â 5/5. While these reactions would be negligible for CH 3 O 2 and aliphatic RO 2 at room temperature, this might not be the case for acyl-, and perhaps hydroxy-, allyl-and other substituted RO 2. Combining our results with the Structure-Activity Relationship (SAR) predicts k II (298 K)$10 À14 molecule À1 cm 3 s À1 for hydroxy-and allyl-RO 2 from isoprene oxidation, potentially accounting for up to 14% of their sinks in biogenic-rich regions of the atmosphere and much more in laboratory studies. Scheme 1 General scheme for the reaction of RO 2 with unsaturated compounds

The Cloud Condensation Nuclei (CCN) properties of 2-methyltetrols and C3–C6 polyols from osmolality and surface tension measurements (Discussion paper)

A significant fraction of the organic material in aerosols is made of highly soluble compounds such as sugars (mono- and polysaccharides) and polyols, including the 2-methyltetrols, methylerythritol and methyltreitol. The high solubility of these compounds has brought the question of their potentially high CCN efficiency. For the 2-methyltetrols, this would have important implications for cloud formation at global scale because they are thought to be produced by the atmospheric oxidation of isoprene. To investigate this question, the complete Köhler curves for C3–C6 polyols and the 2-methyltetrols have been determined experimentally from osmolality and surface tension measurements. Contrary to what expected, none of these compounds displayed a critical supersaturation lower than those of inorganic salts or organic acids. Their Raoult terms show that this limited CCN efficiency is due to their absence of dissociation in water, this in spite of slight surface-tension effects for the 2-methyltetrols. Thus, compounds such as sugars and polyols would not contribute more to cloud formation in the atmosphere than any other organic compounds studied so far. In particular, the presence of 2-methyltetrols in aerosols would not particularly enhance cloud formation in the atmosphere, contrary to what has been suggested

Extraction and Characterization of Surfactants from Atmospheric Aerosols

The authors also warmly thank Marija Margus, Ana Cvitesic, Sanja Frka Milosavljevic and Irena Ciglenecki, from Rudjer Boskovic Institute of Zagreb, Croatia for the help with the aerosol sampling at Marina Frapa, Rogoznica, Croatia.International audienceSurface-active compounds, or surfactants, present in atmospheric aerosols are expected to play important roles in the formation of liquid water clouds in the Earth's atmosphere, a central process in meteorology, hydrology, and for the climate system. But because specific extraction and characterization of these compounds have been lacking for decades, very little is known on their identity, properties, mode of action and origins, thus preventing the full understanding of cloud formation and its potential links with the Earth's ecosystems.In this paper we present recently developed methods for 1) the targeted extraction of all the surfactants from atmospheric aerosol samples and for the determination of 2) their absolute concentrations in the aerosol phase and 3) their static surface tension curves in water, including their Critical Micelle Concentration (CMC). These methods have been validated with 9 references surfactants, including anionic, cationic and non-ionic ones. Examples of results are presented for surfactants found in fine aerosol particles (diameter < 1 mu m) collected at a coastal site in Croatia and suggestions for future improvements and other characterizations than those presented are discussed

Salting out, non-ideality and synergism enhance surfactant efficiency in atmospheric aerosols

Abstract In Earth’s atmosphere, the surface tension of sub-micron aerosol particles is suspected to affect their efficiency in becoming cloud droplets. But this quantity cannot be measured directly and is inferred from the chemical compounds present in aerosols. Amphiphilic surfactants have been evidenced in aerosols but experimental information on the surface properties of their mixtures with other aerosol components is lacking. This work explores experimentally the surface properties of aqueous mixtures of amphiphilic surfactants (SDS, Brij35, TritonX100, TritonX114, and CTAC) with inorganic salts (NaCl, (NH4)2SO4) and soluble organic acids (oxalic and glutaric acid) using pendant droplet tensiometry. Contrary to what could be expected, inorganic salts and organic acids systematically enhanced the efficiency of the surfactants rather than reduced it, by further lowering the surface tension and, in some cases, the CMC. Furthermore, all the mixtures studied were strongly non-ideal, some even displaying some synergism, thus demonstrating that the common assumption of ideality for aerosol mixtures is not valid. The molecular interactions between the mixture components were either in the bulk (salting out), in the mixed surface monolayer (synergy on the surface tension) or in the micelles (synergy on the CMC) and need to be included when describing such aerosol mixtures

Surface tension models for binary aqueous solutions: a review and intercomparison

The liquid–air surface tension of aqueous solutions is a fundamental quantity in multi-phase thermodynamics and fluid dynamics and thus relevant in many scientific and engineering fields. Various models have been proposed for its quantitative description. This Perspective gives an overview of the most popular models and their ability to reproduce experimental data of ten binary aqueous solutions of electrolytes and organic molecules chosen to be representative of different solute types. In addition{,} we propose a new model which reproduces sigmoidal curve shapes (Sigmoid model) to empirically fit experimental surface tension data. The surface tension of weakly surface-active substances is well reproduced by all models. In contrast{,} only few models successfully model the surface tension of aqueous solutions with strongly surface-active substances. For substances with a solubility limit{,} usually no experimental data is available for the surface tension of supersaturated solutions and the pure liquid solute. We discuss ways in which these can be estimated and emphasize the need for further research. The newly developed Sigmoid model best reproduces the surface tension of all tested solutions and can be recommended as a model for a broad range of binary mixtures and over the entire concentration range.ISSN:1463-9084ISSN:1463-907

Vertical distribution of nighttime stratospheric from balloon measurements: comparison with models

International audienceVertical distributions of NO2 and 03 mixing ratios and the aerosol extinction coefficient were measured by night between 18 and 32 km altitude (9-80 hPa) at mid latitudes using the balloon-borne instrument AMON on March 24, 1994. The NO2 profile is compared with results of simulations involving the REPROBUS 3D Chemical-Transport Model and a Lagrangian model in which the ozone mixing ratio and the aerosol surface area are initialized using AMON measurements (other mixing ratios are initialized with REPROBUS). As confirmed by Lidar observations, the surface areas were larger than the monthly and zonally averaged SAGE 2 data available for March 1994 at 45 ø N. The Lagrangian model shows relatively good agreement with AMON results for pressure higher than 40 hPa and smaller than 15 hPa, but the computed NO2 value is too high between 15 and 40 hPa. This seems to indicate that heterogeneous reactions involving the NOv species in the aerosol layer are still incompletely understood

The Reaction of Organic Peroxy Radicals with Unsaturated Compounds Controlled by a non-Epoxide Pathway under Atmospheric Conditions

Today, the reactions of gas-phase organic peroxy radicals (RO2) with unsaturated Volatile Organic Compounds (VOC) are expected to be negligible at room temperature and ignored in atmospheric chemistry. This assumption is based on combustion studies (T ³ 360 K), which were the only experimental data available for these reactions until recently. These studies also reported epoxide formation as the only reaction channel. In this work, the products of the reactions of 1-pentylperoxy (C5H11O2) and methylperoxy (CH3O2) with 2,3-Dimethyl-2-butene (“2,3DM2B”) and isoprene were investigated at T = 300 ± 5 K with Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) and Gas Chromatography/Electron Impact Mass Spectrometry. Unlike what was expected, the experiments showed no measurable formation of epoxide. However, RO2 + alkene was found to produce compounds retaining the alkene structure, such as 3-hydroxy-3-methyl-2-butanone (C5H10O2) with 2,3DM2B and 2-hydroxy-2-methyl-3-butenal (C5H8O2) and methyl vinyl ketone with isoprene, suggesting that these reactions proceed through another reaction pathway under atmospheric conditions. We propose that, instead of forming an epoxide, the alkyl radical produced by the addtion of RO2 onto the alkene reacts with oxygen, producing a peroxy radical. The corresponding mechanisms are consistent with the products observed in the experiments. This alternative pathway implies that, under atmospheric conditions, RO2 + alkene reactions are kinetically limited by the initial addition step and not by the epoxide formation proposed until now for combustion systems. Extrapolating the combustion data to room temperature thus underestimates the rate coefficients, which is consistent with those recently reported for these reactions at room temperature. While slow for many classes of RO2, these reactions could be non-negligible at room temperature for some functionalized RO2. They might thus need to be considered in laboratory studies using large alkene concentrations and in biogenically-dominated regions of the atmosphere.EPHEMERA