The relative importance of competing pathways for the formation of high-molecular-weight peroxides in the ozonolysis of organic aerosol particles

High-molecular-weight (HMW) organic compounds are an important component of atmospheric particles, although their origins, possibly including in situ formation pathways, remain incompletely understood. This study investigates the formation of HMW organic peroxides through reactions involving stabilized Criegee intermediates (SCI's). The model system is methyl oleate (MO) mixed with dioctyl adipate (DOA) and myristic acid (MA) in submicron aerosol particles, and Criegee intermediates are formed by the ozonolysis of the double bond in methyl oleate. An aerosol flow tube coupled to a quadrupole aerosol mass spectrometer (AMS) is employed to determine the relative importance of different HMW organic peroxides following the ozonolysis of different mixing mole fractions of MO in DOA and MA. Possible peroxide products include secondary ozonides (SOZ's), α-acyloxyalkyl hydroperoxides and α-acyloxyalkyl alkyl peroxides (αAAHP-type compounds), diperoxides, and monoperoxide oligomers. Of these, the AMS data identify two SOZ's as major HMW products in the ozonolysis of pure methyl oleate as well as in an inert matrix of DOA to as low as 0.04 mole fraction MO. In comparison, in mixed particles of MO and MA, αAAHP-type compounds form in high yields for MO mole fractions of 0.5 or less, suggesting that SCI's efficiently attack the carboxylic acid group of myristic acid. The reactions of SCI's with carboxylic acid groups to form αAAHP-type compounds therefore compete with those of SCI's with aldehydes to form SOZ's, provided that both types of functionalities are present at significant concentrations. The results therefore suggest that SCI's in atmospheric particles contribute to the transformation of carboxylic acids and other protic groups into HMW organic peroxides

Density changes of aerosol particles as a result of chemical reaction

International audienceThis paper introduces the capability to study simultaneously changes in the density, the chemical composition, the mobility diameter, the aerodynamic diameter, and the layer thickness of multi-layered aerosol particles as they are being altered by heterogeneous chemical reactions. A vaporization-condensation method is used to generate aerosol particles composed of oleic acid outer layers of 2 to 30 nm on 101-nm polystyrene latex cores. The layer density is modified by reaction of oleic acid with ozone for variable exposure times. For increasing ozone exposure, the mobility diameter decreases while the vacuum aerodynamic diameter increases, which, for spherical particles, implies that particle density increases. The aerosol particles are confirmed as spherical based upon the small divergence of the particle beam in the aerosol mass spectrometer. The particle and layer densities are calculated by two independent methods, namely one based on the measured aerodynamic and mobility diameters and the other based on the measured mobility diameter and particle mass. The uncertainty estimates for density calculated by the second method are two to three times greater than those of the first method. Both methods indicate that the layer density increases from 0.89 to 1.12 g·cm?3 with increasing ozone exposure. Aerosol mass spectrometry shows that, concomitant with the increase in the layer density, the oxygen content of the reacted layer increases. Even after all of the oleic acid has reacted, the layer density and the oxygen content continue to increase slowly with prolonged ozone exposure, a finding which indicates continued chemical reactions of the organic products either with ozone or with themselves. The results of this paper provide new insights into the complex changes occurring for atmospheric particles during the aging processes caused by gas-phase oxidants

Design of a new multi-phase experimental simulation chamber for atmospheric photosmog, aerosol and cloud chemistry research

A new simulation chamber has been built at the Interuniversitary Laboratory of Atmospheric Systems (LISA). The CESAM chamber (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) is designed to allow research in multiphase atmospheric (photo-) chemistry which involves both gas phase and condensed phase processes including aerosol and cloud chemistry. CESAM has the potential to carry out variable temperature and pressure experiments under a very realistic artificial solar irradiation. It consists of a 4.2 m<sup>3</sup> stainless steel vessel equipped with three high pressure xenon arc lamps which provides a controlled and steady environment. Initial characterization results, all carried out at 290–297 K under dry conditions, concerning lighting homogeneity, mixing efficiency, ozone lifetime, radical sources, NO<sub>y</sub> wall reactivity, particle loss rates, background PM, aerosol formation and cloud generation are given. Photolysis frequencies of NO<sub>2</sub> and O<sub>3</sub> related to chamber radiation system were found equal to (4.2 × 10<sup>−3</sup> s<sup>−1</sup>) for <i>J</i><sub>NO<sub>2</sub></sub> and (1.4 × 10<sup>−5</sup> s<sup>−1</sup>) for <i>J</i><sub>O<sup>1</sup>D</sub> which is comparable to the solar radiation in the boundary layer. An auxiliary mechanism describing NO<sub>y</sub> wall reactions has been developed. Its inclusion in the Master Chemical Mechanism allowed us to adequately model the results of experiments on the photo-oxidation of propene-NO<sub>x</sub>-Air mixtures. Aerosol yields for the α-pinene + O<sub>3</sub> system chosen as a reference were determined and found in good agreement with previous studies. Particle lifetime in the chamber ranges from 10 h to 4 days depending on particle size distribution which indicates that the chamber can provide high quality data on aerosol aging processes and their effects. Being evacuable, it is possible to generate in this new chamber clouds by fast expansion or saturation with or without the presence of pre-existing particles, which will provide a multiphase environment for aerosol-droplet interaction

Relating hygroscopicity and optical properties to chemical composition and structure of secondary organic aerosol particles generated from the ozonolysis of α-pinene

Secondary organic aerosol (SOA) were generated from the ozonolysis of α-pinene in the CESAM (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) simulation chamber. The SOA formation and aging were studied by following their optical, hygroscopic and chemical properties. The optical properties were investigated by determining the particle complex refractive index (CRI). The hygroscopicity was quantified by measuring the effect of relative humidity (RH) on the particle size (size growth factor, GF) and on the scattering coefficient (scattering growth factor, f(RH)). The oxygen to carbon atomic ratios (O : C) of the particle surface and bulk were used as a sensitive parameter to correlate the changes in hygroscopic and optical properties of the SOA composition during their formation and aging in CESAM. The real CRI at 525 nm wavelength decreased from 1.43–1.60 (±0.02) to 1.32–1.38 (±0.02) during the SOA formation. The decrease in the real CRI correlated to the O : C decrease from 0.68 (±0.20) to 0.55 (±0.16). In contrast, the GF remained roughly constant over the reaction time, with values of 1.02–1.07 (±0.02) at 90% (±4.2%) RH. Simultaneous measurements of O : C of the particle surface revealed that the SOA was not composed of a homogeneous mixture, but contained less oxidised species at the surface which may limit water absorption. In addition, an apparent change in both mobility diameter and scattering coefficient with increasing RH from 0 to 30% was observed for SOA after 14 h of reaction. We postulate that this change could be due to a change in the viscosity of the SOA from a predominantly glassy state to a predominantly liquid state

Improving the design of industrial microwave processing systems through prediction of the dielectric properties of complex multi-layered materials

Rigorous design of industrial microwave processing systems requires in-depth knowledge of the dielectric properties of the materials to be processed. These values are not easy to measure, particularly when a material is multi-layered containing multiple phases, when one phase has a much higher loss than the other and the application is based on selective heating. This paper demonstrates the ability of the Clausius-Mossotti (CM) model to predict the dielectric constant of multi-layered materials. Furthermore, mixing rules and graphical extrapolation techniques were used to further evidence our conclusions and to estimate the loss factor. The material used for this study was vermiculite, a layered alumina-silicate mineral containing up to 10 % of an interlayer hydrated phase. It was measured at different bulk densities at two distinct microwave frequencies, namely 934 and 2143 MHz. The CM model, based on the ionic polarisability of the bulk material, gives only a prediction of the dielectric constant for experimental data with a deviation of less than 5 % at microwave frequencies. The complex refractive index model (CRIM), Landau, Lifshitz and Loyenga (LLL), Goldschmidt, Böttcher and Bruggeman-Hanai model equations are then shown to give a strong estimation of both dielectric constant and loss factor of the solid material compared to that of the measured powder with a deviation of less than 1 %. Results obtained from this work provide a basis for the design of further electromagnetic processing systems for multi-layered materials consisting of both high loss and low loss components

Experimental Uptake Study of Ethanol by Water Droplets and Its Theoretical Modeling of Cluster Formation at the Interface

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