Distribution of gaseous and particulate organic composition during dark alpha-pinene ozonolysis

Secondary Organic Aerosol (SOA) affects atmospheric composition, air quality and radiative transfer, however major difficulties are encountered in the development of reliable models for SOA formation. Constraints on processes involved in SOA formation can be obtained by interpreting the speciation and evolution of organics in the gaseous and condensed phase simultaneously. In this study we investigate SOA formation from dark α-pinene ozonolysis with particular emphasis upon the mass distribution of gaseous and particulate organic species. A detailed model for SOA formation is compared with the results from experiments performed in the EUropean PHOtoREactor (EUPHORE) simulation chamber, including on-line gas-phase composition obtained from Chemical-Ionization-Reaction Time-Of-Flight Mass-Spectrometry measurements, and off-line analysis of SOA samples performed by Ion Trap Mass Spectrometry and Liquid Chromatography. The temporal profile of SOA mass concentration is relatively well reproduced by the model. Sensitivity analysis highlights the importance of the choice of vapour pressure estimation method, and the potential influence of condensed phase chemistry. Comparisons of the simulated gaseous- and condensed-phase mass distributions with those observed show a generally good agreement. The simulated speciation has been used to (i) propose a chemical structure for the principal gaseous semi-volatile organic compounds and condensed monomer organic species, (ii) provide evidence for the occurrence of recently suggested radical isomerisation channels not included in the basic model, and (iii) explore the possible contribution of a range of accretion reactions occurring in the condensed phase. We find that oligomer formation through esterification reactions gives the best agreement between the observed and simulated mass spectra

Développement d'un modèle déterministe pour la formation des aérosols organiques secondaires (application à la sensibilité du système AOS/COV/NOx)

Les composés organiques volatils (COV) émis dans l'atmosphère sont oxydés selon un mécanisme complexe. Cette oxydation est progressive et met en jeu une myriade de composés organiques intermédiaires. Ces COV secondaires sont généralement considérablement moins volatils que leurs précurseurs primaires et peuvent ainsi participer à la formation d'aérosols organiques secondaires (AOS). Cette matière organique représente l'une des composantes majeures du mode fin (rayon < 1 m) des aérosols atmosphériques. Cette formation d'AOS est ainsi supposée modifier significativement les propriétés physiques et chimiques des aérosols notamment la distribution en masse, le comportement hygroscopique (et donc la capacité des particules à agir comme noyau de condensation nuageuse), l'acidité, la réactivité chimique, les propiétés radiatives... Malgré l'impact potentiel considérable des AOS sur le climat, l'état des connaissances relatives à leur formation, leur composition et leur évolution reste parcellaire. Dans le cadre de ce travail, nous avons développé un modèle totalement explicite pour représenter la formation de l'AOS dans l'objectif (i), d'évaluer notre degré de compréhension des mécanismes liés à la formation de l'AOS, (ii) d'identifier les sensibilités du système aux différents paramètres contrôlant la production d'AOS et (iii) de disposer de schémas de référence pour valider le développement de schémas optimisés en taille pour la modélisation tridimensionnelle. Nous avons ensuite utilisé ce modèle de formation de l'AOS pour explorer la sensibilité du système AOS/COV/NOx.Progressive gas-phase oxidation of volatile organic compounds (VOC) leads to the formation of a myriad of intermediate species. These secondary organics are more functionalized than their parent compounds, and the numbe of functions typically increases as oxidation preceeds. Highly functionalized species have lower vapour pressures and/or higher polarities, allowing substantial gas/particle partioning, thus leading to secondary organic aerosol (SOA) formation. This organic particulate matter constitutes a large and variable fraction of fine particles (Dp <1 m). Therefore SOA is suspected to modify significantly physical and chemical aerosol properties, such as mass distribution, acidity, reactivity, hygroscopic and optical properties... Despite the impacts of SOA on climate, our current understanding of SOA formation, composition and evolution remains incomplete. We develop in thisstudy an explicit SOA formation model enabling (i) to evaluate our understanding of mechanisms involved in SOA formation, (ii) to explore the sensitivity of SOA production to external parameters and (iii) to serve as a benchmark for the systematic development and testing of simplified chemical mechanisms for use in three-dimensional models. The explicit SOA formation model has been used to explore the evolution of the SOA/VOC/NOx system.PARIS12-CRETEIL BU Multidisc. (940282102) / SudocSudocFranceF

Assessment of vapor pressure estimation methods for secondary organic aerosol modeling

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Using GECKO-A to derive mechanistic understanding of secondary organic aerosol formation from the ubiquitous but understudied camphene

International audienceAbstract. Camphene, a dominant monoterpene emitted from both biogenic and pyrogenic sources, has been significantly understudied, particularly in regard to secondary organic aerosol (SOA) formation. When camphene represents a significant fraction of emissions, the lack of model parameterizations for camphene can result in inadequate representation of gas-phase chemistry and underprediction of SOA formation. In this work, the first mechanistic study of SOA formation from camphene was performed using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A). GECKO-A was used to generate gas-phase chemical mechanisms for camphene and two well-studied monoterpenes, α-pinene and limonene, as well as to predict SOA mass formation and composition based on gas/particle partitioning theory. The model simulations represented observed trends in published gas-phase reaction pathways and SOA yields well under chamber-relevant photooxidation and dark ozonolysis conditions. For photooxidation conditions, 70 % of the simulated α-pinene oxidation products remained in the gas phase compared to 50 % for limonene, supporting model predictions and observations of limonene having higher SOA yields than α-pinene under equivalent conditions. The top 10 simulated particle-phase products in the α-pinene and limonene simulations represented 37 %–50 % of the SOA mass formed and 6 %–27 % of the hydrocarbon mass reacted. To facilitate comparison of camphene with α-pinene and limonene, model simulations were run under idealized atmospheric conditions, wherein the gas-phase oxidant levels were controlled, and peroxy radicals reacted equally with HO2 and NO. Metrics for comparison included gas-phase reactivity profiles, time-evolution of SOA mass and yields, and physicochemical property distributions of gas- and particle-phase products. The controlled-reactivity simulations demonstrated that (1) in the early stages of oxidation, camphene is predicted to form very low-volatility products, lower than α-pinene and limonene, which condense at low mass loadings; and (2) the final simulated SOA yield for camphene (46 %) was relatively high, in between α-pinene (25 %) and limonene (74 %). A 50 % α-pinene + 50 % limonene mixture was then used as a surrogate to represent SOA formation from camphene; while simulated SOA mass and yield were well represented, the volatility distribution of the particle-phase products was not. To demonstrate the potential importance of including a parameterized representation of SOA formation by camphene in air quality models, SOA mass and yield were predicted for three wildland fire fuels based on measured monoterpene distributions and published SOA parameterizations for α-pinene and limonene. Using the 50/50 surrogate mixture to represent camphene increased predicted SOA mass by 43 %–50 % for black spruce and by 56 %–108 % for Douglas fir. This first detailed modeling study of the gas-phase oxidation of camphene and subsequent SOA formation highlights opportunities for future measurement–model comparisons and lays a foundation for developing chemical mechanisms and SOA parameterizations for camphene that are suitable for air quality modeling

Modeling organic aerosol over Europe in summer conditions with the VBS-GECKO parameterization : sensitivity to secondary organic compound properties and IVOC (intermediate-volatility organic compound) emissions

The VBS-GECKO (volatility basis set - Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere) parameterization for secondary organic aerosol (SOA) formation was integrated into the chemistry-transport model CHIMERE. Concentrations of organic aerosol (OA) and SOA were simulated over Europe for the July-August 2013 period. Simulated concentrations with VBS-GECKO were compared to results obtained with the former H2O parameterization implemented in CHIMERE and to observations from EMEP, ACTRIS and other observations available in the EBAS database. The model configuration using the VBS-GECKO parameterization slightly improves the performances compared to the model configuration using the former H2O parameterization. The VBS-GECKO model configuration performs well for stations showing a large SOA concentration from biogenic sources, especially in northern Europe, but underestimates OA concentrations over stations close to urban areas. Simulated OA was found to be mainly secondary (similar to 85 %) and from terpene oxidation. Simulations show negligible contribution of the oxidation of monoaromatic compounds to SOA production. Tests performed to examine the sensitivity of simulated OA concentrations to hydro-solubility, volatility, aging rates and NOx regime have shown that the VBS-GECKO parameterization provides consistent results, with a weak sensitivity to changes in the parameters provided by the gas-phase mechanism included in CHIMERE (e.g., HOx or NOx concentrations). Different scenarios considering intermediate-volatility organic compound (IVOC) emissions were tested to examine the contribution of IVOC oxidation to SOA production. At the continental scale, these simulations show a weak sensitivity of OA concentrations to IVOC emission variations. At the local scale, accounting for IVOC emissions was found to lead to a substantial increase in OA concentrations in the plume from urban areas. This additional OA source remains too small to explain the gap between simulated and measured values at stations where anthropogenic sources are dominant

Estimation of secondary organic aerosol viscosity from explicit modeling of gas-phase oxidation of isoprene and &lt;i&gt;α&lt;/i&gt;-pinene

International audienceAbstract. Secondary organic aerosols (SOA) are major components of atmospheric fine particulate matter, affecting climate and air quality. Mounting evidence exists that SOA can adopt glassy and viscous semisolid states, impacting formation and partitioning of SOA. In this study, we apply the GECKO-A (Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere) model to conduct explicit chemical modeling of isoprene photooxidation and α-pinene ozonolysis and their subsequent SOA formation. The detailed gas-phase chemical schemes from GECKO-A are implemented into a box model and coupled to our recently developed glass transition temperature parameterizations, allowing us to predict SOA viscosity. The effects of chemical composition, relative humidity, mass loadings and mass accommodation on particle viscosity are investigated in comparison with measurements of SOA viscosity. The simulated viscosity of isoprene SOA agrees well with viscosity measurements as a function of relative humidity, while the model underestimates viscosity of α-pinene SOA by a few orders of magnitude. This difference may be due to missing processes in the model, including autoxidation and particle-phase reactions, leading to the formation of high-molar-mass compounds that would increase particle viscosity. Additional simulations imply that kinetic limitations of bulk diffusion and reduction in mass accommodation coefficient may play a role in enhancing particle viscosity by suppressing condensation of semi-volatile compounds. The developed model is a useful tool for analysis and investigation of the interplay among gas-phase reactions, particle chemical composition and SOA phase state