The research in this dissertation explored the sources and chemistry of organic nitrogen aerosols in the atmosphere. Two approaches were employed: field measurements and laboratory experiments. In order to characterize atmospheric aerosol, two ambient studies were conducted in Cache Valley in Northern Utah during strong winter inversions of 2004 and 2005. The economy of this region is heavily dependent on agriculture. There is also a fast growing urban population. Urban and agricultural emissions, aided by the valley geography and meteorology, led to high concentrations of fine particles that often exceeded the national ambient air quality standards. Aerosol composition was dominated by ammonium nitrate and organic species. Mass spectra from an aerosol mass spectrometer revealed that the organic ion peaks were consistent with reduced organic nitrogen compounds, typically associated with animal husbandry practices. Although no direct source characterization studies have been undertaken in Cache Valley with an aerosol mass spectrometer, spectra from a study at a swine facility in Ames, Iowa, did not show any evidence of reduced organic nitrogen species. This, combined with temporal and diurnal characteristics of organic aerosol peaks, was a pointer that the organic nitrogen species in Cache Valley likely formed from secondary chemistry. Application of multivariate statistical analyses to the organic aerosol spectra further supported this hypothesis. To quantify organic nitrogen signals observed in ambient studies as well as understand formation chemistry, three categories of laboratory experiments were performed. These were calibration experiments, smog chamber studies, and an analytical method development. Laboratory calibration experiments using standard calibrants indicated that quantifying the signals from organic nitrogen species was dependent on whether they formed through acid-base chemistry or via secondary organic aerosol pathway. Results from smog chamber reactions of amines with ozone, nitrogen oxides, nitrate radical, and nitric acid showed that the secondary organic aerosol pathway was more plausible than acid-base chemistry, thus making the contribution of the organic nitrogen species to the total aerosol mass in Cache Valley significant. Gas phase and aerosol products formed from the smog chamber reactions were identified and used to devise reaction mechanisms. Finally, an ion chromatographic method for detecting and quantifying some key organic nitrogen species in aerosol was developed and tested.

La connaissance de l'impact des aérosols sur le climat, au cours de leur cycle de vie, est aujourd'hui un enjeu majeur de la communauté scientifique. En particulier, les aérosols organiques secondaires (AOS) constituent une part importante de la fraction fine des aérosols et pourtant leurs propriétés optiques et hygroscopiques présentent encore des fortes incertitudes. Cette étude avait pour objectif de comprendre et d'évaluer l'évolution des propriétés optiques et hygroscopiques de l'AOS produit dans la chambre de simulation atmosphérique CESAM à partir de l'ozonolyse de l'α-pinène au cours de sa formation et de son vieillissement dans l'atmosphère. Afin de mener à bien ce projet, un HTDMA a été développé et validé. Cette approche a été complétée en mesurant les propriétés hygroscopiques de l'ensemble de la population polydispersé d'aérosols par humidification au sein de la chambre de simulation. Afin de mettre en évidence des modifications de propriétés optiques, l'évolution temporelle de l'indice complexe de réfraction a été déterminée. La méthodologie a été validée et l'effet du temps de contact des particules avec la vapeur d'eau a été investigué. Cette méthodologie a ensuite été appliquée à l'étude des propriétés optiques et hygroscopiques de l'AOS généré par ozonolyse de l'α-pinène au cours de sa formation et du vieillissement par différents processus: dans le noir, en présence d'un excès d'ozone et par photolyse. Les liens avec des modifications possibles de la composition chimique ont été investigués par des observations et par modélisation / The impact of aerosols on climate represents a major challenge in atmospheric science. This is particularly true for secondary organic aerosols, representing a major fraction of the fine aerosols. However, their optical and hygroscopic properties are poorly understood. The present work investigates the optical and hygroscopic properties of SOA generated from the ozonolysis of α-pinene at first and after having undergone atmospheric ageing reactions using the laboratory smog chamber CESAM. For that purpose, an HTDMA has been built and a new approach has been developed to measure hygroscopic properties of polydispersed aerosols by humidifying them directly in the smog chamber. The refractive index has been calculated to investigate the optical properties changes. The methodology has been validated and the residence time of particles with water vapor has been investigated. Then, it has been applied to study the optical and hygroscopic properties of α-pinene SOA during its formation and during ageing by various processes: reaction in the dark, with ozone, and photolysis. The link with changes in the composition has been investigated by measurements and modeling

La combustion de bois, ou plus largement de la biomasse, est une source de pollution très importante en particules atmosphériques en hiver, particulièrement en France. Si les émissions primaires ont été étudiées depuis de nombreuses années, il existe de grandes incertitudes sur le devenir de ces émissions dans l’atmosphère. Le travail de thèse a donc porté sur l’étude du vieillissement en chambre de simulation atmosphérique de l’aérosol émis par 3 appareillages pour le chauffage au bois (conçus entre 2000 et 2010 et représentatifs de la politique de renouvellement mis en place par l’ADEME), avec un intérêt particulier pour le potentiel de formation d’aérosol organique secondaire (SOA) et la modification de l’empreinte chimique à l’échelle moléculaire au cours du transport atmosphérique. Les expériences ont montré un potentiel de formation de SOA très important. La concentration en OA peut ainsi être multipliée par 7 (1.5 – 7.1) entre l’émission et après un temps de résidence atmosphérique équivalent à 5 h. Cette étude met également en évidence l’impact de l’efficacité de combustion sur les facteurs d’émission et par conséquent le rôle crucial de l’opérateur. L’étude à l’échelle moléculaire a mis en évidence la formation de composés susceptibles de servir de marqueurs de combustion de biomasse âgée, principalement des nitrocatéchols. Enfin, cette étude démontre le rôle clé de la volatilisation du lévoglucosan, principal marqueur organique de la combustion de biomasse, au cours du processus de dilution dans l’atmosphère, et pose clairement la question de la pertinence des constantes cinétiques de dégradation préalablement calculées en chambre de simulation atmosphérique. / Biomass burning is in winter a main source of air pollution by particulate matter, especially in France. While primary emissions have been characterized extensively before, few studies have addressed the aging of these emissions in the atmosphere and large uncertainties remain. Therefore, the objectives of this thesis was to study in a smog chamber the aging of the aerosol emitted by 3 different woodstoves used for residential heating (fabricated from between 2000 and 2010, and representative of the policy engaged by the French environmental agency to renew the appliances across the country), with a specific focus on the Secondary Organic Aerosol (SOA) production potential and the modification of the chemical fingerprint of the emissions at the molecular level during their transport in the atmosphere. The experiments showed the SOA production potential can be significant. The OA concentration can be increased by up to a factor of 7 (1.5 – 7.1) after being aged in the smog chamber with a time equivalent to 5 hours in the atmosphere. The study also further demonstrated the influence of the combustion efficiency on the emissions and implicitly the role of the operator. The study of the composition of the aerosol at the molecular level showed the formation of compounds, likely to serve as markers for aged biomass burning, mainly nitrocatechols. Finally, the work also illustrates the influence of the volatilization of levoglucosan, main marker of biomass burning, during the dilution process occurring in the atmosphere, and challenge the pertinence of the degradation rate constant determined previously in smog chamber.

This thesis describes the investigation of three aspects of the formation of secondary organic aerosol (SOA): * Aerosol formation from mixed precursors * Global modelling of SOA formation * Modelling of dynamics of SOA formation based on empirical data collected from smog chamber experiments. The formation and growth processes of secondary organic aerosol were investigated using smog chamber experimentation and modelling techniques to gain a better understanding of the application of SOA yield values in modelling both SOA mass and dynamics. Published SOA yields from a range of volatile organic compounds (VOCs) are used to model SOA mass on a local, regional or global scale, based on the assumption that the SOA yield of a mixture is the sum of the yields of the components. Experimental investigations into SOA yield from mixtures of VOC revealed potential uncertainties that would result from applying these yields to systems containing multiple VOCs. SOA formation in systems of toluene or m-xylene, compared with systems of these VOCs and propene, have shown that the introduction of propene (which has a zero SOA yield) to smog chamber photo-oxidations of toluene or m-xylene delays the formation and suppresses the overall yield of SOA from 450 to 90 µg m-3 ppm-1 for the toluene system and from 325 to 125 µg m-3 ppm-1 for the mvxylene system compared with systems of individual species without propene. The SOA partitioning yield data also indicates that partitioning of species to existing aerosol is suppressed in the mixed systems. Gas-phase modelling of these experiments showed that potential SOA species were expected to be formed sooner due to the increased system reactivity provided by propene. The observed delay in SOA nucleation, similar consumption rates of toluene and m-xylene in both the single and mixed systems and the gas-phase modelling results suggest that the addition of propene to hydrocarbon SOA systems modifies the gas-phase chemistry leading to the formation of potential SOA species from toluene and m-xylene. This result calls into question the bulk and partitioning yield values that have been published for pure substances as well as the validity of applying individual VOC yields to VOC mixture. Application of SOA yields to the global scale provides estimates of annual global SOA formation, global contributions from various VOCs and regional SOA distributions. Two SOA modules, using bulk and partitioning yield methods, were added to a global atmospheric chemical transport model, MOZART-2. The bulk yield method, representing the maximum possible global SOA burden, gave an annual production of 24.5 Tg of SOA, which is slightly lower than previous estimates (30 - 270 Tg yr-1). The partitioning method, which gives a more realistic estimate of SOA formation, produced 15.3 Tg yr-1; the biogenic fraction (13.6 Tg yr-1) compares to a previous estimate of biogenic SOA of 18.5 Tg yr-1 and 2.5 to 44 Tg yr- 1 using the partitioning method. Anthropogenic SOA contributions of 1.1 Tg yr-1 from MOZART-2 compared to recent estimates of 0.05 -2.62 Tg yr-1. SOA production was found to be dependent on oxidant availability and VOC emissions in South America and Asia. The partitioning method produced significantly less SOA due to limited availability of OC. Thepartitioning method also produced a peak SOA concentration of 10 µg m-3 over South America in September and showed that SOA is at maximum production for most of the year in Asia and Europe. The two SOA formation methods also provides data to analyse the restrictions to SOA formation in particular regions, based on the maximum amount of SOA able to form (bulk yield method) and the more realistic partitioning estimate from the same region. Limitations to SOA formation in a particular region can be attributed to deficiencies in OC availability or VOC oxidant concentrations. Comparisons to limited observational and modelled data suggest that the MOZART-2 SOA model provides a good representation of global averaged SOA. SOA mass concentrations, predicted by models such as MOZART-2, can be used in part to model the dynamics of an SOA population (e.g. size of particles, number concentrations etc.). Aerosol properties such as size and number concentration can then be used to estimate their effect on climate and health. The explicit representation of the processes that affect aerosol dynamics, such as nucleation, condensation, evaporation and coagulation can be complex and use significant computational resources. Simplification of the discrete coagulation equation and empirical coagulation coefficients for continuum and non-continuum regime diffusion kinetics provided a simplified method of coagulation capable of predicting the evolution of inert sodium chloride aerosol in chamber experiments. A variable coagulation coefficient (linked to the mean particle number concentration of each experiment) was developed. This method is an empirical surrogate for the standard coefficient corrections applied to Brownian based diffusion in the continuum regime to account for the different kinetic effects within the transition and free molecular diffusion regimes. This method removes the need for calculating individual coefficients for each particle interaction. Estimates of modeluncertainty show that within uncertainty limits the model provides a good representation of experimental data. Correlation and index of agreement (IOA) calculations revealed good statistical agreement between modelled and experimental. Some experiments showed degrees of coagulation under prediction using the variable coefficient technique. Investigations into the effect of aerosol type and size, temperature and humidity may be necessary to refine the variable coefficient calculation technique. The model showed little sensitivity to model time step and is capable of high resolution representation of the aerosol. Mass concentration is conserved within the model whereas some error due to numerical diffusion within the number concentrations results from the bin sectioning technique used. The simplicity of this sectioning method over other methods and the minimal effect of numerical diffusion establishes a simplified method of modelling relative to the high resolution of the aerosol distribution the model achieves. It is suggested that the efficiency improvements introduced by the approaches used in developing this model provide an efficient ultra-fine coagulation modelling for atmospheric models. A semi-empirical model for SOA dynamics (SPLAT) incorporating coagulation, nucleation, condensation and evaporation was developed. The aim of the model and the development process was to predict, with high resolution and minimal computational expense, the formation and growth of SOA given a SOA mass input as a function of time. The average size distribution profile from chamber experimental data was used as part of the nucleation module. This technique provided an alternative method of representing the particle distribution compared to those models that assume a single diameter of nucleated particle or a fixed log-normal mode for the entire evolution of SOA. All SPLAT simulations assume organic nucleation events within the experiments modelled, although it is stilluncertain whether they occur in the atmosphere. The modelled nucleation events have produced a single nucleation burst, a result of immediate domination of condensation as soon as nucleation occurs. This deficiency is likely to be a result of the assumption of free molecular diffusion for condensation. The rate of condensation, calculated at every time step, is based on the aerosol size distributed surface area and the particle-size-dependent saturation mass concentrations. The SPLAT coagulation module was a version of the model developed in Chapter 6. Comparisons between experimental and modelled data showed good agreement. These comparisons revealed the shortcomings in the nucleation module while a statistical analysis of the modelled and experimental data has shown SPLAT to be effective in modelling a range of SOA systems. The complexity introduced in modelling aerosol dynamics in high resolution is offset in SPLAT by efficiency improvements due to the insensitivity of the model to time step size and simplified methods of bin sectioning, nucleation, coagulation, condensation and evaporation. Published SOA yields can be applied to predict SOA mass at local, regional or global scales. Although previously unreported uncertainties in these yields have been shown to exist, the MOZART-2 global chemical transport model has shown that SOA mass concentration can be predicted with reasonable quality, considering the scale of the model and limited observational data. These global scale SOA mass predictions can be used purely for global burden and occurrence, or as the input for modelling the dynamics of an aerosol population, which is significant for estimating an aerosol population's effect on climate change and health. SOA mass concentrations from chamber experiments were used as input to a SOA dynamics model. This model (SPLAT) then predicted the evolution of particle number concentrations and size within these experiments based on this mass input. Application of the dynamics model to the output of the MOZART-2 model could then provide a comprehensive global scale SOA modelling package.