Atmospheric aerosols are the major component in the shorter-term variability governing the radiative balance of the climate system, particularly on regional scales. However, knowledge of the myriad of properties and processes associated with aerosols is often limited, which results in major uncertainties when assessing their climate effects. One such aspect is the chemical make-up of the atmospheric aerosol burden.Airborne measurements of aerosol properties across Northern Europe are presented here in order to facilitate constraint of the properties, processes and effects of aerosols in this highly populated and industrialised region. An Aerodyne Aerosol Mass Spectrometer (AMS) delivered highly time-resolved measurements of aerosol chemical components, which included organic matter, sulphate, nitrate and ammonium.The chemical composition of the aerosol burden was strongly determined by the dominant meteorological conditions in Northern Europe. Pollution loadings in North- Western Europe were strongly enhanced when air masses originated from Continental Europe. Conversely, much cleaner conditions were associated with air masses from the Atlantic Ocean.Organic matter was found to be ubiquitous across Northern Europe and predominantly secondary in nature, which is consistent with other analyses in polluted regions of the Northern Hemisphere. Furthermore, its concentration was generally comparable to, or exceeded that of, sulphate. Significant chemical processing of the organic aerosol component was observed. Highly oxidised secondary organic aerosol dominated, as the distance from source and photochemical processing increased.Ammonium nitrate was found to be a major component of the aerosol burden in Northern Europe, with peak contributions occurring in North-Western Europe, due to the co-location of its emission precursors (NH3 and NOx) in the region. Ammonium nitrate was found to be the dominant sub-micron chemical constituent during periods associated with enhanced pollution episodes. Its concentration was shown to be modulated by the thermodynamic structure of the lower troposphere, with enhanced concentrations prevalent at the top of the boundary layer. This phenomenon greatly enhanced the radiative impact of the aerosol burden; the increased mass and water uptake by the aerosol significantly amplified the aerosol optical depth in the region.The results presented in this thesis highlight a highly dynamic region, where major changes in emissions have played a significant role in determining the chemical composition of the aerosol burden. As substantial reductions in sulphur dioxide emissions have occurred over the past two decades in Northern Europe, the relative contribution of sulphate aerosols to the regional aerosol burden has decreased. Consequently, it is more pertinent to consider the roles of organic matter and ammonium nitrate, as their influence becomes more pronounced than sulphate on regional and global climate.

La pollution particulaire recouvre de nombreux enjeux sanitaires et climatiques en Europe et dans le monde. Les aérosols organiques, qui représentent une fraction importante de la pollution particulaire, sont une matière complexe, émise directement sous forme condensée ou résultant de l'oxydation d'un très grand nombre de composés organiques volatiles dans l'atmosphère. En raison de la grande variété de ses sources d'émissions, qu'elles soient d'origine biogénique ou anthropique, la composition et les processus de formation et de vieillissement chimique des aérosols organiques restent aujourd'hui associés à de nombreuses interrogations. L'observation sur le terrain de la formation de la matière organique et de son évolution au sein des masses d'air étant difficile à mettre en œuvre, la modélisation reste un outil complémentaire indispensable pour simuler l'aérosol organique et suivre son transport sur de longues distances. C'est pourquoi ces travaux se sont attachés dans un premier temps à identifier les processus majeurs gouvernant aussi bien la formation que le vieillissement photochimique de la fraction organique des particules. En se basant sur une méthodologie innovante, un schéma chimique opérationnel dédié à l'émission et à l'oxydation de composés semi-volatiles a été développé pour les modèles de qualité de l'air. Son implémentation et son évaluation au sein du modèle CHIMERE a été réalisée par confrontation aux données des campagnes estivales et hivernales de la campagne MEGAPOLI. Enfin ces travaux se sont également attachés à implémenter dans CHIMERE et à discuter des mécanismes d'oligomérisation des composés organiques en phase condensée, phénomène qui est aujourd'hui reconnu comme l'un des processus majeur du vieillissement de l'aérosol organique / Nowadays, particular pollution is at the center of many environmental concerns because of its health effects and of its interactions with climate issues. Organic aerosols represent an important part of particular matter. They can be directly emitted as particulate matter or results from the oxidation of many different volatile organic compounds in the atmosphere. Owing to the large variety of its biogenic and anthropogenic emission sources, organic aerosol composition as well as its formation and chemical aging processes are still questioning. Field observations of organic aerosol aging and transport are quite complex to set up. This is why modeling tools remain a necessary complementary approach to describe the formation and long-range transport of the organic fraction of particulate matter. For these reasons, this work focused on the identification and the simulation of some of the main processes controlling the formation and photochemical aging of organic aerosols. Based on an innovating approach, we developed an operational chemical scheme for air quality models dedicated to the simulation of semi-volatile primary compound emission and oxidation. Its implementation and its evaluation in the CHIMERE model have been conducted through a comparison with the summer and winter MEGAPOLI campaign database. In parallel, one objective of this work was to implement oligomerization processes in our model, since it is now recognized that this process is one major pathway for organic aerosol aging

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.

Tropospheric fine particulate matter (PM) poses serious health risks and has a significant impact on global climate change. The measurement of various aspects of PM is challenging due to its complex chemical nature. This dissertation addresses various aspects of PM, including composition, measurement, and visibility. The U.S. Environmental Protection Agency (EPA) proposed a new secondary standard based on visibility in urban areas using 24-h averaged measurements of either light scatter or PM concentration. However shorter averaging times may better represent human perception of visibility. Data from two studies conducted in Lindon, UT, 2012, and Rubidoux, CA, 2003, were used to compare different techniques to estimate visibility, particularly the effect of relative humidity on visibility estimations. Particle composition was measured in Salt Lake City during January-February of 2009. One-hour averaged concentrations of several gas phase and particle phase inorganic species were measured. The results indicate ammonium nitrate averages 40% of the total PM2.5 mass in the absence of inversions and up to 69% during strong inversions. Also, the formation of ammonium nitrate is nitric acid limited, while the formation of ozone appears to be oxidant and volatile organic carbon (VOC) limited. Reduction of NOx will reduce ammonium nitrate secondary particle formation, however, a decrease in NOx may increase ozone concentration.Due to the complexity of PM it is poorly characterized. A large fraction of PM is composed of organic compounds, but these compounds are not regularly monitored due to limitations in current sampling techniques. The GC-MS Organic Aerosol Monitor (OAM) combines a collection device with thermal desorption, gas chromatography and mass spectrometry to quantitatively measure the carbonaceous components of PM on an hourly averaged basis. A compact GC and simple pre-concentrator were developed for the system to decouple separation from manual injection and enhance separation of environmentally-relevant polar organic compounds, such as levoglucosan. The GC-MS OAM is fully automated and has been successfully deployed in the field. It uses a chemically deactivated filter for collection followed by thermal desorption and GC-MS analysis. Laboratory tests show that detection limits range from 0.2 to 3 ng for many atmospherically relevant compounds. The GC-MS OAM was deployed in the field for semi-continuous measurement of the organic markers, levoglucosan, dehydroabietic acid, and several polycyclic aromatic hydrocarbons (PAHs) during winter (January to March), 2015 and 2016. Results illustrate the significance of this monitoring technique to more fully characterize the organic components of PM and identify sources of pollution.

Emissions from natural ecosystems, broadly classified as biogenic volatile organic compounds (BVOCs), contribute 90\% to the VOC budget. Individual BVOCs vary widely in their reaction rates with atmospheric oxidants, making their atmospheric impact highly dependent on VOC composition. Their emissions are also dependent on vegetative make up and a number of meteorological and ecological variables. However, the ecological and physical drivers of their emissions is becoming more variable in a changing climate, leading to greater uncertainties in models. Increasing the monitoring of individual compounds can improve our understanding of the drivers of these emissions and the impact of individual chemical species on atmospheric composition. Improved understanding of BVOC composition can better emission models and, SOA and ozone formation predictions. To study the atmospheric impacts and physical drivers of BVOCs, a GC-FID was adapted for automated hourly sampling and analysis. The details of the hardware and software used for the system are described in detail to enable future long-term BVOC measurements in additional locations. The instrument was deployed at a measurement tower in a forest in central Virginia for year-round collection of BVOC concentrations. Using two years of collected hourly data, this work assesses the chemical impacts of individual BVOCs on time scales ranging from hour to year. This work identifies the importance of both concentration and chemical structure in determining atmospheric impacts. Additionally, seasonality in the concentration of some biogenic species has large implications for atmospheric reactivity in the warmest months of the year, particularly ozone reactivity. Using ecological and meteorological data collected at the site in conjunction with the BVOC data, the drivers of BVOC concentrations and their seasonality are identified. Comparison between this data and current models, reveal important deviations which may lead to large modeled uncertainties. Furthermore, the collected data has been made publicly available to aid in future research regarding BVOCs. / Doctor of Philosophy / The earth hosts a number of sources of atmospheric emissions. These range from human-driven sources such as vehicles and factories, to natural sources such as trees and grass. The content of these emissions, amongst others, become a part of a large reactor (the atmosphere), that interact with each other. The interaction of these emissions with atmospheric oxidants forms a gas (ozone) with implications for human and ecosystem health, and secondary organic aerosol (the leading component to smog). However, the extent to which these emissions react with atmospheric oxidants is largely dependent on the structure of individual compounds. A major focus of this dissertation is to show that compounds with reactive structures can have a large impact on atmospheric composition, and that the quantity of emissions can be as important as compound structure. Understanding the impact of individual compounds in the atmosphere requires improved measurement techniques, capable of detecting the compounds of interest over long time periods. Therefore, another focus of this work was the adaptation and deployment of an instrument capable of detecting some of the most reactive species in the atmosphere, volatile organic compounds emitted from forests. The instrument deployed in this work was a gas chromatography flame ionization detector (GC-FID), which detects compounds largely composed of carbon and hydrogen. The instrument was adapted to run automatically through the development of an electronics box and software program interfaced with the GC-FID. Following development, the instrument was deployed to a remote forest research site for two years. The data collected from this work was used to determine the impact of individual compounds on atmospheric composition. Findings from this work could be used to improve a range of atmospheric models. Small changes in emissions (human or plant) contribute to the total VOC budget which can have large implications for the formation of ozone and SOA. Therefore, increased understanding of the BVOC concentrations and emission driver will aid in predicting these atmospheric components.

This dissertation is a collection of studies that investigates the issue of air pollution in the field of environmental chemistry. My thesis consists of research works done to measure the concentration of particulate matter (PM) and gas-phase species in ambient air. High concentrations of PM is a significant problem in Utah and in other regions of the world. Particles having an aerodynamic diameter of 2.5 micrometers and smaller play a crucial role in air pollution and pose serious health risks when inhaled. PM is composed of both organic and inorganic components. The organic fraction in PM ranges from 10-90% of the total particle mass. Several methods have been employed to measure the organic fraction of PM, but these techniques require extensive laboratory analysis, expensive bench top equipment, and do a poor job of capturing diurnal variations of the concentrations of ambient organic compounds. The Hansen Lab has developed a new instrument called the Organic Aerosol Monitor (OAM) which is based on gas chromatography followed by mass spectrometry detection platform for measuring the carbonaceous component of PM2.5 on an hourly averaged basis. Organic marker data collected in 2016 using the OAM was used in a Positive Matrix Factorization (PMF) analysis to identify the sources of PM in West Valley City, Utah. Additionally, data was collected in Richfield and Vernal, UT in 2017 - 2018 to quantitatively monitor the composition of organic markers of PM2.5. Some previously unidentified organic compounds in PM were successfully identified during this study, including terpenes, polycyclic aromatic hydrocarbons (PAHs), diethyl phthalate, some herbicides, and pesticides. Gas-phase species play a significant role in driving the formation of air pollutants in Earth's atmosphere. Traditional gas detection methods do not provide high temporally and spatially resolved data; therefore, it becomes important to detect and measure gas-phase species both qualitatively and quantitatively to better understand the sources of air pollution. An incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) combines a broadband incoherent light source, a stable optical cavity formed by two highly reflective mirrors and a charged-coupled device (CCD) detector to quantitatively measure the gas-phase compounds present in the atmosphere. The concentrations of formaldehyde (HCHO) were measured using IBBCEAS to investigate the sources of this hydrocarbon in Bountiful, Utah during 2019. Another important species is OH radical. It is one of the most predominant oxidizing species present in the atmosphere. It is found in low concentrations, 0.1 ppt. Detecting concentrations this low is challenging. A new IBBCEAS instrument has been designed and elements of this instrument were tested by measuring the OH overtones in a variety of short chained alcohols. A set of experiments were conducted to measure the absorption cross-sections for the 5th and 6th OH vibrational overtones in a series of short chained alcohols by IBBCEAS. Because OH radical's lowest energy electronic state occurs in the same wavelength region (i.e., 308 nm) that SO2 absorbs (300-310 nm), a study was conducted in which the concentrations of SO2 were measured using an IBBCEAS and compared with a commercially available SO2 monitor.

Recent research has shown that secondary organic aerosols (SOA) are major contributors to ultrafine particle growth to climatically relevant sizes, increasing global cloud condensation nuclei (CCN) concentrations within the continental boundary layer (BL). This thesis contains two separate studies investigating SOA characteristics and the implications of SOA on global climate. The first study investigates two critical, but uncertain, characteristics of SOA: (1) the amount of SOA available to condense and (2) the volatility or condensational behavior of SOA. The second study investigates the effect of biological volatile organic compound (BVOC) emission changes on SOA formation from preindustrial to present day, and the effect on CCN concentrations using BVOC emission estimates over the last millennium.

Mechanistic aspects of carbonaceous aerosol toxicity were examined with respect to the ability of particles to catalyse reactive oxygen species-generating redox cycling reactions. To investigate the role of secondary organic material, we examined two systems. In the first, two-stroke engine exhaust particles were found to increase their ability to catalyse redox cycling in the presence of a reducing agent, dithiothreitol (DTT), when the exhaust was exposed to ozone. This occurred through deposition of redox-active secondary organic aerosol (SOA) onto the particle that was ten times more redox active per microgram than the primary engine particle. In the second system, naphthalene SOA formed highly redox active particles. Activity was strongly correlated to the amount of the 1,4- and 1,2-naphthoquinone measured in the particle phase. However, these species and the newly quantified naphthalene oxidation product 5-hydroxy-1,4-naphthoquinone accounted for only 30% of the observed DTT decay from the particles. Gas-particle partitioning coefficients suggest 1,4- and 1,2-naphthoquinone are not strong contributors to ambient particle redox activity at 25°C. However, a large number of redox active species are unidentified. Some of these may be highly oxidised products of sufficiently low vapour pressure to be atmospherically relevant. DTT activity of diesel particles was found to be high per unit mass. The activity was found to be associated with the insoluble fraction as filtration of the particles nearly eliminated DTT decay. Neither methanol nor dichloromethane extracts of diesel particles exhibited redox activity, indicating that the redox active species are associated with the black carbon portion of the particles. Examination of particle concentration techniques found that use of water condensation to grow and concentrate particles introduced a large organic artefact to the particles. Experiments with concentrated inorganic particles suggest that the source of this artefact is from irreversible uptake of water-soluble volatile organic compounds. Overall, carbonaceous redox active species can be thought of as a continuum from small, water-soluble species to redox active functionalities on elemental carbon backbones. In addition to clearly defined, quantifiable species, future research may need to consider examining broader chemical classes or redox-active chemical functionalities to overcome the inherent complexity of these constituents.

Mechanistic aspects of carbonaceous aerosol toxicity were examined with respect to the ability of particles to catalyse reactive oxygen species-generating redox cycling reactions. To investigate the role of secondary organic material, we examined two systems. In the first, two-stroke engine exhaust particles were found to increase their ability to catalyse redox cycling in the presence of a reducing agent, dithiothreitol (DTT), when the exhaust was exposed to ozone. This occurred through deposition of redox-active secondary organic aerosol (SOA) onto the particle that was ten times more redox active per microgram than the primary engine particle. In the second system, naphthalene SOA formed highly redox active particles. Activity was strongly correlated to the amount of the 1,4- and 1,2-naphthoquinone measured in the particle phase. However, these species and the newly quantified naphthalene oxidation product 5-hydroxy-1,4-naphthoquinone accounted for only 30% of the observed DTT decay from the particles. Gas-particle partitioning coefficients suggest 1,4- and 1,2-naphthoquinone are not strong contributors to ambient particle redox activity at 25°C. However, a large number of redox active species are unidentified. Some of these may be highly oxidised products of sufficiently low vapour pressure to be atmospherically relevant. DTT activity of diesel particles was found to be high per unit mass. The activity was found to be associated with the insoluble fraction as filtration of the particles nearly eliminated DTT decay. Neither methanol nor dichloromethane extracts of diesel particles exhibited redox activity, indicating that the redox active species are associated with the black carbon portion of the particles. Examination of particle concentration techniques found that use of water condensation to grow and concentrate particles introduced a large organic artefact to the particles. Experiments with concentrated inorganic particles suggest that the source of this artefact is from irreversible uptake of water-soluble volatile organic compounds. Overall, carbonaceous redox active species can be thought of as a continuum from small, water-soluble species to redox active functionalities on elemental carbon backbones. In addition to clearly defined, quantifiable species, future research may need to consider examining broader chemical classes or redox-active chemical functionalities to overcome the inherent complexity of these constituents.

Dans ce travail de thèse, des méthodes de simulation numérique ont été utilisées afin de modéliser, à l’échelle moléculaire,l’interaction entre des aérosols carbonés et diverses molécules de l’atmosphère.L’objectif était de caractériser les propriétés physico-chimiques de ces aérosols, qui jouent un rôle majeur dans le climat, en particulier via leur capacité à former des noyaux de condensation pour les molécules d’eau. En premier lieu, les techniques de la dynamique moléculaire ont été appliquées à la détermination de l’organisation d’agrégats d’acide carboxylique sur lesquels sont adsorbées des molécules d’eau. Les résultats montrent une influence à la fois de la température, du taux d’humidité et du type d’acide considéré sur le comportement global des agrégats. Par contre, il a été montré qu’un mélange binaire d’acides n’a que peu d’influence sur ce comportement. En second lieu, des techniques de simulation moléculaires quantiques, à la fois statiques et dynamiques, ont été appliquées à l’étude de la chloration de surfaces de suie, modélisées par de grands HAP, ainsi qu’à la caractérisation de leur affinité pour les molécules d’eau. Les résultats ont montré une forte propension de Cl, Cl2 et HCl à former des HAP chlorés à la faveur de la présence de défauts structurels. Ces structures chlorées présentent une forte hydrophilicité, ce qui pourrait expliquer la nature fortement hygroscopique de suies émises lors d’incendies industriels. / In this PhD work, molecular simulation methods have been used in order to model, at the molecular scale, the interaction between carbonaceous aerosols andvarious atmospheric species. The aim wasto characterize the physico-chemical properties of these aerosols, which play a major role in climate forcing, in particular through their ability to act as cloud condensation nuclei.First, molecular dynamics techniques have been applied to determine the structure of carboxylic acid aggregates on which water molecules are adsorbed. The results of our simulations show an influence of the temperature, of the humidity and of the type of carboxylic acid considered on the global behavior of the aggregates. By contrast,a mixture of different types of acid molecules has no influence on this behavior. Secondly, quantum molecular simulation techniques have been applied to study the chlorination of soot surfaces, modeled by large PAHs, and their behavior with respect to water molecules adsorption. The results show a strong propensity of Cl, Cl2and HCl species to form chlorinated PAHsthanks to the presence of structural defects.These chlorinated structures show as trong hydrophilicity, which may explain the strong hygroscopic nature of soots emitted by industrial fires.