Aerosols are a major source of uncertainty in estimates of anthropogenic effects on global radiative forcing and can pose serious health concerns. While many instrumental techniques capable of analyzing aerosol samples are available, individual-particle spectromicroscopic techniques like the ones presented here are the only ones to offer morphological and compositional measurements together. Studying the composition and mixing state of aerosol populations allowed for important aspects to be uncovered, such as: aerosol source, formation mechanism, hygroscopicity, optical properties, level of aging, and inhalation dangers. Ambient aerosols from the Amazon, both biogenic and anthropogenic, were apportioned based on their individual composition. Recently discovered organic aerosols from the central United States were identified and their chemical properties were characterized. The lead fraction of mixed lead- and zinc-rich particles from Mexico City was speciated to determine the lead’s solubility and possible bioavailability. It is through the use of these powerful spectromicroscopic techniques that a better understanding of complex mixed aerosols was achieved.

Thesis advisor: Paul Davidovits / Aerosol particles are an important component of the Earth-Atmosphere system because of their influence on the radiation budget both directly (through absorption and scattering) and indirectly (through cloud condensation nuclei (CCN) activity). The magnitude of the raditaive forcing attributed to the direct and indirect aerosol effects is highly uncertain, leading to large uncertainties in projections of global climate change. Real-time measurements of aerosol properties are a critical step toward constraining the uncertainties in current global climate modeling and understanding the influence that anthropogenic activities have on the climate. The objective of the work presented in this thesis is to gain a more complete understanding of the atmospheric transformations of aerosol particles and how such transformations influence the direct and indirect radiative effects of the particles. The work focuses on real-time measurements of aerosol particles made with the Aerodyne Aerosol Mass Spectrometer (AMS) developed in collaboration with the Boston College research group. A key feature of the work described is the development of a lightscattering module for the AMS. Here we present the first results obtained with the integrated light scattering – AMS system. The unique and powerful capabilities of this new instrument combination are demonstrated through laboratory experiments and field deployments. Results from two field studies are presented: (1) The Northeast Air Quality Study (NEAQS), in the summer of 2004, conducted at Chebogue Point, Nova Scotia and (2) The Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign conducted in and around Mexico City, Mexico in March of 2006. Both field studies were designed to study the transformations that occur within pollution plumes as they are transported throughout the atmosphere. During the NEAQS campaign, the pollution plume from the Northeastern United States was intercepted as it was transported towards Europe. In this study, particles were highly processed prior to sampling, with residence times of a few days in the atmosphere. The MILAGRO campaign focused on the evolution of the Mexico City plume as it was transported north. During this study, regional and locally emitted particles were measured with residence times varying from minutes to days in the atmosphere. In both studies, the light scattering – AMS system provided detailed information about the density and composition of single particles, leading to important insights into how atmospheric processing transforms the particle properties. In Mexico City, the light scattering-AMS system was used for the first time as a true single particle mass spectrometer and revealed specific details about the atmospheric processing of primary particles from combustion sources.To quantify the radiative effects of the particles on climate, the processing and ultimate fate of primary emissions (often containing black carbon or soot) must be understood. To provide a solid basis for the interpretation of the data obtained during the field studies, experiments were conducted with a well characterized soot generation-sampling system developed by the Boston College research group. The laboratory soot source was combined with the light scattering – AMS system and a Cloud Condensation Nuclei Counter (CCNC) to measure the change in cloud-forming activity of soot particles as they are processed in the atmosphere. Because of the importance of black carbon in the atmosphere, several instruments have been developed to measure black carbon. In July of 2008, an intercomparison study of 18 instruments was conducted in the Boston College laboratory, with soot particles produced and processed to mimic a wide range of atmospherically-relevant conditions. Transformations in the physical, chemical, and optical properties of soot particles were monitored with the combined suite of aerosol instrumentation. Results from the intercomparison study not only calibrated the different instruments used in the study, but also provided critical details about how atmospheric processing influences the radiative effects of primary combustion particles. / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Aerosol particles affect the Earth’s climate, although their impact is associated with large uncertainties. Primary marine aerosol represents a significant fraction of the global aerosol budget considering the Earth’s 70-percentage coverage by oceans. They are produced when bubbles burst at the ocean surface and can consist of sea salt, organic matter and bacteria. An experimental approach was here used to investigate the primary marine aerosol production from the bubble-bursting mechanism using water from four different geographical locations. The main findings include: Similar and stable aerosol number size distributions at all locations, centered close to 0.2 μm. Largely varying aerosol organic fractions, both with size and location. Clear tendency for increased water temperature to negatively influence the aerosol production. No covariance between surface water chlorophyll α and aerosol production on a 10-minute time scale, although decreased aerosol production was observed at times of elevated phytoplankton activity on longer time scales. Mainly external mixtures of sea salt and organics was observed. A high tendency for colony-forming marine bacteria to use bubble-bursting to reach the atmosphere. A clear diurnal cycle in aerosol production was found for both laboratory produced aerosol and in-situ aerosol fluxes, probably biologically driven. The first near coastal sea spray fluxes with limited fetch and low salinity. While the primary marine aerosol spectral shape is stable, emission concentration varies with environmental parameters. Above that, the organic fraction of the aerosol varies largely between locations. This shows that observations of primary marine aerosol emissions not necessarily can be applied to large time- or spatial scales. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: In press. Paper 3: Manuscript. Paper 4: Manuscript.

Lors de leur séjour dans l’atmosphère, les aérosols sont soumis, entre autres, à des processus d’agrégation, ainsi que de condensation sur leurs surfaces. Ces processus, dit de vieillissement, dépendent du temps de résidence des particules dans l’atmosphère, des conditions météorologiques et de l’environnement chimique rencontré. Cette étude vise à caractériser l’aérosol inorganique et étudier son évolution physico-chimique sur quelques dizaines de milliers de mètres, dans les panaches industriels et urbains où les concentrations atmosphériques en particules fines (PM₁₀) sont relativement élevées. Il s’agit notamment de rendre compte de l’évolution des particules d’aérosol primaire lors d’épisodes de formation d’aérosols secondaires inorganiques.Dans ce cadre, dans un premier temps, une nouvelle méthodologie d’analyse des aérosols inorganiques, à basse température, par cryo-microscopie électronique (cryo-TSEM-EDX) a été mise au point. L’enjeu était notamment de rendre compte de l’état de mélange des composés atmosphériques d’origine secondaire (composés semi-volatils), avec l’aérosol primaire. Ces développements analytiques ont tout d’abord été réalisés à l’aide de composés modèles, avant d’être validés sur particules environnementales. Dans un second temps, l’étude des processus physico-chimiques mis en jeu lors du vieillissement des aérosols, à l’échelle locale (quelques kilomètres), a été réalisée au cours d’une campagne intensive de terrain sur le dunkerquois, visant à étudier plus particulièrement l’évolution des émissions industrielles en milieu urbain. Des prélèvements ont ainsi été réalisés en bordure de zone industrielle et sur de sites "récepteurs" sous l’influence potentielle des émissions industrielles. Les analyses réalisées sur ces particules par cryo-SEM-EDX ont notamment montré qu’en zone péri-urbaine, à quelques kilomètres de la zone industrielle, des particules émises par la sidérurgie, comme les oxydes de fer, évoluaient rapidement, pour se retrouver, en mélange interne, associés à de la matière organique particulaire. En parallèle, nous avons pu caractériser, sur ces sites récepteurs, la présence d’aérosols inorganiques secondaires absents de la zone source et donc formés au sein de l’air ambiant, lors du survol de l’agglomération dunkerquoise. / During their transport in the atmosphere, aerosols are subject, for example, to aggregation and condensation processes on their surfaces. These processes, so-called aging, depend on particle residence time in the atmosphere, meteorological conditions and chemical environment. This study aims to characterize inorganic aerosols and to highlight their physico-chemical evolution on a few tens of thousands meters, from an industrial area to the urban environment of Dunkirk (Northern France), in which PM₁₀ concentrations are quite important. It notably includes reporting on the evolution of primary particles during the formation of secondary inorganic aerosols. First, a new analytical methodology of inorganic aerosols, at low temperature, with cryo-electronic microscopy (cryo-TSEM-EDX) has been developed. Our goal was to characterize the mixing state of secondary atmospheric components (semi-volatile components) with primary aerosols. These analytical developments have been realized with model particles, before validation on real atmospheric particles. In a second time, the study of physico-chemical processes involved in the aging of industrial inorganic aerosols has been undertaken through an intensive field campaign. The objective is to describe the particles evolution between the industrial zone and receptor sites located in the suburb of Dunkirk. Our main results show that Fe-rich particles (Fe oxides), released in the atmosphere by steelworks, incorporate particulate organic matter in a few kilometers, between the source and receptor sites. In addition, the formation of secondary inorganic aerosols (SIA), not present at the source, has been evidenced. Clearly, these SIA have been formed during the transport of air masses over the urban area.