The focus of this work has been to quantify important processes for climatically relevant aerosols, and to improve our understanding of, and ability to accurately model, aerosols in the atmosphere on a large scale. This thesis contains five papers focused on different parts of the life cycle of atmospheric aerosol particles. Two papers describe the physical process of emission of primary marine aerosols. The large uncertainties in these processes are demonstrated by examining the diversity of existing parameterizations for emissions. Building from laboratory experiments to validation of model results with observations, new parameterizations are suggested. These take into account also effects of water temperature on primary marine aerosol production. In the third paper the main focus was to develop a new aerosol wet removal scheme in the Lagrangian transport and dispersion model FLEXPART. Removal timescales and atmospheric concentrations are found to be close to observation based estimates. The final two papers focus on atmospheric black carbon aerosols at high latitudes. As an example of increased human activities in the Arctic, local emissions from cruise ships visiting the research base in Ny Ålesund had demonstrable effects on the level of pollutants measured there. In contrast, inland Antarctic air was shown to be clean compared to the Arctic, due to the extremely long transport time from any major aerosol sources. The work done in this thesis has addressed critical uncertainties regarding the aerosol lifecycle, by better constraining aerosol emissions and atmospheric lifetimes. The development of the new wet removal scheme has improved FLEXPART model accuracy, which will be beneficial in future applications of the model. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.</p>

L'amplitude du forçage radiatif direct par les aérosols dépend d'un grand nombre de facteurs, incluant entre autre leur nature chimique, leurs variations de concentrations temporelles et spatiales, et leur distribution en taille. Les grandes hétérogénéités de ces différents paramètres rendent les études de modélisation de l'impact radiatif délicates. Ces difficultés sont accentuées aux hautes latitudes, avec conditions environnementales particulières et un manque de données d'observation, alors même que les changements climatiques potentiels sont renforcés dans l'Arctique par certains effets de rétroactions, incluant par exemple les changements d'albédo résultants de la fonte des glaces polaires. Dans ce cadre, le programme EAAS (European Arctic Aerosol Study) avait pour objectif l'étude des propriétés physico-chimiques et optiques des aérosols dans l'Arctique européen afin d'en évaluer le forçage radiatif à l'échelle régionale. Cette étude s'est déroulée à Sevettijärvi (69°35' N., 28°50' E., 130 m au-dessus du niveau des mers), en Laponie finlandaise, de Juillet 1997 à Juin 1999. En vue de l'évaluation du forçage radiatif, nous nous sommes ainsi intéressés à la charge des différentes composantes de l'aérosol dans la basse troposphère en prenant en compte l'aérosol carboné (carbone suie et matière organique particulaire) afin de proposer un bilan de masse le plus précis possible. Nous avons ensuite, à des périodes clés de l'année, étudié plus en détail les distributions en taille des composantes de l'aérosol selon le type de masse d'air rencontré. Certaines espèces chimiques présentes au sein de l'aérosol étant issues de précurseurs gazeux, les échanges et les équilibres entre ces deux phases ont été étudiés. Un intérêt particulier a été porté à l'influence de la vapeur d'eau sur les distributions en taille des composantes particulaires. Enfin, nous avons introduit les propriétés optiques des aérosols et les paramètres importants pour une modélisation précise du forçage radiatif à l'échelle régionale.

Atmospheric particles can affect climate directly, by scattering solar radiation, or indirectly, by acting as the seed upon which cloud droplets form. These clouds can then cool the earth's surface by reflecting incoming sunlight. In order to constrain the large uncertainties in predicting the ultimate effect of aerosol on climate, the sources of atmospheric particles and their subsequent ability to turn into cloud droplets needs to be better understood. This thesis addresses two parts of this issue: the sources of Arctic aerosol and the hygroscopicity of continental organic aerosol. Small particles were observed in Baffin Bay during September 2008 that coincided with high atmospheric and ocean surface dimethyl sulphide (DMS) concentrations suggesting that the aerosol formed from oceanic sources. An aerosol microphysics box model confirmed that local DMS could have produced the observed particles. In addition, the particle chemical composition was measured using aerosol mass spectrometry in the central Arctic Ocean in August 2008 and particles were found to be 43% organic and 46% sulphate. Factor analysis further apportioned the aerosol mass to marine biogenic and continental sources 33% and 36% of the time, respectively, with the source of the remaining mass unidentified. The second part of the study parameterises the hygroscopicity of the ambient organic aerosol fraction (κorg) at Egbert, Ontario and Whistler, British Columbia. This was done using two methods: 1) by assuming that the oxygenated organic component was hygroscopic and that the unoxygenated organic component was non-hygroscopic, κ of the oxygenated component was found to be 0.22 ± 0.04, and 2) by assuming that κorg varied linearly with the atomic oxygen to atomic carbon ratio, it could be parameterised as κorg = (0.29 ± 0.05) × (O/C). Calculations predict that knowing κorg is important in urban, semi-urban, and remote locations whenever the inorganic mass fraction is low.

Atmospheric particles can affect climate directly, by scattering solar radiation, or indirectly, by acting as the seed upon which cloud droplets form. These clouds can then cool the earth's surface by reflecting incoming sunlight. In order to constrain the large uncertainties in predicting the ultimate effect of aerosol on climate, the sources of atmospheric particles and their subsequent ability to turn into cloud droplets needs to be better understood. This thesis addresses two parts of this issue: the sources of Arctic aerosol and the hygroscopicity of continental organic aerosol. Small particles were observed in Baffin Bay during September 2008 that coincided with high atmospheric and ocean surface dimethyl sulphide (DMS) concentrations suggesting that the aerosol formed from oceanic sources. An aerosol microphysics box model confirmed that local DMS could have produced the observed particles. In addition, the particle chemical composition was measured using aerosol mass spectrometry in the central Arctic Ocean in August 2008 and particles were found to be 43% organic and 46% sulphate. Factor analysis further apportioned the aerosol mass to marine biogenic and continental sources 33% and 36% of the time, respectively, with the source of the remaining mass unidentified. The second part of the study parameterises the hygroscopicity of the ambient organic aerosol fraction (κorg) at Egbert, Ontario and Whistler, British Columbia. This was done using two methods: 1) by assuming that the oxygenated organic component was hygroscopic and that the unoxygenated organic component was non-hygroscopic, κ of the oxygenated component was found to be 0.22 ± 0.04, and 2) by assuming that κorg varied linearly with the atomic oxygen to atomic carbon ratio, it could be parameterised as κorg = (0.29 ± 0.05) × (O/C). Calculations predict that knowing κorg is important in urban, semi-urban, and remote locations whenever the inorganic mass fraction is low.