Chemical characteristics of atmospheric aerosols : the influence of natural and man-made sources and sinks
Sammanfattning: In order to improve the understanding of the climatic and environmental effects of atmospheric aerosol particles, knowledge about their sources, sinks, chemical and physical characteristics as well as transportation in the atmosphere is needed. This thesis examines the water-soluble chemical characteristics of aerosol particles and precipitation and their interaction with trace gases. One main objective was to study the influence from man-made sources in comparison to natural sources. Another objective was to study the main causes of depletion and chemical transformation of aerosol particles during transport. Measurements were performed during field experiments in marine areas like over the Indian, Atlantic and Arctic Oceans as well as over the Indian continent.Large differences in the chemical characteristics of aerosols and precipitation were observed when comparing air mainly influenced by continental sources with air influenced by natural marine sources only. Chemical constituents used as tracers for fossil fuel combustion (non-sea-salt sulfate), biomass burning (potassium), soil dust (non-sea-salt calcium) and agriculture (ammonium) all showed to be elevated in air with recent contact with estimated large continental source areas. These constituents were more than a factor of three higher in air being transported from Africa or the Indian subcontinent out over the Atlantic or Indian Ocean, in comparison to clean marine air, despite more than 3 days since last contact with the land. However, in the case of northern Indian Ocean, the intertropical convergence zone limited the spatial spreading of polluted air during the winter monsoon season (February-March).Biogenic gases, like dimethyl sulfide and ammonia, emitted from the ocean surface water were identified as the major source for the submicrometer natural marine aerosol. Non-sea-salt sulfate was the most abundant constituent while the content of methane sulfonate (MSA, only from marine biogenic sources) increased with latitude and available aerosol surface area. The contribution from surface water bubble bursting to the primary sea salt aerosol was found to be dependent on the local wind speed. The bubble bursting process was also found to be a significant source for organic constituents to the atmosphere.
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