Properties and Origin of Arctic Aerosols

Abstract: The present thesis deals with the origin and physics of aerosols in the Arctic atmosphere. These show a large annual variability due to changes of the photochemical and cloud processes as well as of the synoptic-scale atmospheric pressure patterns. High concentrations of anthropogenic trace gases and particles are found in the atmosphere during winter and spring, whereas the summer period is least affected as regards human impact. The thesis is based on a synthesis of aerosol observations from ground stations as well as research aircraft. A major goal was to study the shift that the Arctic aerosol-size distribution undergoes from spring to summer, a transition that takes place during a rather short period of around 10 days. Six years of aerosol, chemical, and transport data are investigated for the April-June period. This analysis indicates that the rapid transition is governed by a delicate balance between insolation and the source and sink processes affecting the aerosol. In-situ observations show that exchange processes between the boundary layer and the free troposphere may be a key component governing the temporal evolution of the aerosol during summer. It has been concluded that air-borne measurements are essential for establishing the vertical distribution of the aerosol (knowledge of which may be essential when analysing long-term and point measurements). As emphasized in the thesis, insights concerning this vertical structure are especially valuable when layers aloft show concentrations of soot or light-absorbing aerosol and, in addition, the environment is highly reflecting, as is the case in the Arctic. Such plumes, transported from lower latitudes and difficult to detect from the surface, are suggested to have contributed to the high-altitude Arctic warming trend observed during the last two decades. The results in this thesis underline that merging long-term observations with aircraft measurements is highly useful when studying aerosol and its effects.