Direct aerosol radiative forcing is modulated by many factors, including the chemical nature of the components and their size distribution, and the variability of concentrations with time and space. Estimation of the forcing is therefore difficult, particularly in the high latitudes where few field data are available while the environmental conditions are significantly different from that in other regions. Further, the high northern latitudes are very sensitive to climate change, with potentially several positive feedback loops in the climate system, including the change in surface albedo related to the decrease of surfaces covered with snow and ice. In this context, the EAAS program (European Arctic Aerosol Study) has been devoted to the study of aerosol physical, chemical and optical properties in order to model radiative forcing at regional scale. This study took place at Sevettijärvi (69°35' N., 28°50' E., 130 m asl), Finnish Lapland, from July 1997 to June 1999. In order to evaluate radiative forcing, we focused on lower tropospheric burden of aerosol components, taking into account carbonaceous species (black carbon and particulate organic matter) for drawing a more accurate aerosol mass balance depending on seasons and type of air masses. At key seasons, we studied more precisely the size distribution of the chemical constituents depending on the air mass origin. Since many chemical species in the particulate phase originate from gaseous precursors, exchanges and equilibriums on these two phases have been studied. The emphasis has been put on water vapour influence on particulate components size distributions. Finally, we introduced aerosol optical properties and important parameters for a more accurate modelling of the radiative forcing at a regional scale.
