Aerosol Exchange between Forests and the Atmosphere : Fluxes over a Tropical and a Boreal Forest

Sammanfattning: The main goal of this thesis was to investigate primary biogenic aerosol emission from the Amazon rain forest through measurements of vertical turbulent aerosol number fluxes. In addition, the particle dry deposition sink has been analysed and quantified, and is also compared with the dry deposition sink at a boreal forest site.The total aerosol number flux of particles with diameter larger than 10 nm was dominated by downward fluxes at the rain forest site, even in the most pristine conditions in the wet season. This is an indication that the primary biogenic aerosol number source is small when considering the total particle size spectrum. However, size resolved aerosol number fluxes indicated net emission for particles with dry diameter 0.5-2.5 ?m in clean conditions. These emission fluxes are likely explained by a primary biogenic aerosol source from the rain forest and seemed to be best correlated with horizontal wind speed, peaking during afternoon. Even though there are few particles in this diameter interval, typically one particle per cm3, they could potentially play an important role as giant nuclei in warm rain initiation.Average particle number based dry deposition velocities over the whole aerosol population were lower at the rain forest site than at the boreal forest site. The reasons are likely the high fraction of accumulation mode particles at the rain forest site and low wind speeds in the tropics compared to the midlatitudes.This thesis provides a relation describing emission of particles with diameter 0.5-2.5 ?m from the rain forest, as a function of wind speed. In addition, linear equations relating average dry deposition velocity of the total aerosol number population to friction velocity are suggested for both the wet and dry season at the rain forest site. Finally, this thesis provides a relationship between dry deposition velocity of particles within the diameter range 0.25-0.45 ?m, friction velocity and particle diameter.