The contributions of wet, fog and dry deposition to the summer SO4 2- flux at Summit, Greenland

Abstract : Experiments were performed during May-July of the 1993 field season at Summit, Greenland. Real time concentrations of particles greater than 0.5 mm and greater than 0.01 µm were measured with continuous monitors. Filter samplers were used to determine the daily average aerosol SO42- concentrations, and impactors were used to determine mass size distributions. Dry deposition velocities for SO42- were estimated using surrogate surfaces (symmetric airfoils) and the airborne size distribution data. Snow and fog samples from nearly all of the events occurring during the field season were collected on polyethylene trays. Impactor and real time concentration data indicate that particles > 0.5 µm efficiently serve as nuclei to form fog droplets. Results also show that condensation nuclei > 0.01 µm (CN) are not as greatly affected by fog. Dry deposition velocity estimates using the airfoils are in the range 0.023 cm/s to 0.062 cm/s, 60% greater than values calculated using the airborne size distribution data with a model for deposition to snow. This could be due to differences in the boundary layer resistances of the airfoils and the modeled snow surface; furthermore, calculations using the impactor results assume no particle growth in the viscous sub layer. The contribution of wet, fog, and aerosol dry deposition to the seasonal SO42- inventory is estimated as 58% ± 6%, 25% ± 4%, and 17% ± 7%, respectively. These values do not take into consideration the spatial variability caused by the blowing and drifting of surface snow. Results indicate that all three processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.