ATMOSPHERIC DEPOSITION OF CHEMICAL-SPECIES TO POLAR SNOW

Particles and gases can deposit from the atmosphere to polar snow by s everal mechanisms. Dry deposition can be considered to consist of thre e steps: aerodynamic transport from the free atmosphere to the viscous sublayer near the surface, boundary layer transport across the sublay er, and interactions with the surface. The particle dry deposition mas s flux is dominated by the largest particles present in a size distrib ution. Wet deposition includes in-cloud and below-cloud scavenging, wh ere the former refers to uptake of particles during nucleation of clou dwater as well as scavenging of particles and gases by existing drople ts and ice crystals. Of all the wet deposition mechanisms, nucleation scavenging is often the most important mechanism for particles in the polar regions. Finally, incorporation of particles and gases into fog droplets and subsequent settling of the fog to the snow surface can be an important removal process in regions of frequent fog. For Summit, Greenland, the total deposition of MSA, SO42-, Na+, K+, and Ca2+ durin g May 24-July 13, 1993 was dominated by wet deposition: this mechanism accounted for an average of 62% of the total deposition for these spe cies. Fog and dry deposition accounted for 21% and 17% of the total, r espectively. These results suggest that all three mechanisms may need to be considered when estimating total deposition of certain chemical species to polar snow.