A review of the current status of knowledge on dry deposition

Dry deposition can account for a large portion of the removal of trace chemicals from the troposphere. Resistance schemes used in modeling often perform quite well in daytime conditions over flat terrain. Model results for hilly or mountainous areas, however, are considerably less reliable than those for flat terrain. Even for homogeneous atmospheric and surface conditions and flat terrain, an inadequate model description of surface properties such as vegetative species and soil moisture stress can lead to large differences between modeled and measured fluxes. Third-generation models of mesoscale meteorology and atmospheric chemistry have the potential to achieve several advances, but scaling up of local to regional flux information remains a subject of research. Also, the integrated modeling of gaseous emissions and deposition, which need to be tied together at a low level of model structure, has not yet been accomplished. Many of the processes affecting dry deposition of O-3 over individual types of surfaces an fairly well understood. The role of rapid in-air chemical reactions involving NO, NO2, and O-3 are difficult to quantify comprehensively, and the effects of water from rain or dew on uptake of gases can be highly variable. The influence of lipid solubility on the uptake of organic substances is not well understood. For large bodies of water, the dry deposition rate of most gases appears to be determined largely by water solubility. Parameterizations for the deposition of fine particles tend to be empirical or based on theories untested in natural settings outdoors. Direct measurements of fluxes are required for improved parameterizations for gases and particles and have been made successfully in many past experiments. Micrometeorological approaches have been used extensively, but they are sometimes limited by chemical instrumentation. Long-term flux measurements for diverse terrain and relatively large areas remain difficult. (C) 2000 Elsevier Science Ltd. All rights reserved.