Combining biology,chemistry, and meteorology in modeling and measuring dry deposition

The range of chemical, biological, and meteorological processes contributing to the net exchange of trace chemical species between the atmosphere and the underlying surface is examined, in the context of a multiple-resistance exchange model. For those chemical species known to be always depositing, the resistance model provides a means to formulate appropriate deposition velocities in a convenient manner; however, extension to other situations is not straightforward. Field data indicate that the multiple-resistance approach is appropriate for application to assess the dry deposition of ozone and sulfur dioxide, but is not as appropriate for nitrogen dioxide. Data obtained over agricultural crops suggest that canopy factors frequently dominate the overall exchange process. A trial program of dry deposition measurement based on application of parameterized deposition velocities was initiated late in 1984, and has demonstrated shortcomings in under-standing concerning several factors, most importantly the roles of surface emissions and wetness, and the scaling-up of laboratory results to describe vegetative canopies.