Measuring dry deposition: A re-assessment of the state of the art

The exchange process known as dry deposition encompasses the dynamic exchange of trace gases and aerosol particles, and the gravitational settling of large particles. Except for particles large enough that their sedimentation velocity exceeds turbulent velocities, the rate of deposition is mostly determined by surface properties, such as roughness, stickiness, and wetness, and by atmospheric stability. Thus, it is difficult to interpret results obtained using collection devices having surfaces different from those of nature. Other methods for measuring dry deposition exist, mostly micrometeorological, but these are sufficiently complicated that routine application as in a monitoring network is not yet feasible. For some chemical species and in some locations, inferential methods offer considerable promise. These methods measure atmospheric concentrations (C) of the relevant chemical species, and derive relevant deposition velocities (v_d) on a site-specific, species-dependent, and time-evolving basis. The dry deposition rates of interest are then evaluated as the product v_d.C. A major goal of current research programs is to provide the knowledge necessary to evaluate v_d. Experimental methods are reviewed, and potential sources of error are examined, for both the research methods and the “concentration monitoring” methodologies presently being advocated for use in numerical models as well as for routine monitoring.