Dry deposition of particles to canopies-A look back and the road forward Journal of Geophysical Research: Atmospheres

The so-called accumulation-size range of airborne particles is the center of a continuingdisagreement about the formulation of dry deposition. Some contemporary meteorological and air qualitymodels use theoretical developments based on early wind tunnel and other controlled experiments, whileother models consider the bulk properties of the underlying surface and the ability of atmosphericturbulence to deliver particles to it. This dichotomy arose when thefirst micrometeorological measurementsof particle deposition velocities became available, yielding numbers exceeding the highest expectations ofthe then-current models based on assumptions about inertial impaction and interception. The modelpredictions had previously been shown to be in accord with theoretical treatments offiltration. A commonreaction was to distrust thefield experimental results, but the experimentalfindings were supported bysubsequent studies. The difference between model predictions andfield measurements appears greatest fordensely vegetated canopies. Ongoing research is investigating factors that could give rise to the discrepancy,e.g., turbulence intermittency, leaf orientation, leaf morphology, leafflutter, electrical charges, and a numberof phoretic effects. In the meantime, many investigators are faced with a decision as to whether to makeuse of parameterizedfield results or theoretical descriptions of behaviors that are not yet well examined. Herethe history of the ongoing disagreement is reviewed, and some possible resolutions are presented.