Dry deposition of particles to canopies—A look back and the road forward

Abstract

The so‐called accumulation‐size range of airborne particles is the center of a continuing disagreement about the formulation of dry deposition. Some contemporary meteorological and air quality models use theoretical developments based on early wind tunnel and other controlled experiments, while other models consider the bulk properties of the underlying surface and the ability of atmospheric turbulence to deliver particles to it. This dichotomy arose when the first micrometeorological measurements of particle deposition velocities became available, yielding numbers exceeding the highest expectations of the then‐current models based on assumptions about inertial impaction and interception. The model predictions had previously been shown to be in accord with theoretical treatments of filtration. A common reaction was to distrust the field experimental results, but the experimental findings were supported by subsequent studies. The difference between model predictions and field measurements appears greatest for densely vegetated canopies. Ongoing research is investigating factors that could give rise to the discrepancy, e.g., turbulence intermittency, leaf orientation, leaf morphology, leaf flutter, electrical charges, and a number of phoretic effects. In the meantime, many investigators are faced with a decision as to whether to make use of parameterized field results or theoretical descriptions of behaviors that are not yet well examined. Here the history of the ongoing disagreement is reviewed, and some possible resolutions are presented.

The particle dry deposition component of total deposition from air quality models: right, wrong or uncertain?

“…Several studies have noted over recent years that the algorithms describing the size-dependent dry deposition of atmospheric particles over vegetative canopies do not agree very well with measurements, especially in the accumulation mode, and particularly for canopies with high surface roughness (Zhang and Vet, 2006;Petroff et al, 2008a;Pryor et al, 2008a). Recently, a review by Hicks et al (2016) surveyed the historical development of both the algorithms and measurements of this phenomenon, noting that measurements as far back as 1977 suggested a pronounced difference between measurements and modelderived expectations. Pryor et al (2008a) provided a thorough summary of potential explanations for the discrepancies, ranging from observational errors, to chemical RELATIONSHIP BETWEEN TURBULENT ENERGY DISSIPATION AND GAS TRANSFER 5 flux divergences, faulty model assumptions, or the neglect of important deposition processes (e.g.…”

Evaluation and Intercomparison of Five North American Dry Deposition Algorithms at a Mixed Forest Site

“…However, dry deposition estimates for these species still have large uncertainties due to the many chemical, biological, and meteorological factors affecting dry deposition processes. Uncertainties for other chemical species are even larger (Fowler et al, 2009;Hicks et al, 2016;Mohan, 2016;Wesely & Hicks, 2000;Wright et al, 2016).…”

Impacts of Aerosol Dry Deposition on Black Carbon Spatial Distributions and Radiative Effects in the Community Atmosphere Model CAM5

“…Large uncertainties exist in modeled V d for fine particles (diameter around 0.1–1.0 μm) based on the comparisons of several dry deposition schemes (Hicks et al, ; Petroff et al, ; Pryor et al, ; Ruijgrok et al, ; Zhang & Vet, ), which can vary over 2 orders of magnitude. Previous studies that used wind tunnel measurements to optimize model simulations suggested that particles in the accumulation mode should have V d around 0.01 cm s −1 (e.g., Giorgi, ; Slinn, ), whereas studies based on field observations suggested much higher V d values (e.g., Allen et al, ; Gallagher et al, ; Grönholm et al, ; Sievering, ; Wesely et al, ).…”