Deposition of Trace Metal-Containing Aerosols on Smooth, Flat Surfaces and on Wild Oat Grass (Avena fatua)

Author: Davidson, Cliff Ian

Year: 1977

Degree: Dissertation (Ph.D.)

Advisor: Friedlander, Sheldon K.

Committee Member: Unknown, Unknown

Option: Environmental Science and Engineering; Electrical Engineering

DOI: 10.7907/cb91-ae70

Abstract

A new model for the dry deposition of aerosol particles on fields of vegetation has been developed. The model, which includes diffusion, sedimentation, and impaction as transport mechanisms, has been applied to deposition on a field of wild oat grass, Avena fatua. The model was tested experimentally for the diffusion range, and applied to predict the deposition of lead for all mechanisms for typical wind conditions. Inputs to the model include the airborne chemical species distribution with respect to particle size, the windfield above and within the canopy, and details of the vegetation elements.

Deposition is considered to be a two-step process, namely transport from the open atmosphere into the vegetation followed by filtration by the vegetation elements. Each step is described by an expression for particle flux. The equation for transport from the atmosphere relates the flux to the sedimentation velocity and the Brownian and eddy diffusivities. This relation is assumed to apply both above and within the canopy. The filtration equation is used only within the canopy, incorporating either convective diffusion or impaction. The two equations are solved simultaneously for the effective concentration sink and particle flux.

According to the model, the deposition of unit density, spherical particles with diameters smaller than 0.001 μm and larger than 10 μm is controlled by the rate at which particles are delivered to the canopy. For particles with diameters between these values, the rate of deposition is control led by relatively inefficient filtration within the canopy.

The model was tested by measuring profiles of condensation nuclei above a field of Avena fatua. The atmospheric transport equation was used with the profiles to determine the flux. This value compared well with fluxes calculated by solving the equations for atmospheric transport and filtration simultaneously. The latter calculation used only one concentration measurement at a known height rather than the entire profile.

The deposition of lead, zinc, and cadmium on flat, smooth surfaces exposed to the free atmosphere was also measured. Simultaneous isokinetic measurements of the airborne size distributions of these trace metals provided data which were used to calculate the sedimentation fluxes. About 70% of the total mass deposition of each trace metal was due to sedimentation of particles with aerodynamic diameter greater than 10 μm. The airborne fractions of these large particles for lead, zinc, and cadmium were 17%, 32'%, and 37%, respectively. Because large airborne particles control led the mass deposition, isokinetic sampling was essential.

The isokinetically measured distribution of lead with respect to particle size was used to calculate deposition on a field of Avena fatua. Ninety percent of the lead deposition resulted from impaction of particles greater than 10 μm. As with the flat, smooth surfaces, the relatively high concentrations of small airborne lead particles did not contribute significantly to mass deposition.

Comparing the smooth surface data with the calculations for Avena fatua fields suggested that the deposition of lead on such fields is two to three times greater than on a flat, smooth surface.

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