Dry Deposition of Particles to Vegetative Canopies

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Three transport processes of fine particles need to be considered when modeling the dry deposition of fine particles to vegetative canopies: the aerodynamic transport, boundary layer transport with collection of particles by canopy elements, and surface interactions including particle rebound. The aerodynamic transport of fine particles occurs by turbulent diffusion and gravitational sedimentation with minor influence of Brownian diffusion. The turbulent diffusion coefficient of a given chemical above the vegetation canopy is assumed to be the same as the kinetic eddy viscosity of the air.

The transport due to sedimentation by gravity for particles of sizes typically found in the atmosphere is estimated by taking into account the Stokes settling velocity. This transport is proportional to the particle density which is assumed to be much larger than the density of the air, thus the buoyant force can be neglected. The particle flux due to settling is then estimated as the sedimentation velocity multiplied by the particle concentration.

The Brownian diffusivity is proportional to the Boltzmann constant and the air temperature and also depends on the size of the particles and the dynamic viscosity of the air. The aerodynamic diameter is often used for describing the size of real particles which are often nonspherical and with different densities. The aerodynamic diameter is defined as the diameter of a spherical, unit density particle with the same motion characteristics as the actual particle.

The wind and particle concentration profiles are needed for proper modeling and analysis of dry deposition fluxes and dry deposition velocities. The velocity profiles above the canopy are estimated by analyzing momentum transport dependent on the wind speed, roughness height, and the height of canopy. The shape of the wind and concentration profiles can be used to identify the region with the greatest resistance to air momentum flux and concentration flux, respectively.

The transport of particles through the vegetative canopies is governed by the wind speed and particle concentrations. The collection efficiency within the canopy depends on the area available for the collection and the efficiency of the mechanisms which deposit the particle on the receptor. A few models have been developed to describe the deposition velocity to the vegetative surfaces with different approaches to estimate the collection area and collection efficiency of particles within the canopies. Some of these models use a Gaussian distribution of foliage, others apply a leaf area index. The ranges of deposition velocities obtained from various models are presented in Figure 3.

The collection efficiency for particles within various canopies depends also on the transport mechanisms across the viscous sublayer, including Brownian diffusion, interception, and impaction. Thus, the collection efficiency for Brownian diffusion depends on the diffusion coefficient, mentioned earlier in this article, for interception of the ratio of particle to receptor size, and for impaction on the wind speed and specific characteristics of the particle and surface.

In some cases, the kinetic energy of the particle after collection is larger than the surface attraction forces and then the bounce-off occurs. This is particularly the case for energetic particles driven by turbulent fluctuations. The bounce-off process increases with the increase of either wind speed or particle size. Some approaches have been made to assess the critical rebound velocity.

Figure 3 Deposition velocities from the models of Davidson et al., Slinn, Bache, and Wiman and Agren. Reproduced with permission from Atmospheric Particles, Harrison RM and van Grieken R (eds.), Copyright 1998, © John Wiley & Sons Limited.

Particles can also be transported in the air by turbulent bursts which can be eddies from the free atmosphere or eddies created due to surface roughness. The rate of particle transport by turbulent bursts is comparable with the rate of aerodynamic transport in the region and is not affected by the viscous sublayer. Dry deposition of small particles is much more affected by the turbulent bursts than the deposition of coarse particles.

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