Abstract
Abstract. In existing particle dry deposition schemes, the effects of gravity and surface roughness elements on particle motion are often poorly represented. In this study, we propose a new scheme to overcome such deficiencies. Particle deposition velocity is a function of aerodynamic, surface-collection and gravitational resistances. In this study, the effect of gravitation settling is treated analytically. More importantly, the new scheme takes into consideration the impacts of roughness elements on turbulent particle diffusion and surface particle collection. A relationship between aerodynamic and surface-collection processes is established by using an analogy between drag partition and deposition-flux partition. The scheme is then tested against a wind-tunnel data set for four different surfaces and a good agreement between the scheme predictions and the observations is found. The sensitivity of the scheme to the input parameters is tested. Important factors which affect particle deposition in different particle size ranges are identified. The scheme shows good capacity for modeling particle deposition over rough surfaces.
Highlights
Particle dry deposition, the removal of particles from the atmosphere to the surface in absence of precipitation (Seinfeld and Pandis, 2006), can be divided into several sub-processes, including turbulence diffusion, surface collection and gravitational settling (Droppo, 2006)
The effects of the sub-processes are represented with the corresponding resistances, i.e., turbulence diffusion, surface collection and gravitational settling are respectively related to the aerodynamic resistance, surface collection resistance and gravitational resistance
The usual approach to estimating deposition velocity is in analogy to electrical circuits: deposition velocity is considered to be the inverse of the deposition resistance which comprises of the contributions of the aerodynamic and surface collection resistances in series and the gravitational resistance in parallel (Hicks et al, 1987; Seinfeld and Pandis, 2006)
Summary
The removal of particles from the atmosphere to the surface in absence of precipitation (Seinfeld and Pandis, 2006), can be divided into several sub-processes, including turbulence diffusion, surface collection and gravitational settling (Droppo, 2006). Slinn (1982) deduced an analytical expression for particle deposition velocity over canopy surface based on the particle concentration equation. In his approach, the gravitational effect was not considered at first, but later added to the result. Kouznetsov and Sofiev (2012) reported a more de- the particle concentration equation with a boundary conditailed scheme for particle dry deposition, but the parameter tion which involves the surface-collection process. As particle concentration is in steady state and hor- where α is a dimensionless coefficient and σ the standard izontally homogeneous, the particle deposition flux, Fd, is vertically constant and obeys the following equation The value of B1 is estimated to be 0.45, based on the wind-tunnel measurements of Zhang et al (2014)
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