Dry Deposition above Smooth Surfaces - A Numerical Investigation for the Concentration Boundary Layer

Abstract

Objective: Dry deposition velocity towards a surface is commonly investigated by modelling. However, there is still a lack of understanding about the nature of the concentration boundary layer (CBL). Methods: We aimed at acquiring in-depth description of the particle concentration profile within the CBL by investigating the layer height and the concentration profile. The particle concentration, as a solution to the particle flux equation, is obtained and modeled numerically by performing left Riemann sum using MATLAB software. The friction velocity u^* and the particle diameter D_p are the major parameters taken into consideration when characterizing the concentration boundary layer above a surface. The particle concentration profile depends on the friction velocity; the concentration gradient starts from zero at the surface and reaches its maximum in the middle of the layer and then reaches zero again at the top of the boundary layer Results: The concentration profile is slightly altered with a sudden increase in the concentration gradient at the surface when considering large particles or when the friction velocity is has extreme values. Conclusion: The boundary layer height (y+cbl) varied with the particle diameter, and a proper value is 100 to ensure accurate calculations for the dry deposition velocity (diameter 0.01 – 100 µm) above a smooth surface. From a numerical point of view, the numerical setup of the calculation required y+ divisions to be more than 1000 for all particle diameters included in the investigation. In addition, y+max = 104 is important for ultrafine particles (diameter smaller than 0.1 µm). Nevertheless, y+max does not need to be investigated beyond 100 when the friction velocity is below 10 cm/s.