Large-Eddy Simulation of Particle Dispersion above and Downstream of Area Sources in the Unstable Atmospheric Boundary Layer

Models of particle dispersion from area sources (e.g., deserts, forests, and agricultural crops) are essential to estimate the atmospheric concentrations and transport distances of mineral aerosols (e.g., sand and dust), biological aerosols (e.g., seeds, pollens, and spores), and materials that can adhere to suspended particles (e.g., toxic chemicals, radionuclides, viruses, and bacteria). Chamecki and Meneveau (2010) have obtained analytical solutions of the mean particle concentration field, the height of the particle concentration plume, and the total amount of particles transported downstream of area sources in the neutrally stratified atmosphere using boundary-layer approximations. Results are consistent with numerical simulations using a three-dimensional (3-D) pollen dispersion model based on large-eddy simulation (LES) technique (Chamecki et al., 2009). In this work the numerical results obtained by Chamecki and Meneveau (2010) are further extended to unstable atmospheric stratifications. The LES model for pollen dispersion is modified to account for the effects of atmospheric stability on the pollen boundary conditions using the vertical concentration profiles obtained by Chamecki et al. (2007). Numerical experiments are conducted for area sources of different sizes under a range of atmospheric conditions (characterized by z_i/L). Results are compared to numerical simulations and theoretical results for neutral conditions. In particular, the effects of atmospheric stability on the growth of the particle plume and the ground deposition downstream of the source field are investigated. In addition, theoretical results are extended to account for effects of stability, generalizing the results of Chamecki and Meneveau (2010).