Particle boundary layer above and downstream of an area source: Scaling, simulations, and pollen transport

Dispersion of small particles emitted from an area source at the surface into a fully developed high-Reynolds-number boundary layer flow is studied as a theoretical model for pollen dispersion in the neutral atmospheric boundary layer. The particle plume above the area source is assumed to behave as a particle concentration boundary layer. Boundary layer scaling and the assumption of self-preservation lead to an analytical solution in the form of a similarity function that has an additional dependence on the ratio of gravitational settling and turbulent diffusion velocities. Similar arguments are used to predict patterns of deposition onto the surface downstream of the source. Theoretical predictions are tested using a suite of large-eddy-simulation numerical experiments, with good agreement. The combined analysis of theoretical and numerical results reveals interesting features in the patterns of downstream deposition, such as non-monotonic trends in isolation distance with particle settling velocity and surprisingly large isolation distances for practically relevant parameter ranges. Possible effects of turbulence on effective settling velocity are highlighted.