A large-eddy simulation study of pollen dispersal from field crops: effects of source size and boundary-layer scaling

The recent development of genetically modified crops and questions about cross-pollination and subsequent contamination of natural plant populations enhanced the importance of understanding wind dispersion of airborne pollen. A critical question to be addressed is how far from the source field pollen grains will be transported.

In order to study pollen dispersion in the atmospheric boundary layer, a framework to simulate pollen dispersal based on the large eddy simulation (LES) technique is developed. Pollen is represented by a continuum concentration field and is evolved following an advection-diffusion equation including a gravitational settling term. Pollen emission and ground deposition are parameterized by the lower boundary condition. The approach is validated against classical data on point-source releases and our own field data for a natural ragweed field.

The LES is further used as a tool to investigate the effect of the size of the source field on the patterns of pollen dispersion and ground deposition, an issue of fundamental importance in the development of policies for genetically modified crops. Simulation results show that the pollen concentration field above the source can be described as a pollen boundary layer. An expression for the pollen boundary layer growth is obtained by extending the usual approach for internal boundary layers to the concentration field of settling particles. Results of numerical simulations are used to validate the proposed expression. In addition, the pollen boundary-layer height at the trailing edge of the field is shown to be the relevant scale to compare dispersion patterns from different source field sizes: the cross-wind integrated deposition and the fraction of pollen that remains airborne at a given distance form the source field are shown to scale with the BL height for different source field sizes. Amongst other results, the scaling yields a simple expression to scale results from small test fields to realistic agricultural conditions. Finally, the use of LES allows quantification of important intermittent deposition events far from the source field. The implications for atmospheric transport of biological particles and particulate matter in general are discussed.3 on 8-6-2009-->