Measurements and Large-Eddy Simulations of Particulate Matter Dispersion Over a Vegetative Wind-break

We measure and model dispersion of PM10 aerosols over a vegetative windbreak. Field observations of micrometeorological conditions and PM concentrations using both direct point measurements and lidar-based remote sensing were made at two agricultural facilities. We observed reduction of wind speed and increase in turbulence kinetic energy downwind of the windbreak. However, marked differences in PM concentrations on opposing sides only appeared during nighttime under stable boundary layer conditions. Concentrations during convective boundary conditions were surprisingly low, both upwind and downwind, given the strong source nearby. Observations indicate that large uplift of the dispersing plume already occurs near the source due to heat flux from the facility. Results from a large eddy simulation affirm this uplift of the dispersing plume from the source under fairly low wind conditions, but also indicate a potential for plume uplift at the windbreak if the plume is at low elevations. Both of these were confirmed by lidar observations. These results suggest that gaussian plume models, which cannot resolve interactions with vegetation, can be easily improved in cases where the plume is uplifted near the source. This can be achieved by prescribing a strong heat flux or virtual chimney vertical velocity at the scalar emission point, or by prescribing the emission at a high effective virtual release height. The large-eddy simulations of dispersing scalar plumes from this experiment serve as a foundation for modeling the chemistry and dispersion of Biogenic Volatile Organic Compounds (BVOCs) in and above forest canopies. We are using the Ocean Land Atmosphere Model (OLAM) and appending it to include the option for handling multiple additional scalars, which will be the species of BVOCs in which we are interested. The ultimate goal of this work is to produce a simulation tool that can simulate emission, chemistry and dispersion of BVOCs and their reaction products, focusing on the interaction with small-scale processes inside and above forest canopies, such as light- dependent oxidation of VOC and NO_X reactions, and turbulent mixing and segregation. These processes are important in determining the rate and type of chemical species that will be ejected from the canopy top into the atmospheric boundary layer.