Size-Resolved Particle Fluxes and Vertical Gradients over and in a Sparse Pine Forest

Abstract

Although a number of flux data sets indicate apparent upward fluxes of particles over forests, the causes of such fluxes remain only partially understood. Using data collected during the 2011 BEACHON experiment, it is shown that over one third of fluxes of both sub- and super-30-nm diameter particles over this sparse pine forest are upward. Quadrant analysis, timescale analysis using a particle dynamics model, and frequency power spectra of particle concentrations demonstrate that, in this environment, the morning upward fluxes tend to be associated with downward “sweeps” of particle depleted air during break down of the nocturnal inversion, and it is the most common mechanism resulting in upward fluxes of particle size distributions with diameters above 30 nm. Upward fluxes of particles later in the day are more strongly linked to “ejections” of particle enriched air from the canopy that are attributable to growth of fairly recently nucleated particles by both addition of oxidation products of biogenic volatile organic compounds and coagulation. This mechanism appears to dominate upward fluxes of sub-30-nm particles, although the resulting destabilization of the particle size distribution can result in upward fluxes of larger particles. Vertical gradients of particle size distribution above, through, and below the canopy are also analyzed to investigate the size dependence of canopy uptake of particles and indicate that, in accord with wind tunnel analyses, penetration efficiencies are lower for smaller geometric mean diameters (∼15–20 nm) and increase with diameter up to approx. 80 nm (the largest diameter considered here). Copyright 2013 American Association for Aerosol Research