What is the fate of nanoscale particles over a forest?

A key determinant of the atmospheric burden and ultimately climate relevance of recently nucleated nano-particles is the rate at which those particles are removed by atmosphere-surface exchange. Given the importance of forests to (i) land surface cover, (ii) particle removal and (iii) both the production and growth of ultrafine particles, there is a specific need to understand removal to, and in, forests. This presentation describes an ongoing experiment in which we are measuring both total number and size-resolved particle fluxes above and below the canopy of a dense deciduous forest and size-resolve particle profiles at multiple levels through the canopy. The total number fluxes are being measured using two Gill 3-D WindMaster Pro sonic anemometers and data from an Ultrafine Condensation Particle Counter (UCPC) operated at 10 Hz. Size-resolved fluxes of particles in the size range 6 to 560 nm are being computed from the same sonic anemometers and a Fast Mobility Particle Sizer (FMPS) operated at 1 Hz. The particle profiles derive from a separate FMPS scanning at three measurement heights across the canopy (top, middle and bottom), and are analyzed to compute penetration efficiencies using the data both at the original discretization of the FMPS and using a modal fitting approach to reduce the influence of electrometer noise. Three methods are being applied to derive the total number and size-resolved fluxes from the UCPC and FMPS respectively; eddy covariance, inertial dissipation and the co-spectral approach. Initial data analysis has focused on periods during the leaf-on season just subsequent to nucleation to ensure high concentrations of the ultra-fine particles. The results indicate that at the upper-most flux sampling level the total number flux concentrations and the size-resolved concentrations derived using the three different approaches show a high degree of accord, but that the eddy-covariance fluxes are generally of smaller magnitude than those derived using the spectral methods. Fewer of the computed fluxes from the below-canopy level are statistically significant even for the total particle number concentrations from the UCPC. The penetration efficiencies from the gradient system indicate the expected, nearly exponential, decrease with increasing particle size, and considerably higher capture efficiencies in the upper-half of the canopy. This presentation will provide details regarding the experimental approach, flux and penetration estimation methodologies and compare and contrast the canopy and ground partitioning of the particle fluxes during leaf-on and leaf-off periods.