Submicron Particle Number Fluxes and Size Distributions above a Loblolly Pine Forest in the Southeast U.S.

John T. Walker*
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
Research Triangle Park, NC

Representative parameterizations of particle dry deposition are required for accurate air quality model prediction of particulate matter concentrations. While several models of size-specific particle deposition velocity are available, observations vary significantly across vegetation types within size categories. Additional data are needed to further test and refine these models for surface types and chemical environments not previously characterized. We present bulk particle number fluxes measured above a Loblolly pine canopy (Forest-Atmosphere Carbon Transfer and Storage site, Chapel Hill, NC) using the eddy covariance technique. Size-specific deposition velocities for particles between 20 and 70 nm aerodynamic diameter (Dp) are inferred by combining bulk number fluxes (3 nm 3 m Dp) and particle size distributions (10 400 nm). Particle number concentrations and size distributions are controlled by primary emissions from local mobile sources and nucleation events. Particle number fluxes are largely controlled by turbulent mixing and deposition velocities are within the range of values observed over other forests. Deposition velocity and particle size vary inversely between 20 70 nm Dp, indicating the importance of the boundary layer resistance (Rb), particularly for the smallest particles at low friction velocities.