Size‐resolved fluxes of sub‐100‐nm particles over forests

Abstract

Dry deposition of atmospheric particles is critically dependent on particle size and plays a key role in dictating the mass and number distributions of atmospheric particles. However, modeling dry deposition is constrained by a lack of understanding of controlling dependencies and accurate size‐resolved observations. We present size‐resolved particle number fluxes for sub‐100‐nm particle diameters (Dp) over a deciduous forest derived using eddy covariance applied to data from a fast mobility particle sizer. The size‐resolved particle number fluxes in 18 diameters between 8 and 100 nm were collected during leaf‐on and are statistically robust. Particle deposition velocities normalized by friction velocity (vd+) are approximately four times smaller than comparable values for coniferous forests reported elsewhere. Comparison of the data with output from a new one‐dimensional mechanistic particle deposition model designed for broadleaf forest exhibits greater accord with the measurements than two previous analytical models, but modeled vd+ underestimate observed values by at least a factor of two for all Dp between 6 and 100 nm. When size‐resolved particle deposition velocities for Dp < 100 nm are normalized by friction velocity, the key controlling role of particle diffusivity is strongly manifest. On the basis of analyses of these new measurements and recently published size‐resolved particle number fluxes from a conifer forest, we present working parameterizations for size‐resolved particle deposition velocities over forests that could reasonably be applied in regional and global atmospheric chemistry transport models.