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

Abstract

[1] 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 sizeresolved 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 (v d + ) 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 v d + 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 sizeresolved 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.

Aerosol flux measurements above a mixed forest at Borden, Ontario

“…In the 2009 study, size-resolved flux measurements were made with an FMPS. To the authors' knowledge, there has been only one study of size resolved fluxes over forests using EC with an FMPS (Pryor et al, 2009). The results of this study demonstrate a predominance of apparent upward fluxes at this location, allowing for further exploration of the root cause of this phenomenon.…”

“…The majority of these studies measured aerosol fluxes using the eddy covariance (EC) technique with Condensational Particle Counters (CPCs), which measure the total particle number concentration over a particle size range typically between 10 nm and 1 µm. Size-resolved fluxes have been measured using optical particle counters (Gallagher et al, 1997;Ahlm et al, 2010), relaxed Eddy Accumulation (REA) with a Differential Mobility Particle Sizer (DMPS; Grönholm et al, 2007), and EC with a Fast Mobility Particle Sizer (FMPS; Pryor et al, 2009). Lamaud et al (1994) found that particle deposition velocity (V d ) is related to friction velocity (u * ) and atmospheric stability.…”

“…There have been a number of studies which have measured size-resolved aerosol fluxes over forests. Forest types and locations include: mixed deciduous in eastern Tennessee (Hicks et al, 1989) and southern Indiana, USA (Pryor et al, 2009); pine in Hyytiälä, Finland (e.g. Buzorius et al, 1998;Grönholm et al, 2007); maritime pine in Les Landes, France (Lamaud et al, 1994); douglas fir in Northern Holland (Gallagher et al, 1997); spruce in Germany, at Solling (Bleyl, 2001) and near Münchberg (Held and Klemm, 2006); beech in Soro, Denmark (Pryor, 2006); and Amazonian rain forest near Manaus, Brazil (Ahlm et al, 2010).…”

Evidence of an elevated source of nucleation based on model simulations and data from the NIFTy experiment

“…Within the original UHMA model dry deposition processes are described by a semiempirical parameterization validated for the boreal forest in Hyytiälä, Finland (Rannik et al, 2003). Deposition velocities over deciduous forests have been shown to be lower than those measured over boreal forests (Pryor et al, 2009), thus we implemented a physical parameterization of dry deposition (Slinn, 1982;Pryor and Binkowski, 2004) constrained to match observed sizeresolved particle deposition velocities of sub-100 nm diameter particles as measured at the MMSF site (Pryor et al, 2009). No removal by wet deposition was parameterized.…”

Environmental Impacts—Atmospheric Chemistry

“…sub-micron) (e.g. Pryor et al 2009) rather than aerosol particles in sizes that may dominate the chemical flux. Those recent studies have tended to indicate a very strong influence of vegetation canopy morphology and aerosol properties on deposition velocities, and thus, they explain-at least to some degree-the large variability in measurement data sets of particle number fluxes to vegetated surfaces taken under superficially similar atmospheric conditions ).…”