A review of measurement and modelling results of particle atmosphere–surface exchange

Abstract

A B S T R A C T Atmosphere-surface exchange represents one mechanism by which atmospheric particle mass and number size distributions are modified. Deposition velocities (v d ) exhibit a pronounced dependence on surface type, due in part to turbulence structure (as manifest in friction velocity), with minima of approximately 0.01 and 0.2 cm s −1 over grasslands and 0.1-1 cm s −1 over forests. However, as noted over 20 yr ago, observations over forests generally do not support the pronounced minimum of deposition velocity (v d ) for particle diameters of 0.1-2 μm as manifest in theoretical predictions. Closer agreement between models and observations is found over less-rough surfaces though those data also imply substantially higher surface collection efficiencies than were originally proposed and are manifest in current models. We review theorized dependencies for particle fluxes, describe and critique model approaches and innovations in experimental approaches, and synthesize common conclusions of experimental and modelling studies. We end by proposing a number of research avenues that should be pursued in to facilitate further insights and development of improved numerical models of atmospheric particles.

Particle concentration and flux dynamics in the atmospheric boundary layer as the indicator of formation mechanism

“…Dividing the total flux by the particle concentration above the canopy we can compare the results with observed deposition velocities. In the simulations the median normalised deposition velocity v d /u * is 8 × 10 −3 for 10 nm particles and the minimum 6 × 10 −4 is achieved Pryor et al, 2008).…”

Size-dependent aerosol deposition velocities during BEARPEX'07

“…It was found that all 30 min BEARPEX data had a result that V ds ≥0.93 V d . Since V d ≈V ds , measurements of accumulation mode aerosol V d ought to display a diameter dependence that depends primarily on impaction and interception processes that occur near the needle surface (Pryor et al, 2008;Gallagher et al, 1997). Particle inertial impaction ought to depend on aerosol diameter because larger accumulation mode particles that move at the fluid velocity (as assumed in EC) have more momentum than smaller particles in the same fluid parcel for a constant particle density.…”

“…Both the transport of accumulation mode particles by turbulence through the atmospheric surface layer and their subsequent removal by inertial impaction ought to depend on friction velocity (Pryor et al, 2008;Gallagher et al, 1997). As shown in Fig.…”

“…However, rates and mechanisms for the removal of accumulation mode particles by turbulence are not well known nor is the dependence of particle deposition velocity (V d ) on diameter (Pryor et al, 2008;Seinfeld and Pandis, 1998). This study particularly addresses particle fluxes to a second growth forest which is typical of those that have been planted across the western United States.…”

Aerosol flux measurements above a mixed forest at Borden, Ontario

“…Petroff et al, 2008). In a review of particle atmosphere-surface exchange, Pryor et al (2008b) conclude that: few studies have sought to quantify the dependence of flux on particle size; observations over forests do not support the theoretical minimum depositions rate manifest in models; and that more research is required to determine if dry deposition of aerosols is an important removal mechanism for ultrafine particles, especially over surfaces with high roughness, such as forests. There have been a number of studies which have measured size-resolved aerosol fluxes over forests.…”