Micrometeorological measurements of particle deposition velocities to moorland vegetation

Size-segregated particle number flux measurements using a micrometeorological technique, over the diameter (d(p)) range 0.1 to 3 mum and total particle number fluxes (d(p) > 11 nm) are reported for Scottish moorland vegetation. Mean particle deposition velocities (V-d) range from 0.3 mm s(-1) for 0.1 mum particles to more than 10 mm s(-1) for 3 mum particles. On average, the measured size-dependence of V-d is almost identical with the prediction by the widely used Slinn model, using the original parameters derived from wind-tunnel studies. Within each size-band V-d increased linearly with the friction velocity (u(*)). Total particle number fluxes measured with a condensation particle counter are dominated by the high concentration of small particles; therefore, this provides the first estimate of V-d for particles smaller than those that can be sized by optical techniques, for short vegetation. Values of V-d ranged from 0.2 to 0.6 mm s(-1) as u* increased from 0.15 to 0.55 m s(-1). These deposition rates are one order of magnitude smaller than have been observed for a pine forest stand. The analysis shows that, at least for super-micron particles, the emission fluxes that were frequently observed at the study site need to be included in the computation of average values. At this site, upward fluxes are therefore more likely to be an artefact due to statistical constraints than a systematic physical phenomenon.