Micrometeorological measurements of particle deposition velocities to moorland vegetation

Abstract

SUMMARYSize-segregated particle number ux measurements using a micrometeorological technique, over the diameter (d p ) range 0.1 to 3 ¹m and total particle number uxes (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 ¹m particles to more than 10 mm s ¡1 for 3 ¹m 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 uxes measured with a condensation particle counter are dominated by the high concentration of small particles; therefore, this provides the rst 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 uxes that were frequently observed at the study site need to be included in the computation of average values. At this site, upward uxes are therefore more likely to be an artefact due to statistical constraints than a systematic physical phenomenon.

Eddy correlation measurements of aerosol deposition to grass

“…The EFLAT 0.52 µm diameter value of 0.44 cm s −1 for V dt during unstable conditions is a little larger than values reported by Wesely et al (1983Wesely et al ( , 1985 while the EFLAT value of 0.3 cm s −1 during near-neutral conditions is similar to experimental values reported by Hummelshoj (1994), Gallagher et al (1997), and Nemitz et al (2002) for particles of this size.…”

“…As discussed in section 4.4, uncorrelated noise in the aerosol data (Fig. 4) should not degrade the covariance data (flux) if that noise is sufficiently small (Nemitz et al, 2002). Using the approach of Lenschow and Kristensen (1985), we find that the fluxes for 0.34 and 0.52 µm diameter particles are dominated by atmospheric variability and are therefore, valid.…”

“…Thus, counting noise usually did not significantly affect the 30-min fluxes for 0.34 and 0.52 µm diameter particles but it was an important uncertainty in the fluxes of 0.84 and 1.5 µm diameter particles. During low-concentration periods (NW winds), the counting uncertainties for aerosol deposition velocity averaged 0.22, 0.16, 0.65 and 1.1 cm s −1 (Fairall, 1984;Nemitz et al, 2002) those for the 0.52 µm diameter particles because the 0.34 µm diameter particles were undercounted by the FAST; the PCASPcalibration corrected the FAST 0.34 µm diameter aerosol concentrations but could not change the counting rate. Counting uncertainties for 30-min values of flux of the 0.84 µm diameter particles during EFLAT were of the same magnitude as V dt while the 1.5 µm diameter particles had counting uncertainties that were often larger than the EC measured V dt .…”

“…There are random errors ("white noise") in aerosol fluxes measured by EC due to the discrete nature of the "counts" produced by aerosol instruments such as the FAST. The minimum counting error in aerosol deposition velocity is σ w /N 0.5 , where N is the number of particles counted and σ w is the standard deviation of the vertical wind velocity w for the flux time interval (Fairall, 1984;Nemitz et al, 2002;Buzorius et al, 2003). During EFLAT, the FAST particle counts typically ranged from 2 × 10 3 to 6 × 10 5 per 30 min, depending on the time of day and the particle diameter.…”

“…The noise in particle concentration due to counting is uncorrelated to vertical velocity and, thus, will affect the flux and concentration similarly (Nemitz et al, 2002). One approach to aerosol flux uncertainty due to counting noise is to compare atmospheric and counting variability.…”