Measurements and parameterization of particulate sulfur dry deposition over grass

Abstract

Eddy correlation measurements of particulate sulfur fluxes at the 1981 and 1982 Dry DepositionIntercomparison Experiments indicate a strong diurnal variation in deposition velocity (downward flux divided by concentration, at a height near 6 m). Most of the run-to-run variability for data collection periods of 30 min seems to be due to unwanted sensor noise. Systematic changes of fluxes wth height due to possible advection from strong point sources of sulfur can occur but should be quite small. A parameterization for deposition velocity is derived in terms of friction velocity u, and the Obukhov stability length scale L. When the parameterization is applied in conjunction with nearly continuous measurements of mean micrometeorological variables such as wind speed and temperature difference, the resulting long-term mean deposition velocity found is 0.22 + 0.06 cm/s with a variation greater than + 50% from day to day, depending on local atmospheric conditions. Peak deposition velocities greater than 0.5 cm/s occur on windy afternoons. Such values are considerably greater than suggested from wind-tunnel and theoretical investigations, but are similar to past results obtained by use of the same techniques over lush vegetation. Boundary layer convective motions that increase wind gustiness near the surface seem to increase particle deposition velocities considerably. Relatively small deposition velocities are found in neutral and stable atmospheric conditions or over surfaces that lack complex fine structure. INTRODUCTIONSulfur in the lower atmosphere exists in particulate and gaseous forms and is deposited on the surface of the earth by both wet and dry processes [e.g., Garland, 1978]. Since the sulfur component exists in substantial quantities as sulfate aerosol above eastern North America [e.g., Hidy et al., 1978], airborne particulate material could provide a significant portion of the sulfur delivered to sensitive receptors if the deposition velocity, or downward flux divided by concentration at a specified height, were large. The value of the deposition velocity averaged over typical meteorological and surface conditions is a matter of debate [e.g., Sehmel, 1980], especially for particles with diameter 0.05-1.0 um where most of the sulfate associated with long-range transport is found in ambient aerosols. For such particles, the rates of Brownian diffusion very close to surface elements, gravitational settling, and deposition by impaction and interception are thought to be small [e.g., Chamberlain, 1975]; mechanisms that provide a means for deposition velocities to exceed 0.1 cm/s have not been explained and widely accepted. However, as suggested by direct eddy correlation measurements of vertical fluxes at heights of 5-10 m above surface [e.g., Hicks et al., 1982], there is the possibility that deposition velocities for ambient particulate sulfur can be considerably larger than 0.1 cm/s in certain field situations.Argonne National Laboratory (ANL) on sulfate particle fluxes measured by eddy correlation. First, m...

Gas-particle interactions above a Dutch heathland: II. Concentrations and surface exchange fluxes of atmospheric particles

“…It is possible that the morphology of heather is comparably efficient in collecting aerosols, but the somewhat large average V d found at Elspeet were probably a consequence of the strong increase in V d during unstable conditions, which often occurred during the warm measurement period. This increase is qualitatively consistent with former observations (Wesely et al, 1985;Vong et al, 2004).…”

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.…”

Size-dependent aerosol deposition velocities during BEARPEX'07

“…particle fluxes (top panel) and for mean values (for data binned into ten equal increments of the RHS of eq.5; bottom panel). Gallagher et al (1997) and Wesely et al, (1985) previously fit this parameterization to their measured aerosol V d for unstable to neutral conditions. While the numerical coefficient (C) in Eq.…”