Comparison between knife-edge and frisbee-shaped surrogate surfaces for making dry deposition measurements: Wind tunnel experiments and computational fluid dynamics (CFD) modeling

Dry deposition is a major pathway for atmospheric contaminant movement from the atmosphere to the earth surface. Despite its importance, there is no generally accepted direct method to measure dry deposition. Recently, the interest in using surrogate surfaces to measure dry deposition is growing, primarily because of their ease of use. However, a problem with these surfaces is extrapolating the results obtained to natural surfaces. There are two popular surrogate plates used to measure dry deposition. One had a sharp leading edge (knife-edge) (KSS), and the other has a smooth-edge (frisbee-shaped) (FSS). In this study, the performances of these two surrogate surfaces to directly measure gas dry deposition were explored using wind tunnel experiments and two-dimensional (2D) computational fluid dynamic (CFD) models. Although the fluid fields above these two plates were different, both created laminar boundary layers (distance above the surface where the velocity gradient is constant) with a constant thickness after approximately five cm. In the wind tunnel, gaseous elemental mercury (GEM) deposition to gold-coated filters was used to measure deposition velocities (Vd) in part because for this combination deposition is air-side controlled. The GEM Vd to both surfaces increased with increasing wind speeds. Based on both measurements and CFD simulations, the Vds to the FSS were approximately 30% higher and more variable than to the KSS when the wind flow was parallel to the surfaces. However, when the angle between the surfaces and the wind was varied the Vds to the FSS were less dependent on the incident angle than to the KSS.

► Knife-edge (KSS) and frisbee-shaped (FSS) surrogate surfaces were tested. ► The boundary layer height was constant starting 5 cm from the leading edges. ► GEM dry deposition rates were more variable and higher to the FSS than to the KSS. ► GEM dry deposition rates increased when flow was not parallel to the surfaces. ► The reaction constant between Hg(aq) and BrCl(aq) was 6.7 ± 2.3 × 105 M−1 s−1.