Surrogate surfaces

Dasch, J. M Direct Measurement of Dry Deposition to a Polyethylene Bucket and Various Surrogate Surfaces, Environ. Sci. Tech-nol., 19, 721-725 (1985). 

Vandenberg, J. J., and K. R. Knoerr, Comparison of Surrogate Surface Techniques for Estimation of Sulfate Dry Deposition, Atmos. Environ., 19, 627-635 (f985). 

Particle deposition velocities depend on a number of factors, including wind speed, atmospheric stability, relative humidity, particle characteristics (diameter, shape, and density), and receptor surface characteristics. Recent studies on dry particle deposition to surrogate surfaces and derived from atmospheric particle size distributions and micrometeorology suggest that a V equal to about 0.5 cm s 1 is applicable to urban/industrial regions [116-120]. 

In August, 1983, members of the Research Staff of Ford Motor Company carried out a field experiment at two rural sites in southwestern Pennsylvania involving various aspects of the acid deposition phenomenon. This presentation will focus on the wet (rain) deposition during the experiment, as well as the relative importance of wet and dry deposition processes for nitrate and sulfate at the sites. Other aspects of the experiment have been discussed elsewhere the chemistry of dew and its role in acid deposition (1), the dry deposition of HNO3 and SO2 to surrogate surfaces (2), and the role of elemental carbon in light absorption and of the latter in visibility degradation (3). 

Measurement of Dry Deposition onto Surrogate Surfaces A Review... 

A review of dry deposition measurement techniques is presented, focusing on surrogate surfaces. Detailed field data from the literature are available for three types of collectors filter paper with rainshields positioned overhead, Petri dishes, and flat Teflon or polyethylene plates. The data suggest that deposition velocities of submicron particles increase in the order flat plates < Petri dishes < filter paper. For supermicron particles, the order is filter paper < Petri dishes < flat plates. These results are interpreted in terms of the geometry of the collector, surface roughness, and peripheral shielding of the surface. 

One of the simplest and most straightforward methods of measuring dry deposition is with surrogate surfaces. Table 1 identifies the primary disadvantage of this technique deposition on artificial collectors may be quite different from that on natural surfaces of interest Furthermore, it may be difficult to calibrate surrogate surface fluxes with those on natural surfaces, since deposition rates vary with meteorological and surface parameters in... 

Long-term trends in dry deposition onto natural surfaces may be reflected, at least semi-quantitatively, to similar trends measured with surrogate surfaces. 

Certain pollutant species may have fluxes onto natural surfaces which are rate-limited by delivery from the atmosphere, and thus have deposition rates which are relatively independent of surface characteristics. Examples include large particles, which deposit primarily by sedimentation, and nitric acid vapor which is highly reactive with some natural and surrogate surfaces. 

Many specific processes of dry deposition in the field are best studied with surrogate surfaces which can be modified to suit the design of the experiment. Examples include investigations into the effects of obtacle geometry and surface roughness. 

Relating fluxes on surrogate surfaces to those on some natural surfaces may be feasible for certain designs of collectors. Research to assess this possibility is needed (11). 

This paper addresses the use of surrogate surfaces to assess dry deposition of atmospheric pollutants. Several of the designs of artificial... 

Many different designs of surrogate surfaces have been used to measure dry deposition in the field. These include smooth flat surfaces, rough flat surfaces, and collectors with complex geometries. Examples of additional design modifications include application of an adhesive coating to minimize particle resuspension, covering the surface with a film of water to study... 

Table 2 describes many of the surrogate surface designs reported in the literature. The descriptions are listed in order of increasing rim height. Unless otherwise noted, these collectors are generally positioned horizontally, facing... 

The surrogate surfaces summarized in Table 2 have been used to measure dry deposition fluxes for a number of species. In many cases, airborne... 

Results of those studies reporting deposition velocities are shown in Table 3. For three types of surrogate surfaces, a sufficiently large database exists to... 

Table Hit Dry Deposition Data for Surrogate Surfaces (continued)...

Figure 7. Deposition Velocity vs. Particle Aerodynamic Diameter, Type Surrogate Surface Filter Paper (23-28).

This paper has presented a review of the various methods of monitoring dry deposition using surrogate surfaces, and has summarized data obtained using some of these surfaces. The lack of a standardized surface design and standardized field deployment procedures make comparison of the results obtained by the surfaces difficult. Nevertheless, the data show some consistencies deposition velocity increases with increasing MMD of depositing species, and there is a definite pattern when data for filter paper, flat plates, and Petri dishes are compared. 

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