Scavenging ratio

Scavenging of particles or gases may take place in clouds (rainout) by cloud droplets or below clouds(washout) by precipitation. A scavenging ratio or washout ratio W can be defined as... 

For wet deposition, it is assumed that the rain scavenges Q (the scavenging ratio) or about 200,000 times its volume of air. Using a particle concentration (volume fraction) vQ of 2 x 10 n, this corresponds to the removal of Qvq or 4 x 1CF6 volumes of aerosol per volume of rain. The total rate of particle removal by wet deposition is then QvqUrAw m3/h, thus the wet transport velocity QvqUr is 4 x 1(T10 m/h. 

Rain Scavenging Ratios (RS) and Percent Washout Due to Particulates (%W) for CDDs and CDFs in Ambient Air in Two Midwest Cities... 

Tucker, W.A., Lyman, W.J., Preston, A.L. (1983) Estimation of the dry deposition velocity and scavenging ratio for organic chemicals. In Precipitation Scavenging, Dry Deposition, and Resuspension. Pruppacher, et al., Editors, pp. 1242-1256, Elsevier Science Publishing Co., New York. 

Scavenging Ratios for SO/, and NOj by Rain. If it is assumed that the concentration of a pollutant in precipitation is dependent on its concentration in the air in which the precipitation forms, then the scavenging ratio, W, can be defined as Wi = Cir/Cia... 

These results are similar to daily-averaged values reported by Barrie (18) for sites in eastern Canada (1978-1981). His mass scavenging ratios, multiplied by 890 (the ratio between the density of water and that of air at the 838-m altitude of Allegheny Mountain in order to match units), give the following averages for 4 remote locations ... 

It is also possible that snow scavenges particulate S04-- less efficiently than rain, but this cannot be determined from this data set. However, indications from the literature suggest that the reverse is true. Knutson et al. (9) reviewed several studies showing that snow scavenged particles faster than rain. Chan and Chung (10) also found a higher scavenging ratio for S04-- particles by snow than rain. 

Typical values of scavenging ratio lie within the range 300-2000. Scavenging ratios are rather variable, dependent upon the ehemieal nature of the trace substance (particle or gas, soluble or insoluble, etc) and the type of atmospheric precipitation. Incorporation of gases and particles into rain can occur both by in-cloud scavenging (also termed rainout) and below-cloud scavenging (termed washout). 

Note that if there is no production or removal of i in the cloud, then C, 0 = C, int + C, cioud. The mass scavenging ratio defined above may vary from zero to unity. The number scavenging ratio FN can be defined as... 

These theoretical estimates are in good agreement with the high mass scavenging efficiencies measured in the atmosphere. Ten Brink et al. (1987) observed nearly complete scavenging of aerosol sulfate in clouds. The data of Daum et al. (1984) also showed that the bulk of the sulfate mass is incorporated into cloud droplets. Hegg and Hobbs (1988) reported scavenging ratios for sulfate of 0.5 0.2. 

Another empirical parameter that has been used in wet removal studies is the scavenging ratio defined as the ratio of species concentration in collected precipitation divided by that in air ... 

Barrie, L. (1992) Scavenging ratios a useful tool or black magic, in Precipitation Scavenging and Atmosphere-Surface Exchange Processes, Hemisphere Publishing, Washington, DC. 

If Cj Q is the concentration (in mass per volume of air) of an aerosol species in clear air before cloud formation (e.g., at the cloud base), and C/, cloud and C/ im are its concentrations again in mass per volume of air in the aqueous phase and in the interstitial aerosol, respectively, one can define the cloud mass scavenging ratio for species i, Fi, as... 

The direct measurement of contaminants in precipitation samples is certainly the best approach for determining wet deposition, but problems with rain sampling, contamination, and the natural variability of the concentration of trace substances in precipitation often make representative flux estimates difficult using this approach. Studies have shown that the concentration of a substance in rain is related to the concentration of that substance in the atmosphere. This relationship can be expressed in terms of a scavenging ratio, S ... 

Therefore, a comprehensive model could be developed with inclusion of all the factors affecting the total scavenging ratio of SVOCs as given below ... 

Henry s law constants are temperature-dependent and they have to be corrected to the ambient air temperatures during the samphng period by using pubhshed enthalpy and entropy data for PAHs (Hulscher et al. 1992 Bamford et al. 1999b Monte et al. 2006) and correlation between H and temperature for OCPs (Sahsuvar et al. 2003 Cetin et al. 2006). Equihbrium gas scavenging ratios (Wg,diss) were calculated for PAHs and OCPs using the temperature-corrected Henry s law constants. 

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