Washout 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... 

Denison (12) gave a table of field observations of washout ratios. The values for various pollutants range from less than 100 to more than 4000. These values are a function of particle size and rainfall intensity, generally decreasing with the latter and increasing with the former. 

This model has been further expanded [92] to include a broader range of particle sizes, each exhibiting its own washout ratio, yet application of this expanded model is difficult due to the complexities in attributing particles of various sizes to their original atmospheric diameter and inefficiencies in separating small particles in rainwater. As such, the simpler model of Poster and Baker [93] is sufficiently specific and useful to examine the relative influences of operationally defined small and large particles on the total contaminant removal from the atmosphere by precipitation. 

Wet deposition of a chemical compound is characterized in terms of the overall washout ratio, W, which is given by... 

Table 2.12 shows washout ratios of radioactive and stable nuclides as measured in the UK. Fission products from distant bomb tests become attached to natural condensation nuclei in the atmosphere, and enter the accumulation mode of particle sizes (approximately 0.02 to 0.2 pm diameter). Washout ratios in the range 250typically present as particles in the 1-5 pm range, outside the normal accumulation mode, and this explains... 

It is likely that both the above mentioned effects - larger particle sizes and increase in airborne activity with height - contributed to the high values of W. At later times, the airborne activity from Chernobyl was mainly submicrometre in size and had equilibrated with the accumulation mode of natural nuclei. Over the period 10-90 d from the emission, 137Cs disappeared from the atmosphere with a half-life of 6 d, or mean life of 9 d (Fig. 2.8). The mass of air in the troposphere is 9000 kg per m2 of the earth s surface, and the average daily rainfall in the northern hemisphere is 3.1 mm. Using these data, it can be deduced that the washout ratio of 137Cs was... 

Sr calculated by dividing the annual deposit gauge collection by the average air concentration. The contribution of dry deposition to the deposit gauge is in fact usually ignored in calculating the washout ratio. Hence the velocity of deposition by washout is... 

Only limited information is available on the washout of radioiodine vapour by rain. Measurements of stable iodine in rain, compared with iodine vapour in air (Whitehead, 1984), give a value about 30 for the washout ratio W, defined as the ratio (I per kg rain)/(I per kg air). For a moderately heavy rainfall of 1 mm h-1, this would imply a velocity of deposition in rain of 7 mm s-1, which is the same order of magnitude as the velocity of dry deposition. The washout ratio of particulate fission products is typically about 500 (Table 2.12), so radioiodine deposited in... 

The Henry s law constants for the gas-phase PCBs are generally in the range 20-100Pa m3 mol-1 at 298K and show little variation with the number of chlorine atoms, and the average value is H = 48 Pa m3 mol 1 at 298 K.65 This value of H corresponds to a washout ratio for PCBs of Wg = 50 at 298 K and an atmospheric residence time due to wet deposition of several tens of years (for removal from the well-mixed troposphere at an average temperature of 298 K... 

Washout ratios for aerosol S04= and for total NO3" (HNO3 + aerosol NC ") are presented in Table 3. These washout ratios presuppose, in accordance with Barrie s treatment (18), that SO2 does not contribute to the rain SO and that both HNO3 and aerosol NO3" contribute to the rain NO3. Accordingly, the sulfate ratios are upper limits on W and the NC " ratios reported are close to, and only slightly less than, WNQ for HNO3 alone since HNO3 is highly soluble and dominates the total NC ". Thus from Table 3,... 

Table 4-10 presents values for washout ratios for many common atmospheric gases. 

Particles, unlike gases, do not partition between air and water in a consistent and thermodynamically predictable way. Nevertheless, some workers have empirically constructed particle washout ratios analogous to those used for wet deposition of gases and vapors, as shown in Eq. [4-28], These particle washout ratios are calculated as the ratio of the measured particle concentration in near-surface precipitation to the average measured concentration of particles in near-surface air. Examples of such washout ratios for fine atmospheric particles are shown in Table 4-11. 

Table XIV. Soil Concentration C, Dry-Deposition Velocities V, Washout Ratios W, and Aerosol Concentrations x Used in Model Calculations...

In the second approach, the trace-metal flux from the atmosphere may be divided into two components wet and dry deposition. The sum of these two components of the atmospheric flux will be calculated, using local atmospheric concentration and precipitation data, estimates of dry deposition velocity as defined by Chamberlain (1960) and reported by Cawse (1974), and washout ratios from Gatz (1975), all listed in Table XV. 

The wet-deposition contribution, due to the combined effects of rainout and washout, was calculated as WxMp, where W is the washout ratio,... 

Wet deposition encompasses the removal of gases and particles from the atmosphere by precipitation events, through incorporation into rain, snow, cloud, and fog water, followed by precipitation (Hales, 1986). As in the case of dry deposition, wet deposition is a complex phenomenon which in this particular case involves transport to the surface of a droplet, absorption, and possible aqueous-phase chemical conversion. Wet removal of gases is frequently approximated by assuming that the species is in equilibrium between the gas and aqueous phases. The equilibrium partitioning is represented in terms of a washout ratio, Wg = [C]drop/[C]air, where [C]drop and [C]ajr are the concentrations of the chemical in the aqueous and gas phases (Mackay, 1991). 

Wet deposition is an important removal route for highly water-soluble species with washout ratio values of W% > 104. Examples of such species are HN03 and hydrogen peroxide (H202). Particles and particle-associated species are efficiently removed from the atmosphere by precipitation washout ratios for particles, Wp, are typically in the range of 104 to 106 (Eisenreich, 1987 Bidleman, 1988). Note that the importance of wet deposition as a removal process may depend on whether the chemical is present in the gas phase or is associated with particles. For example, gas-phase alkanes have low values of Wg and are inefficiently removed by wet deposition, but if the same alkane is absorbed by a particle it can be efficiently removed through removal of the host particle. 

Note that according to the definitions of (20.7) and (20.8) the relation between the wet deposition velocity uw and the washout ratio wr is... 

The washout ratios of SVOCs can be described by standard equations (Ligocki et al. 1985a, b) ... 

If T is the mean residence time of a radioactive nuclide associated with aerosol particles, in s or in d, that is the inverse of the fractional rate of removal of the radionuclide, X in s or d then Equation (3.4) becomes for the washout ratio... 

Table 3.2 shows data for the washout ratio, W, of atmospheric aerosol particles associated with Be, Pb and Cs. Regarding the Be-associated particles, the washout ratio varied from 103 to 948 (Todd et al., 1989 Papastefanou and loannidou, 1991 McNeary and... 

Baskaran, 2003). For the Pb-associated particles, the washout ratio varied from 203 to 637 (Chamberlain, 1960 Peirson et al., 1966 Todd et al., 1989 McNeary and Baskaran, 2003). For the Cs-associated particles, the washout ratio varied from 230 to 6600 (Chamberlain, 1960 Peirson and Keane, 1962 Peirson and Cambray, 1965 Cambray et al., 1970 Clark and Smith, 1988 Garland and Playford, 1991 Papastefanou and loannidou, 1991). 

PHYSICAL REMOVAL OF CHEMICALS FROM THE ATMOSPHERE TABLE 4.9 Washout Ratios for Several Chemicals ... 

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