Clouds scavenging coefficients

Below-Cloud Scavenging Coefficient The concentration of gas A is lOpgm 3 below a raining cloud. Assuming a constant scavenging coefficient of 3.3 h "1, calculate the concentration of A in the atmosphere after 30 min of rain, and the overall wet deposition flux. Assume a cloud base at 2 km. 

An in-cloud scavenging coefficient follows by using Eq. (4.298) and (4.300) lifetime of the cloud. Table 4.22 shows the relationship between A values for SO2 and H2O2 as an example for enhancement due to chemical reactions. 

GEM-AQ only has a simplified aqueous phase reaction module for oxidation of SO2 to sulphate. Thus, for the gas phase species, wet deposition processes are treated in a simphfied way. Only below-cloud scavenging of gas phase species is considered in the model. The efficiency of the rainout is assumed to be proportional to the precipitation rate and a species-specific scavenging coefficient. The coefficients apphed are the same as those used in the MATCH model (Multiscale Atmospheric Transport and Chemistry Model) used by the Swedish Meteorological and Hydrological Institute (SMHl) (Langner et al. 1998). 

In theory, the removal of fine particles by collision processes results in chemical concentrations in the air mass following an exponential decay law. In actual clouds, both collision and nucleation may contribute to particle removal, and their combined effects often are lumped into a single scavenging coefficient, A [T 3] ... 

Assume the dry deposition velocity over a New England forest for particles emanating from a midwestern U.S. power plant averages 0.5 cm/sec on a certain day. The scavenging coefficient in a rain-forming cloud over the forest is 10 3/sec. If the cloud is 500 m in vertical extent, what is the equivalent wet deposition velocity, Vw, for rainout of the particles ... 

Chamberlain, 1960 Engelmann, 1968). The quantity A(r2) is called the washout coefficient. It depends on r2 in much the same fashion as Ec. Figure 8-7 shows sample calculations for two drop size spectra with maxima at radii of 0.2 and 1 mm, respectively, according to Zimin (1964) and Slinn and Hales (1971). For a given particle size, the washout coefficient is a constant only if the rain drop spectrum does not change with time. This ideal situation is rarely met in nature, due to the usual variation of the rainfall rate. If this assumption is nevertheless made, the contribution of below-cloud scavenging to the total concentration of particulate matter in rainwater is... 

Fig. 8-7. Washout coefficients according to Slinn and Hales (1971) are shown in curves A and B (left-hand scale). They are based on rain drop size spectra of Zimin (1964) with r,max = 0.2 and 1 mm, respectively, and a precipitation rate of 10 mm/h (10 kg/m2 h). Curve C represents the first term and curves D and E the second term in the bracket of Eq. (8-6) in nonintegrated form (right-hand scale applies). These latter three curves are based on the mass-size distribution for the rural continental aerosol in Fig. 7-3. Curve C was calculated with eA(r2)=l for r2>0.5 ra and eA < I for r2<0.5(im, decreasing linearly toward zero at r2 = 0.06 p.m. This leads to eA = 0.8. Curves D and E were obtained by using the washout coefficients of curves A and B, respectively. Note that below-cloud scavenging (curves D and E) affect only giant particles, whereas nucleation scavenging (curve C) incorporates also submicrometer particles.

Table 8-2. Henry s Law Coefficients for Several Atmospheric Gases, In-Cloud Scavenging Efficiencies eg, Concentrations cs of Dissolved Substances in Cloud Water for Initial Gas-Phase Mixing Ratios m0, and Residence Times t for Rainout"... 

Table 8-4 lists in a fashion similar to Table 8-2 modified Henry coefficients, the resulting in-cloud scavenging efficiencies, concentrations in the aqueous... 

If the atmosphere below the cloud is homogeneous, then one can also define an average scavenging coefficient Ag so that... 

Because of the reversibility of the scavenging process, one can define a scavenging coefficient only if CgH RT This is valid when the initial raindrop species concentration is much lower than the equilibrium concentration corresponding to the below-cloud atmospheric conditions. If this is valid, then... 

TABLE 20.2 Scavenging Coefficients A(h for Below-Cloud Irreversible Scavenging of HNO3 Based on a Marshall-Palmer Raindrop Size Distribution... 

Using expressions obtained for the collision efficiency for E(Dp, dp) in(20.53)and (20.54), one can estimate the scavenging coefficient, and the scavenging rate for a rain event. The calculation requires knowledge of the size distributions of the raindrops and the below-cloud aerosols. 

The older literature is full of experimental estimations of X, also called the washout coefficient, by the measurement of the gas and rainwater concentration of soluble gases. However, that approach is wrong because a) A is a function of height (it should measured as the vertical gas phase concentration profile and not the surface concentration) and b) a dominant part of the dissolved matter arises from in-cloud scavenging. For sub-cloud scavenging, assuming the washout process to be a first-order process (rfc/rft) = Ac, we can describe the sub-cloud process for gases as well as particles indexes g and p denote the gas and particle, respeetively ... 

Table 4.22 Mean scavenging coefficients A (in 10" s" ) of H2O2 in dependence on the SO2 gas phase concentration c (in ppb), calculated from a cloud model with coupled gas-aqueous chemistry after Moller (1995b).

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