In-cloud scavenging

Total aerosol mass scavenging rate is given by (20.51). Replacing the aerosol mass distribution with the number distribution, the overall mass scavenging rate is given by 

Our calculations can be simplified by defining a mean mass scavenging coefficient Am so that  

Note that the denominator is equal to 6Vaer/n, where Vaer is the particle volume concentration and assuming a particle density pp the denominator is equal to 6Maer/npp. Therefore, by definition, we have  

Species can be incorporated into cloud and raindrops inside the raining cloud. These processes determine the initial concentration Caq° of the raindrops, before they start falling below the cloud base, and have been discussed previously. Let us summarize the rates of in-cloud scavenging for gases and aerosols. 

Gases like HN03, NH3, and S02 can be removed from interstitial cloud air by dissolution into clouddrops. For a droplet size distribution N(DP), the local rate of removal of an irreversibly soluble gas like HN03 with a concentration Cg is given by 

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Pathways and In-Cloud-Scavenging of Sulfur Dioxide in the Atmosphere, Geophys. Res. Lett., 21, 1519-1522 (1994). 

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). 

Model system Aerosol water uptake Aerosol activation aero-CCN/IDN In-cloud scavenging Below-cloud scavenging Sedimentation of aerosols and cloud droplets... 

The current version of GEM-AQ has five size-resolved aerosols types, viz. sea salt, sulphate, black carbon, organic carbon, and dust. The microphysical processes which describe formation and transformation of aerosols are calculated by a sectional aerosol module (Gong et al. 2003). The particle mass is distributed into 12 logarithmically spaced bins from 0.005 to 10.24 pm radius. This size distribution leads to an additional 60 advected tracers. The following aerosol processes are accounted for in the aerosol module nucleation, condensation, coagulation, sedimentation and dry deposition, in-cloud oxidation of SO2, in-cloud scavenging, and below-cloud scavenging by rain and snow. 

Precipitation Transfer In cloud scavenging Dry deposition Precipitation... 

Tanaka N., Rye D. M., Xiao Y., and Lasaga A. C. (1994) Use of stable sulfur isotope systematics for evaluating oxidation pathways and in-cloud-scavenging of sulfur dioxide in the atmosphere. Geophys. Res. Lett. 21, 1519-1522. 

The relative quantum yield of H02 from formaldehyde, 2yb(HCHO)/ j(HCHO), is about 0.8 at ground level. Since photodecomposition is the dominant loss process for formaldehyde in the atmosphere, the oxidation of methane ultimately produces more than one H02 radical for each OH radical entering into reaction with methane, providing enough NO is present to convert all the CH302 radicals to formaldehyde. Losses of formaldehyde from the atmosphere due to in-cloud scavenging and wet precipitation amount to less than 15% of those caused by photolysis and reaction with OH radicals (Warneck et al., 1978 Thompson, 1980). 

Let cA (in xg/m3) = cA exp(-zcb/h) denote the aerosol concentration at the cloud condensation level zcb (cloud base). Here cA may refer either to an individual constituent of the aerosol or to its entire mass concentration. The aqueous concentration cw resulting from in-cloud scavenging can then be estimated from a relation first formulated by Junge (1963),... 

This value has the same magnitude as the residence time for the bulk aerosol derived in Section 7.6 in a different manner. The congruence of results shows that in-cloud scavenging and precipitation is the dominant process for the removal of particulate matter from the troposphere. 

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... 

Fig. 8-9. Enhancement of in-cloud scavenging efficiencies for formaldehyde and S02 due to their interaction in aqueous solution.

Saxena, V. K., and A. H. Hendler (1983). In-cloud scavenging and resuspension of cloud active aerosols during winter storms over Lake Michigan. In Precipitation Scavenging, Dry Deposition and Resuspension (H. R. Pruppacher, R. G. Semonin, and W. G. N. Slinn, eds.), pp. 91-101. Elsevier, New York. 

Wet deposition refers to the natural processes by which material is scavenged by atmospheric hydrometeors (cloud and fog drops, rain, snow) and is consequently delivered to the Earth s surface. A number of different terms are used more or less synonymously with wet deposition including precipitation scavenging, wet removal, washout, and rainout. Rainout usually refers to in-cloud scavenging and washout, to below-cloud scavenging by falling rain, snow, and so on. 

From a mass transfer point of view, the heterogeneous nucleation is also a nucleation scavenging of PM, the first process of in-cloud scavenging, following by a gas uptake. Junge (1963) described it by the simple equation ... 

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 ... 

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. 

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