Below-cloud scavenging

BELOW CLOUD SCAVENGING OF ACIO GASES AND FINE PARTICLES... 

PAHs present in the atmosphere enter rain as a result of in-cloud and below-cloud scavenging (van Noort and Wondergem 1985). Total PAHs deposited on land and water are almost equivalent... 

Levine, S. Z. Schwartz, S. E. In-cloud and below-cloud scavenging of nitric acid vapor. Atmos. Environ. 1982, 16, 1725-1734. 

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 fri-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. 

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). 

Dry deposition is parameterized via a resistance approach in which resistances depend on particle size and density, land-use classification and atmospheric stability (Wesely 1989 Zanetti 1990). Wet deposition is included via below cloud scavenging (washout), using a parameterization based on precipitation rates (Baklanov and Sprensen 2001) and scavenging by snow is parameterized using the scheme by Maryon and Ryall (1996). The terminal settling velocity is considered in both the laminar case, in which Stake s law is used and the mrbulent case in which a iterative procedure is employed (Naslund and Thaning 1991). For very small particles a correction for non-continuum effects is used. 

Processes needed in-/below-cloud scavenging and droplet sedimentation... 

Fo6i below-cloud scavenging of gaseous reactants or reactant products. 

F76 below-cloud scavenging of coarse-mode particles. Rns occult precipitation (deposition of cloud droplets direcily to the Earth s surface, trees, etc.). 

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 8-6 presents an overview on the concentrations of the major ions in rainwater observed at various locations. Table 8-7 provides some information on cloud and fog waters. In maritime regions seasalt is an important source of cloud condensation nuclei, and it undergoes effective below-cloud scavenging as well. Sodium chloride accordingly contributes the largest fraction of all ions in rainwater. Some of the other ions usually are somewhat enriched in comparison with their relative abundances in seasalt. The enrichment of potassium and calcium is due to the admixture of aerosol from continental sources, and that of sulfate arises from the oxidation of gaseous precursors such as dimethyl sulfide of S02- This excess sulfate is associated almost exclusively with submicrometer-sized particles (see Section 7.5.1). 

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. 

If Cg(z, t) is the concentration of a species in a horizontally homogeneous atmosphere, washed out by rain, the below-cloud scavenging rate F will be equal to... 

The overall wet flux of a species is the sum of transfer of the species from the cloud to rain plus the below-cloud scavenging. The rate of removal of a species from the cloud is often referred to as the rainout rate and the rate of below-cloud scavenging, as the washout rate. 

If the species exists only in the gas phase (not in the aerosol), and if the contribution of rainout is negligible compared to washout," the wet deposition flux of the species will be equal to the below-cloud scavenging rate as given by (20.4). 

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. 

The overall below-cloud scavenging rate, Wbc, can be calculated once more by multiplying the per droplet scavenging rate by the number of droplets per volume of air... 

The total scavenging rate by the rain from the below-cloud atmosphere (equal to the wet deposition rate due to below-cloud scavenging) is... 

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