Below-cloud scavenging particles

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

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). 

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. 

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. 

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). 

Therefore the below-cloud scavenging (rainout) rate of aerosol particles of diameter dp can be written as... 

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... 

As discussed above, SVOCs are removed from the atmosphere and transported to the waters by precipitation scavenging of atmospheric vapors and particles, which are incorporated into the rain within or below the clouds. After SVOCs are deposited into the bulk seawater, partitioning in water column can affect the distribution of pollutants between the dissolved aqueous and the solid phases and eventually impact the fate of these compounds in oceans (Luo et al. 2004). In addition, air-sea exchange can make SVOCs diffuse across the air-sea interface however, the sea surface microlayer (SML), a unique compartment at the air-sea boundary defined operationally as the upper millimeter (1-1,000 pm)... 

This is in an unscrubbed plume. Now, concerning ammonia, you ae not really talking about ammonia as a catalyst for sulfate formation. The actual process is SO2 to sulfuric acid, followed by ammonia neutralization. The theory about SO2, water, ammonia catalysis is being questioned. The real process seems to be SO2 to sulfuric acid, then reaction with ammonia to form ammonium sulfate or ammonium bisulfate. In fact, in scavenging the sulfate, as the particle size increases, the scavenging efficiency also increases. Sulfuric acid aerosols are submicron particulates for which scavenging is very inefficient (Marsh, Atmos. Environ. 12 401-406, 1978). So what you are finding in a rain droplet is perhaps what is below the cloud or before the condensation nucleus stabilizes. 

Recent field observations [15, 45] suggest that there is an important role for heterogeneous ice nucleation in the upper troposphere, where 1 -10 IN per liter were found at temperatures below —30 C. Examination of the solid residues found in cirrus cloud particles over the North Sea showed the particles were typically several tenths of microns in linear dimension [46]. These were probably involved in the ice nucleation process since their concentrations are sufficiently low at cirrus cloud levels that it is not likely they are scavenged. 

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