Nucleation scavenging

Since the production rates of the cosmic ray radionuclides increase rapidly with increasing altitude in the lower atmosphere, the atmospheric concentrations and ratios of short lived cosmic ray radionuclides can be used to study rapid vertical air motions if the equilibrium concentrations of the radionuclides are known. For example, the concentrations of the short lived cosmic ray radionuclides in air which has moved upward recently from a lower altitude will be less than the equilibrium concentrations. The concentrations of the radionuclides will therefore increase with time until equilibrium is reached. However, the concentration of the shorter lived of two short lived radionuclides will increase more rapidly initially, causing the ratio of the two radionuclides of different half-lives to change with time until equilibrium is reached. Therefore, the time since the air moved from a lower altitude, the speed of the upward motion, and the altitude from which the air originated could be calculated from the concentrations and concentration ratios of cosmic ray radionuclides of different half-lives. Vertical motions of different speeds could be studied since several cosmic ray radionuclides of different half-lives are present in the atmosphere (Table I). Many other radionuclides are produced by cosmic rays in the atmosphere, but they have not yet been detected. Some of these with half-lives of a few minutes could serve as tracers of very short term processes such as post-nucleation scavenging. 

The S0 ==/N03 concentration ratio in the rain was about 3.7 equivalents per equivalent vs. a ratio of about 2.5 in the dew. This is not surprising since significant amounts of sulfate are introduced into rain by nucleation scavenging (18) while aerosol sulfate deposition to dew is minimal (1). (In fact, at Allegheny Mountain (1) SO2 was responsible for about 80% of the dew... 

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.

Nucleation scavenging of aerosols in clouds refers to activation and subsequent growth of a fraction of the aerosol population to cloud droplets. This process is described by (17.70) and has been discussed in Section 17.5. 

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 ... 

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