Aerosol particles, washout with precipitation

Rain and snow remove a significant fraction of Cd from the atmosphere either through the solubilization of Cd aerosols in water droplets or the washout of particles associated with precipitation events [17,39]. Concentrations of Cd in rainwater show a considerable range that is related to air mass history and proximity to anthropogenic point sources. Rainwater Cd concentrations over Europe tend to track emissions and have declined from maximum values of 0.7 pg L in the mid-1980 s to 0.1 pg in 2004 [40]. Similar Cd concentrations exist in rainwater over the North Atlantic spanning a wide range from 0.07 to 0.95 pg L [38]. In the North Pacific (37° 7°N) much lower concentrations were measured in... 

Particles in the accumulation range tend to represent only a small portion of the total particle number (e.g., 5%) but a significant portion (e.g., 50%) of the aerosol mass. Because they are too small to settle out rapidly (see later), they are removed by incorporation into cloud droplets followed by rainout, or by washout during precipitation. Alternatively, they may be carried to surfaces by eddy diffusion and advection and undergo dry deposition. As a result, they have much longer lifetimes than coarse particles. This long lifetime, combined with their effects on visibility, cloud formation, and health, makes them of great importance in atmospheric chemistry. 

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.

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