Aerosol particles, washout with

Table 3.2 shows data for the washout ratio, W, of atmospheric aerosol particles associated with Be, Pb and Cs. Regarding the Be-associated particles, the washout ratio varied from 103 to 948 (Todd et al., 1989 Papastefanou and loannidou, 1991 McNeary and... 

The change of the mass concentration of aerosol particles (M) caused by washout can also be calculated easily. Let us designate by v(R) the falling speed of the drops with number concentration N(R). Suppose that this speed is much higher than the deposition velocity of the particles. Under these conditions the particle mass loss in the air per unit time is... 

If T is the mean residence time of a radioactive nuclide associated with aerosol particles, in s or in d, that is the inverse of the fractional rate of removal of the radionuclide, X in s or d then Equation (3.4) becomes for the washout ratio... 

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... 

Deposition. The products of the various chemical and physical reactions in the atmosphere are eventually returned to the earth s surface. Usually, a useful distinction is made here between wet and dry deposition. Wet deposition, ie, rainout and washout, includes the flux of all those components that are carried to the earth s surface by rain or snow, that is, those dissolved and particulate substances contained in rain or snow. Dry deposition is the flux of particles and gases, especially SO2, FINO, and NFl, to the receptor surface during the absence of rain or snow. Deposition can also occur through fog, aerosols and droplets which can be deposited on trees, plants, or the ground. With forests, approximately half of the deposition of SO(, NH+,andH+ occurs as dry deposition. 

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.

Washout by rain greatly reduces the Aitken nuclei mode and the coarse particle mode but has little effect on the accumulation mode in the trimodal size distribution (Whitby, 1975). The origin of each mode of atmospheric aerosol size distribution can be associated with various aerosol formation mechanisms, such as Brownian motion of the particles smaller than 0.1 pm in diameter, which causes the particles to diffuse and by collisions to coagulate to larger sizes. Coagulation generates multimodal distributions and affects the shape and the chemical composition of the particles. 

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