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For the experimental measurement of washout of Asian dust particle, a laboratory-scale experimental setup was designed. It consists of a chamber (D 0.2 m, H 9.5 m), an artificial Asian dust particle distributer, a drop generator, a drop collector, and an optical particle counter. For the rainfall generation in the laboratorial experiment, the height of experimental chamber has to be the height required to leach to terminal velocity of drop.

A quantitative theoretical and experimental investigation of Wang and Pruppacher (1977) showed that at 1000 mb and 20˚C, the drops of equivalent radius with 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 and 2000 μm require 0.15, 0.71, 1.6, 2.8, 3.9, 5.2, 6.3, 7.3, 8.4, 9.5 and 14.0 m to accelerate up to 99% of their terminal velocity, respectively. Accordingly, 9.5 m of height is not sufficient for the experimental chamber to thoroughly assess wet scavenging of Asian dust particle compared to ones with at least several tens of meters for a quasi-real scale model.

As mentioned earlier, because the analysis of rain as bulk phase will not necessarily provide detailed information of droplet scavenging, the size-classified droplets were collected and analyzed individually in this study. Our raindrop collector illustrated at the bottom leftside of Fig. 1 consists of a Dewar vacuum flask (Jencons Co.) filled with liquid nitrogen and a stainless steel sieve (Nonaka Rikaki Co.) with 0.2 mm mesh size. Raindrops fallen into the liquid nitrogen were frozen and handled for subsequent elemental analysis.

Fig. 1. 
Experimental set up to study the scavenging of dust particles by rainfall.

The procedures adopted in this study for the laboratory-scale experiment were as follows:

(1) As the artificial Asian dust particle, CaCO₃ particles (98.8% of assay, Linzunyaku co.) fractioned into 2 size classes (2-5 μm and >5 μm) were diluted by 99% ethyl alcohol.

(2) The particle number concentration in a chamber was continuously measured by the end of each experimental period.

(3) The diluted CaCO₃ particles were brought into a chamber through a nebulizer.

(4) The number concentration of CaCO₃ particles was maintained constantly in a chamber prior to dropping of artificial raindrops.

(5) Raindrops were generated at the upper side of a chamber and then fell through a chamber.

Artificial raindrops were collected during about three minutes. After collection, a sieve was pulled out from the dewar vacuum flask, and the raindrops frozen on the sieve were placed on Nuclepore filter (10 μm thickness) by using a vacuum pipette. The frozen raindrops were melted and dried under an infrared lamp for five minutes. Every handling process was performed in a clean air system filled with the cool nitrogen gas. Then the residual solid particles on Nuclepore filter were subsequently analyzed.

The elemental concentration of Ca on the residues of individual drop was determined by the particle induced X-ray emission (PIXE) installed at the Cyclotron Research Center of Iwate Medical University. As widely known, this PIXE system has the great advantages such as an excellent sensitivity, a nondestructive technique for multielement with a wide range of elements (Z >10). The sensitivity, if defined by the ratio between PIXE yield per unit dose and mass thickness, can be determined for all objective elements both experimentally and theoretically. For instance, the sensitivity of calcium was calculated to be 1700 (counts∙cm²/μC∙ μg)with a detection limit of 9.4×10^-3 (μg/cm²). The more detailed analytical procedures and experimental setup for PIXE analysis were described elsewhere (Sera et al., 1999).

In this work, the number concentrations of two-size particle fractions (i.e., 2.0-5.0 μm and >5 μm) were adjusted to include those of real ambient Asian dust particles measured in Kyoto Japan (Ma and Choi, 2007; Ma et al., 2005). On the average of five-time model experiments in which CaCO₃ particles were nebulized into a chamber, the number concentrations of CaCO₃ particles with 2.0-5.0 and >5 μm diameter ranges were between 3.7×102 and 6.1×105 and between 10 and 5.5×104 particles L^-1, respectively.

The variation of particle number concentrations at two-size fractions with relative humidity during a period of model experiment is displayed in Fig. 2. This time serial particle number concentration severely fluctuated throughout the whole experimental period (initial condition - CaCO₃ nebulizing - stable (uniform CaCO₃ particle number concentration) - raindrop falling (drop falling with 12 mm hr^-1 rainfall intensity)) of five-time model experiments. The experiments were carried out at relative humidity varying from 48 to 51%. The particles showing a slight varying of number concentration at initial condition in Fig. 2 mean the initial background particles in the chamber of model experiment. The number concentrations of both sizes of CaCO₃ particles fell right down to the level of initial condition during the 5 minutes. Therefore, although CaCO₃ particles are not very soluble and do not easily react in water, nearly all of them including a portion of background particles in the experimental chamber were scavenged. Gravitational settling of large particles (>5 μm) can be negligible due to the facts: (1) the CaCO₃ particles simulated for the Asian dust particle are smaller than 7 μm, and (2) two small scale fans were operated to distribute CaCO₃ particles uniformly in our experimental chamber.

Fig. 2. 
Variation of particle number concentration and relative humidity during model experiment.

The real size of CaCO₃ particles retained in a drop could be identified by the photograph of a scanning electron microscope (SEM, KYENCE, VE-7800) (Fig. 4).

Fig. 3 shows the individual frozen drops collected on a stainless steel sieve (top) and a drop placed on a filter media (bottom). The raindrops maintained their spherical shape during the freezing process. Consequently, it was possible to separate the frozen raindrops according to their size for the further analysis. The size fractions of individual drops classified as <1.0 mm, 1.0-2.0 mm, and >2.0 mm were 67.5%, 20.5%, and 12%, respectively.

Fig. 3. 
Spherical frozen raindrops artificially dropped at the top of experimental chamber.

In order to certify the existing of CaCO₃ particles in the melted and dried drop, a microscopical observation was carried out by means of a SEM. As shown in Fig. 4, the detailed morphology of the residual CaCO₃ particles indicates a large number of CaCO₃ residual particles retained in a drop, while most of them were distributed at the rim of a dried drop. It thus suggests undoubtedly that raindrop should be capable of scavenging significant quantities of Asian dust particles in the ambient atmosphere too.

Fig. 4. 
Residual CaCO₃ particles distributed at a portion of the rim of a dried drop.

In order to quantitatively describe the raindrop scavenging of CaCO₃ particle, an analysis was made to determine chemical components in a single and sizefractionated drop by use of a PIXE. Fig. 5 shows the PIXE spectra of the residual particles embedded in a drop as well as filter blank. It was possible to resolve the Ca peaks corresponding to channel number of PIXE spectra, whereas no meaningful peaks of other elements were detected. Although the lighter elements (Z<10) cloud not be measured by a PIXE, the analysis of PIXE spectra for residual particles indicates that the peak of Ca-K_α coexisting with Ca-K_β, drawn between 200 to 300 channel numbers, was derived from CaCO₃ particles nebulized into a chamber instead of chamber background particles.

Fig. 5. 
PIXE spectrum of CaCO₃ particles embedded in a drop.

Fig. 6 shows the variation of Ca concentration as a function of rain drop diameter. Ca concentration of individual drop shows a continuous decrease with increasing drop size.

Fig. 6. 
Drop size dependence of Ca concentration estimated by PIXE analysis. The numbers inside the figure indicate the number concentration of CaCO₃ particle (# L^-1) at each drop size.

Based on the representative diameter of CaCO₃ particle (3.5×10^-4 cm), the density of CaCO₃ (2.93 g cm^-3), and the ratio of CaCO₃ molecular weight of to Ca atomic weight, the number concentration of CaCO₃ particle (# L^-1) at each drop size was also calculated (see Fig. 6). It is expected that several mechanisms should be responsible for the Ca enrichment in the smaller drop (<1.0 mm). In the ambient atmosphere, it is acknowledged that smaller raindrops should have higher elemental concentration because of their low settling velocities and associated longer lifetimes. Such pattern may also be accounted for by the effect of evaporation. As small raindrop shows a much higher degree of evaporation than larger ones, the former is likely to exhibit an increase of the elemental concentration. However, in this study, both longer lifetime and active evaporation of small size drop cannot justify enhanced Ca concentrations. This is because the height of experimental chamber is not more than 9.5 m in consideration of the terminal velocity of a drop. The humidity in

the chamber was also not so low (around 50% through a whole experimental period). Therefore, the drop size dependence of Ca concentration might be accounted for by the combined effects of such factors as: the processes of Brownian diffusion, interception, and impaction between CaCO₃ particles and falling drops. Meanwhile, Brownian diffusion might not play a critical role in CaCO₃ scavenging because, as mentioned above, the size of CaCO₃ particles nebulized into the experimental chamber were larger than 2 μm.

Kim et al. (2007) carried out the study on the number size distribution of atmospheric aerosols during both the Asian dust and precipitation events. These authors reported that during precipitation particles in the coarse mode were scavenged by impaction mechanism and the degree of scavenged particle varied depending on the rainfall rate, raindrop size distribution and aerosol size distribution.

In order to make a quantitative estimation of the washout properties of CaCO₃ particles as a function of raindrop size, a theoretical model calculation was carried out. The schematic of model concept and three scavenging processes are illustrated in Fig. 7. This model is a Lagrange type model that can set the special coordinates as the one-dimensional vertical direction from the top to bottom of the experimental chamber (H_T). For a model calculation, we assumed several parameters as follows: the uniform distribution of CaCO₃ particles existing in a volume (V=h_t×πd_r²/4 in Fig. 7) swept by falling drop, no diffusion by the operating of fans, and no evaporation and coalescence of drops.

Fig. 7. 
The conceptional illustrations of the model calculation of dust particles by rainfall scavenging (left) and three scavenging processes (right).

Let us define the mass concentration of CaCO₃ particles with diameter d_CaCO3 in the experimental chamber as C_CaCO3,0 (d_CaCO3) and the collection efficiency of drop with d_r for CaCO₃ particles with diameter d_CaCO3 as E₀ (d_r, d_CaCO3). Then, the mass (m_CaCO3) of CaCO₃ particles (d_CaCO3) in drop (d_r), which falls through V for one second, can be written as following.

Therefore, the total mass (M_CaCO3) of CaCO₃ particles (d_CaCO3) in drop (d_r), which falls in the chamber from top to bottom, can be rearranged into the following equation.

where υ_t (h_t, d_r) is the terminal velocity of drop (d_r) at height (h_t), C_CaCO3 (h_t, d_CaCO3) is the mass concentration of particles existing at height (h_t), and E(h_t, d_r, d_CaCO3) is the collection efficiency of raindrop (d_r) for CaCO₃ particle (d_CaCO3) at height (h_t).

Slinn and Hales (1971) proposed three particle collection efficiencies (E), i.e., Brownian diffusion (E_dif), E by interception (E_int), and E by inertial impaction processes (E_imp). Furthermore, Strauss (1975) suggested the following combined particle collection efficiency (E_com) equation:

Meanwhile, to calculate the CaCO₃ particle collection efficiency (E(d_r, d_CaCO3)), Strauss’s E_com (Strauss, 1975) was modified further because Brownian diffusion is not meaningful for the CaCO₃ particles in this study with large diameter (>2 μm).

The following are E_int and E_imp applied in this work.

where r_CaCO3 is CaCO₃ particle radius (μm), r_r is drop radius (μm), μ_a is dynamic viscosity of air (Pa∙s), μ_r is dynamic viscosity of water (Pa∙s), Re is the Reynolds number of drop based on its radius ((r_r∙υ_t∙ρ_a (air density kg m^-3))/μ_a), St is the Stokes number of collected CaCO₃ particle ((2C (Cunningham slip factor)∙ ρ_CaCO3 (CaCO₃ particle density kg m^-3)∙r_CaCO3∙ υ_t)/(9μ_a∙r_r)), and S^* is the critical Stokes number ((1.2+(1/12)ln(1+Re))/(1+ln(1+Re))).

The result of model calculation of Ca concentration in the fractionated drop size (i.e., 1.0 mm and 2.0mm drop diameter) is plotted along with the actual PIXE results in Fig. 8. Here, the theoretically estimated Ca concentration was determined by CaCO₃ mass concentration. The result of this intercomparision exhibits the severe drop size fluctuation, although both theoretical and measured Ca concentrations were enriched consistently in small drop size (1.0 mm drop diameter). The theoretical Ca concentrations accounted for 92.1% and 83.2% of those measured from small and large size drops, respectively. This result suggests that the processes of interception and inertial impaction should be the essential components of scavenging mechanisms of CaCO₃ particles. One of the reasons for these discrepancies between measured and calculated Ca concentrations might be the limitation on our experimental chamber (i.e., partially inhomogeneous particle distribution from top to bottom, insufficient H_T, etc.). The Ca amount, underestimated by model calculation, can nonetheless be compensated by other mechanisms like electrostatic force and thermophoresis between CaCO₃ particles and drops.

Fig. 8. 
A comparison between experimentally and theoretically estimated Ca concentrations.