Deposition velocity of aerosols

Equations [5.3] and [5.4] can be applied in the case of relatively simple and sensitive radioactivity measurements. The use of long-lived artificial fission products in intervals without nuclear tests seemed particularly suitable in the past since they have a relatively constant vertical distribution in the troposphere (e.g. Ishii, 1960) which facilitates the determination of H. In this way Small (1960), by using total -activity measurements, calculated that the overall dry deposition velocity of aerosol particles over Norway is 0.50 km day-1. On the other hand, Stewart found (see Small, 1960) a much smaller value in England. According to artificial / -activity measurements of E. Meszarosand Simon (1967), carried out near... 

Peters, K., and Eiden, R. (1992) Modeling the dry deposition velocity of aerosol particles to a spruce... 

Table 4.12. Mean concentrations and depositional velocities of aerosols considered and dry deposition rates of combined nitrogen over the ECS (Nakamura et al., 2005) (With permission from Elsevier s Copyright Clearance Center)...

Fig. 3.2. Relationship between deposition velocities of aerosol particles to grass and particle diameter.

Porstendorfer, 1984). Knutson et al. (1983) measured similar results in their chamber investigation. The results show that the values of the deposition velocity of the free radon daughters are about 100 times those of the aerosol radon progeny. But there are no information about the effective deposition surface S of a furnished room for the calculation of the plateout rates qf and qa by means of Vg and Vg. For this reason the direct measurements of the plateout rates in rooms are necessary. Only Israeli (1983) determined the plateout rates in houses with values between qf = 3-12 h"1 and qa = 0.4-2.0 h"1, which give only a low value of the... 

Rangarajan C, Eapen CD, Gopalakrishnan SS. 1986. Measured values of the dry deposition velocities of atmospheric aerosols carrying natural and fallout radionuclides using artificial collectors. Water Air Soil Pollut 27 305-314. 

THE AMBIENT ATMOSPHERIC AEROSOL consists of liquid and solid particles that can persist for significant periods of time in air. Generally, most of the mass of the atmospheric aerosol consists of particles between 0.01 and 100 xm in diameter distributed around two size modes a coarse or mechanical mode centered around 10- to 20- xm particle diameter, and an accumulation mode centered around 0.2- to 0.8- xm particle diameter (1). The former is produced by mechanical processes, often natural in origin, and includes particles such as fine soils, sea spray, pollen, and other materials. Such particles are generated easily, but they also settle out rapidly because of deposition velocities of several centimeters per second. The accumulation mode is dominated by particles generated by combustion processes, industrial processes, and secondary particles created by gases converting to par-... 

Inputs and outputs to the lake have been measured to calculate net retention for the pre-acidified lake. Precipitation inputs of sulfate were based on data from wet collectors (1980-1983) compiled by the National Atmospheric Deposition Program (NADP). SO2 inputs were calculated from regional ambient air concentrations (22) usinga deposition velocity of 0.5 cm/sec. Aerosol sulfate was estimated from NADP dry bucket measurements and from dry bucket and snow core measurements made in this study (22). Groundwater inputs occur largely at the southeast corner of the lake and were calculated from modeled inseepage (21) and measured sulfate concentrations in a well located in the major inseepage area. Sulfate output was estimated from mean lakewater sulfate concentration and modeled outflows. 

The seas may also act as a receptor for depositing aerosol. Deposition velocities of particles to the sea are a function of particle size, density, and shape, as well as the state of the sea. Experimental determination of aerosol deposition velocities to the sea is almost impossible and has to rely upon data derived from wind tunnel studies and theoretical models. The results from two such models appear in Figure 4, in which particle size is expressed as aerodynamic diameter, or the diameter of an aero-dynamically equivalent sphere of unit specific gravity.If the airborne concentration in size fraction of diameter d is c then... 

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... 

The dimensionless time has once more been defined relative to the characteristic time for vapor deposition, o is the ratio of the mass transfer rate to the gas-phase deposition rate, oj is the ratio of the initial aerosol concentration of AB to the corresponding concentration of the rest of the aerosol species, o is the ratio of the deposition velocities of the two gas-phase species, 04 is the ratio of the aerosol deposition velocity to the deposition velocity of A(g), o is the ratio of the emission (or gas-phase chemical reactions) of A to its initial deposition rate, 6 is the ratio of the emission rates of A and B, (77 is the ratio of the initial gas-phase concentrations of A and B, and finally o is the ratio of the initial concentrations of gas species A and aerosol AB. 

The first case discussed is for rapid mass transfer between the gas and aerosol phases, a — 1000. The deposition ratios are presented in Figure 19.8 as a function of and y. When no source of the organic species A is present (8 =0), and because the system starts from equilibrium, the gas-phase concentration of A cannot exceed its saturation value. Therefore in this case A does not condense at any time to the aerosol phase and the particles evaporate in an attempt to maintain equilibrium. This nonsymmetry in the system is depicted in Figure 19.8a, and for / > 1 (deposition velocity of the particles exceeds the deposition velocity of the vapor) the equilibration process does not significantly affect the deposition. The material in the aerosol phase is preferentially deposited... 

Giorgi, E (1988) Dry deposition velocities of atmospheric aerosols as inferred by applying a particle dry deposition parameterization to a general circulation model, Tellus, 40B, 23-41. 

Surface deposition is the most important parameter in reduction of the free and aerosol attached radon decay products in room air. If V is the volume of a room and S is the surface area available for deposition (walls, furniture etc), the rate of removal (plateout rate) q is vg S/V, always assuming well mixed room air. vg is the deposition velocity. 

Following Barry, James et al (1972), and Thomas and Hinchliffe (1972) investigated the use of wire screens for collecting 218Po atoms or ions. Experiments were done in the absense of aerosol particles, yielding collection efficiency as a function of screen dimensions and face velocity. Information was developed on the fraction of deposited o-activity that could be counted from the front and back sides of the screens. 

For wet deposition, it is assumed that the rain scavenges Q (the scavenging ratio) or about 200,000 times its volume of air. Using a particle concentration (volume fraction) vQ of 2 x 10 n, this corresponds to the removal of Qvq or 4 x 1CF6 volumes of aerosol per volume of rain. The total rate of particle removal by wet deposition is then QvqUrAw m3/h, thus the wet transport velocity QvqUr is 4 x 1(T10 m/h. 

In general, slow, deep inhalation followed by a period of breath holding increases the deposition of aerosols in the peripheral parts of the lungs, whereas rapid inhalation increases the deposition in the oropharynx and in the large central airways. Thus, the frequency of respiration (the flow velocity) and the depth of breath (tidal volume) influence the pattern of pulmonary penetration and deposition of inhaled aerosols. Therefore, an aerosol of ideal size will penetrate deeply into the respiratory tract and the lungs only when the aerosols are inhaled in the correct manner (Sackner, 1978 and Sackner et al., 1975). 

A different approach which also starts from the characteristics of the emissions is able to deal with some of these difficulties. Aerosol properties can be described by means of distribution functions with respect to particle size and chemical composition. The distribution functions change with time and space as a result of various atmospheric processes, and the dynamics of the aerosol can be described mathematically by certain equations which take into account particle growth, coagulation and sedimentation (1, Chap. 10). These equations can be solved if the wind field, particle deposition velocity and rates of gas-to-particle conversion are known, to predict the properties of the aerosol downwind from emission sources. This approach is known as dispersion modeling. 

Quinn, T. L., and J. M. Ondov, Influence of Temporal Changes in Relative Humidity on Diy Deposition Velocities and Fluxes of Aerosol Particles Bearing Trace Elements, Atmos. Environ., 32, 3467-3479 (1998). 

Bigu, J. (1985) Radon daughter and thoron daughter deposition velocity, and unattached fraction under laboratory-controlled conditions and in underground uranium mines. Journal of Aerosol Science, 16,157-65. 

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