The size distribution of atmospheric aerosol particles

Very early on, Aitken (1923) showed that most particles in the atmosphere are smaller than 0.1 pm diameter and that their concentrations vary from some hundreds per cm over the ocean to millions per cm in urban areas. Junge (1955,1963,1972) measured the atmospheric aerosol number size distribution and concentration in urban and non-urban areas as functions of altitude and site. He established the standard form for plotting size distribution data log of AN/ADp versus logD, where N = number and Dp = particle diameter. He observed that this plot was a straight line that could be described by the equation AN/ADp = AD, where A and k were constants. He also noted that in the range from 0.1 to 10.0 pm particle diameter, k was approximately equal to 4.0. This distribution mode was widely known as the Junge distribution or the power law distribution. 

Friedlander (1961) later showed that by balancing aerosol source and removal rates a portion of the resultant theoretical number distribution steady state could be fitted reasonably well by the Junge distribution. Clark and Whitby (1967), by fitting the Junge distribution to 52 at- 

Whitby et al. (1972) found that the number size distribution established by Junge was not a good model for the surface and mass or volume size distribution which was normally bimodal, with one mode being around about 0.3 pm diameter and the other ranging from 5.0 to 15.0 pm diameter. 

Depending on their source there may be from one to three distinct maxima in the surface and volume or mass distributions. The activity size distribution of a radionuclide-associated aerosol particle is a surface distribution (Papastefanou and Bondietti, 1987). 

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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Meszaros a., On the size distribution of atmospheric aerosol particles of different composition. Atmos. Environ. , 11, 1075-1081 (1977). 

Curve J of Fig. 28 represents the size distribution of atmospheric aerosol particles obtained by A. Meszaros and Vissy (1974) from samples collected on membrane filters over the oceans of the Southern Hemisphere. The samples were evaluated by optical and electron microscopy in the radius range of 0.03-64 /tm8. The total concentration of these particles is also shown. It can be seen that the maximum of the distribution occurs around 0.1 /Jir> radius, a value in the range of the... 

A method for estimating the residence time of tropospheric aerosol particles associated with the cosmic-ray produced radionuclides, such as Be, is based on the aerosol particle growth rate, which is the change of particle diameter with time, which was estimated to be 0.004 to 0.005 pmh (McMurry and Wilson, 1982) and the difference between the activity median aerodynamic diameter, AMAD, of a radionuclide, e.g. Be, and the size of the Aitken nuclei in the size distribution of the aerosol particles, which is 0.015 pm (NRC, 1979). The AMAD of all radionuclides is in the accumulation mode of the size distribution of atmospheric aerosol particles which ranges between 0.1 and 2.0 pm (NRC, 1979 Papastefanou and Bondietti, 1987). 

The climate effects of atmospheric aerosol particles are a matter of continuous interest in the research community. The aerosol-climate effects are divided into two groups The direct effect represents the ability of the particle population to absorb and scatter short-wave radiation - directly affecting the radiation balance. These direct effects depend primarily on the aerosol optical properties and particle number size distribution, as the particle size significantly affects the scattering efficiency of... 

Sweepout and interception most efficiently remove larger particles (particle diameter > 1 pm). Diffusion is most efficient for very small particles (particle diameter < 0.1pm). Consequently, very fine and very large aerosol particles are efficiently removed from the containment atmosphere by sprays. There is, however, an intermediate size of particle that is minimally affected by sprays. The decontamination of the atmosphere of these intermediate sized particles is increased by decreasing the size of the spray droplets, which are typically between 250 and 2000 pm in diameter. Because of the particle size dependence of spray effectiveness, the spray not only changes the concentration of particles in the atmosphere, it also changes the size distribution of these aerosols. The residual aerosol has a narrow distribution of sizes centered around the size minimally affected by the spray. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

To better understand the effects of atmospheric processes (reactions, gas-particle partitioning, etc.) on the size distributions of PAHs in ambient aerosols, Venkataraman and Friedlander (1994b) carried out measurements of gases and particles during winter and... 

On-line measurements of the sulfur content of atmospheric aerosols have been made by removing gaseous sulfur species from the aerosol and then analyzing the particles for sulfur with a flame photometric detector (24) or by using an electrostatic precipitator to chop the aerosol particles from the gas so that the sulfur content could be measured by the difference in flame photometric detector response with and without particles present. These and similar methods could be extended to the analysis of size-classified samples to provide on-line size-resolved aerosol composition data, although the analytical methods would have to be extremely sensitive to achieve the size resolution possible in size distribution analysis. 

The concentration of metals in atmospheric aerosols and rainwater (Table 7.1) is therefore a function of their sources. This includes both the occurrence of the metals in combustion processes and their volatility, as well as their occurrence in crustal dust and seawater. As a result of this, the size distribution of different metals is very different and depends on the balance of these sources. For a particular metal this distinction is similar in most global locations (Table 7.2), although some variability does occur as wind speed and distance from source exert an influence on the particle size distribution spectrum (Slinn, 1983). Once in the atmosphere particles can change size and composition to some extent by condensation of water vapour, by coagulation with other particles, by chemical reaction, or by activation (when supersaturated) to become cloud or fog droplets (Andreae et al., 1986 Arimoto et al., 1997 Seinfeld and Pandis, 1998). 

In the fourth type of identification the chemical composition of particles is studied in situ. By suitable chemical aerosol instruments the concentration and the size distribution of certain elements can be continuously monitored. The flame photometry of sodium containing particles (e.g. Hobbs, 1971) is a good example for such a method. Recently flame photometric detectors have also been developed to measure aerosol sulfur in the atmosphere (e.g. Kittelson et at., 1978). 

The first challenge concerns the involvement of multiple phases in wet deposition. Not only does one deal with the three usual phases (gas, aerosol, and aqueous), but the aqueous phase can be present in several forms (cloudwater, rain, snow, ice crystals, sleet, hail, etc.), all of which have a size resolution. To complicate matters even further, different processes operate inside a cloud, and others below it. Our goal will initially be to create a mathematical framework for this rather complicated picture. To simplify things as much as possible we consider a warm raining cloud without the complications of ice and snow. There are four media or phases present, namely, air, cloud droplets, aerosol particles, and rain droplets. A given species may exist in each of these phases for example, nitrate may exist in air as nitric acid vapor, dissolved in rain and cloud droplets as nitrate, and in various salts in the aerosol phase. Nonvolatile species like metals exist only in droplets and aerosols, while gases like HCHO exist only in the gas phase and the droplets. The size distribution of cloud droplets, rain droplets, and aerosols provides an additional complication. Let us initially neglect this feature. For a species i, one needs to describe mathematically its concentration in air C(,air, cloudwater C,[C 0ud, rainwater C .rain, and the aerosol phase Qpan- We assume that all concentrations are expressed as moles of i per volume of air (e.g., mol m 3 of air). These concentrations will be a function of the location (x,y,z) and time and can be described by the atmospheric diffusion equation... 

It is out of the scope of this book to describe the AP mechanics, i. e. microphysics and dynamics (Friedlander 1977, Hinds 1882, Kouimtzis and Samara 1995, Harrison and van Grieken 1998, Meszaros 1999, Spumy 1999, 2000, Baron and Willeke 2001). Here, we only summarize the important topic of atmospheric aerosol size distribution (Jaenicke 1999). Fig. 4.15 shows that the size range covers several orders of magnitude. Therefore, the common logarithm of the radius is useful to describe the different distribution functions dN r)ld gr = f( gr) or dN r)ldr =/(Igr)/2.302 r. N r) cumulative number size distribution (or the integral of radii) having dimension cm , r radius of particle ... 

FIGURE 9 Left distribution of particle number concentration versus aerodynamically equivalent radius for the rural continental aerosol. Right The corresponding size distributions for the concentrations of particle surface and volume. The size distribution of mass concentration is nearly equivalent to that of volume. [From Zellner, R., ed. (1999). Global Aspects of Atmospheric Chemistry, Steinkopff/Springer, Darmstadt, Germany.]... 

Highlights During the execution of industrial operations involving UFg, release of some material is practically inevitable. The aerosols formed in the atmosphere initially contain particulate UO2F2 in which the F U atom ratio should theoretically be 2 1. The size distribution of these particles depends on the relative humidity and the temperature, and experimentally the diameter of most particles was found to be between 0.4 and 1.0 pm. The aging of the particles would lead to gradual loss of fluorine and decrease in the F U ratio. However, the presence of other fluorides. 

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