Density distributions, aerosol

Besides mass concentration, atmospheric particles are often characterized by their size distribution. Aerosols are typically sized in terms of the aerodynamic equivalent diameter (dae) of the particle, usually expressed in micrometer (pm) or nanometer (nm) (Mark, 1998). Atmospheric particles are usually nonspherical and with unknown density. Therefore, the r/ae of a particle is usually defined as the diameter of an equivalent unit density sphere (p = 1 gctrf3) having the same terminal velocity as the particle in question (Mark, 1998 Seinfeld and Pandis, 1998). 

Mathai CV (1990) Visibility and fine particles. 4. A summary of the A WMA EPA International Specialty Conference. J Air Waste Manage Assoc 40 1486-1494 Matteson MJ, Preiming O, Fox JF (1972) Density distribution of sodium-chloride aerosols formed by condensation. Nature-Phys Sci 238 61... 

Fig. 7-3. Average volume size distributions for continental aerosols. [Adapted from Whitby and Sverdrup (1980).] The measurement data were smoothed and idealized by fitting to them additive log-normal distributions. (1) Background aerosol, very clean (2) normal background aerosol (3) background aerosol disturbed by an urban plume (these data from measurements at Goldstone, California). (4) Average urban aerosol (from data taken at Minneapolis, Minnesota, Denver, Colorado, and various locations in California). The dashed curve gives the volume distribution resulting from the number density distribution for the rural continental aerosol shown in Fig. 7-1. The integrated volumina, given by the area underneath each curve, are shown in the insert.

Fig. 1. Number density distribution of maritime and continental aerosol in the surface boundary layer. (Straight Line power law distribution with exponent s = 3.4, see text.)...

Fig. 1.2. Normalised frequency plots of number, surface, and volume (particle volume times particle density) distributions for the grand average 1969 Pasadena, California smog aerosol. Note the bimodal distribution of mass. Each weighting shows features of the distribution not shown by the other plots. From Whitby (1975, p. II-ll) in NRC (1979).

Table VIII. Physical Processes Affecting the Evolution of Aerosol Number Density Distributions...

Activity Median Aerodynamic Diameter (AMAD)—The diameter of a unit-density sphere with the same terminal settling velocity in air as that of the aerosol particle whose activity is the median for the entire size distribution of the aerosol. 

The contributions of aerosol chemical species to the extinction coefficient can be estimated from knowledge of their mass distributions, densities, and refractive indices. It is assumed that the particles can be represented as spheres. For a mixture in which the composition is a function of particle size and all particles of a given size have the same composition, defined here as a specific mixture, the contribution of species i becomes (4 ) ... 

In practice, when one measures the size distributions of aerosols using techniques discussed in Chapter 11, one normally measures one parameter, for example, number or mass, as a function of size. For example, impactor data usually give the mass of particles by size interval. From such data, one can obtain the geometric mass mean diameter (which applies only to the mass distribution), and crg, which, as discussed, is the same for all types of log-normal distributions for this one sample. Given the geometric mass mean diameter (/) ,) in this case and crg, an important question is whether the other types of mean diameters (i.e., number, surface, and volume) can be determined from these data or if separate experimental measurements are required. The answer is that these other types of mean diameters can indeed be calculated for smooth spheres whose density is independent of diameter. The conversions are carried out using equations developed for fine-particle technology in 1929 by Hatch and Choate. 

Particle Measurements. A variety of instruments is available for measuring the number density and size distribution of particles sampled from airborne platforms. This discussion is restricted to instruments that measure particles smaller than 50 xm (cloud droplets and aerosol particles) because these particles are of most interest to atmospheric chemists. 

The additional channels in the lower portion of the size spectrum overlap the sizes measured by the ASASP and the PCASP probes and have allowed intercomparisons between aerosol size distributions measured by these two probes (90). Dye et al. have evaluated the capabilities of this probe for measuring the number density and size distribution of atmospheric aerosols (98). 

Accomplishment of the complex observational experiment LACE-98 made it possible to obtain extensive information about atmospheric aerosol (aircraft measurements of the size distribution and number density of fine aerosols, coefficients of aerosol absorption, backscattering and depolarization, chemical composition of aerosol, as well as surface observations of the spectral optical thickness of the atmosphere, coefficients of extinction and backscattering). Fiebig et al. (2002) compared the observational data on optical parameters obtained from the results of numerical modeling for total H2S04 aerosol near the tropopause as well as for the ammonium sulfate/soot mixture in the remainder of the air column (Osborne et al., 2004). 

Barr et al. (2003) performed an analysis of the impact of phytogenic aerosol (PhA) which is defined as forming mainly due to monoterpene oxidation (primarily, a- and /3-pinenes), on the radiative regime of the ABL over the forest in the eastern part of Canada. In the forest ecosystem the level of emissions to the atmosphere of biogenic hydrocarbons is moderate, with the concentration of a- and /3-pinenes constituting about 1.6 ppb. NMHC oxidation resulted in the formation of PhA at a number density of particles of about 5 108 cm 3. For a given concentration and size distribution of aerosol, its impact on the short-wave radiation transfer in the ABL was assessed. 

Aspiration rate is only a small part of the overall transport process in flame spectrometry. The production of aerosol and its transport through the spray chamber are also of great importance. The size distribution of aerosol produced depends upon the surface tension, density, and viscosity of the sample solution. An empirical equation relating aerosol size distribution to these parameters and to nebulizer gas and solution flow rates was first worked out by Nukiyama and Tanasawa,5 who were interested in the size distributions in fuel sprays for rocket motors. Their equation has been extensively exploited in analytical flame spectrometry.2,6-7 Careful matrix matching is therefore essential not only for matching aspiration rates of samples and standards, but also for matching the size distributions of their respective aerosols. Samples and standards with identical size distributions will be transported to the flame with identical efficiencies, a key requirement in analytical flame spectrometry. 

In general, particles or droplets in the size range 5-10 wm tend to deposit in the nasal passages. Although the extent and site of particle deposition can be estimated from a knowledge of the aerodynamic size distribution of the aerosol, the situation can be complicated by the fact that the size of the particle can increase (and possibly its density decrease) as a result of water condensation, due to the humidity change upon entering the nasal cavity. 

Example 8.6 An aerosol made up of unit-density spheres is lognormally distributed with a geometric mean diameter of 2.0 pm and a geometric standard deviation of 2.2. Calculate the respirable fraction of this aerosol as sampled by a sampler which follows the BMRC curve and a sampler which follows the ACGIH curve. 

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