Ambient Aerosol Size Distributions

As a result of paiticle emission, in situ formation, and the variety of subsequent processes, the atmospheric aerosol distribution is characterized by a number of modes. The volume or 

FIGURE 8.10 Typical number and volume distributions of atmospheric particles with the different modes. 

Particles with diameters larger than 2.5 pm are identified as coarse particles, while those with diameters less than 2.5 pm are called fine particles. The fine particles include most of the total number of particles and a large fraction of the mass. The fine particles with diameters smaller than 0.1 pm are often called ultrafine particles. 

Atmospheric aerosol size distributions are often described as the sum of n lognormal distributions 

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Using a number of size bins to describe an aerosol size distribution generally results in loss of information about the distribution structure inside each bin. While this may be acceptable for some applications, our goal in this chapter is to develop a rigorous mathematical framework for the description of the aerosol size distribution. The issues discussed in the preceding example provide valuable insights into how we should express and present ambient aerosol size distributions. 

The modified gamma distribution (Deirmendjian 1969) has been proposed as another function that approximates ambient aerosol size distributions, nN(Dp) = AD exp (—BLfp), where A, b, B, and c are all positive constants. 

A measured aerosol size distribution can be reported as a table of the distribution values for dozens of diameters. For many applications carrying around hundreds or thousands of aerosol distribution values is awkward. In these cases it is often convenient to use a relatively simple mathematical function to describe the atmospheric aerosol distribution. These functions are semiempirical in nature and have been chosen because they match well observed shapes of ambient distributions. Of the various mathematical functions that have been proposed, the lognormal distribution (Aitchison and Brown 1957) often provides a good fit and is regularly used in atmospheric applications. A series of other distributions are discussed in the next section. 

Pandis, S. N., Wexler, A. S., and Seinfeld, J. H. (1993) Secondary organic aerosol formation and transport II. Predicting the ambient secondary organic aerosol size distribution, Atmos. Environ., 27A, 2403-2416. 

Since the 1970s laser sizers have offered convenient and rapid estimation of the optical size distributions of nebulized aerosol size distributions. However, just as weight loss as a measure of aerosol output is confounded by evaporation, droplet size distributions are now clearly understood also to be affected by evaporation once the aerosol cloud is mixed with drier ambient air. The methodology adopted within the European standard was inevitably a compromise but may arguably present the most representative clinical compromise. 

The European Standard presents a set of in vitro methods that are expected to reflect in vivo deposition and have been shown to provide repeatable and consistent results. It is clear that some nebulizer designs cannot easily be adapted into this or any other standard and that some flexibility is required in interpreting and applying such to these systems. In particular, obtaining a realistic profile of aerosol size distribution from the small aerosol boluses mixed with entrained ambient air from the Halolite, Circulaire, and AERx is particularly problanatic. However, such technical difficulties can be overcome and realistic measures of... 

Particles in the atmosphere come from different sources, e.g., combustion, windblown dust, and gas-to-particle conversion processes (see Chapter 6). Figure 2-2 illustrates the wide range of particle diameters potentially present in the ambient atmosphere. A typical size distribution of ambient particles is shown in Fig. 2-3. The distribution of number, surface, and mass can occur over different diameters for the same aerosol. Variation in chemical composition as a function of particle diameter has also been observed, as shown in Table 4-3. 

Table II. Summary of Mean Monthly Activity Median Aerodynamic Diameters (AMAD) and Geometric Standard Deviations (tfg) of Radon and Thoron Daughter Size Distributions in Ambient Aerosols...

In exposures of humans to artificially generated aerosols, where the information is to be relevant to ambient aerosols, several factors are important the particle diameter distribution must be fairly constant and fall within size ranges typical for the given compound in the ambient air, the chemical composition of the aerosol must be stable and predictable, and the electric charge distribution of the aerosol must simulate that of normal atmospheric aerosols. 

For example, Fig. 9.23a shows the measured volume distribution of one ambient aerosol sample. When this volume distribution is multiplied by the size distribution of the scattering coefficient per unit volume in Fig. 9.22, one obtains the calculated curve for light scattering in Fig. 9.23b. It is seen that the particles in the 0.1-to 1-yu.m-diameter range, that is, in the accumulation mode, are clearly expected to predominate the light scattering. 

An important aspect of inhalable PAHs is their distribution as a function of particle size in ambient aerosols since size is a key parameter in determining aerosol lung deposition efficiencies (see Chapter 2.A.5). 

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... 

FIGURE 10.15 Mass distributions as a function of particle size of fluoranthene, winter (C = 1.01 ng/m1), and benzo[r ]pyrene, winter (C = 0.060 ng/m1) and summer (C = 0.21 ng/m1), in ambient aerosols sampled at an urban/suburban air monitoring station in Pico Rivera, near downtown Los Angeles (adapted from Venkataraman and Friedlander, 1994b). 

Van Vaeck, L., and K. A. Van Cauwenberghe, Characteristic Parameters of Particle Size Distributions of Primary Organic Constituents of Ambient Aerosols, Environ. Sci. Technol., 19, 707-716... 

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