Size distribution coarse mode

Fig. 6. Size distribution of an urban aerosol showing the three modes containing much of the aerosol mass. The fine mode contains particles produced by condensation of low volatility gases. The mid-range, or accumulation mode, results from coagulation of smaller aerosols and condensation of gases on preexisting particles. Coarse particulates, the largest aerosols, are usually generated mechanically.

However, this mode of flow usually is limited to conveying distances of L < 500 m. The Group D materials that possess wide size distributions (especially a considerable amount of fines) and/or unusual particle shape (e.g., crushed coal, petroleum coke, crushed bath, rice hulls) usually are not suited to low-velocity conveying. In these cases, the options are dilute-phase or single-slug dense-phase (i.e., these materials usually are too coarse for bypass conveying). However, when long distances are involved, dilute-phase may be the only practical option. 

Figure 4 presents particle size distributions for six elements which differ among themselves and also from those in Figure 3. Somewhat subjectively, we may identify three patterns in these distributions (a) A coarse mode, typified by Ca and the other elements of Figure 3, which may represent a terrestrial dust origin. This mode can account for coarse particle concentrations observed for Fe, K, Mn, and S. (b) A fine mode with somewhat greater concentrations in the 0.5-1 ymad fraction than in 1-2 ymad particles. The amounts in this <2 ymad range, in excess of those which can be attributed to a coarse crustal aerosol tail with the Ca distribution, show similarities in particle size distributions for Zn, Mn, and possibly Fe. Since the trends shown in Figure 2 point to these elements being characteristic of large scale air masses, their fine modes may be principally due to natural processes.

Figure 4. Particle size distributions of 6 elements that exhibit fine-mode concentrations, as distinct from a coarse-mode typified by Ca.

Concentrations K, Fe, Mn represent excess over that attributed to a coarse mode with Ca particle size distribution. 

A word of caution is also in order with respect to assigning a particular particle to the fine or coarse particle modes. Since the size distributions can generally be described as log-normal, they do not have sharp cutoffs. A few particles at the top end of the fine mode distribution will have diameters larger than 2.5 [jlm and a few at the bottom end of the coarse mode will have diameters smaller than this. For example, as Lodge (1985) points out, for a coarse particle distribution with a geometric mean diameter of 15 fim and a geometric standard deviation of 3, about 5% of the particles will have diameters below the 2.5-gm fine particle cutoff. This may be responsible for observations that while Si and Ca dominate the coarse particle mode, they are also often found at significant levels in fine particles (e.g., see Katrinak et al., 1995). 

Figure 9.35 shows a typical set of mass size distributions for total suspended particles (TSP), Na, Cl, Al, V, NO-, S04, and NH4 at Chichi in the Ogasawara (Bonin) Islands, about 1000 km southeast of the main island of Japan (Yoshizumi and Asakuno, 1986). As expected for a marine site such as this, Na and Cl from sea salt predominate, and both the TSP and Na and Cl components peak in the coarse particle range. Al is also found primarily in the larger particles and is attributed to a contribution from soil dust. On the other hand, vanadium, non-sea salt sulfate (nss-S04 ), and ammonium are primarily in the fine particles. The vanadium levels are extremely low and likely reflect long-range transport of an air mass containing the products of combustion of fuel oil, which contains V because it is likely associated with a combustion source, it would be expected in the fine particle mode, consistent with Fig. 9.35. 

In a similar study, Allen and co-workers (1996) determined the particle size distribution for 15 PAHs with molecular weights ranging from 178 (e.g., phenan-threne) to 300 (coronene) and associated with urban aerosols in Boston, Massachusetts. As for BaP in the winter (Venkataraman and Friedlander, 1994b), PAHs with MW >228 were primarily present in the fine aerosol fraction (Dp < 2 /Am). A study of 6-ring, MW 302 PAH at the same site showed bimodal distributions, with most of the mass in the 0.3- to 1.0-/zm particle size size range a smaller fraction was in the ultrafine mode particles (0.09-0.14 /xm) (Allen et al., 1998). For PAHs with MW 178—202, the compounds were approximately evenly distributed between the fine and coarse (D > 2 /am) fractions. Polycyclic aromatic hydrocarbons in size-segregated aerosols col-... 

Figure 1. Example of compositionally resolved bimodal and monomodal distributions of aerosols. The ordinate gives the percent of the species found in the given size fraction of the impactor. The mode near 0.3 xm is the accumulation mode , and that above 8 xm is the coarse mode The minimum of mass between 1 and 2 xm is typical the chlorine distribution is anomalous. Chlorine is in fact a coarse-mode marine aerosol that has lost its larger particles during transport from the ocean to Davis, California, a distance of roughly 100 km. (Reproduced with permission from reference 15. Copyright 1988.)...

Number concentrations are dominated by submicron particles, whereas the mass concentrations are strongly influenced by particle concentrations in 0.1-10 pm diameter range [13]. Similarly, the variability of the number-based measurements is strongly dominated by variability in smaller diameter ranges, whereas the variability of mass-based properties, such as PM10, are dominated by variability in the accumulation mode (usually around 500 nm of mass mean diameter) and in the coarse mode. This means the variabilities of these properties are not necessarily similar in shorter timescales, due to sensitivity of variance from very different air masses and thus aerosol types. This is demonstrated in Fig. lb, where the variance of the each size class of particle number concentrations between 3 and 1,000 nm is shown for SMEAR II station in Hyytiala, Finland. The variance has similarities to the particle number size distribution (Fig. la), but there are also significant differences, especially on smaller particles sizes. Even though in the median particle number size distribution the nucleation mode is visible only weakly, it is a major contributor to submicron particle number concentration variability. 

Most of the studies on size-resolved aerosol mass concentrations in areas with different levels of pollution show that particulate matter typically exhibit a bimodal distribution, with most of their mass being found in the submicron size range (dae < 1pm) and an additional minor mode in the coarse fraction (1 < dae < 10 pm) (Maenhaut et al., 2002 Smolfk et al., 2003 Wang et al., 2003 Gajananda et al., 2005 Samara and Voutsa, 2005). However, with instrumentation allowing more precise measurements, the aerosol mass size distribution was found to be multimodal with the preponderance of a fine mode (dae < 0.2 pm) and an accumulation mode (dae 0.5pm), with a minor coarse mode at d 3-4pm (Raes et al., 2000 Pillai and Moorthy, 2001 Berner et al., 2004). Traditionally, atmospheric researchers classify airborne particles into three size classes coarse (2.5 < c/ ie < 10pm), fine... 

Figure 12.1. Theoretical aerosol mass size distribution profile showing a typical segmentation of chemical species into fine (dae < 2.5 pm) and coarse (2.5 < dae < 10 pm) modes. [Adapted from Seinfeld (1986), Seinfeld and Pandis (1998), Krivdcsy and Molnar (1998), and Samara and Voutsa (2005).]...

Figure 6 Several views of the size distribution of a hypothetical aerosol consisting of three modes denoted nuclei, accumulation, and coarse. Panel (a) is a log-log plot of number distribution of the three modes and their sum as a function of particle diameter. Dp. Panels (b)-(d) are semi-log plots of number N), surface area (5), and volume (V) distributions, respectively (Whitby, 1978) (reproduced by permission of Elsevier from Atmos. Environ. 1978, 72,...

The residence time distribution curve (Fig. 13.4) provides further insight into the origins of the bimodal distribution. The peak in the residence time curve falls in the size range 0.1 < dp < 1.0 (im corresponding to the accumulation mode. Although the coarse mode has a short residence time, it is continually reinforced by fresh injections of crustal material and. perhaps, anthropogenic sources. Thus the two modes are essentially uncoupled. 

Figure 13.8 Size distribution for ammonium nitrate in the Claremont. California aerosol, 13 July 1987 (0600-0930) (John ct a]., 1990). Strong nitrate peaks are frequently observed in the coarse mode. This is not usually the case for sulfates (Fig. 13.7).

Atmospheric Aerosol Size Distribution 360 General Features 360 Coarse Mode (dp > 2.S pm) 361 Accumulation Mode fO.l < dp 2.5 pm) 364... 

Size distribution of the volume of aerosol particles (solid line) according to Whitby (1978). Dotted line corresponds to the power law of Fig. 26. V particle volume n nuclei mode a accumulation mode < coarse particle mode tlrGV geometric volume mean size. (By courtesy of Atmospheric EnvironmentI... 

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