Aerosol trimodal

Whitby and Cantrell (16) report that aerosols collected near the surface of the earth are trimodal in nature. As listed in Table IV, these consist of distributions arising from (1) the irect emission of primary products from combustion having d = 0.02 un (2) coagulation of the aerosols from (1), or the c6ndensation of reaction products or water on the aerosols from (1), having = 0.16 and (3) larger particles arising from... 

Trimodal Nature of Surface Aerosols as Proposed by Whitby and Cantrell (16)... 

FIGURE 9.7 Schematic of an atmospheric aerosol size distribution showing four modes. The original hypothesis of Whitby and co-workers is shown by the solid, trimodal curves, and the fourth, ultrafine particle mode, as well as the two peaks sometimes observed in the accumulation mode are shown by the dashed lines (adapted from Whitby and Sverdrup, 1980). 

With increased sensitivity, the size-segregating samplers have produced data that are most helpful in characterizing the size and composition of aerosols. The data from these samplers have been used to resolve a bimodal, and sometimes a trimodal, distribution of particles. 

As noted above, the LUT algorithm assumes a unimodal lognormal functional form to describe stratospheric aerosols. This approximation is well suited for most non-volcanic stratospheric aerosols as shown by Pueschel et al. [7] and Yue et al. [8]. Volcanic size distributions, however, are typically bi- or trimodal. This raises the question of whether the assumption of unimodality in the LUT can introduce bias into retrieved values of Rt//, S and V. Russell et al. [1] have shown that retrieved unimodal distributions accurately describe the second, larger mode of several measured bimodal size distributions, but fail to account for the smaller particles in the first mode. The smaller particles, which contribute little to the measured extinction spectra, are not accounted for in the LUT retrievals. Unless this bias is accounted for, the values of Rtff retrieved under the assumption of a unimodal distribution will be overestimated. 

We routinely use nose-only inhalation exposure of B(a)P aerosol to evaluate the consequence of prenatal exposure to this toxicant on physiological and behavioral endpoints. The properties of this B(a)P aerosol are shown in Figure 17.4. The aerosol typically exhibits a trimodal distribution with a 93% cumulative mass less than 5.85 pm, 89% cumulative mass less than 10 pm, 55.3% cumulative mass less than 2.5 pm, and 38% less than 1 pm. Fifty-five percent of the aerosol generally has a cumulative mass less than PM2.5 and the mass median aerodynamic diameter (MMAD) + geometric standard deviation for this mode is consistently 1.7 =E 0.085 pm. For several years we employed a rat model exposing timed pregnant dams to inhalation concentrations of 25, 75, and 100 pg/m. ... 

Figure 2. Trimodal stmcture of the submicron particle number size distribution observed at a boreal forest in Hyytia la, Finland on June 17, 1996, 08 09-08 19. The total particle number concentration of the submicron aerosol is 1011 particles cm. From Ma kela et al. (1997). Used by permission of the American Geophysical Union.

There is now considerable evidence to show that due to the individual source processes, the atmospheric aerosol as a whole has a size distribution that is basically trimodal, even though it is rare for all three modes to show... 

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. 

The aerodynamic size distribution of radionuclide-associated aerosol particles is, as mentioned in Section 2, a surface distribution, and so it is trimodal, the first mode being the so-called Aitken nuclei mode, the second the accumulation mode and the third the coarse particle mode. Analytically, these modes can be summarised as follows ... 

Tables 8 and 9 show that only the plasma torch results in a trimodal distribution of aerosols, these aerosols being less for stainless steel than for mild steel. This may be explained by the vaporization of the stainless steel ingredients (chrome, etc.) and the greater oxidation of mild steel.

Figure 3 A trimodal number-size distribution of an Arctic aerosol measured during the International Arctic Ocean Expedition 1991 (1). The solid squares represent the measurement, and the solid line the sum of log-normal distributions (dotted lines) of the ultrafine, the Aitken, and the accumulation mode.

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