Growth by coagulation

As indicated earlier, primary exhaust lead particulates are of approximate diameter 0.015 fim and are subject to growth by coagulation processes [7]. In a polluted urban atmosphere, growth is rapid and continues until particle sizes of ca. 0.5 iim are reached by which time the Brownian diffusion mechanism of particle collisions becomes unimportant. Typical size distributions measured with Andersen impactors, which determine aerodynamic particle sizes, are shown in Fig. 2.10. 

Because of the stability and low moisture content of the stratosphere, clouds do not form in it. The absence of cloud formation prevents particle removal by rainout. Also, since particle concentrations are only a few per cubic centimeter, growth by coagulation with subsequent removal by settling is negligible. Similarly, growth by condensation is slow because of low vapor concentration. Consequently, particle lifetimes in the stratosphere are 1—2 years, compared with 1—2 weeks in the lower troposphere. Climatic and global effects of the stratospheric aerosol are described in Section 14.3. 

Number of k-fold Precursor Particles. Dynamic differential equations were written for the concentration of the k-fold precursors to account for birth and death by coagulation, growth by propagation, and the formation of primary precursors by homogeneous nucleation. There... 

Soybeans have been used as food in the Orient since ancient times and various methods have been developed to make soybeans as palatable as possible. In recent years, a large number of these simply processed soyfoods are emerging in the West. Tofu and tempeh are the most popular and have the fastest growth rate of any soyfood in America. Tofu is made by coagulating the protein with a calcium or magnesium salt from a hot-water extracted, protein-oil emulsion of whole soybeans. 

In addition to particle growth by accretion of mass by chemical reaction, growth can occur as well by coagulation. This process reduces the number concentration of particles, while preserving mass concentration. Coagulation is especially important for small particles shortly after nucleation (Kerminen et al., 2001). Likewise, unactivated particles in clouds can be substantially scavenged by activated cloud droplets because of their high surface areas. 

The change in the discrete distribution function with time and position is obtained by generalizing the equation of convective diffusion (Chapter 3) to include terms for particle growth and coagulation ... 

Figure 11.4 Evolution of the moments of the size distribution function for the aerosols shown in Fig. 11.3, The peak in the number distribution probably results when formation by homogeneous nudeaiiun is balanced by coagulation. Total aerosol volume increases with time as gas-tO partide conversion takes place. Total. surface area, A, increases at first and then approaches an approximately constant value, due probably toa balance between growth and coagulation (Husarand Whitby, 1973). The results should be compared with Pig. 11.2.

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