Aitken chemical composition

When considering the impact of uptake by aerosol, the chemical composition of the aerosol is also likely to be significant. Bates et al. (1998, 2001) measured strong variations in the chemical composition of the Aitken, accommodation and sea-salt dominated coarse modes that would influence the free radical uptake rates, particularly the extent of aerosol acidification. Without data on the size segregated aerosol chemical composition during SOAPEX-2 and the relevant laboratory data, it is not possible to calculate accurate accommodation coefficients. 

The most common way in aerosol science to represent PND data is in terms of various modes. Generally, these modes are nucleation (typically in the 1-30 nm range), Aitken (typically in the 20-100 nm range), accumulation (typically in the 30-300 nm range) and coarse (typically over 300 nm size range). Each mode contains different sources, size range, formation mechanisms, and chemical compositions [30],... 

The study of Reiter et al. (1976) at the Wank Mountain Observatory distinguished days with strong and weak vertical exchange by means of Aitken nuclei counts and concentrations of radon decay products. The main difference between both meteorological situations is a shift in the total aerosol concentration. Variations in chemical composition are minor. This indicates that at the height of the Wank Mountain [1780 m above sea level (a.s.l.)] the continental aerosol is already well homogenized. Gillette and Blifford (1971) and Delany et al. (1973) have reached similar conclusions from aircraft samples over the North American continent. 

The sources and chemical compositions of the fine and coarse urban particles are different. Coarse particles are generated by mechanical processes and consist of soil dust, seasalt, fly ash, tire wear particles, and so on. Aitken and accumulation mode particles contain primary particles from combustion sources and secondary aerosol material (sulfate, nitrate, ammonium, secondary organics) formed by chemical reactions resulting in gas-to-particle conversion (see Chapters 10 and 14). 

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 variation of Tg of the soft matrix in segmented polyurethanes as a function of composition or segmental chemical structure has been monitored and used as an indicator of the degree of microphase separation. Factors influencing the phase-separation process in these MDI-based polyurethanes have been summarized by Aitken and Jeffs (69) as follows (a) crystallization of either component, (b) the steric hin-... 

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