Profile, smog

Fig. 12-3. Concentration versus time profiles of propene, NO, NO, -NO, and Oj from smog chamber irradiation /Cj = 0.16 min. Source Akimoto, H., Sakamaki, F., Hoshino, M, Inoue, G., and Oduda, M., Emiiron. Set. Technol. 13, 53-58 (1979).

FIGURE 2 3 Typical concentration-time profiles for irradiation of a propylene-NO, mixture in a smog chamber. Reprinted with permission from Niki et ai. 

FIGURE 3-15 Smog-chamber profiles. Ltfi, cyclohexene. Right, tetramethylethylene. Initial concentrations hydrocarbon, 1 ppm NO, 0.33 ppm NO, 0.16 ppm. 

Perhaps the most direct experimental means of examining the relationship between emissions and air quality is to simulate atmospheric conditions using large chambers. Measured concentrations of the primary pollutants are injected into these environmental (or smog) chambers, as they are called. These are then irradiated with sunlight or lamps used to mimic the sun, and the time-concentration profiles of the primary pollutants as well as the resulting secondary pollutants are measured. The primary pollutant concentrations as well as temperature, relative humidity, and so on can be systematically varied to establish the relationship between emissions and air quality, free from the complexities of continuously injected pollutant emissions and meteorology, both of which complicate the interpretation of ambient air data. 

FIGURE 16.7 Typical primary and secondary pollutant profiles in a propene-NO, irradiation in a smog chamber (adapted from Pitts et al., 1975). 

FIGURE 16.11 Ozone time profiles from the irradiation of a hydrocarbon-NO, mixture (initial concentrations were 0.4 ppm propene, 2.0 ppm n-butane, 0.4 ppm NO, and O.f ppm N02) in a smog chamber where the light source has been filtered with Pyrex of thickness jy or y in., respectively (adapted from Winer el al., f979). 

However, without knowledge of the source of the increased OH flux, extrapolation of the concentration-time profiles of both the primary and secondary pollutants observed in such smog chamber studies to real atmospheres becomes less certain. For example, the reactions leading to the unknown precursor(s) to OH may occur only in smog chambers. Extrapolation to ambient air would thus require subtracting out this radical source. On the other hand, the same reactions may occur in ambient air where surfaces are available in the form of particulate matter, buildings, the earth, and so on if this is true, then the rates would be expected to depend on the nature and types of surfaces available and may thus differ quantitatively from the smog chamber observations. 

Figure 9.9 Typical profiles of primary (green lines), secondary (red lines) and tertiary pollutants (blue lines) of the photochemical smog. PAN, peroxyacetyl nitrate, CH3C03N02...

The overall effect of humidity on the smog reactions of propylene has been estimated, based on Reactions 48-54 and 58. Figure 9 shows a comparison of reaction profiles computed for zero and 50% relative humidity. According to the present kinetic model, the humidity effect seems to be rather slight. However, this prediction should be further verified experimentally. Existing smog chamber results on the humidity effect are not definitive and are frequently conflicting (66, 67, 68). 

Solar Radiation. Of all the factors which collectively determine the amount and spectral distribution of the radiation entering a surface layer of the atmosphere, the best established appear to be the spectral irradiance outside the atmosphere and the attenuation by molecular scattering. The absorption coefficients of ozone are well established, but no easy method exists for determining the amount of ozone in a vertical profile of the atmosphere at a given time. The measurement of the particulate content of the atmosphere and its correlation with atmospheric transmission is a field in which much remains to be accomplished. Surprisingly few data exist on the spectral distribution of sky radiation and its variation with solar elevation and atmospheric conditions. The effect of clouds is of secondary importance, as intense smog generally occurs under a clear sky. 

Figure 1. Experimental ( ) and simulated ozone profiles for different initial concentrations of nitrous acid and formaldehyde for the smog chamber run using fuel 1.

Figure 1. Typical smog profile—mineral spirits (4.0 ppm) + NO (2.0...

F. 7.7 Time profiles of NO, NO2 and O3 mixing ratios under the irradiation of NO2 in the smog ehamher. The values of ki in the figure is an experimentally obtained photolysis rate constant of NO2 from the stationary state equation (see text) (Adapted from Akimoto et al. 1979a)... 

Fig. 7.8 Time profiles of the mixing ratios of CaH, NO, NOx-NOandOjin the irradiation of the mixture of CaH. and NOx in air in the smog chamber (Adapted from Akimoto et al. 1979b)...

Laboratory chamber studies utilizing PTR-MS for VOC measurements have also been used to elucidate the chemical routes for the formation of secondary organic aerosols [191-197]. An example is aerosol formation from the 1,3,5-trimethylbenzene-propene-NO c-water vapour system, which was investigated at the PSI smog chamber [ 192,196]. This work classified the VOCs according to their temporal profile and identified oligimerization as a key secondary organic aerosol formation mechanism. 

An interesting reaction is the oxidation of CO to CO2 and the reduction of NO to N2 in automotive exhaust gases as a smog-control measure. Figure 20-18 shows how this reaction is thought to occur on the surface of rhodium metal in a catalytic converter. In general, the reaction profile for a surface-catalyzed reaction resembles that shown in Figure 20-19. 

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