Smog chamber

M. C. Dodge, Combined Use of Modeling Techniques and Smog Chamber Data to Derive O ne-Precursor Relationships,Repott No. EPA-600/3-77-001a, U.S. Environmental Protection Agency, Research Triangle Park, N.C., 1977. 

Similar chemical steps occur in the ambient air and in laboratory smog chamber simulations. Initially, hydrocarbons and nitric oxide are oxidized... 

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).

Equation (12-17) is called the photostationary state expression for ozone. Upon examination, one sees that the concentration of ozone is dependent on the ratio NO2/NO for any value of k. The maximum value of k is dependent on the latitude, time of year, and time of day. In the United States, the range of k is from 0 to 0.55 min T Table 12-5 illustrates the importance of the NO2/NO ratio with respect to how much ozone is required for the photostationary state to exist. The conclusion to be drawn from this table is that most of the NO must be converted to NO2 before O3 will build up in the atmosphere. This is also seen in the diurnal ambient air patterns shown in Fig. 12-2 and the smog chamber simulations shown in Fig. 12-3. It is apparent that without hydrocarbons, the NO is not converted to NO2 efficiently enough to permit the buildup of O3 to levels observed in urban areas. 

In addition to possible errors due to the steps in the kinetic mechanisms, there may be errors in the rate constants due to the smog chamber data... 

Existence of the PSS was predicted theoretically by Leighton (61), and experimental studies of this relationship date back almost 20 years. These experiments have been accomplished in smog chambers (62), polluted urban air (63,64,65), rural environments (66), and in the free troposphere (67). The goal of these experiments has been to verify that our understanding of NOjj chemistry is fundamentally correct, and to ver the role of H02 and R02 in ozone formation. Studies in polluted air seem to confirm the dominance... 

Models of chemical reactions of trace pollutants in groundwater must be based on experimental analysis of the kinetics of possible pollutant interactions with earth materials, much the same as smog chamber studies considered atmospheric photochemistry. Fundamental research could determine the surface chemistry of soil components and processes such as adsorption and desorption, pore diffusion, and biodegradation of contaminants. Hydrodynamic pollutant transport models should be upgraded to take into account chemical reactions at surfaces. 

Nolting F, Behnke W, Zetzsch C. 1988. A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH. Journal of Atmospheric Chemistry 6 47-59. 

The principal pathway leading to degradation of acrylonitrile in air is believed to be photooxidation, mainly by reaction with hydroxyl radicals (OH). The rate constant for acrylonitrile reaction with OH has been measured as 4.1 x 10" cm /molecule/second (Harris et al. 1981). This would correspond to an atmospheric half-life of about 5 to 50 hours. This is consistent with a value of 9 to 10 hours measured in a smog chamber (Suta 1979). 

Oxidation rate constant k for gas-phase second order rate constants, koe for reaction with OH radical, kN03 with N03 radical and ko3 with 03 or as indicated, data at other temperatures see reference k0H = 19.3 x 10-12 cm3 molecule-1 s-1 at 312 K in a smog chamber (Nolting et al. 1988)... 

Air estimated photooxidation t,/2 = 0.24-24 h (Darnall et al 1976) for the reaction with hydroxyl radical photooxidation with OH radicals with an estimated lA 3.1 h (Lyman et al. 1982 quoted, Howard 1989) completely degraded within 6 h in a smog chamber irradiated by sunlight (Kopcynski et al 1972 quoted, Howard 1989) ... 

Nirmalakhandan, N.N., Speece, R.E. (1988b) QSAR model for predicting Henry s law constant. Environ. Sci. Technol. 22,1349-1357. Nolting, F., Behnke, W., Zetzsch C. (1988) A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH. J. Atmos Chem. 6, 47-59. 

Winer, A.M., Damall, K.R., Atkinson, R. Pitts, Jr., J.N. (1979) Smog chamber study of the correlation of hydroxyl radical rate constants with ozone formation. Environ. Sci. Technol. 7, 622-626. 

The time dependence of the oxidant concentrations shown in Figure 2-1 can be mimicked in laboratoiy studies. The results of a typical smog-chamber experiment are shown in Figure 2-3. A sample of air initially... 

Laboratory experiments of this type have the great advantage that the initial conditions can be well defined (although often they are not ), in contrast with the average sample of urban air, which is a mixture of new and old pollutants. Also, in laboratory experiments, the same sample of air is observed over a long period, which is not possible with most air pollution monitoring networks. For these reasons, most attempts to understand the chemistry of oxidant formation have concentrated on smog-chamber experiments, rather than the real atmosphere. 

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. 

Both the modeling studies and smog-chamber simulations show significant oxidant formation with NO -h aldehydes, NO, + alkanes (except methane), or even NO, -i- carbon monoxide in moist air. The development of significant oxidant from NO + aldehydes is particularly ominous, because aldehyde emission is not now controlled. As the modelers state [Pg.27]

The success of these computer simulations must be rated as quite good. Figure 2-8 compares concentration-time measurements from a smog-chamber study of NO,-propylene-air with computer-calculated results based on the same initial conditions. The time dependences and absolute concentrations agree fairly well, but not perfectly. Note that the... 

FIGURE 2-8 Photooxidation of propylene in irradiated CjH -NO-NO, mixtures in moist air. A. experimental rate data from smog-chamber experiment of Altshuller et al. Initial concentrations C,H, 2.09 ppm NO. 0.90 ppm NOj. 0.09 ppm. Relative humidity at 31.S C, 50%. B. computer simulation of product concentration-tinte curves for same initial conditions. Reprinted with permission from Demeijian et al. 

However, the number of concentrations being fitted is fairly small, compared with the number of parameters going into the model. The computer calculations can be used to predict the concentrations of other trace compounds that should be present in smog chambers. If these compounds are later found at approximately the predicted concentrations, the model will be strengthened. If not, changes will have to be made in the model. A more stringent test of the model will occur when it becomes... 

Table 2-6 is a list of some compounds that may be present in photochemical smog, but have not yet been reported. The presence of some of these compounds (such as PBzN and ketene) seems very probable, in diat they have been observed in smog-chamber studies, whereas others are... 

TABLE 3-5 Compounds Investigated in Smog-Chamber Studies, with References... 

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