Smog chamber experiment

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 2-8 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. 

Recent chemical modeling studies have been reasonably successful in reproducing the concentration-time histories of smog-chamber experiments. An examination of these models shows a need for much more detailed chemical knowledge. Modeling studies also point out the necessity of carefully defining the initial conditions of smog-chamber experiments. Some observations that have been made with these models are ... 

Smog-chamber experiments on aerosol formation, 54-72 applied to atmosphere, 66-67 compounds studied in, 57-59 on chemistry of oxidant formation, 16, 27... 

Chemical/Physical. Gaseous products formed from the reaction of cyclohexene with ozone were (% yield) formic acid (12), carbon monoxide (18), carbon dioxide (42), ethylene (1), and valeraldehyde (17) (Hatakeyama et al., 1987). In a smog chamber experiment conducted in the dark at 25 °C, cyclohexane reacted with ozone. The following products and their respective molar yields were oxalic acid (6.16%), malonic acid (6.88%), succinic acid (0.63%), glutaric acid (5.89%), adipic acid (2.20%), 4-hydroxybutanal (2.60%), hydroxypentanoic acid (1.02%), hydroxyglutaric acid (2.33%), hydroxyadipic acid (1.19%), 4-oxobutanoic acid (6.90%), 4-oxopentanoic acid (4.52%), 6-oxohexanoic acid (4.16%), 1,4-butandial (0.53%), 1,5-pentanedial (0.44%), 1,6-hexanedial (1.64%), and pentanal (17.05%). 

In a smog chamber experiment, 2-methylphenol reacted with nitrogen oxides to form nitrocresols, dinitrocresols, and hydroxynitrocresols (McMurry and Grosjean, 1985). Anticipated products from the reaction of 2-methylphenol with ozone or OH radicals in the atmosphere are hydroxynitrotoluenes and ring cleavage compounds (Cupitt, 1980). 

Eberhard, J., C. Muller, D. W. Stocker, and J. A. Kerr, The Photo-oxidation of Diethyl Ether in Smog Chamber Experiments Simulating Tropospheric Conditions Product Studies and Proposed Mechanism, lnt. J. Chem. Kinet., 25, 639-649 (1993). 

FIGURE 7.2 Ratio of final concentrations of PAN to HN03 (PAN/HNO3) versus initial VOC/NOv in a series of smog chamber experiments. The HNO-, includes both that in the gas phase and that estimated to be adsorbed on chamber walls (from Spicer, 1983). 

Akimoto, H H. Takagi, and F. Sakamaki, Photoenhancement of the Nitrous Acid Formation in the Surface Reaction of Nitrogen Dioxide and Water Vapor Extra Radical Source in Smog Chamber Experiments, lnt. J. Chem. Kinet., 19, 539-551 (1987). 

It is apparent from (VIII- 18a) that a rapid conversion of NO to N02 is needed for the buildup of O, concentration levels. It was recognized that the conversion rate of NO to N02, observed in the real atmosphere, was several hundred times as fast as the rate of the reaction 2NO + 02 — 2NOz in the range of NO concentrations of 0.05 to 0.5 ppm. Furthermore, in simulated smog chamber experiments it was found that the rate of butene consumption by illumination of a NO- N02-H20-butene mixture in air was much faster than that calculated on the basis of reactions of O(3P) atoms and 02 with butene. Apparently a new mechanism based on radical chain reactions was needed to explain these observations. Two groups of scientists, Heicklen and coworkers and Weinstock and coworkers, were the first to propose independently the following sequence of reactions in 1970. 

Heterogeneous reactions on the surface of seasalt aerosols have been suggested as a potential source of atomic chlorine in the marine boundary layer [10,105]. The tropospheric relevance of the reaction of nitrogen oxide species N203 with Nad and NaBr was demonstrated in a smog chamber experiment on dry and deliquiescent NaCl aerosol and on salt solutions [74,78,106,107] ... 

NHa, S0E, HCHQ and H sQs and measurements of these gases have been made under field conditions and in smog chamber experiments. A modified version of the system in which a liquid Na Dewar was substituted for the helium cryostat was flown successfully in and above the boundary layer over the eastern Pacific to measure N0a and HNOa. 

Smog chamber experiments have shown that the addition of aldehyde significantly increases the formation rates of ozone and the conversion rates of NO and NO2 under simulated urban conditions. ... 

Atkinson, Pitts, and coworkers at the University of California at Riverside are in the process of publishing a series of papers on NO3 chemistry. I will briefly describe a few of their results. Their method involves the use of a smog chamber experiment with long path infrared absorption measurements of reactant concentrations [26]. Their kinetic measurements are based on the competition between two different reactants for the NO3 radical. N2O5 is used as a source of NO3. Table 2 summarizes some of their data for reactions of NO3 with selected organic compounds. Most of these materials are present in significant concentrations only in polluted urban environments. For the alkene compounds the rate coefficients follow the general trend found few the addition reactions of O and OH. This supports the proposal that the mechanism involves addition to the double bond. A dramatic increase in reactivity is observed with the... 

The stabilized products, propionaldehyde and propylene oxide, have not yet been observed in smog chamber experiments of this system. However, this is understandable because as has been discussed above, Reaction 4 accounts for only a small fraction of the total CsHc consumption. 

The mechanism described has been the subject of numerous validation exercises. The smog chamber experiments against which the mechanism has been tested include the following hydrocarbons propylene, isobutylene in the presence and absence of CO, n-butane in the presence and absence of CO, and a mixture of propylene and n-butane. Predicted concentrations generally match well with experimental results for all systems studied. See Hecht and Seinfeld 45) for a detailed description of the validation procedure and results.)... 

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