Advantages and Limitations of Environmental Chambers

Clearly, environmental chamber studies are very useful tools in examining the chemical relationships between emissions and air quality and for carrying out related (e.g., exposure) studies. Use of these chambers has permitted the systematic variation of individual parameters under controlled conditions, unlike ambient air studies, where the continuous injection of pollutants and the effects of meteorology are often difficult to assess and to quantitatively incorporate into the data analysis. Chamber studies have also provided the basis for the validation of computer kinetic models. Finally, they have provided important kinetic and mechanistic information on some of the individual reactions occurring during photochemical smog formation. 

Indeed, offgassing of nitrogenous inorganics has been measured from the Teflon wall of the evacuable 

Clearly, such adsorption-desorption processes on the surfaces of chambers potentially can have substantial effects on the observed levels of 03 and other trace pollutants and on their rates of formation. While such effects can be minimized using bake-out while pumping if the chamber is evacuable, relatively few smog chambers have such capabilities at present. Even for evacuable chambers, contamination from adsorption on, and desorption from, the walls occurs. How to correct the results for this and reliably extrapolate the data to real atmospheres remains problematical. 

A second concern has been indirect evidence pointing to the occurrence of chemical reactions on the walls of the chamber, followed by desorption of some of the products into the gas phase. These reactions manifest themselves by changing the observed gas-phase concentrations of one or more species in a manner that cannot be explained solely on the basis of known homogeneous gas-phase reactions. 

OH as a function of time when a VOC-NOx mixture was irradiated in the evacuable smog chamber shown in Fig. 16.3 (Carter et al., 1981, 1982). The rates of decay of propane and propene were used to estimate the concentration of OH shown by the horizontal bars in Fig. 16.12 as a function of reaction time. 

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