Environmental chamber experiments are currently the primary method used for evaluating mechanisms for predicting ozone impacts of volatile organic compounds (VOC) emissions. However, such experiments are costly, difficult for low volatility compounds, and do not provide a complete test of all of the important aspects of a VOC's mechanism. The use of nitrous acid (HONO) + VOC irradiation experiments to provide an alternative reactivity measurement that can reduce ambiguities in mechanism evaluations, and can potentially be carried out at lower cost and applied to a wider variety of VOCs than currently practical was investigated in this project. Calculations indicate that such experiments provide a better measure of the direct effects of VOCs on the NO to NO2 conversions that cause ozone formation than obtained from environmental chamber experiments.
A nitrous acid generator was constructed that produces a continuous and stable output of ~90 percent HONO at the levels needed for continuous flow experiments. Alternative approaches based on static irradiations, stirred flow and plug flow systems were examined, with the best results being obtained with a plug flow system using a 0.7" x 3' quartz tube reactor with a residence time of about 30 seconds. This system was tested by conducting experiments with the homologous n-alkanes up to n-hexadecane, and results in good agreement with model predictions were obtained. Experiments were conducted on several other representative compounds, and the results indicated potential problems with the mechanisms for 2,2,4-trimethyl pentane and the aromatics. However, experimental problems were encountered in reliably injecting and analyzing the lower volatility compounds, and experiments with some low volatility compounds could not be successfully reproduced.
Improvements are needed to the system used to inject and analyze low volatility compounds before the method can be reliably applied to low volatility compounds or complex mixtures such as petroleum distillates. Work on improving this method is underway as part of a new project for the Air Resources Board and some progress has been made. Preliminary results of this ongoing work will be discussed.
William P. L. Carter holds a joint appointment at the Air Pollution Research Center and the College of Engineering Center for Environmental Research and Technology (CE-CERT) at UC Riverside. His research concerns the gas-phase atmospheric reactions of VOCs and the assessment of ozone and other impacts of VOCs in the atmosphere. This includes developing chemical mechanisms for airshed models, conducting environmental chamber experiments to evaluate and improve these mechanisms, and utilizing them in airshed models to develop ozone reactivity scales for VOCs. Ozone reactivity scales he developed have been incorporated in several VOC emissions regulations in California. Dr. Carter's current projects include developing a new environmental chamber facility for more comprehensive evaluation of gas-phase and particle formation mechanisms, developing new methods for VOC reactivity assessment, experimental and modeling studies to reduce uncertainties in ozone impacts of architectural coatings VOCs, and evaluating and improving mechanisms for low pollution conditions.
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