Research Program Area: Atmospheric Processes
Topic Areas: Chemistry & Reactivity
This project was aimed at developing improved and lower-cost alternative experimental procedures for evaluating chemical mechanisms for predicting ozone impacts of volatile organic compounds (VOCs). More precise measurements of effects of VOCs on OH radicals in chamber experiments could be obtained if 1,3,5-trimethylbenzene is used instead of m-xylene as the radical tracer, but our ability to model reactions of 1,3,5-trimethylbenzene need improvement before the data will reduce uncertainties in evaluations. The use of HONO + VOC irradiations to provide an alternative reactivity measurement to reduce ambiguities in mechanism evaluations, and extend the range of compounds that can be studied, was investigated. Calculations indicate that such experiments are more sensitive to the direct effects of VOCs on ozone formation than environmental chamber experiments. Plug flow experiments with short reaction times should provide the best measurement of direct reactivity of the VOC itself, while stirred flow experiments with longer reaction times are more useful for evaluating effects of the VOC’s reactive products. A HONO generator was constructed that produces a continuous and stable output of HONO at the needed levels. Use of static, stirred flow, and plug flow systems were examined, with best results being obtained with a plug flow system using a 0.7” x 3’ quartz tube reactor with a residence time of ~30 seconds. This was tested using most of the homologous n-alkanes through n-hexadecane, with results generally in good agreement with model predictions. However, low volatility compounds could not always be reliably and reproducibly injected and measured. Experiments with CO,
2-2-4-trimethylpentane, methyl ethyl ketone, ethyl acetate, propene, benzene, toluene, and 1,3,5- trimethylbenzene were also conducted, with the results indicating potential problems with the mechanisms for 2,2,4-trimethyl pentane and the aromatics. Improvements are needed to the system used to inject low volatility compounds, and a total carbon analysis system needs to be integrated into the experiment before the HONO flow method can reliably applied to low volatility compounds. Work on improving this method is underway as part of a new project for the CARB.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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