Research Projects

Project at a Glance

Project Status: complete

Title: Environmental chamber studies for development of an updated photochemical mechanism for VOC reactivity assessment.

Principal Investigator / Author(s): Carter, William P L

Contractor: UC Riverside

Contract Number: 92-345


Research Program Area: Atmospheric Processes


Abstract:

A series of indoor environmental chamber experiments were conducted to fill gaps in the data base needed for evaluation of gas phase photochemical mechanisms for assessing the effects of emissions of volatile organic compounds (VOCs) on ambient air quality. Two large dual-mode indoor Teflon bag chambers, one irradiated by blacklights and the other by xenon are light, were employed. Alternative methods for measuring light intensity in these chambers were evaluated. It was found that quartz tube NO2 actinometry provides satisfactory data for the blacklight chamber, but that CL2 -n-butane irradiations provide a better method for the xenon arc chamber. The effects fo varying humidity on results and reproducibility of chamber experiments were examined. It was found that differences between dry runs and runs at ~50 percent RH were minor and should not significantly affect mechanism evaluation results, but that runs with humidities approaching 100 percent RH may have problems. Incremental reactivity experiments, where a test compound is added to NOx-air irradiations of reactive organic gas (ROG) surrogates representing ambient pollution, were conducted for representative compounds using the xenon arc chamber. These were needed to supplement the much larger data base of incremental reactivity experiments in blacklight chambers. The results were consistent with model predictions and the large data base of incremental reactivity experiments in blacklight chambers, if the aromatics mechanisms was modified to account for differences in light source. An extensive series of single aromatic NOx experiments were carried out using both light sources to provide data needed to develop, adjust and evaluate mechanisms for benzene, toluene, ethylbenzene, o-, m- and p-xylenes, and all three trimethylbenzene isomers. The current version of the detailed SAPRC mechanism (SAPRC-93) did not correctly account for isomeric differences and tended to underpredict reactivities in the xenon arc chamber. Much better fits could be obtained if yields of two lumped fragmentation products, representing different photodecomposition action spectra, are optimized separately for each isomer. However, such adjustment still did not provide satisfactory fits for benzene, and could not simulate the data for the other aromatics in all respects. The data base obtained in this study will be an important resource for evaluating updated mechanisms which are under development.


 

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