2002 Annual

Thursday, 17 January 2002: 9:45 AM
PANs measured over Greater Houston during TexAQS 2000 and their implications for hydrocarbon precursors to ozone formation
A.J. Weinheimer, NCAR, Boulder, CO; and F. Flocke, E. L. Atlas, S. G. Donnelly, S. Schauffler, A. Fried, B. P. Wert, J. M. Roberts, M. Trainer, G. J. Frost, D. T. Sueper, C. Wiedinmyer, G. Hubler, T. B. Ryerson, D. K. Nicks Jr., W. C. Custer, P. D. Goldan, D. D. Parrish, and J. S. Holloway
Several PAN-type compounds were measured aboard the NCAR Electra during TexAQS 2000: PAN, APAN, PPN, PiBN, and MPAN. These organic nitrates are co-produced with ozone. Their mixing ratios, and how they correlate with each other and with ozone, have implications for the hydrocarbon precursors to ozone formation. For example, high ozone during the TexAQS project was found to be co-located with high levels of anthropogenic PANs (PAN, PPN, APAN, PiBN). On the other hand, the very high ozone was not co-located with high MPAN, which is biogenic. Thus we can conclude that biogenic hydrocarbons such as isoprene do not contribute significantly to high ozone events in Houston. Additionally, based on PAN-PPN, ozone-PAN, and ozone-PPN correlations in the Greater Houston area, the mix of hydrocarbons contributing to ozone formation in Houston does not appear atypical in an overall sense (though the case of APAN is unique). Moreover, the same appears to be true of the very high ozone events as well, at least to the extent that these have been examined on a case-by-case basis. Further work along these lines will be pursued. In at least some cases, PAN samples corresponding to high ozone values (~200 ppbv) show PAN/ozone ratios and PAN/PPN ratios that are consistent with data from other locales (based on data obtained in these projects: SOS, TOPSE, TRACE-P). There are cases where PAN/PPN ratios are anomalous, but these occur generally when ozone is not extreme.

This PAN perspective, which is "business-as-usual" in many respects, appears to conflict with the proposal that a unique hydrocarbon mix, dominated by ethene and propene, is responsible for ozone formation in Houston in the high-ozone events. A dominance by ethene and propene should lead to unusual correlations of the PAN-type compounds with each other and with ozone, because ethene oxidation results in the formation of none of the PAN-type compounds measured, while the oxidation of propene leads to the formation of PAN but not PPN. The observations of large amounts of ethene and propene downwind of petrochemical industrial sources in the Houston area no doubt suggest that these compounds play a significant role as ozone precursors. Furthermore this is corroborated by the high rate of ozone formation that is found in these cases, the high efficiency of ozone production that is inferred, and by the large amounts of formaldehyde that are observed. Thus we have a dilemma. There is a strong case for the importance of ethene and propene, yet this appears not to be consistent with the perspective portrayed by the PAN-type compounds. A resolution to this dilemma will be pursued by a careful examination of the data on a case-by-case basis, and by consideration of the effects of mixing and loss processes on the correlations of the PAN-type compounds among themselves and with ozone.

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