Research Program Area: Atmospheric Processes
Topic Areas: Chemistry & Reactivity
A combined experimental and chemical computer modeling research program has been carried out to develop and evaluate an experimental protocol for the determination of organic reactivities.
While it is well recognized that organics emitted into the atmosphere have a wide range of reactivities, the precise definition and ranking of organic reactivities is still a matter of debate, and in fact depends on the actual physical and chemical environment considered. Moreover, because of the recently recognized influence of chamber-dependent radical initiation processes on the gas phase chemistry of irradiated NOx- organic-air mixtures, data obtained from single NOx-organic-air systems are of only qualitative value. Thus, we have employed the irradiation of NOx-multicomponent organic-air mixtures to which incremental additions or deletions of the test organic were made.
Based upon a previous SAPRC / ARB 13-component surrogate hydrocarbon mixture designed to account for organic emissions from all sources into the South Coast Air Basin, we first developed a four-component surrogate mixture (the "mini-surrogate") which mimicked the more complex mixture in terms of maximum O3 yields, NO oxidation rates and organic consumption rates. In all subsequent NOx-organic-air irradiations incremental additions or deletions of the test organic were made to this four-component mini-surrogate mixture. Irradiations were carried out under either dual-mode (in a 40,000 liter outdoor all-Teflon chamber) conditions. The majority of the irradiations were carried out under single-mode conditions and the effects of addition (or in some cases removal) of toluene, benzaldehyde, propene, n-butane, trans-2-butene, methanol, ethanol and t-butyl methyl ether were determined in consecutive single-mode indoor chamber irradiations. In this series the standard mini-surrogate-NOx run was alternated with runs with added or removed test organics.
As an integral part of this overall program, chemical kinetic computer modeling studies were also carried out using the most up-to-date kinetic and mechanistic data. Within the experimental uncertainties, the data from the chamber irradiations were in agreement with the computer predictions. Of particular interest was the observation that, with respect to maximum O3 yields, negative reactivities were observed for toluene and benzaldehyde, (i.e, the maximum O3 concentrations were reduced in the presence of these two organics), while propene and trans-2-butene exhibited slightly negative or zero reactivities under the particular experimental conditions employed. However, in terms of NO oxidation, all of the organics studied, apart from benzaldehyde, enhanced the oxidation rates.
The chemical reasons for these observed effects are interpreted in terms of our current knowledge of the atmospheric chemistry of the organics studied, and the relevance of this research program to the development of emission control strategies by the CARB is discussed.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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