Gary Z. Whitten

The carbon-bond mechanism: a condensed kinetic mechanism for photochemical smog

Efforts to develop a model that can simulate photochemical smog with kinetic mechanisms are discussed. The carbon-bond mechanism is a set of generalized reactions that can be used to model photochemical oxidant formation. The theoretical framework of carbon-bond mechanism is outlined. Chemical variables that are incorporated into the carbon-bond mechanism model are described. Further work that is needed on the carbon-bond mechanism model is considered. (1 diagram, 13 graphs, 30 references, 2 tables)

Impact of an Updated Carbon Bond Mechanism on Predictions from the CMAQ Modeling System: Preliminary Assessment

Abstract An updated and expanded version of the Carbon Bond mechanism (CB05) has been incorporated into the Community Multiscale Air Quality (CMAQ) modeling system to more accurately simulate wintertime, pristine, and high-altitude situations. The CB05 mechanism has nearly 2 times the number of reactions relative to the previous version of the Carbon Bond mechanism (CB-IV). While the expansions do provide more detailed treatment of urban areas, most of the new reactions involve biogenics, toxics, and species potentially important to particulate formation and acid deposition. Model simulations were performed using the CB05 and the CB-IV mechanisms for the winter and summer of 2001. For winter with the CB05 mechanism, ozone, aerosol nitrate, and aerosol sulfate concentrations were within 1% of the results obtained with the CB-IV mechanism. Organic carbon concentrations were within 2% of the results obtained with the CB-IV mechanism. However, formaldehyde and hydrogen peroxide concentrations were lower by 25...

A mechanism describing the photochemical oxidation of toluene in smog

The photo-oxidation of toluene/NOx exhibits several features that distinguish it from olefin and paraffin smog systems: highly photolytic products, a relatively low production rate of peroxy radicals, and strong sinks for NOx. The underlying chemical behavior of the toluene smog system is discussed and a kinetic simulation mechanism is presented. The mechanism simulates toluene smog chamber experiments conducted at two facilities: the University of California at Riverside evacuable chamber, and the outdoor smog chamber at the University of North Carolina.

The Ozone Formation Potential of 1-Bromo-Propane

Abstract 1-Bromo-propane (1-BP) is a replacement for high-end chlorofluorocarbon (HCFC) solvents. Its reaction rate constant with the OH radical is, on a weight basis, significantly less than that of ethane. However, the overall smog formation chemistry of 1-BP appears to be very unusual compared with typical volatile organic compounds (VOCs) and relatively complex because of the presence of bromine. In smog chamber experiments, 1-BP initially shows a faster ozone build-up than what would be expected from ethane, but the secondary products containing bromine tend to destroy ozone such that 1-BP can have a net overall negative reactivity. Alternative sets of reactions are offered to explain this unusual behavior. Follow-up studies are suggested to resolve the chemistry. Using one set of bromine-related reactions in a photo-chemical grid model shows that 1-BP would be less reactive toward peak ozone formation than ethane with a trend toward even lower ozone impacts in the future.

Comment on "Photochemical reactivity and ozone formation in 1-olefin-nitrogen oxide-air systems".

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The Ozone Productivity of n-Propyl Bromide: Part 2-An Exception to the Maximum Incremental Reactivity Scale

Abstract In an earlier paper the ozone-forming potential of n-propyl bromide (NPB) was studied with a new methodology designed to address issues associated with a marginal smog-forming compound. However, the U.S. Environmental Protection Agency (EPA) subsequently revised its policy and now recommends using the Maximum Incremental Reactivity (MIR) scale to rank the ozone-forming potential of all volatile organic compounds (VOCs), including those of marginal ozone productivity. Nevertheless, EPA contemplated exceptions to the box-model-derived MIR scale by allowing use of photochemical grid-model simulations for case specific reactivity assessments. The California Air Resources Board (CARB) also uses the MIR scale and CARB has a Reactivity Scientific Advisory Committee that can consider exceptions to the MIR scale. In this study, grid-model simulations that were recommended by EPA are used to evaluate the incremental ozone impacts of NPB using an update to the chemical mechanism developed in an earlier paper. New methods of analysis of the grid-model output are further developed here to quantify the relative reactivities between NPB and ethane over a wide range of conditions. The new grid-model-based analyses show that NPB is significantly different and generally less in ozone-forming potential (i.e., reactivity) than predicted by the box-model-based MIR scale relative to ethane, EPA’s “bright-line” test for non-VOC status. Although NPB has low reactivity compared to typical VOCs on any scale, the new grid-model analyses developed here show that NPB is far less reactive (and even has negative reactivity) compared to the reactivity predicted by the MIR scale.