Chemical oxidant mechanisms for air quality modeling: critical review

Chemical oxidant mechanisms suitable for use in grid-based air quality simulation models are reviewed. Included are (1) a lumped structure mechanism, known as the Carbon Bond Mechanism, that is used for both urban- and regional-scale modeling, (2) a lumped species mechanism, called the SAPRC mechanism, which was developed for urban-scale modeling, (3) two versions of a lumped species mechanism (the RADM and RACM) developed for regional-scale applications, and (4) a new mechanism now under development, called the Morphecule Mechanism, that uses a novel approach for representing organic species. Included in the review are descriptions of the mechanisms and the results of evaluating the mechanisms against smog chamber data. Because of the importance that smog chamber data play in the development and evaluation of chemical mechanisms, a description of the chamber data available for this purpose is also presented. The shortcomings of both the smog chamber data and the existing chemical mechanisms are described. These shortcomings include difficulties in characterizing chamber artifact effects and chamber photolysis rates, uncertainties associated with kinetic and mechanistic data used to develop the mechanisms, and the assumptions and approximations used to condense mechanisms to a size appropriate for use in air quality models. Recommendations are offered for research activities to reduce these uncertainties. An effort is also made to translate the uncertainties and information gaps associated with the mechanisms into uncertainties in ozone predictions and ozone control strategies.