Comparison of the carbon bond and SAPRC photochemical mechanisms under conditions relevant to southeast Texas

Gridded, regional photochemical models use simplified photochemical reaction mechanisms, and two commonly used mechanisms are the SAPRC and the carbon bond (CB) mechanism. Versions of the mechanisms currently in use include SAPRC99 and the CB-IV mechanism. For most urban areas, the CB-IV and SAPRC mechanisms yield similar results, but for the modeling done of the summer of 2000 in southeast Texas, the SAPRC mechanism leads to concentrations of ozone that are 30–45 ppb higher than with CB-IV. The differences are due to differences in both reaction rate/stoichiometry parameters and condensation methods in the mechanisms. Major reasons for the differences are: (1) the products of the reactions of aromatics with hydroxyl radical, which are more reactive in the SAPRC formulation, (2) the overall balancing of radical generation and termination reactions, which lead to higher radical concentrations in the SAPRC formulation, and (3) the production of higher aldehydes, which is greater in the SAPRC formulation. The differences between the mechanisms is particularly relevant for evaluating attainment with the National Ambient Air Quality Standard (NAAQS) for ozone concentrations averaged over 8 h.