Evaluating photochemical mechanisms for use in Southeast Texas
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Gridded, regional photochemical models use simplified photochemical reaction mechanisms, and two commonly used mechanisms are the [California] Statewide Air Pollution Research Center (SAPRC) mechanism and the Carbon Bond (CB) mechanism. Versions of the mechanisms currently in use include SAPRC99 and the CB-IV version from 1996. For the modeling done of the summer of 2000 in southeast Texas, the SAPRC99 mechanism leads to concentrations of ozone that are 30-45 ppb higher than with CB-IV, and is more sensitive to reductions in NOx emissions. Differences between the mechanisms could have significant consequences for determining the levels of emission reductions that will be required to demonstrate attainment with the National Ambient Air Quality Standard (NAAQS) for ozone, with concentrations averaged over 8 hours. Therefore, various modeling tools, together with chamber experiments were used to diagnose the differences between the mechanisms. These differences are due to differences in both reaction rate parameters/stoichiometry and the condensation methods in the mechanisms. Major reasons for the differences are differences in aromatics and free radicals chemistries, which lead to higher radical concentrations in the SAPRC formulation.