Comparison of SAPRC99 and SAPRC07 mechanisms in photochemical modeling for central California

A condensed version of the SAPRC07 photochemical reaction mechanism is compared to an earlier version, SAPRC99, in a 3-dimensional air quality model applied to central California. The updated mechanism results in lower predicted ozone concentrations in all locations. A sensitivity analysis shows the 19% increase (at 1 atm and 300 K) in the OH + NO₂ → HNO₃ reaction rate coefficient contributes to decreases in predicted ozone concentrations in our study domain. This revision to SAPRC07 results in the largest ozone change of all the rate coefficient changes we examined. Also, lumping species to achieve a more computationally efficient mechanism, with respect to SAPRC99, has the effect of lowering ozone concentrations in the region studied. Although ozone concentrations in SAPRC07 simulations are decreased with respect to SAPRC99, the NO_x and VOC emission sensitivities of the new mechanism are on the same order or greater than those in SAPRC99. The new mechanism is less sensitive to climate change related perturbations including increased temperature, water vapor concentration, and biogenic VOC emissions.