Comparison of the SAPRC07 and SAPRC99 photochemical mechanisms during a high ozone episode in Texas: Differences in concentrations, OH budget and relative response factors

The latest SAPRC photochemical mechanism SAPRC07 (S07), an update from SAPRC99 (S99), is implemented into the Community Multi-scale Air Quality (CMAQ) model and applied to simulate a high ozone episode during the Texas Air Quality Study 2000 (TexAQS 2000). The results from the S07 mechanism are compared with those from the S99 mechanism. In general, the predicted O₃ concentrations by S07 are lower than those of S99 with maximum difference as high as 20%. The two mechanisms also show significant differences in the predicted OH, HNO₃, PAN and HCHO concentrations. S99 predicts higher OH concentration by 7-20%, higher PAN by approximately 20-25% and higher HCHO by approximately 15% than S07. OH budget analysis suggests that OH propagation factor (P_OH) is higher in urban and industrial areas but lower in rural areas. Although S07 generally predicts similar or higher P_OH, S99 predicts higher new OH formation rate from H₂O + O(¹D) and leads to higher OH and thus higher O₃ in most part of the domain. OH reactions with VOCs and CO accounts for a majority part of reacted OH. Secondary VOCs photolysis is not a main path of regenerated OH. New OH mainly comes from H₂O + O(¹D) reaction. Although the two mechanisms predict different ozone concentrations, the relative response factor (RRF) of ozone at rural, urban and industrial sites under emission controls of anthropogenic NO_x and VOC controls by factors 0.6-1.4 predicted by the two mechanisms are very similar. This suggests that although the absolute predictions are different, both models will provide nearly identical attainment assessment in regulatory applications.