Interpreting predictions from the SAPRC07 mechanism based on regional and continental simulations

The SAPRC07T mechanism is implemented and evaluated in the CMAQ air quality model. The implementation is described and tested with simulations over the United States for two periods. The evaluation compares results against observations for ozone and particulate matter as well as against predictions from the SAPRC-99 mechanism for the same simulation periods and model domain. Comparisons against SAPRC-99 results reveal largest differences over urban areas with low VOC to NOx ratios. Integrated rate analysis is used to investigate the chemical reactions responsible for these differences. Comparison against observations shows over predictions of ozone concentrations over the southeastern United States. The errors appear linked to failing to match the observations during early morning hours. We discuss the source of the errors in terms of both simulation inputs and the photochemical mechanism and recommend potential directions for improving applications with regional photochemical models.