Multipollutant modeling of ozone, reactive nitrogen and HAPs across the continental US with CMAQ-CB6

The accuracy of atmospheric chemical mechanisms used in air quality models is critical for robustly predicting the production and decay of air pollutants and developing strategies to reduce concentrations that are above levels harmful to humans and ecosystems. In this study we document, evaluate and analyze an application of the CB6r3 chemical mechanism in a 3-D air quality model, the Community Multiscale Air Quality (CMAQ) model, and demonstrate the impact of this updated chemical mechanism on predictions of ozone, oxidized nitrogen and formaldehyde. In general, CMAQ-CB6 slightly improves the predictions of ozone over much of the dynamic range, while providing updates that are more consistent with current scientific understanding. For these reasons, CMAQ-CB6 is recommended in place of earlier versions available in CMAQ. Nitric acid is generally overpredicted in both winter and summer, and ongoing work continues to address this overprediction and update other aspects of the mechanism.