Coupling CMAQv5.3 with FV3 and Its Intercomparison with FV3-CMAQv5.0.2 for the Next Generation of the National Air Quality Forecasting Capability

The next generation of the National Air Quality Forecast Capability (NAQFC) will use a new Finite Volume Cube-Sphere Global Forecast System (FV3GFS) to drive the Community Multiscale Air Quality (CMAQ) modelling system. An experimental version of the offline-coupled FV3-CMAQ v5.0.2 has been applied for real-time air quality forecasting over the U.S. since July 2017. However, its preliminary evaluation by NOAA shows moderate-to-large biases, e.g., ozone (O₃) and fine particulate matter (PM_2.5) underpredictions over the western U.S. during wildfires, O₃ overprediction but PM_2.5 underprediction over eastern U.S., and large, diurnally compensating errors in PM_2.5 (e.g., daytime underpredictions but nighttime overpredictions). Since v5.0.2, significant updates in the gas and aerosol chemistry of CMAQ have occurred. For example, CMAQ v5.3 updates the 2005 carbon-bond 5 gas-phase mechanism (CB05) to the carbon-bond gas-phase mechanism version 6 (CB6). Compared to CB05, CB6 contains more detailed halogen and dimethyl sulfoxide chemistry as well as new organic chemistry pathways, products, and updated reaction rates. A new aerosol module aero7 in v5.3 contains multiple changes for secondary organic aerosols (SOA) compared to aero6 in v5.0.2. These changes include improved uptake of isoprene-derived intermediates and dicarbonyls onto aqueous aerosol particles, new mechanisms for biogenic SOA from monoterpenes, reorganization of anthropogenic SOA using a volatility basis set-like speciation, and semivolatile partitioning and gas-phase aging of primary organic aerosols. Further, CMAQ v5.3 offers an alternative aqueous chemistry module, AQCHEM-KMT2, that contains 53 extended aqueous-phase reactions previously not included in AQCHEM. updates, CMAQ v5.3 is expected to have an improved forecasting capability for both O₃ and PM_2.5, especially SOA. In this study, CMAQ v5.3 is first coupled offline with FV3 then evaluated against FV3-CMAQ v5.0.2 and available observations for July 2019 over continental U.S. The evaluation and intercomparison will include O₃, PM_2.5, PM₁₀, and their gaseous precursors. Major differences between the predictions from the two model systems and between the model predictions and observations as well as the underlying causes will be investigated. This work will demonstrate the impacts of mechanism updates in CMAQ v5.3 on its forecasting capability, thus shed light on areas of further model improvements on FV3-CMAQv5.3.