EOMF-25. Experimental Air Quality Forecasting with the Rapid-Refresh Model Coupled to Chemistry (RAP-Chem)

Experimental air quality forecasts with the Rapid-Refresh model coupled to chemistry (RAP- Chem) at NOAA ESRL began in July 2020 in an effort to capture changing atmospheric composition due to the emissions reductions associated with the COVID-19 lockdowns. The full gas-phase and aerosol chemistry mechanism used in the RAP-Chem and proposed for transition into the Unified Forecast System (UFS) offers a potential lower computational cost alternative compared to mechanisms used in similarly capable operational models. Additionally, the RAP-Chem includes wildfire emissions of gases and aerosols, natural emissions of biogenic gases, dust, and sea salt, and simulates aerosol feedback to atmospheric physics allowing evaluation of the impact of changes in atmospheric composition on numerical weather prediction and feedbacks on air quality. Here we will show results of retrospective forecasts of the August-September 2020 wildfire season to highlight key model features and developments in the RAP-chem that are slated for potential implementation into the UFS; specifically, we will demonstrate the suitability of a reduced complexity gas-phase chemical mechanism and its coupling to a VBS-SOA aerosol module, the improvements associated with the use of inline non-local mixing of chemical species with the MYNN PBL scheme, coupling of the full TUV photolysis module, and a simple parameterization of aerosol indirect effects.