Evaluation of NAQFC Performance during an Air Pollution Episode in Maryland and the "Postmortem" Analysis Using WRF-CMAQ Simulations

A severe air pollution episode developed on June 26-28, 2019 in Maryland, with maximum hourly ozone concentrations higher than 100 ppbv. The Maryland Department of the Environment (MDE) air quality forecaster consulted results from the NOAA National Air Quality Forecast Capability (NAQFC) to issue air quality alerts during this air pollution episode. The NAQFC successfully predicted the Code Orange (Unhealthy to Sensitive Groups, USG) conditions, but the locations and peak surface ozone concentrations were imperfectly simulated. To investigate this episode, the University of Maryland (UMD) conducted aircraft measurements of ozone and its major precursors aloft such as CO, VOCs, and NO₂ on June 27 and 28, which showed up to 65 ppbv O₃ in the residual layer (> 1000 m AGL) in the morning and up to 85 ppbv O₃, 2 ppbv NO₂, 6 ppbv HCHO, and 180 ppbv CO within the well-mixed PBL in the afternoon. To improve our understanding about the development of this episode, we conducted a “post-mortem experiment” with a coupled WRF-CMAQ modeling system. First, we reproduced the NAQFC forecast using the NAM 12 km forecast meteorology, assimilated emissions developed by the NAQFC team, as well as initial concentrations and boundary conditions from previous NAQFC runs using CMAQ. Model performance of NAQFC was evaluated with MDE surface observation and UMD aircraft measurements in the lower troposphere. Then we conducted two sensitivity experiments: 1) swap the NAM forecasting meteorology with WRF simulations driven by the NCEP GFS reanalysis; 2) swap the NAQFC emissions with the latest EPA 2016 Beta emissions. Analyses of these CMAQ runs can identify the impacts of meteorology and emissions on air quality for this episode. The results provide guidance for understanding the origins of ozone episodes which will help improve air quality forecasting in Maryland and help determine the reduction in emissions sources needed to potentially avoid similar severe pollution episodes in the future.