Twelve-month, 12 km resolution North American WRF-Chem v3.4 air quality simulation: performance evaluation

Abstract. We present results from and evaluate the performance of a 12-month, 12 km horizontal resolution year 2005 air pollution simulation for the contiguous United States using the WRF-Chem (Weather Research and Forecasting with Chemistry) meteorology and chemical transport model (CTM). We employ the 2005 US National Emissions Inventory, the Regional Atmospheric Chemistry Mechanism (RACM), and the Modal Aerosol Dynamics Model for Europe (MADE) with a volatility basis set (VBS) secondary aerosol module. Overall, model performance is comparable to contemporary modeling efforts used for regulatory and health-effects analysis, with an annual average daytime ozone (O₃) mean fractional bias (MFB) of 12% and an annual average fine particulate matter (PM_2.5) MFB of −1%. WRF-Chem, as configured here, tends to overpredict total PM_2.5 at some high concentration locations and generally overpredicts average 24 h O₃ concentrations. Performance is better at predicting daytime-average and daily peak O₃ concentrations, which are more relevant for regulatory and health effects analyses relative to annual average values. Predictive performance for PM_2.5 subspecies is mixed: the model overpredicts particulate sulfate (MFB = 36%), underpredicts particulate nitrate (MFB = −110%) and organic carbon (MFB = −29%), and relatively accurately predicts particulate ammonium (MFB = 3%) and elemental carbon (MFB = 3%), so that the accuracy in total PM_2.5 predictions is to some extent a function of offsetting over- and underpredictions of PM_2.5 subspecies. Model predictive performance for PM_2.5 and its subspecies is in general worse in winter and in the western US than in other seasons and regions, suggesting spatial and temporal opportunities for future WRF-Chem model development and evaluation.