Overprediction of aerosol nitrate by chemical transport models: The role of grid resolution

This study examines the role of grid resolution on particulate nitrate predictions over the Eastern US during the summer using the three-dimensional chemical transport model (CTM) PMCAMx. The Base Case simulation with coarse resolution (36 × 36 km) often predicts high nighttime nitrate production rates thus leading to overprediction of aerosol nitrate levels. This overprediction is due to the artificial mixing of NOx-rich plumes from major point and area sources with the background atmosphere. Three different horizontal grid resolutions were tested (12 × 12 km, 4 × 4 km and telescoping 12x12/4 × 4 km) for parts of the northeastern US. The bias for PM2.5 nitrate decreased by 65% when the grid resolution was increased to 4 × 4 km. However, the remaining discrepancies between nitrate predictions and measurements indicate the need for additional improvements including better simulation of the total emissions of ammonia and their temporal evolution, improved description of nighttime chemistry and mixing etc.