Modeling urban pollutant transport at multi-resolutions: Impacts of turbulent mixing

Abstract. Air pollution in cities seriously impacts public health and regional climate. Turbulent mixing plays a crucial role in pollutant formation and dissipation, yet current atmospheric models struggle to accurately represent it. The intensity of turbulent mixing varies with model resolution, which has rarely been analyzed. To investigate turbulent mixing variations at multi-resolutions and their implications for urban pollutant transport, we conducted numerical experiments using WRF-Chem at 25 km, 5 km, and 1 km resolutions. The simulated meteorological fields and black carbon (BC) concentrations are compared with observations. Differences in turbulent mixing across multi-resolutions are more pronounced at night, resulting in noticeable variations in BC concentrations. BC surface concentrations decrease as resolution increases from 25 km to 5 km and further to 1 km, but are similar at 5 km and 1 km resolutions. Enhanced planetary boundary layer (PBL) mixing coefficients and vertical wind flux at higher resolutions reduce the overestimation of nighttime BC surface concentrations. The 1 km resolution parameterized lower PBL mixing coefficients than 5 km but resolved more small-scale eddies, leading to similar near-surface turbulent mixing at both resolutions, while the intensity at higher altitudes is greater at 1 km. This caused BC to be transported higher and farther, increasing its atmospheric lifetime and column concentrations. Variations in mixing coefficients are partly attributed to differences in land use and terrain, with higher resolutions providing more detailed data that enhanced PBL mixing coefficients. This study interprets the impacts of turbulent mixing on simulated urban pollutant diffusion at multi-resolutions.