Contributions of local weather characteristics to elevated urban air pollution in northern cities in Canada

This research aims to investigate the effects of severe cold climates on the air quality of urban areas by developing and using a high-resolution (time and space) integrated modeling framework. Simulations were conducted for January and July 2019 for the City of Edmonton. The modeling system couples the WRF and CMAQ models in a nested configuration with a 1 km resolution. The model outputs were validated against surface observations using established benchmarks, confirming the robustness of the model. The results indicate that while average emission rates do not show a significant seasonal difference, average winter concentrations of NO2 (9.9 ppb) and PM 2 . 5 ( 10 . 5 μ g /m 3 ) are more than double those of summer, whereas O 3 average concentrations are marginally higher in summer. The results show that persistent temperature inversions and stratified air flow induce a predominant positive Monin-Obukhov length which severely inhibits vertical mixing and accumulates pollutants such as NO2 and PM 2 . 5 near the surface during the cold season. In contrast, summer conditions display a typical daily pattern of stability, showing an unstable condition driven by low-level jets and high shear wind profiles in the early morning. However, changes in O 3 follow incoming solar radiation rather than stability. Analyzing the VOC/NO 2 and POA/TOA ratios indicates that the winter conditions correspond to a transitional/VOC-limited regime, while the summer conditions reflect NO x -limited chemistry and enhanced SOA formation. Analysis of the vertical dispersion of pollutants using penetration height (PH) shows that winter’s consistently low turbulence intensity (TI) restricts the upward transport of pollutants to higher altitudes. In contrast, summer’s significantly higher and diurnally varying TI enhances PH.