Quantitative and mechanistic study of the effect of river valley topography on urban scale pollution dispersion

This study investigates the influence of complex terrain on air pollution dispersion during a valley-basin pollution episode in Lanzhou, northwestern China, using the WRF-CALPUFF modeling system and terrain-modified sensitivity experiments to quantify these topographic effects and reveal their underlying mechanisms. Results indicate that orographic features increase NO₂ concentrations in urban areas by approximately 30 % on average, while channeling effects enhance ventilation and reduce pollutants in northwestern valleys. Analysis of atmospheric dynamics and thermodynamics reveals that thermally-induced local circulations combined with topographic stagnation create recirculation zones that effectively trap pollutants, as evidenced by recirculation factor analysis showing pronounced accumulation in these zones compared to well-ventilated areas. Valley heat deficit (VHD) demonstrates a strong correlation with NO₂ levels (R = 0.77), where elevated VHD coincides spatially with recirculation zones and pollution hotspots. The study identifies a critical VHD threshold of 1.4 MJ/m², beyond which pollution episodes occur due to suppressed vertical mixing from enhanced atmospheric stability and restricted horizontal transport from terrain-induced flow blockage. These findings highlight how the combined effects of thermal stabilization and flow obstruction work synergistically to sustain pollution accumulation in mountainous regions. The research establishes a comprehensive framework that elucidates the coupled dynamic-thermodynamic mechanisms through which terrain characteristics modulate pollution dispersion patterns, offering valuable insights into boundary layer processes and airflow dynamics during pollution events, with important implications for understanding urban air quality deterioration in complex terrain environments.