A Positive Urbanization–Pollution–Fog Feedback Loop Revealed by High-Resolution Modeling over Eastern China

Urbanization and atmospheric aerosols individually influence fog, yet their synergistic effects and coupled feedback mechanisms remain poorly quantified. This study aims to systematically disentangle the thermal, dynamic, and aerosol-radiative contributions of urbanization and pollution to a persistent dense fog event, and to elucidate the positive feedback loop connecting urban development, pollutant emissions, fog evolution, and boundary layer processes. A heavy fog episode over the Yangtze River Delta during 23–30 November 2018 was simulated by using the Weather Research and Forecasting model with Chemistry (WRF-Chem) over three two-way nested domains (finest resolution: 3 km) and 40 vertical levels. Four sensitivity experiments were designed against a baseline simulation (BASE): replacing urban with cropland (CTL1), turning off anthropogenic heat (CTL2), reducing building height (CTL3), and switching off aerosol radiative effects while retaining urban surfaces (CTL4). The model was validated against surface observations from eight national stations and ERA5 reanalysis, showing correlation coefficients of 0.85 for both 2-m temperature and relative humidity. The results show that urbanization significantly suppresses fog development. Relative to suburbs, the urban heat island weakens the nocturnal near-surface inversion by 0.3°C and elevates the mean planetary boundary laye height (H_PBL) by 20 m. Consequently, urban areas exhibit lower nocturnal relative humidity (94% vs 98%), reduced liquid water content (LWC; 0.21 vs 0.25 g kg⁻¹), lower maximum fog-top height (162 vs 231 m), and shorter (by ~10.6 h) cumulative fog duration. Sensitivity experiments quantify the individual LWC reductions: integrated urban surface modification contributes ~0.04 g kg⁻¹; anthropogenic heat flux contributes ~0.02 g kg⁻¹; and increased building height contributes ~0.01 g kg⁻¹. Aerosol radiative effects (CTL4 vs BASE) further reduce the mean urban LWC by ~0.03 g kg⁻¹, while their daytime solar attenuation delays dissipation and nocturnal blanketing reinforces the urban heat island, slightly prolonging fog duration. It is concluded that urbanization and aerosol pollution jointly establish a positive “urbanization–pollution–fog–boundary layer” feedback loop: urban development drives pollutant emissions; aerosols modify the boundary layer thermal structure through radiative effects; this in turn suppresses vertical fog development (lower LWC and top height) but modestly extends fog lifespan and enhances pollutant retention. This feedback provides a quantitative, process-level insight for improving fog forecasting and transportation risk assessment in heavily polluted urban agglomerations.