Chalmers Climate Dynamics Group

Old towns, with dense populations and compact urban forms, are highly vulnerable to Urban Heat Island (UHI) effects. This study defines and examines a specific UHI subtype—the Old Town Urban Heat Island (OTUHI)—to assess how large-scale atmospheric regulation policies affect thermal conditions in dense historic urban cores. While aerosols influence UHI by modulating solar radiation, the thermal impacts of recent air quality improvements remain unclear. Using multi-source satellite data, we analyze 278 old towns in eastern China from 2001 to 2021. Applying a combined approach of panel fixed-effects modeling and causal inference, we identify significant thermal effects linked to China’s 2013 Air Pollution Prevention and Control Action Plan. Results show OTUHI intensity in southeastern regions nearly doubled post-policy, rising from 0.010 °C to 0.018 °C per year. Mechanistic analysis indicates that reduced aerosol levels increased solar irradiance, which, under southern humid conditions, promoted suburban vegetation growth. This vegetation cooling expanded the temperature gap between urban and suburban areas, intensifying OTUHI. Between 2014 and 2021, the Enhanced Vegetation Index (EVI) in southern suburbs increased by 13.7 %, 1.3 times faster than northern areas. Crucially, the study reveals regional disparities in OTUHI responses to air quality improvements and highlights the need for enhanced urban greening in southern regions to mitigate intensified OTUHI. By defining OTUHI and using a rigorous empirical framework, this work provides new evidence on unintended climate effects of environmental policies, offering insights for climate-adaptive urban planning in similar rapidly urbanizing regions worldwide.