Mechanisms of Aerosol Composition Effects on Multi‑Layer Urban Heat Islands: A Case Study of Beijing

Severe PM_2.5 pollution (particles with an aerodynamic diameter ≤ 2.5 μm) and the urban heat island (UHI) effect pose serious threats to human health and living environments in densely populated cities. However, the specific role of aerosols in shortwave and longwave radiation transfer, as well as the mechanisms through which radiation processes affect urban heat island intensity (UHII), remain insufficiently understood. In this study, the WRF-Chem model was employed to simulate several typical pollution episodes during 2016–2017. We quantitatively assessed aerosol radiative forcing and further distinguished the contributions of different aerosol components to shortwave and longwave radiation, systematically analyzing their impacts on surface UHI, canopy-layer UHI, and boundary-layer UHI. The results show that, overall, boundary-layer UHI increases with worsening pollution, while the peak intensities of surface and canopy-layer UHIs are significantly weakened under polluted conditions. However, during sustained pollution episodes, as pollution intensifies, the maximum UHI intensities of both tend to increase. To exclude the influence of indirect aerosol radiative effects, periods with high pollution but low cloud cover were selected for further analysis. Comparative sensitivity experiments reveal that absorbing aerosols enhance UHIs at all levels, particularly daytime canopy-layer UHI (by 14.65%) and nighttime boundary-layer UHI (by 20.04%). In contrast, scattering aerosols weaken boundary-layer UHI and daytime surface UHI, while strengthening canopy-layer UHI and nighttime surface UHI. By comparing radiative heating profiles in urban and rural areas, we found that absorbing aerosols absorb more radiation in urban areas during the day, resulting in a markedly higher heating rate than in rural areas; at night, urban areas also exhibit slightly stronger heat retention. Decomposing the radiative heating profiles into shortwave and longwave components further indicates that absorbing aerosols strongly absorb shortwave radiation during the day and subsequently heat the near-surface layer via longwave radiation at night. Scattering aerosols reduce radiation received by the surface and boundary layer during the day, while at night they intercept longwave radiation in the upper boundary layer, leading to warming above and cooling below the boundary layer. In summary, absorbing aerosols enhance UHIs at all levels by absorbing shortwave radiation during the day and continue to intensify UHI through longwave radiation release at night. Scattering aerosols, by scattering solar radiation, weaken boundary-layer UHI and reduce daytime surface heating, while scattering radiation toward the canopy enhances canopy-layer UHI. This study distinguishes between the radiative effects of absorbing and scattering aerosols, revealing their differential impacts on multi-layer urban heat islands and providing new insights into pollution-climate interactions. The findings offer relevant implications for urban climate adaptation planning and synergistic air quality management.