Compound urban extremes weaken the cooling efficacy of super cool broadband materials in cities

Delhi, one of the fastest-warming megacities globally, frequently experiences compound urban extremes. This study evaluates the radiative–dynamic performance of super cool broadband materials under compound urban extremes. During peak heat episodes, super cool broadband materials can reduce surface temperature by up to 6.3 °C, roof temperature to 34.8 °C, and ambient air temperature by 3.2 °C, with an urban-average cooling of 2.9 °C. Reduced net radiation limits sensible heat flux, weakens buoyant turbulence, and suppresses vertical momentum exchange, resulting in enhanced mesoscale stabilization following surface cooling. Additionally, convective fluxes decline, lowering wind speed by 3.2 m s⁻¹ and planetary boundary-layer height by 752.6 m. Under high-humidity conditions (>75 %), elevated atmospheric water vapor narrows the radiative window and increases downward longwave flux, while enhanced latent heat flux reduces sensible heat release. Net radiation decreases by 276 W m⁻², surface temperature falls by 5.2 °C, roof temperature reaches 30.4 °C, and ambient air cools by 2.4 °C, accompanied by a 9.6 % rise in relative humidity and reductions in wind speed and boundary-layer height to 2.7 m s⁻¹ and 677.1 m. During severe PM₂.₅ pollution (>100 μg m⁻³), aerosol scattering and absorption further reduce insolation, lowering net radiation by 265 W m⁻² and causing 4.7 °C surface and 2.2 °C ambient cooling. The optical design of super cool broadband materials sustains reflectivity and emissivity under heat, humidity, and aerosol-laden skies, maintaining a net radiative deficit and converting urban surfaces from heat sources into radiative sinks, enabling robust, condition-sensitive cooling under Delhi's combined extremes.