Subambient daytime radiative cooling to mitigate haze-induced amplification of urban heat islands.

Abstract

High concentrations of particulate matter (PM) in cities intensify the urban heat island (UHI) effect by significantly altering the optical and thermophysical properties of the atmosphere. Although passive daytime radiative cooling technology (PDRC) offers a potential mitigation strategy, its performance under haze-polluted urban conditions remains largely unexplored. Here, we develop a general framework to model PDRC in the presence of haze pollution. Our analysis reveals that, due to the selective scattering of PM-which affects solar irradiation and infrared emission differently-the optimal photonic design for haze-polluted atmospheres should prioritize thermal emission within the atmospheric window while reducing emphasis on minimizing solar absorption. Guided by this principle, we optimized the thickness of a polydimethylsiloxane-based transparent thermal emitter by carefully balancing the tradeoff between solar absorption and thermal radiative emission. Experimental results highlight a critical shift: the performance advantage shifts from the thinner cooler under clear skies to the thicker design under haze-polluted conditions, which confirms our design strategy.