City expansion alters the land surface and can cause an urban heat island (UHI), where a metropolitan area is warmer than its surroundings, particularly during the daytime and under weak winds. Cities are also known for their substantial aerosol emissions, generally thought to absorb and reflect sunlight, shade the land surface and offset daytime UHI intensity. However, these impacts can vary throughout the year, and seasonal differences are not well documented.
Wenchao Han from Beijing Normal University, China, and co-authors used satellite and ground-based observations for 35 Chinese cities during 2001–2010, as well as a regional climate model, to study the seasonality of aerosol–UHI interaction. During the summer, aerosols typically decrease UHI intensity, as expected from the aerosol radiative effect that shades the surface. In contrast, aerosol pollution tends to increase UHI intensity during the winter. The authors attribute this to an aerosol dynamic effect, where aerosols absorb sunlight aloft and stabilize the atmosphere, preventing vertical mixing and cooling. These results add critical understanding to the interactions between urban development, air pollution and heat exposure.
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Langenbrunner, B. Aerosol-driven seasonality. Nat. Clim. Chang. 10, 708 (2020). https://doi.org/10.1038/s41558-020-0868-z