Note: Yale School of the Environment (YSE) was formerly known as the Yale School of Forestry & Environmental Studies (F&ES). News articles and events posted prior to July 1, 2020 refer to the School’s name at that time.

A new Yale-led study published in the journal Nature Communications sheds light on the surprising role that haze in China plays in promoting the urban heat island effect [UHI], a process whereby city centers tend to be significantly warmer than surrounding rural areas.

Scientists have always suspected that aerosol particles, or haze, played a role in amplifying UHI, said Xuhui Lee, the Sara Shallenberger Brown Professor of Meteorology at the Yale School of Forestry & Environmental Studies [F&ES] and senior author of the study. Now, they have evidence that in China — unlike the U.S. — haze is a significant driver of UHI.

The study also highlights the paradoxical nature of haze, and the challenges scientists face in isolating its effects on climate change. Smaller aerosol particles that pose public health risks such as asthma actually cool surfaces and reduce UHI by blocking sunlight. But larger aerosol particles radiate heat in the form of long-wave radiation, thus increasing local temperatures.

“When people talk about aerosols, particle size matters a lot,” Lee said.

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