Researchers at the Yale School of Forestry and Environmental Science discovered that haze pollution caused by aerosol particles plays a surprising role in the urban heat island effect, a phenomenon in which city centers are significantly warmer than surrounding rural areas. UHI, which is more apparent during the night, can contribute to heat stress on urban residents.

The study, published in August, analyzed satellite data to show that haze in Chinese cities is a significant factor in UHI and that, for aerosols, size matters — larger particles can amplify UHI at night. Previous studies about the urban heat island effect had focused largely on vegetation density and albedo, the ability of an area to reflect radiation rather than absorbing it, as they affect UHI. Aerosols, which cause haze pollution, consist of very fine particles suspended in gas. The resulting haze has largely gone unstudied as a factor in UHI in Chinese cities. The researchers hope their findings will help combat UHI-related public health issues, including heat stress and heatstroke.

“We’re focusing on urban heat islands from a new perspective, a biochemical perspective, because the former studies usually focus on biophysical [factors] like albedo change, and vegetation,” said Chang Cao , first author of the paper. “But here, we are looking at the aerosol effect on the urban heat island, and our findings have shown that the aerosols … can actually enhance the incoming longwave radiation, especially at nighttime in China.”

While shortwave, or solar, radiation that only occurs during the day is traditionally studied by researchers for its role in UHI, longwave radiation is energy re-emitted by the ground at night as infrared radiation, or heat, after having absorbed the shortwave radiation during the day.

While scientists have long suspected that haze pollution amplified UHI, previous findings have been complicated by the fact that while smaller aerosol particles can scatter shortwave radiation to have a cooling effect, larger particles can actually contribute to the warming effect of UHI by amplifying longwave radiation.

Geographical variations also contributed to the new findings. A previous study conducted by members of the research group had focused on studying North American countries, namely the U.S. and Canada, finding that localized biophysical factors such as heat convection efficiency largely explained differences in temperature between urban and rural areas. Specific recommended strategies for combating UHI resulted from these targeted research factors. The researchers initially sought to compare the analyses of the North American cities to those of the Chinese cities, expecting them to be very similar models.

“To some degree China is similar [to the] U.S.; it is a large country and it has different climate zones,” Chang said.

However, the group realized that some cities in China did not fit the North American model. In North American cities, larger population was linked to more severe UHI, but in China, city size did not correlate to UHI severity. Another contrast the researchers found was that while North American cities suffered from more serious UHI in the daytime, in China, the phenomenon was more pronounced during the nighttime.

On further analysis, the researchers found that the main difference in UHI effect between cities in China and their North American counterparts could be attributed to serious haze pollution caused by aerosols. Chang said the team was the first to quantify the aeresol effect on urban heat islands.

The researchers hope to eventually alleviate the negative public health effects of UHI, including decreasing overall heat stress on the population.

“We are interested in the UHI effect because it affects a large percent of the world’s population, keeping in mind that over 50 percent of the population lives in urban land,” said Xuhui Lee, senior author of the study and professor of forestry and environmental studies. “The UHI increases the number and the intensity of heat waves in cities, thus aggravating the heat stress problem on urban residents.”

Reducing haze pollution in cities, Chang notes, will have multiple benefits for human health, decreasing both respiratory issues resulting from pollution and heat stress caused by the increased urban temperature. The results may also have implications past the traditional contrast between urban and rural areas — because haze pollution is a factor in an extensive part of China, it may also contribute to the accelerated warming of the country’s climate, Chang said.

Future directions for the study include refining climate models to produce more accurate simulations of the nighttime urban heat island effect. According to Chang, the model the researchers used to simulate UHI tended to greatly underestimate the nighttime surface temperature, due to the focus on the harder-to-quantify longwave radiation.

“There are still a lot of uncertainties regarding the effect of haze or aerosol pollution on the climate system. We don’t have good experimental data for checking theoretical predictions,” Lee said. “Cities in China are burdened with unprecedented levels of aerosol pollution. We hope to fill this knowledge gap.”