Air pollution control is unambiguously good for health, but aerosols also reflect sunlight, and removing them changes the surface energy balance. Researchers in the Department of Geography in the HKU Faculty of Social Sciences, with collaborators at the University of Illinois Urbana-Champaign, University College Dublin, Pacific Northwest National Laboratory and Yale, used large-ensemble global climate simulations to ask how that change plays out differently in cities and countryside.
The answer depends on local hydroclimate. Lower aerosol loading allows more solar radiation to reach the surface, warming urban and rural areas alike. In humid regions such as East Asia and Indonesia, urban areas warm faster, because impervious surfaces limit evaporative cooling while surrounding rural land benefits from rainfall, vegetation and evapotranspiration. In arid regions the pattern reverses and the urban heat island weakens, as rural land without much moisture to evaporate heats more quickly.
Observations provide a test case. Data covering China's Air Pollution Prevention and Control Action Plan show that the period of stringent pollution controls coincided with a marked rise in urban heat island intensity, consistent with the simulated mechanism. The study was published in Nature Cities, with funding from the HKU-100 Scholars Fund, a Social Sciences seed grant and a Research Grants Council Strategic Topics Grant.
The authors are careful that this is not an argument against emission controls, whose health benefits are large and well established. It is an argument for coupling air quality policy with urban heat adaptation, since cities that successfully clean their air may find themselves managing more severe heat exposure. For urban climate researchers it also underlines how poorly aerosol and land-surface effects have been considered together.