Pollution severity-regulated effects of roof strategies on China’s winter PM_2.5

Urbanization took place rapidly over recent decades and is expected to continue in the future, producing a series of environmental issues, including heat stress. Cool roof and green roof strategies have been adopted in a number of megacities to mitigate urban heat and carbon emissions, yet China is lagging behind developed countries in the implementation. One reason is the lack of careful and thoughtful assessment of potential effects of roof strategies, including their influences on winter PM_2.5. With numerical simulations in this study, we assess how cool and green roof strategies affect winter PM_2.5 pollution in North China, and we find that adoptions of cool roofs tend to aggravate PM_2.5 pollution in lightly polluted regions. When PM_2.5 pollution worsens, the negative effects of cool roofs are likely to be diminished. Green roofs cause less enhancements of PM_2.5 pollution as a result of inhibited evapotranspiration in winter. We demonstrate that the effects of roof strategies are regulated by pollution severity and conclude that green roofs with suppressed evapotranspiration and thus weaker penalty on winter PM_2.5 pollution seem to be better choices given the current pollution severity level in China, especially for regions suitable for growth of broadleaf plants.