The mediating effect of air pollution in the impacts of urban form on nighttime urban heat island intensity.

• Much of the urban form's impact on nighttime UHI is mediated by air pollution. • Urban fractal dimension affects the UHI intensity almost through air pollution. • Urban population density has direct effect on heat island effect. • Urban elongation and continuity have both direct and indirect effects on UHI. Urban form studies on urban heat island (UHI) are mostly from the microscale perspective of thermodynamics or fluid mechanics, lacking of consideration of the impact of urban form on traffic behavior or air pollution, which is also proved to be influential on nighttime urban thermal environment. The objective of this paper is to quantitatively reveal the mediating role of air pollution in the impacts of urban form on nighttime UHI intensity based upon an empirical research of 150 cities of China. We constructed multivariate regression models, structural equation models and scenario simulations to verify and predict the mediating effect of air pollution. Results showed that the fractal dimension of urban planar shape almost all influences UHI intensity through air pollution. The mediating effect related to urban continuity and urban elongation also accounts for 25–28 % and 33–40 % of the total effect, respectively. Controlling urban population density can play an important role on UHI mitigation, but optimization of urban geometry can bring co-benefits on urban air and thermal environments. For the purpose of improving urban thermal environment, we advocate the proper elongation and decentralization of urban form, and control urban sprawl disorderly, especially in cities with severe air pollution. [ABSTRACT FROM AUTHOR]