Do Urban Heat Island Intensity, Urban Breeze Intensity, and Urban Air Pollutant Concentration Linearly Change with Roof Albedo?

Cool roof (reflective roof) is a widely adopted strategy to mitigate the urban heat island (UHI). Previous studies demonstrated that the cool roof effectively alleviates the UHI, but how the UHI and urban breeze circulation (UBC) respond to an increase in roof albedo (α_r) and how this affects urban air pollutant dispersion were not systematically studied. To investigate these, idealized ensemble simulations are conducted using the Weather Research and Forecasting (WRF) model coupled with an urban canopy model. The increase in α_r from 0.20 to 0.65 decreases the UHI intensity, UBC intensity, and urban planetary boundary layer (PBL) height averaged over the daytime (1200–1700 LST) by 47%, 36%, and 6%, respectively. As the UBC intensity and PBL height decrease, the daytime mean urban near-surface passive tracer concentration increases by 115%. The daytime UHI intensity, UBC intensity, and urban tracer concentration nonlinearly change with α_r: For 0.10 ≤ α_r ≤ 0.80, the rates of changes in the UHI intensity, UBC intensity, and urban tracer concentration with α_r overall increase as α_r increases. For α_r > 0.80, the UHI intensity, UBC intensity, and urban tracer concentration slightly change with α_r. This study is expected to provide insights in finding an appropriate trade-off between the UHI mitigation and its side effects on air quality.