Modeling Impacts of Urbanization and Urban Heat Island Mitigation on Boundary Layer Meteorology and Air Quality in Beijing Under Different Weather Conditions

Beijing has experienced a rapid urbanization in the last few decades and has been suffering from serious air pollution during recent years. The Weather Research and Forecasting-Chem model is used to quantify the effects of urbanization on regional climate and air quality and those of urban heat island (UHI) mitigation strategy on urban air quality in Beijing, with a special focus on the impacts under different weather conditions (heat waves in summer and polluted days in winter). The modification of rural land use into urban impervious surface significantly increases 2-m temperature (T-2) and planetary boundary layer height but decreases 2-m relative humidity (RH2) and 10-m wind speed (WS10) in urban Beijing, which further leads to the increases in surface-layer O-3 concentrations of 9.5ppbv in summer and 1.8ppbv in winter and the decreases in PM2.5 concentrations of 16.6gm(-3) in summer and 26.2gm(-3) in winter. Compared with normal days (clean days), the UHI intensity is enhanced by 11.1% during heat waves in summer (by 16.7% during polluted days in winter). Although increasing urban albedo is an effective mitigation strategy to decrease UHI intensity, it worsens the urban air quality. When the urban albedo increases from 0.2 to 0.85, the daily average PM2.5 concentrations are increased by 10.2 (6.1)gm(-3) in summer (in winter), and the daily maximum O-3 concentrations are increased by 12.8ppbv under heat waves in summer.