Impacts of urbanization on air quality and related health risks in a city with complex terrain

Abstract. Urbanization affects air pollutants by urban expansion and emission growth, and thereby inevitably changes health risks of air pollutants. However, the health risks related to urbanization are rarely estimated, especially for cities with complex terrain. In this study, a highly urbanized city with severe air pollution and complex terrain (Chengdu) is selected to explore this issue. The effects of urban expansion are further compared with emission growth as air quality management is mainly to regulate emissions. Air pollution in Chengdu is mainly caused by PM2.5 and O3. PM2.5 pollution tends to appear in cold months (November to February) due to the secondary circulation forced by complex terrain and the frequent temperature inversion, while O3 pollution is likely to occur in warm months (April to August) because of high temperature and strong sunlight dominated by high-pressure systems. From 2015 to 2021, the annual total premature mortalities from all non-accidental causes (ANAC) attributed to PM2.5 and O3 exposure are 9386 and 7743, respectively. Based on the characteristics of PM2.5 and O3, six numerical experiments are conducted to investigate the impacts of urban expansion and emission growth on health risks. The results show that urban expansion causes an increase in air temperature and the boundary layer height, which is conducive to the diffusion of PM2.5. Thus, surface PM2.5 concentrations decrease by 11.7 μg m-3 in January. However, the MDA8 O3 concentrations increase by 10.6 μg m-3 in July due to the stronger photochemical production and better vertical mixing during the daytime. Correspondingly, the total premature mortalities from ANAC attributed to PM2.5 exposure decrease by 182 (6.9 %) in January, but those attributed to O3 exposure increase by 203 (9.5 %) in July. As for the effects of emission growth, PM2.5 and MDA8 O3 concentrations can increase by 26.6 μg m-3 and 4.8 μg m-3 when anthropogenic emissions are taken into account. The total premature mortalities from ANAC attributed to PM2.5 and O3 exposure then increase by 424 (16.0 %) and 87 (4.1 %), respectively. The effect of urban expansion on health risks of PM2.5 is about half that of anthropogenic emissions. Whereas the effect of urban expansion on health risks of O3 can be 2.3 times that of anthropogenic emissions. This reminds us that the development of cities is also important for the urban air quality apart from the emissions reduction.