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

Abstract

Urbanization affects air pollutants via urban expansion and emission growth, thereby inevitably changing the health risks involved with 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 because air quality management is usually achieved by regulating anthropogenic emissions. Air pollution in Chengdu was mainly caused by PM2.5 and O3 from 2015 to 2021. PM2.5 pollution tended to appear in cold months (November to February) owing to the blocking of air and the stable atmospheric layer, whereas O3 pollution was likely to occur in warm months (April to August) that experience high-temperature and strong-sunlight conditions and are dominated by high-pressure systems. From 2015 to 2021, the 7-year annual average of premature mortality from all non-accidental causes (ANACs) due to PM2.5 and O3 was 9386 (95 % confidence interval (CI) of 6542–11 726) and 8506 (95 % CI of 4817–11 882), respectively. Based on the characteristics of PM2.5 and O3, six numerical experiments were conducted to investigate the impacts of urban expansion and emission growth on the health risks related to air pollutants. The results show that urban land use led to an increase in the air temperature and boundary layer height compared with cropland, which was conducive to the diffusion of PM2.5. Thus, the monthly average surface PM2.5 concentrations decreased by 10.8 µg m−3 (7.6 %) in January. However, the monthly average daily maximum 8 h average (MDA8) O3 concentrations increased by 10.6 µg m−3 (6.0 %) in July owing to the stronger photochemical production and better vertical mixing during daytime. In this case, premature mortality from ANACs due to PM2.5 decreased by 171 (95 % CI of 129–200, or about 6.9 %) in January, and that due to O3 increased by 203 (95 % CI of 122–268, or about 9.5 %) in July. As for the effects of emission growth, the monthly average PM2.5 and MDA8 O3 concentrations increased by 23.9 (16.8 %) and 4.8 µg m−3 (2.7 %), respectively, when anthropogenic emissions were taken into account. Premature mortality from ANACs due to PM2.5 and O3 then increased by 388 (95 % CI of 291–456, or about 15.7 %) and 87 (95 % CI of 54–112, or about 4.1 %), respectively. From a health risk perspective, the effects of urban land use on the health risks related to PM2.5 are about half that of anthropogenic emissions, whereas the effects of urban land use on the health risks related to O3 can be 2 times that of anthropogenic emissions. This emphasizes that, in addition to regulating anthropogenic emissions, urban planning is also important for urban air quality, especially for secondary pollutants like O3.