Temporal and spatial trends in air quality in Beijing during 2001-2017

Air quality is one of the most critical environmental issues in Beijing, China, and the government act vigorously to reduce air pollutants and improve air qualiy. Over the past decade, numerous studies uesd single air pollutant, such as PM_2.5 and PM₁₀, to assess air qualiy, but these mothod neglected the comprehensive status of air qualiy. In this study, temporal and spatial variations in air quality in Beijing are characterised by two aggregate indexes, the air pollution index (API) and air quality index (AQI). API is calculated from PM₁₀, SO₂, and NO₂ and was used to indicate air quality status before 2013. From then on, API was replaced with AQI, which is calculated from PM_2.5, PM₁₀, SO₂, NO₂, O₃, and CO. The air quality datasets were collected in Beijing Environmental Protection Monitoring Center; the land-use datasets were sourced from the Computer Network Information Center of the Chinese Academy of Sciences; the economic and social datasets were originated from Beijing Statistical Yearbook. Based on the results of correlation and variation analysis, we found API decreased significantly during 2001-2012, and AQI decreased significantly during 2013-2017 (p < 0.001). In addition, these two indexes were negatively correlated to tree cover ratio and positively correlated to energy consumption intensity. In terms of seasonal characteristics, API was highest in spring and lowest in summer, reflecting trends in natural emission sources. AQI was highest in winter and lowest in summer, primarily due to anthropogenic emissions resulting from heating demand and industrial production. Regarding spatial variability, air quality reflects the differences in the different functional zones in Beijing, likely due to the different land-use composition and structure as well as the energy consumption intensity. Moreover, population and automobile density also played a role in the differences in air quality status. Our results indicate that, while air quality in Beijing has improved, seasonal effects, land use, automobile density, and emission intensity are major factors that continue to influence air quality. Therefore, further improvements in Beijing's air quality should take seasonality, urbanized land optimization, and major pollutant sources into account when developing future emission-reduction policies.