Abstract
Atmospheric diffusion is one of the factors affecting local air quality, dominating the evolution of air pollution episodes. Previous work has emphasized the unfavorable diffusion conditions in the Sichuan Basin resulting from its complex terrain. However, the recent spatiotemporal variation in atmospheric diffusion conditions in the basin and their effects on local air quality remain unclear. Based on the wind speed, boundary layer height, vertical potential temperature difference of ERA5 analysis, two independent metrics containing information on horizontal and vertical diffusion ability, i.e., ventilation coefficient (VE) and the air stagnant conditions (ASI), are involved to indicate wintertime atmospheric diffusion conditions in the basin. Both VE and ASI reveal a decrease tendency of atmospheric diffusion condition from the northwestern portion of the basin to the southeast. In terms of the long-term variation in diffusion conditions, VE showed a broader increasing trend from 1979 to 2019, with a distinct increase in the western region. In contrast, the occurrence of air stagnation events has declined −3∼−6%/decade in the basin, more significantly over the western basin. Both the increase in VE and the decrease in air stagnation frequency indicate the improvement of atmospheric diffusion conditions in the Sichuan Basin from 1979 to 2019, which mitigates the effects of air pollutant emissions to some extent. The enhancement of diffusion conditions is due to the improvement of vertical diffusion conditions. The lowest seasonal PM2.5 concentrations occur in the northwestern basin, where VE is highest and ASI is lowest. Atmospheric diffusion conditions can explain approximately 25–50% of the interannual variation in PM2.5 concentrations in Chengdu.
Highlights
Severe and frequent air pollution has become a major environmental concern in China in recent years (Dang and Liao, 2019; Hu et al, 2019; Li et al, 2019; Mu and Zhang, 2014)
The boundary layer height (BLH) and average wind speed in the boundary layer were higher in the northwest portion of the basin, where the elevations are in the range of 500–1,000 m (Figure 1)
Deeper in the basin, where the elevation is lower than 500 m, both BLH and average wind speed declined by approximately 40% compared to the northwest region due to the blocking of terrain and the effect of valleys
Summary
Severe and frequent air pollution has become a major environmental concern in China in recent years (Dang and Liao, 2019; Hu et al, 2019; Li et al, 2019; Mu and Zhang, 2014). The Sichuan Basin comprises the greater part of eastern Sichuan Province and the western portion of Chongqing municipality located in southwestern China. It is surrounded by the highlands of the Tibetan Plateau to the west, the YunGui Plateau to the south, the Wu Mountains to the east and the Daba Mountains to the north (Cao et al, 2020; Liu et al, 2021; Zhong et al, 2019). The anthropogenic emissions in the basin are not as much as those in the other three regions, its air pollution ranks in the top four regions in China (Fan et al, 2015; Gui et al, 2019; Liu et al, 2019; Zhang et al, 2019; Zhao S. et al, 2018)
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