Effects of atmospheric circulations on the interannual variation in PM<sub>2.5</sub> concentrations over the Beijing-Tianjin-Hebei region in 2013–2018

Abstract

<jats:p>Abstract. The Chinese government has made many efforts to mitigate fine particulate matter (PM2.5) pollution in recent years by taking strict measures on air pollutants reduction, which has generated the nationwide improvements in air quality since 2013. However, under the stringent air pollution controls, how PM2.5 concentration varies and how much the meteorological conditions contribute to the interannual variations in PM2.5 concentrations are still unclear, which is very important for the local government to assess the emission reduction of previous year and adjust mitigation strategies of next year. The effects of atmospheric circulation on the interannual variation in wintertime PM2.5 concentrations over the Beijing-Tianjin-Hebei (BTH) region in the period of 2013–2018 are evaluated in this study. Generally, the transport of clean and dry air masses and unstable boundary layer working with the effective near-surface horizontal divergence or pumping action at the top of the boundary layer benefit for the horizontal or vertical diffusion of surface air pollutants. Instead, the co-occurrence of a stable boundary layer, frequent air stagnation, positive water vapor advection and deep near-surface horizontal convergence exacerbate the air pollution. Favorable circulation conditions lasting for 2~4 days are beneficial for the diffusion of air pollutants, and 3~7 days of unfavorable circulation events exacerbate the accumulation of air pollutants. The occurrence frequency of favorable circulation events is consistent with the interannual variation in seasonal mean PM2.5 concentrations. There is better diffusion ability in the winters of 2014 and 2017 than in other years. A 76.5 % of the observed decrease in PM2.5 concentrations in 2017 over the BTH region could be attributed to the improvement in atmospheric diffusion conditions. It is essential to exclude the contribution of meteorological conditions to the variation in interannual air pollutants when making a quantitative evaluation of emission reduction measurements. </jats:p>

Three dominant synoptic atmospheric circulation patterns influencing severe winter haze in eastern China

“…Circulation Type1 is associated with the upper troposphere's wave-train structure of "− + −". Unlike previous studies (Zhong et al, 2019;Wang and Zhang, 2020), there are no significant anticyclonic anomalies in the mid-troposphere over the YRV, but there is a substantial northerly wind component in the lower troposphere over northern China. The TIP, sinking movement, and RH anomalies over the YRV are weak (Fig.…”

“…3 Dominant synoptic atmospheric circulation patterns of severe HD EC Figure 2a shows the composite anomalies of 500 hPa GPH during all severe HD EC in 853 stations. Generally, the stations with severe haze are located in the southwestern parts of the anticyclonic anomaly center, which is consistent with previous studies (Zhong et al, 2019;Wang and Zhang, 2020). We then performed the HCA as described in Sect.…”