Impact of atmospheric quasi-biweekly oscillation on the persistent heavy PM2.5 pollution over Beijing-Tianjin-Hebei region, China during winter

In recent years, persistent heavy air pollution (PHP) events occurred frequently over the Beijing-Tianjin-Hebei (BTH) region in China, which posed a great threat to human health. The pollution was characterized by fine particulate matter smaller than 2.5 μm in diameter (PM_2.5). This study investigates the evolution of PHP over the BTH region and its relation to the atmospheric quasi-biweekly oscillation in winters of 2013–2017. A PHP event is defined as three or more consecutive days with daily mean PM_2.5 concentration exceeding 150 μg m^-3. We observed a significant periodicity of 10–16 days of the PM_2.5 concentration, which notably contributes to the occurrence of PHP. According to the quasi-biweekly variation of PM_2.5, the life cycle of PHP events are divided into eight phases. The phase composites of circulation anomalies show that the atmospheric quasi-biweekly oscillation provides favorable conditions for the persistence of wintertime PM_2.5 pollution. During the PHP events, the quasi-biweekly southerly anomalies prevail persistently over eastern China. The East Asian winter monsoon is weakened and more moisture is transported to the BTH region continuously. The anomalous warming in the lower troposphere indicates a stable stratification on the quasi-biweekly time scale. In the mid-troposphere, the oscillation of East Asian trough’s intensity is significantly correlated with the PHP events. Further lead-lag correlation analysis suggested that the quasi-biweekly oscillation of East Asian trough can be traced back to a precursor signal over northwestern Eurasia about 11 days earlier, through a southeastward wave train propagation. Therefore, the meteorological conditions conducive to PHP over the BTH region can be predicted on the quasi-biweekly time scale.