Atmospheric circulation and dynamic mechanism for persistent haze events in the Beijing-Tianjin-Hebei region

In this study, regional persistent haze events (RPHEs) in the Beijing-Tianjin-Hebei (BTH) region were identified based on the Objective Identification Technique for Regional Extreme Events for the period 1980-2013. The formation mechanisms of the severe RPHEs were investigated with focus on the atmospheric circulation and dynamic mechanisms. Results indicated that: (1) 49 RPHEs occurred during the past 34 years. (2) The severe RPHEs could be categorized into two types according to the large-scale circulation, i.e. the zonal westerly airflow (ZWA) type and the high-pressure ridge (HPR) type. When the ZWA-type RPHEs occurred, the BTH region was controlled by near zonal westerly airflow in the mid-upper troposphere. Southwesterly winds prevailed in the lower troposphere, and near-surface wind speeds were only 1-2 m s(-1). Warm and humid air originating from the northwestern Pacific was transported into the region, where the relative humidity was 70% to 80%, creating favorable moisture conditions. When the HPR-type RPHEs appeared, northwesterly airflow in the mid-upper troposphere controlled the region. Westerly winds prevailed in the lower troposphere and the moisture conditions were relatively weak. (3) Descending motion in the mid-lower troposphere caused by the above two circulation types provided a crucial dynamic mechanism for the formation of the two types of RPHEs. The descending motion contributed to a reduction in the height of the planetary boundary layer (PBL), which generated an inversion in the lower troposphere. This inversion trapped the abundant pollution and moisture in the lower PBL, leading to high concentrations of pollutants.