Environmental and Dynamic Conditions for the Occurrence of Persistent Haze Events in North China

This paper presents a concise summary of recent studies on the long-term variations of haze in North China and on the environmental and dynamic conditions for severe persistent haze events. Results indicate that haze days have an obviously rising trend over the past 50 years in North China. The occurrence frequency of persistent haze events has a similar rising trend due to the continuous rise of winter temperatures, decrease of surface wind speeds, and aggravation of atmospheric stability. In North China, when severe persistent haze events occur, anomalous southwesterly winds prevail in the lower troposphere, providing sufficient moisture for the formation of haze. Moreover, North China is mainly controlled by a deep downdraft in the mid-lower troposphere, which contributes to reducing the thickness of the planetary boundary layer, obviously reducing the atmospheric capacity for pollutants. This atmospheric circulation and sinking motion provide favorable conditions for the formation and maintenance of haze in North China.