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