Characteristics and Circulation Classification of Persistent Haze Events in North China during 1981-2015

Based on the surface meteorological observational elements and the 1981-2015 climate daily station data from China Meteorological Administration (CMA), 1400 BST (Beijing Summer Time) moment method was used to select haze days (HDs) in North China，and an objective identification technique for regional extreme events was employed to determine persistent haze events (PHEs). Finally, the major characteristics of HDs and PHEs were obtained. The major characteristics of PHEs are as follows: (1) The spatial distribution of HDs in North China was extremely uneven, with four high- frequency regions: The valley area between Lvliang Mountains and Taihang Mountains; the plain east of Taihang Mountains; northern Henan; the gulf area of Bohai and Yellow Seas. (2) A total of 111 PHEs, with the longest one lasting 12 d, were identified. The PHEs lasting three to five days accounted for 86.5%. (3) The annual and seasonal variation of PHEs was very similar to those of HDs. The number of PHEs increased during 1981-2015, which peaked in winter, followed by autumn, spring, and summer. (4) According to the characteristics of topography, HDs, and PHEs and their circulations, PHEs can be categorized into seven types. Among these, we compared the circulations of the four high-frequency PHE types (North China; northern Henan and the plain east of Taihang Mountains; northern Henan; northern Henan and the gulf area of Bohai and Yellow Seas). The major circulation characteristics of the four PHE types were obtained. In the mid-lower troposphere, North China was controlled by zonal westerlies or northwesterlies in the front of ridges. A sinking motion was produced by a weak convergence—formed by southwesterlies along the west of high pressure with westerlies in North China. A vertical circulation existed near the surface, which is related to the influence of topography. The most severe haze areas generally appeared in the east slope of the topography and the subsidence of the vertical circulation. Southeasterlies or southwesterlies in the lower troposphere transported moisture and pollutants into the haze area. The location of haze in North China was mainly determined by the position and intensity of high pressure in the eastern or southern China, as well as the location of the local sinking motion in vertical circulation. These results provide references for the prevention and control of persistent hazes in North China.