title: Process analysis of characteristics of the boundary layer during a heavy haze pollution episode in an inland megacity, China.
abstract: Ground observation data from 8 meteorological stations in Xi'an, air mass concentration data from 13 environmental quality monitoring sites in Xi'an, as well as radiosonde observation and wind profile radar data, were used in this study. Thereby, the process, causes and boundary layer meteorological characteristics of a heavy haze episode occurring from 16 to 25 December 2013 in Xi'an were analyzed. Principal component analysis showed that this haze pollution was mainly caused by the high-intensity emission and formation of gaseous pollutants (NO2, CO and SO2) and atmospheric particles (PM2.5 (fine particles) and PM10 (respirable suspended particle). The second cause was the relative humidity and continuous low temperature. The third cause was the allocation of the surface pressure field. The presence of a near-surface temperature inversion at the boundary layer formed favorable stratification conditions for the formation and maintenance of heavy haze pollution. The persistent thick haze layer weakened the solar radiation. Meanwhile, a warming effect in the urban canopy layer and in the transition zone from the urban friction sublayer to the urban canopy was indicated. All these conditions facilitated the maintenance and reinforcement of temperature inversion. The stable atmospheric stratification finally acted on the wind field in the boundary layer, and further weakened the exchange capacity of vertical turbulence. The superposition of a wind field with the horizontal gentle wind induced the typical air stagnation and finally caused the deterioration of air quality during this haze event.
authors.affiliation: Xi'an Meteorological Bureau, Xi'an 710016, China.
authors.forename: Shan
authors.initials: S
authors.lastname: Wang
authors.affiliation: Plateau Atmospheric and Environment Key Laboratory of Sichuan Province, College of Atmosphere Sciences, Chengdu University of Information Technology, Chengdu 610225, China.
authors.forename: Tingting
authors.initials: T
authors.lastname: Liao
authors.affiliation: State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100191, China.
authors.forename: Lili
authors.initials: L
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100191, China. Electronic address: suny@mail.iap.ac.cn.
authors.forename: Yang
authors.initials: Y
authors.lastname: Sun
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D000397:Air Pollution / Q000032:analysis*; D002248:Carbon Monoxide / Q000032:analysis; D002681:China; D002947:Cities; D004784:Environmental Monitoring; D009585:Nitrogen Dioxide / Q000032:analysis; D052638:Particulate Matter / Q000032:analysis; D025341:Principal Component Analysis; D013458:Sulfur Dioxide / Q000032:analysis; D013696:Temperature; D014919:Wind
chemical_list: D000393:Air Pollutants; D052638:Particulate Matter; D013458:Sulfur Dioxide; D002248:Carbon Monoxide; D009585:Nitrogen Dioxide
keywords: Air stagnancy; Haze; Radiation; Temperature inversion