Analysis of a winter regional haze event and its formation mechanism in the North China Plain

摘要

A regional haze episode occurred in the Beijing, Tianjin and Hebei province(BTH) area in the North China Plain (NCP) from 16 to 19 January 2010. Datawere collected and analyzed during the time frame of 14 through 23 January2010 to include the haze event. The increase of secondary inorganicpollutants (SO₄2−, NO₃−, NH₄+) in PM_2.5was observed simultaneously at four sites, especially in the plain area ofthe BTH, which could be identified as a common characteristic of pollution hazein east China. The sulfate and nitrate in PM_2.5 were mainly formedthrough the heterogeneous reaction process in the urban area. The organicmatter (OM) increased more significantly at the Chengde (CD) site than the otherthree sites in the plain area. The secondary organic aerosols only existedduring haze days at CD but in both haze and non-haze days at the other threesites, which suggested the greater regional impact of secondary formationprocess during the haze episode. The secondary formation of aerosol was oneimportant formation mechanism of haze. The strong temperature inversion anddescending air motions in the planetary boundary layer (PBL) allowedpollutants to accumulate in a shallow layer. The weak surface wind speedproduced high pollutants concentration within source regions. Theaccumulation of pollutants was one main factor in the haze formation. Theenhanced southwest wind in the last period of this episode transportedpollutants to the downwind area and expanded the regional scope of the haze.