北京市秋冬季大气环流型下的气象和污染特征

Meteorological and pollution characteristics under atmospheric circulation types in autumn and winter in Beijing

分析了北京市2013～2018年秋冬季(即当年11、12月和次年1、2月份)11种环流型的地面和垂直气象特征,归纳出5类大气环流条件,探讨了不同环流型下北京地区的大气传输规律以及环流型与北京PM_(2.5)污染之间的关系.在5类大气环流条件中,第I类(含北(N)、东北(NE)环流型,天数占比28%)和第II类(含西北(NW)、反气旋(A)环流型,占33%)有利于传输扩散,以西北风为主,风向较稳定,风速大,边界层高度高;第III类(含东(E)环流型,占7%)传输扩散条件居中,边界层内以东南风为主,风向变化大,风速中等;第IV类(含西南(SW)、西(W)、南(S)3种环流型,占12%)和第V类(含东南(SE)、均压(UM)、气旋(C)3种环流型,占20%)均不利于传输扩散,边界层内以偏南风为主,风速较小,边界层高度低,低层逆温较强,第IV类近地面风向较稳定,而第V类则风向变化大.不同环流型下气团传输至北京的路径存在差异,对北京空气质量产生潜在影响的周边地区随之发生变化.大气环流型与北京市秋冬季PM_(2.5)污染紧密关联,SW、UM、C、S和W是北京地区最易发生PM_(2.5)污染的环流型(平均污染发生频率>75%,平均重度以上污染发生频率>42%),而在N、A、NE和NW环流型下污染发生频率最低.研究期间,PM_(2.5)污染极端严重的月份存在UM环流型占比显著增加的共同特点,而PM_(2.5)污染水平最低的月份N环流型占比增加近一倍.此外,PM_(2.5)污染变化相对于环流型变化存在一定的滞后性.

The ground and vertical meteorological characteristics associated with 11 atmospheric circulation types during autumns and winters(Nov.-Feb., 2013~2018) in Beijing was summarized. The circulation types were classified into five categories, and the influence of each category on the transport of air pollution as well as fine particulate matter(Pm2.5) pollution was discussed. Among five categories, Category I(including north(N) and northeast(NE) circulation types, which accounted for 28% of days) and Category II(including northwest(NW) and anticyclone(A), which accounted for 33%) were favorable for air pollution diffusion,which were characterized by the dominant northwest winds with steady direction and high speed, as well as high planetary boundary layer(PBL) height. Category III(including East(E), 7%) was dominated by southeast winds with variable directions and moderate speed within PBL, and provided moderate conditions for air pollution diffusion. Category IV(including southwest(SW), west(W)and south(S), 12%) and Category V(including southeast(SE), uniform pressure fields(UM) and cyclone(C), 20%) were unfavorable for air pollution diffusion, which were characterized by the dominant south winds with low speed within PBL, as well as the low PBL height due to strong temperature inversion in low atmosphere. The wind direction near ground was usually steady under Category IV,while varied greatly under Category V. The transport pathways of the air mass arriving in Beijing under different circulation types showed significant difference, thus, resulted in the changing of potential source regions contributing to air pollution in Beijing.Pm2.5 pollution was closely related to the atmospheric circulation types in autumn and winter in Beijing. Pm2.5 pollution was most likely to occur under the types of SW, UM, C, S and W(the average frequency of pollution is larger than 75%, the average frequency of severe pollution is larger than 42%), while the types of N, A, NE and NW were associated with low frequency of occurrence of Pm2.5 pollution. A higher proportion of the UM type was usually observed in the months with extremely severe Pm2.5 pollutions,while the proportion of the N type nearly doubled in the months with the lowest Pm2.5 level. In addition, the change of Pm2.5 concentrations sometimes lagged behind the change of circulation types.