北京冬季不同污染情况下PM_(2.5)传输特征模拟研究

Numerical Study on Transport Characteristics of PM_(2.5) under Different Pollution Conditions in Beijing duringWintertime

近年来北京空气质量持续得到改善,但仍未达国家二级标准,且在减排效果良好情况下,2019年冬季仍有严重的细颗粒物污染发生,所以还需对现阶段北京PM_(2.5)来源进行有针对性的研究。因不同污染阶段,污染物来源和构成均会发生变化,所以本研究利用耦合了源解析模块ISAM的空气质量模式RAMS-CMAQ对2019年12月北京PM_(2.5)来源进行解析,并从非污染、轻度污染和中重度污染3种情况分析PM_(2.5)和其组分及相应前体物的区域传输特征。结果表明,现阶段北京冬季PM_(2.5)以外来输送为主,其比例可由非污染天的69.7%增至中重度污染天的75.7%,主要来自河北和内蒙古自治区。非污染天以偏西风长距离输送为主,当同时有持续的偏南方向气团输入时,北京将出现污染并持续加重。二次无机气溶胶在北京冬季PM_(2.5)中的占比较大,其比例随污染加重而增加,中重度污染天比非污染天高6.7%。其中硫酸盐贡献随污染加重而降低,硝酸盐贡献却持续增加,是污染加重的主导组分。除硝酸盐外,硫酸盐、铵盐、有机碳、一次尘和黒碳均以外来输送为主,相应比例也随污染加重而增加,中重度污染天比非污染天分别高出3.8%,15.7%,9.8%,6.9%和7.4%。此外,作为前体物的SO_(2)、NH_(3)和NO_(2)也以外来输送为主,其中SO_(2)和NH_(3)的外来输送比例随污染加重而增加,中重度污染天比非污染天分别高出5.4%和6.2%,但外来输送对NO_(2)的贡献并没有类似变化。

The air quality in Beijing has been improved in recent years,but still fails to meet the national secondary standard.The heavy pollutions associated with the fine particle matters occurred in the winter of 2019 even though the effective emission reductions were carried out.Thus,it needs to do some researches targeted at the current sources of PM_(2.5)in Beijing.In this study,the regional air quality modeling system RAMS-CMAQ(the Regional Atmospheric Modeling System-Community Multiscale Air Quality)coupled with a source apportionment tool named ISAM(the Integrated Source Apportionment Method)was applied to investigate the variations in the chemical components of PM_(2.5)in Beijing and their sources during three different pollution(nonpolluted,light,moderate and heavy polluted)episodes in December 2019,including the regional transport characters of PM_(2.5)and its components as well as the corresponding precursors.Analysis of the model results shows that the regional transport was a major contributor to PM_(2.5)over Beijing during the winter time,and its contribution increased from 69.7%in the nonpolluted days to 75.7%in the moderate and heavy polluted days,and the major source regions were Hebei and Inner Mongolia.During the nonpolluted days,the westerly winds prevailed,and the pollution might happen and deteriorate in Beijing when the continuous air-mass came from the south appeared at the same time.In winter,the secondary inorganic aerosols played a dominated role in PM_(2.5)over Beijing,and their contributions increased with the aggravation of pollution.The contribution of secondary inorganic aerosols to PM_(2.5)was 6.7%higher in the moderate and heavy polluted days than that in the nonpolluted days.The contribution of sulfate aerosols to PM_(2.5)decreased from the nonpolluted days to the moderate and heavy polluted days,while the contribution of nitrate aerosols increased,which led to the worsening air pollution.Except nitrate aerosols,the regional transport was also a major contributor to sulfate,ammonium,organic carbon,primary dust and black carbon,and its contribution increased with the deterioration of pollution.The corresponding contribution increased by 3.8%,15.7%,9.8%,6.9%and 7.4%in the moderate and heavy polluted days than that in the nonpolluted days,respectively.Furthermore,the regional transport contributed significantly to SO_(2),NH_(3) and NO_(2),and the contribution to SO_(2) and NH_(3) continuously increased by 5.4%and 6.2%in the moderate and heavy polluted days higher than that in the nonpolluted days,but did not show the same feature to NO_(2).