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大气中的污染物成分复杂多变,包含可吸入颗粒物(PM10)、细颗粒物(PM2.5)等固态物质,也可包含氮氧化物(NOx=NO2 + NO)、二氧化硫(SO2)、臭氧(O3)、一氧化碳(CO)、甲醛(HCHO)等有毒有害气体。空气中的污染物能使大气能见度降低[1],通过吸收或散射改变地-气系统辐射平衡[2-3],也可以通过携带有毒有害物质经呼吸道进入人体,从而影响人体健康[4-5]。最新研究[6]表明,悬浮在空气中新型冠状病毒也能够通过气溶胶方式传播,从而引起呼吸道疾病。随着大量工业废气及交通尾气的排放,大气光化学污染加剧,O3已成为造成大气污染尤其是夏季大气中的重要污染物之一。甚至,在一些地区O3已经取代PM2.5成为首要的大气污染物[7-8]。因此,研究O3的污染机制意义重大。
O3是一种典型的二次污染物,主要是由挥发性有机物(VOCS)和NOx在光照条件下通过光化学反应生成[9]。具体过程为:在清洁大气中,NO2可以在光照条件下可以分解成NO与氧原子O。由于该氧原子O活性较高,它能够迅速结合O2形成O3,而形成的O3又可以将NO氧化成NO2,并形成O2[10],此上述过程在清洁大气中保持动态平衡状态。一旦大气被污染,VOCS则会在氧化性自由基的作用下,消耗大气中的NOx,使得O3剩余,进而造成O3和二次PM2.5污染。在此过程中,一次PM2.5质量浓度的降低会使得气溶胶削弱的能量减少,也会减少大气中过氧羟基自由基的沉降,从而促进O3的生成[11-12]。同时,O3亦可以作为氧化剂增加大气中二次颗粒物的生成[13]。作为光化学反应重要的前体物,VOCS的组分复杂,目前无法获得其大范围、长时间的监测数据。但研究也发现,HCHO作为光化学反应的重要中间产物,可以用于估算大气中VOCS的含量[14-15]。因此,为了研究大气O3污染加剧的原因,本文选择用大气中的HCHO的变化趋势表征近年来长三角地区大气中的VOCS变化情况。
中国东部地区是中国大气污染最严重的区域。作为中国最主要的经济区,长江三角洲地区(以下简称长三角地区)则是中国东部大气污染区的重中之重[16-17]。因此,研究长三角地区大气污染机制就显得尤为重要。
长三角地区O3与PM2.5复合污染机制研究
Mechanism of complex pollution of O3 and PM2.5 in Yangtze River Delta region
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摘要: 近年来,长江三角洲(下称“长三角”)地区深受细颗粒物(PM2.5)和臭氧(O3)污染的困扰。基于多源数据和理想模型,文章研究了长三角地区O3和PM2.5的复合污染机理。结果表明,PM2.5污染比O3污染更严重,且2种污染物呈现出不同的时空分布特征。与2015年相比,2018年长三角地区的PM2.5平均质量浓度下降了22%,而O3质量浓度却上升了15%。长三角地区东部的O3质量浓度较高,且主要在夏季;而北部的PM2.5污染在冬季更为严重。PM2.5的减排可通过影响到达地表的短波辐射进而促进O3的形成,同时也会导致更多的二次PM2.5的生成。因此,控制长三角地区夏季大气污染的关键问题是了解光化学反应前体物和PM2.5对光化学反应的相对贡献。Abstract: In recent years, the Yangtze River Delta (YRD) region is suffering from the fine particulate matter (PM2.5) and ozone (O3) pollution. Based on the multi-sourced data and ideal-model, the mechanism of the complex pollution of O3 and PM2.5 in the YRD region is investigated in this study. The results show that the PM2.5 pollution is more serious than O3 pollution, and the two pollutants indicate the different spatial and temporal distribution. The annual mean PM2.5 concentration decreases by 22% in 2018 compared with that in 2015, while the O3 concentration increases by 15%. And the O3 concentration is much higher in the east of the YRD region, mainly in the summer period, while the PM2.5 pollution in the north is more serious in winter. The PM2.5 emission reduction affects the shortwave radiation to the ground and increases the O3 production, thus also resulting in more secondary production of PM2.5. Therefore, the key in controlling the summer air pollution in the YRD region is understanding the precursor of the photochemical reaction and the relative contribution of PM2.5 pollutants to the reaction.
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Key words:
- PM2.5 /
- O3 /
- VOCS /
- shortwave radiation /
- atmosphere oxidation /
- photochemical reaction
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表 1 不同污染物的质量浓度阈值
污染物项目 时效 阈值/µg·m−3 SO2 小时平均 500 NO2 小时平均 200 O3 小时平均 200 CO 小时平均 10 000 PM10 日平均 150 PM2.5 日平均 75 表 2 冬季与夏季OX与O3、NO2质量浓度间关系
季节 OX
/µg·m−3O3/OX R1
(OX, O3)R2
(OX, NO2)样本
数/个夏季 105.1 0.75 0.98 −0.16 7 956 冬季 96.3 0.54 0.78 −0.10 8 119 注:cO3/cOX表示O3占OX的比重,R1和R2表示cOX与cO3和cNO2的相关系数。 -
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