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大气过氧化氢(H2O2)是重要的光化学二次产物,其主要是由HO2与HO2自由基结合生成的[1]。此外,无光情况下已知的能够形成过氧化氢的途径为O3与烯烃的反应以及NO3与有机化合物的反应[2]。过氧化氢唯一的直接排放源为生物质燃烧[3-4]。过氧化氢本身是大气中重要的氧化剂,同时又作为HOx自由基的储库分子,参与HO2和OH自由基的循环[5-6]。作为大气中最重要的氧化剂之一,H2O2在污染大气化学中扮演着重要的作用。一方面,在pH<5的云、雾和雨水中,H2O2被认为是S (Ⅳ)氧化生成硫酸盐最重要的氧化剂,对大气降水酸化和颗粒物的质量增长有重要影响[7-8]。而且,近年来随着研究手段的进步以及对过氧化氢认识的不断深入,有研究发现过氧化氢与气溶胶颗粒物之间存在着复杂的关系。有研究通过外场观测以及模型模拟发现气溶胶表面非均相反应对H2O2可能同时存在源和汇的双重作用[4, 6, 9]。另一方面,有研究将H2O2/HNO3的比值用做评估O3生成对VOCs或NOx敏感性的指示剂,且在多种指示剂评估方法中,认为H2O2/HNO3比值的指示剂方法对O3生成敏感性的评估最合适[10-11]。此外,毒理学研究表明,H2O2能够随细颗粒物进入肺部,进而损伤肺泡,从而对人类健康产生不利影响[12]。因此,对大气中过氧化氢的浓度水平、污染特征、生消过程及影响因素等进行深入研究,对掌握区域光化学氧化剂的污染水平和研究光化学与颗粒物复合污染的形成以及人类健康具有重要意义。
本研究对大气过氧化氢的浓度水平、气象因素和相关环境污染物的大气浓度开展了同步观测,旨在分析影响H2O2生成的因素。着重关注高NOx以及颗粒物污染情况下H2O2的污染特征,通过模式对过氧化氢的源汇进行分析。
大气过氧化氢观测及生成途径的模拟研究
Observation and modeling on the production of atmospheric hydrogen peroxide
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摘要: 大气过氧化氢是大气光化学的重要产物,在大气污染化学中起着重要的作用。为了解不同天气条件下大气过氧化氢的浓度水平及变化规律,探究不同污染状况下大气过氧化氢的主要生成和消耗途径,本研究于2018年秋季开展了北京大气过氧化氢(H2O2)的观测。观测期间H2O2的平均体积浓度为(0.29±0.36 )×10−9,H2O2日变化明显,日间最高峰值出现在午后15:00左右。观测期间夜间偶尔出现H2O2上升的现象,并常伴随O3的同时上升,H2O2与O3的夜间高值似与污染气团的区域传输有关。本研究利用气相RACM化学机理,模拟了基于实测前体物浓度下H2O2浓度和生成速率、不同大气气相反应途径对H2O2的源、汇贡献以及H2O2对光化学前体物变化的敏感性。模拟结果表明,气相中HO2与HO2自由基结合反应是H2O2已知生成途径中最主要源,H2O2干沉降为主要的H2O2汇。H2O2生成速率对光化学前体物的敏感性研究表明,烯烃对H2O2的生成最为敏感。与实测值相比,模式倾向于低估H2O2的浓度水平,研究中认为除了高NOx条件下模式中HO2自由基源的缺失的原因以外,气溶胶颗粒物上H2O2的非均相生成也可能是大气H2O2的重要来源。Abstract: Hydrogen peroxide is a product of atmospheric photochemistry, which plays important role in atmospheric pollution chemistry. In order to characterize the species abundance and variation pattern of atmospheric hydrogen peroxide, and to explore the dominant production and consumption pathways for hydrogen peroxide at Beijing, an observation centered on hydrogen peroxide (H2O2) photochemistry was conducted in autumn 2018. The mixing ratio of H2O2 was (0.29±0.36)×10−9. H2O2 exhibited pronounced diurnal variation, with daytime peaks observed at about 15:00. Air mass transport occasionally resulted in high concentrations of H2O2 and O3 in the night. The H2O2 concentration, gas phase sources and sinks, as well as the sensitivity of H2O2 to photochemical precursors were examined by a box model employing the Regional Atmospheric Chemistry Mechanism (RACM), with the precursors concentration constrained by observations. The modeling results demonstrated that the combination of HO2 and HO2 was the predominant gas phase source of H2O2, and the dry deposition of H2O2 was the predominant gas phase sink of H2O2. The H2O2 formation rate were more sensitive to alkene species increase. Comparing with the observed value, the model underestimates the concentration of H2O2. The underestimation suggested the existence of important H2O2 missing source in atmospheric, such as the heterogeneous formation on aerosol particles or the model’s missing source of HO2 radical under high NOx condition.
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表 1 同步观测的仪器信息
Table 1. The instrument information for observation
污染物Pollutants 型号Instruments O3/NOx /CO/SO2 Thermo Fisher Scientific,49i/42i/48i/43i PM2.5 Thermo Fisher Scientific,TEOM-1405F NH3, HNO3, HNO2, H2SO4, Na+, , K+, Mg2+, Ca2+,${\rm{NH}}_4^{+} $
F−, Cl−, ,${\rm{NO}}_2^{-} $ ,${\rm{NO}}_3^{-} $ ${\rm{SO}}_4^{2-} $ Thermo Fisher Scientific,URG-9000D VOCS Entech Instrument Inc.,Entech 7200;Thermo- fisher USA,Trace 1300 Particle size distribution TSI,TSI-3082,TSI-3321 表 2 以往观测中大气H2O2的浓度范围
Table 2. Concentration range of atmospheric H2O2 in previous observations
观测地点
Observation site观测时期
Observation time体积浓度范围
Concentration range(×10−9)文献
Reference北京,中国 2015—2016,春、夏、冬 0.01—2.08 Qin et al.(2018)[18] 北京,中国 2010—2011,夏 D.L—1.59 Liang et al.(2013)[19] 北京,中国 2017—2018,冬 0—0.68 Ye et al.(2018)[20] 珠三角,中国 2006,夏 0.02—4.6 Hua et al.(2008)[7] 北京,中国 2013,秋 0.10—2.4 Zhang et al.(2018)[21] 华北平原,中国 2014,夏 0.01—11.3 Wang et al.(2016)[4] 泰山,中国 2007,春、夏 D.L —1.28 春、
D.L —3.55 夏Ren et al.(2008)[22] 北京,中国 2008,夏 D.L —2.34 He et al.(2010)[2] 萨格勒布,克罗地亚 2004,夏 0.05—6.2 Acker et al.(2008)[23] 首尔,韩国 2002—2004,秋 0.07—0.87 Lee et al. (2008)[24] Toyama Prefecture,日本 2012—2015,夏、冬 0.01—3.5 Watanabe et al.(2018)[25] Whiteface Mountain,北美 1995,夏 0.15—5.15 Balasubramanianand Husain.(1997)[26] 卡罗莱纳州海岸,美国 1986,春 2.4 (max) Barth et al.(1989)[27] 东部,美国 1984,秋 D.L—4.1 Heikes et al.(1987)[28] 加利福尼亚,洛杉矶 2004—2005,春、夏、秋、冬 0.2—3.0 Arellanes et al.(2006)[29] Nikko,日本 1998—2000,夏 0.2—1.6 Takami et al.(2003)[30] -
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