北京城区冬季空气污染时期C2～C6碳氢化合物含量特征

Characteristics of C2-C6 Hydrocarbons During the Winter Air Pollution Period in Beijing Urban Area

本研究于2015年冬季,在北京市东南城区开展了C2~C6碳氢化合物大气环境浓度在线监测,研究共检出25种C2~C6碳氢化合物,但鉴于分析仪器的局限性,未包含苯这一重要物种.检出的25种碳氢化合物含量之和(C2~C6 HCs)在12.4~297.5×10^(-9)范围内,不同的空气质量条件下,C2~C6 HCs平均含量分别为29.4×10^(-9)(Ⅰ级,PM_(2.5):<35μg·m^(-3))、63.2×10^(-9)(Ⅱ级,PM_(2.5):35~75μg·m^(-3))、85.5×10^(-9)(Ⅲ级,PM_(2.5):75~150μg·m^(-3))、94.9×10^(-9)(Ⅳ级,PM_(2.5):150~250μg·m^(-3))、131.8×10^(-9)(Ⅴ级,PM_(2.5):>250μg·m^(-3)),且碳氢化合物的化学组成亦有所差异,烷烃、烯烃、乙炔的摩尔比分别从Ⅰ级条件的47%、45%、7%,变为Ⅴ级条件的59%、30%、12%,烷烃和乙炔的比重上升;烯烃的比重下降.碳氢化合物日变化规律显示,C2~C6 HCs在优良日(PM_(2.5)小时浓度均低于75μg·m^(-3))和污染日(PM_(2.5)小时浓度均高于75μg·m^(-3)),均在08:00~09:00、17:00~18:00存在两个明显的峰值,与日交通峰值时间一致,显示了道路源对局地碳氢化合物浓度的显著影响.通过HCs与CO浓度比值研究,发现随着污染情况的加重,HCs/CO(×10^(-9)/×10^(-6))呈显著下降趋势:90.6(Ⅰ级)、63.8(Ⅱ级)、56.9(Ⅲ级)、37.4(Ⅳ级)、36.4(Ⅴ级).具体化合物与CO比值在污染条件(Ⅲ~Ⅴ)与优良条件(Ⅰ~Ⅱ)的变化率,与各化合物的OH反应速率关联性很差(R=-0.31),由此判断污染时期C2~C6碳氢化合物并未发生强烈的化学衰减.HCs/CO比值变化更多反映了污染源贡献的变化,后向轨迹分析表明,在优良日北京城区多受北部和西北部清洁气团影响,北部地区燃烧源较少,其气团HCs/CO比值较高;而在污染日北京城区受南部和西南部污染气团输送,南部地区工业燃烧源和散煤燃烧源均偏多,其气团HCs/CO值偏低.综上所述,本研究认为重污染过程,北京城区C2~C6碳氢化合物(未包含苯)未体现出显著的化学衰减,碳氢化合物浓度的大幅提升,不仅源于本地污染源的排放累积,还受到南部污染气团的输送贡献.

A C2-C6 hydrocarbons monitoring campaign was carried out in the Beijing Southeastern Urban Area during December 2015.Twenty-five compounds excluding benzene were detected by an on-line VOCs analyzer; the sum of their concentrations is referred to as C2-C6 HCs in this study. During the monitoring period,C2-C6 HCs ranged from 12. 4 × 10^-9to 297. 5 × 10^-9. The mean value of C2-C6 HCs reached 29. 4 × 10^-9,63. 2 × 10^-9,85. 5 × 10^-9,94. 9 × 10^-9,and 131. 8 × 10^-9,respectively,in AQ Ⅰ（ air quality）（ hourly PM（2.5） 35 μg·m^-3）,AQ Ⅱ（ hourly PM（2.5）： 35-75 μg·m^-3）,AQ Ⅲ（ hourly PM（2.5）： 75-150 μg·m^-3）,AQ Ⅳ（ hourly PM（2.5）： 150-250 μg·m^-3）,and AQ Ⅴ（ hourly PM（2.5）： 250 μg·m^-3）. Moreover,the mole percentage of alkanes,alkenes,and ethyne significantly varied,47% vs. 59%,45% vs. 30%,and 7% vs. 12%（ AQ I vs. AQ V）. The diurnal variation of C2-C6 HCs presented two peaks at 08： 00-09： 00 and 17： 00-18： 00 not only in clean days（ when 24-h PM（2.5） 75 μg·m^-3） but also in polluted days（ when 24-h PM（2.5） 75 μg·m^-3）. This result is consistent with the normal traffic pattern and indicates the significant impact of vehicle emissions on atmospheric hydrocarbon concentrations. Furthermore,we calculated the HCs/CO（ × 10^-9/× 10-（-6）） ratio to prevent the impact of meteorological diffusion on C2-C6 HCs and to trace the physical transport process and the chemical degradation process of hydrocarbons. The C2-C6 HCs/CO ratio and the individual hydrocarbon to CO ratio presented a notable decreasing trend with worsening air quality,90. 6（ AQ Ⅰ）,63. 8（ AQ Ⅱ）,56. 9（ AQ Ⅲ）,37. 4（ AQ Ⅳ）,and 36. 4（ AQ Ⅴ）. However,the rateof decrease in the ratio of individual hydrocarbons to CO in the polluted period（ AQ Ⅲ-Ⅴ） relative to the clean period（ AQ I-Ⅱ） was never effectively related to the kinetic parameters of the reactions with the OH radical. Therefore,the strong chemical degradation of C2-C6 hydrocarbons in the polluted air was denied as the main reason. The HYSPLIT trajectory model showed that the transported air mass from the north and northwest and from the south and southwest prevail in the clean period and in the polluted period,respectively.Compared to the northern region,there were more sources of fossil fuel combustion in the southern region,which led to a lower HCs/CO ratio for the air mass in the southern region. Therefore,the increase in C2-C6 hydrocarbons during the polluted period was not only caused by the accumulation of local emissions but also by the air mass transport from the south.