Li Yue, Wei Wei, Yang Gan, Chen Dong-Sheng, Cheng Shui-Yuan, Han Li-Hui
College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China.
Key Laboratory of Beijing on Regional Air Pollution Control, Beijing University of Technology, Beijing 100124, China.
Huan Jing Ke Xue. 2017 Oct 8;38(10):4084-4091. doi: 10.13227/j.hjkx.201703217.
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 to 297.5×10. The mean value of C2-C6 HCs reached 29.4×10, 63.2×10, 85.5×10, 94.9×10, and 131.8×10, respectively, in AQ Ⅰ (air quality) (hourly PM<35 μg·m), AQ Ⅱ (hourly PM:35-75 μg·m), AQ Ⅲ (hourly PM:75-150 μg·m), AQ Ⅳ (hourly PM:150-250 μg·m), and AQ Ⅴ (hourly PM:>250 μg·m). 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<75 μg·m) but also in polluted days (when 24-h PM>75 μg·m). 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/×10) 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 rate of 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.
2015年12月，在北京东南部城区开展了一次C2 - C6碳氢化合物监测活动。使用在线挥发性有机物分析仪检测到了25种不包括苯的化合物；在本研究中，它们的浓度总和被称为C2 - C6碳氢化合物。在监测期间，C2 - C6碳氢化合物浓度范围为12.4×10至297.5×10。在空气质量（AQ）Ⅰ级（每小时PM<35 μg·m³）、AQⅡ级（每小时PM:35 - 75 μg·m³）、AQⅢ级（每小时PM:75 - 150 μg·m³）、AQⅣ级（每小时PM:150 - 250 μg·m³）和AQⅤ级（每小时PM:>250 μg·m³）时，C2 - C6碳氢化合物的平均值分别达到29.4×10、63.2×10、85.5×10、94.9×10和131.8×10。此外，烷烃、烯烃和乙炔的摩尔百分比有显著变化，分别为47%对59%、45%对30%、7%对12%（AQⅠ级对AQⅤ级）。C2 - C6碳氢化合物的日变化不仅在清洁日（24小时PM<75 μg·m³），而且在污染日（24小时PM>75 μg·m³）的08:00 - 09:00和17:00 - 18:00出现两个峰值。这一结果与正常交通模式一致，表明车辆排放对大气碳氢化合物浓度有显著影响。此外，为了防止气象扩散对C2 - C6碳氢化合物的影响，并追踪碳氢化合物的物理传输过程和化学降解过程，我们计算了碳氢化合物/一氧化碳（×10⁻⁹/×10⁻⁶）比率。随着空气质量恶化，C2 - C6碳氢化合物/一氧化碳比率以及单个碳氢化合物与一氧化碳的比率呈现出显著下降趋势，分别为90.6（AQⅠ级）、63.8（AQⅡ级）、56.9（AQⅢ级）、37.4（AQⅣ级）和36.4（AQⅤ级）。然而，污染期（AQⅢ - Ⅴ级）相对于清洁期（AQⅠ - Ⅱ级）单个碳氢化合物与一氧化碳比率的下降速率与与羟基自由基反应的动力学参数并无有效关联。因此，否认了污染空气中C2 - C6碳氢化合物的强烈化学降解是主要原因。HYSPLIT轨迹模型显示，在清洁期和污染期分别主要是来自北方和西北以及来自南方和西南的传输气团。与北部地区相比，南部地区有更多化石燃料燃烧源，这导致南部地区气团的碳氢化合物/一氧化碳比率较低。因此，污染期C2 - C6碳氢化合物的增加不仅是由于本地排放的积累，还由于来自南方的气团传输。
