北京城区夏冬两季VOCs时间变化和光化学特征的质子转移反应飞行时间质谱(PTR-TOF-MS)观测
The temporal variation and photochemical characters of VOCs in summer and winter of Beijing based on PTR-TOF-MS
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摘要: 为增进对北京地区不同季节大气挥发性有机物（VOCs）变化特征的认识，利用高时间分辨率质子迁移反应-飞行时间质谱（PTR-TOF-MS）于2016年在北京城区开展了VOCs （甲醛、乙醛、丙酮、异戊二烯、苯、甲苯和8碳芳香烃）夏季（6月8日-20日）和冬季（11月22日-12月10日）的连续观测.VOCs体积分数（浓度）的均值为（夏季/冬季，×10-9）：甲醛（8.56/24.58）、乙醛（3.95/7.57）、丙酮（5.06/3.50）、异戊二烯（0.66/0.52）、苯（0.53/1.78）、甲苯（1.03/2.54）、8碳芳香烃（1.34/3.42）.受大气扩散条件的影响，夏冬两季大部分VOCs浓度波动趋势相近，仅异戊二烯在夏季拥有明显的白天浓度高于夜间的时间序列，其白天的高浓度与植被排放较强有关.由日变化可见：冬季，所有VOCs在中午浓度处于全天较低水平，在早高峰期间VOCs浓度上升明显；夏季，甲醛、乙醛和丙酮等3种含氧VOCs （OVOCs）在中午有短暂的浓度峰值，这与它们光化学二次生成加快有关.由VOC与苯浓度比值的日变化可知：冬季与夏季类似，中午前后3种OVOCs （甲醛、乙醛和丙酮）的光化学生成以及甲苯和8碳芳香烃的光化学消耗都会增强，只是冬季增强的程度明显弱于夏季；在夏冬两季，甲醛中午的光化学生成速率均强于乙醛和丙酮.8碳芳香烃光化学消耗速率大于甲苯的速率仅出现在夏季；异戊二烯在冬季白天不存在植被排放增强的现象，但有光化学消耗加快的特征；夏季北京城区VOCs以机动车排放影响为主，而冬季VOCs还可能来自于燃煤排放.Abstract: To better learn the characters of the VOC variations in Beijing in different seasons, the high time resolution proton transfer reaction-time of flight-mass spectrometer (PTR-TOF-MS) was used to measure continuously the VOCs in Beijing during summer (June 8-20) and winter (from November 22 to December 10) of 2016. The mean values of the mixing ratios (concentration) of VOCs were (summer/winter,×10-9):formaldehyde (8.56/24.58), acetaldehyde (3.95/7.57), acetone (5.06/3.50), isoprene (0.66/0.52), benzene (0.53/1.78), toluene (1.03/2.54), C8-aromatics (1.34/3.42). Being impacted by the atmospheric dilution, the concentrations of the VOCs in summer and winter had similar variation trends. Only the time series of isoprene concentration had higher values in daytime than nighttime in summer, and this was related to the daytime higher vegetation emission. From the diurnal variation, the concentrations of all VOCs were at low level in the daytime, and increased evidently during the morning rush-hour in winter. In summer, the three oxygenated volatile organic compounds, formaldehyde, acetaldehyde and acetone had short time concentration peaks in the morning. This was due to the acceleration of the photochemical secondary formation. What can be known from the concentration ratios of VOC versus benzene are:the winter and summer were similar that the photochemical formation of three OVOCs (formaldehyde, acetaldehyde and acetone) and the photochemical consumption of toluene and C8-aromatics both increased around noon hours, although the extent of the increase in winter was obviously weaker than summer. In both summer and winter, the production rate of formaldehyde at noon was larger than acetaldehyde and acetone. The photochemical consumption rate of C8-aromatics was higher than toluene only shown in summer. In the daytime of winter, the vegetation emission enhance of isoprene did not appear, but the photochemical consumption increase was visible. In summer the VOCs were predominantly impacted by the vehicular emission, however the VOCs could also be emitted from the coal burning in winter.
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Key words:
- volatile organic compounds /
- diurnal variation /
- photochemistry /
- summer /
- winter /
- Beijing
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