北京市植物排放的异戊二烯对大气中甲醛的贡献

Contribution of Isoprene Emitted from Vegetable to Atmospheric Formaldehyde in the Ambient Air of Beijing City

2006年3-11月期间利用2,4-二硝基苯肼涂敷的硅胶柱采集了北京市大气中的羰基化合物,并使用高效液相色谱（HPLC）的方法测定了甲醛及异戊二烯光氧化特征产物：甲基乙烯基酮（methyl vinyl ketone,MVK）和甲基丙烯醛（methacrolein,MACR）的大气浓度.研究发现北京市大气中MVK和MACR具有明显的季节变化和日变化：在植物生长季节的4-10月,可在大气中检测出MVK和MACR,它们在大气中的浓度（体积分数）的月平均范围分别为0.11×10^-9-0.67×10^-9和0.19×10^-9-1.36×10^-9,日最高浓度均出现在10：00-14：00（4月除外）,8月的浓度达到最高;3月和11月的大气中未检测出MVK和MACR,可能是因为植物处于枯叶期,异戊二烯的排放较少.本研究通过异戊二烯与其光氧化产物MVK、MACR和甲醛之间的转化产率关系,先利用测定的MVK和MACR的大气浓度反演在光氧化过程中损失的异戊二烯的大气浓度,然后估算异戊二烯光氧化对甲醛形成的贡献量.估算结果显示,4-10月,异戊二烯光氧化所产生的甲醛浓度（体积分数）范围为0.35×10^-9-2.50×10^-9,占北京市大气中甲醛总量（在大气中的体积分数范围为5.49×10^-9-22.04×10^-9）的4.6%-11.5%,在大气光化学活跃的夏季（6-8月）植物排放的异戊二烯对大气中甲醛贡献尤为显著.本研究证实了北京市区植物排放异戊二烯对大气光氧化剂形成有重要贡献.

Methacrolein （MACR） and methyl vinyl ketone （MVK）, which are the characteristic products of isoprene photooxidation in the atmosphere, as well as formaldehyde, were determined during March to November of 2006 in the ambient air of Beijing city. Cartridges coated with 2, 4-dinitrophenylhenylhydrazine （DNPH） were used to collect those carbonyls in ambient air and a high pressure liquid chromatography （HPLC） was used for quantification. Distinct amounts of MACR and MVK were detected during vegetable growing seasons from April to October with ambient levels in ranges of 0.11 × 10^-9-0.67× 10^-9 and 0.19 × 10^-9-1.36 × 10^-9, respectively, with their maximal concentrations appearing in August. MACR and MVK were measured under the detecting limit in March and November, which may be ascribed to low isoprene emission from the broadleaf trees without leaves during two months. According to the stoichiometric coefficients among MVK, MACR, isoprene and formaldehyde, the concentrations of the reacted isoprene due to photo-oxidation can be calculated derived from the concentrations of the measured MVK and MACR. Then, the monthly （April to October） average concentrations of formaldehyde produced by the photo-oxidation of reacted isoprene were roughly estimated to be in the range of 0.35 × 10^-9-2.5 × 10^-9 , which accounted for 4.6%- 11.5 % of total atmospheric formaldehyde. During the period with active atmospheric photo-oxidation （June to August）, the contribution of biogenic isoprene to formaldehyde was predominant. Therefore, the biogenic isoprene is an important source to local photo-oxidant formation in Beijing and should be considered of.