Isoprene in the atmosphere of Beijing was collected and analyzed by GC/MS from January, 2005 to December, 2005. The results showed that the average volume fraction of isoprene in 2005 was 0. 58 ×10-9 and the seasonal average concentrations of isoprene ranged from 0. 1 × 10-9 to 1. 8 × 10-9. The concentration of isoprene in July was highest and that in January was lowest. The diurnal variation of isoprene in Spring, Autumn and Winter showed the trends of three peaks. There were higher levels of isoprene at 14:00、 18:00、02:00; 18 :00、 02: 00、 08 : 00; 02 : 00、 10 : 00、 16 : 00 in Spring, Autumn and Winter respectively. While in Summer the values of concentration of isoprene were higher at daytime and lower at night and the value at 14:00 was highest. The sources of isoprene were effected most by biogenic sources and the diurnal variation was controlled by temperature, total radiation in Summer. In Spring and Autumn, the sources of isoprene were effected by biogenic sources and vehicle exhaust and the diurnal variation was controlled by vehicle exhaust and temperature, total radiation. In winter, the sources of isoprene were effected most by vehicle exhaust which played most important role in the diurnal variation of the concentrations of isoprene. The diurnal variation of the concentrations of isoprene showed the same trend as those of PM2.5 in the atmosphere of Beijing in 2005.%对2005年北京大气中异戊二烯进行了一年的观测分析.结果表明,异戊二烯体积分数年平均值为0.58×10-9,月平均值为0.1×10-9～ 1.8×10-9,7月最高,1月最低.春、秋、冬三季,异戊二烯日变化形式呈三峰形,分别在14:00、18:00、02:00；18:00、02:00、08:00;02:00、10:00、16:00出现峰值；夏季异戊二烯体积分数日变化呈现白天高夜晚低且在14:00出现峰值.夏季异戊二烯源排放主要由生物排放控制,其日变化形式受温度、辐射影响大；春季和秋季异戊二烯源排放受汽车尾气和生物排放共同控制,其日变化形式受汽车尾气影响大,温度、辐射也有一定影响；冬季异戊二烯源排放主要由汽车尾气控制,其日变化形式主要受汽车尾气影响.不同季节北京大气中的异戊二烯体积分数日变化形式与PM2.5浓度日变化形式大致相同.
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