In the present study, atmospheric suspended particles from two representative sites at Shanghai were collected for the determination of stable free radicals in particles surface. Particle samples were extracted with dichloromethane by soxhlet extractor method and analyzed by electron spin resonance ( ESR ) . ESR spectra showed a triple peak signal with g-values of 2�002 7-2�004 7, consistent with the presence of ortho-position semiquinone radicals after comparing with the ESR spectra of freshly prepared, aerated buffered solutions of quinones. The concentration of semiquinone radicals in particles from Minhang and Putuo were estimated by comparing with the standard substance of Manganese ( Mn) . The mean concentration of semiquinone radicals in TSP, PM10 and PM2. 5 from Minhang site was 5�02 × 105 , 5�54 × 105 , 9�43 × 105 g-1 respectively, whereas the mean concentration in TSP, PM10 and PM2� 5 from Putuo site was 2�02 × 105 ,3�50 × 105 ,7�16 × 105 g-1 .%采集上海市城区和郊区2个典型站位的大气颗粒物样品，以二氯甲烷作萃取溶剂，采用索氏抽提法萃取分离吸附在颗粒物表面的稳定自由基，并利用电子自旋共振波谱技术（ ESR）分析测定其种类和浓度。结果表明，测试样品的波谱特征表现为显著的三重信号峰，且g因子值均为2�0027～2�0047，通过与醌类物质的碱性饱和溶液ESR图谱比对，可以推断颗粒物表面至少吸附一种邻位半醌自由基。同时，通过锰标定量的方式估算了闵行和普陀2个采样点颗粒物样品表面的半醌自由基浓度，闵行采样点TSP、PM10和PM2．5表面半醌自由基的平均浓度分别为5�02×105、5�54×105、9�43×105 g-1，而普陀采样点的平均浓度分别为2�02×105、3�50×105、7�16×105 g-1。
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