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Urban air pollution models may not accurately reflect levels of volatile organic compounds (VOCs) in the atmosphere, a new research report suggests. VOCs play a role in the formation of ground-level ozone and secondary organic aerosols. Alastair C. Lewis of the University of Leeds, England, and his coworkers there and at Royal Melbourne Institute of Technology in Victoria, Australia, use "comprehensive" gas chromatography to analyze urban air samples collected in Melbourne [Nature, 405, 778 (2000)]. The authors write that they can isolate more than 550 individual compounds using this method, which employs two chromatographic columns with different properties. Many compounds with more than six carbon atoms, the authors note, can only be isolated by using comprehensive chromatography. The peaks for many of these compounds are hidden in the baseline of conventional single-column chromatography analyses. "If you start to look at the baseline in detail with this technique, you can see that that baseline is actually composed of lots and lots of species," says Nicola Carslaw, a research fellow at the University of Leeds and an author on the paper. "When you add them all up, you are left with much more organic carbon than conventional chromatographic techniques would indicate." The upshot is that conventional chromatography underestimates the total concentration of VOCs in the atmosphere, the authors say. Single-column methods peg the concentration of C2 to C6 compounds at 60% of the total reactive carbon mass, with C6 to C14 compounds at 40%. With comprehensive chromatography, the C2 to C6 figure drops to less than 20%. Thus, conventional chromatography fails to account for almost two-thirds of the total reactive carbon in the atmosphere, according to the researchers. Lewis and coworkers find that about half of the VOCs in the higher molecular weight fraction are aromatic. Oxygen-containing species make up about 2% of the total. The ratio of aromatic to aliphatic species is significantly higher than those reported previously. "Some aromatics are very reactive, and they have the potential to form lots of ozone," Carslaw says. "The other thing that aromatics do when they're oxidized in the atmosphere is form secondary organic aerosols. Recent work has suggested that there may be adverse health effects from these aerosols. If there's a lot more aromatic carbon in the atmosphere, it could have implications for health and also for tropospheric ozone." So far, measurements using the comprehensive method have only been made in Melbourne. "These measurements need to be repeated in cities around the world," Carslaw notes. Roger Atkinson, an atmospheric chemist at the University of California, Riverside, points out that, if the Melbourne results are confirmed in other locations, they will affect emission inventories and modeling results--particularly since current models fit the ambient data with the present VOC concentration data. "If the ambient VOC levels are higher, either the models are incorrect or they are insensitive to VOC levels," Atkinson says. "Either way, this study has potentially important implications for emission inventories, urban and regional airshed modeling, and control strategies." Atkinson also notes that "ambient VOC levels may have been underestimated because certain classes of VOCs are not amenable to gas chromatography without derivatization. There is a need for further analytical methods development to allow a full accounting of organic compounds present in the atmosphere." |
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