ABSTRACT
Introduction Carbon monoxide (CO) is an important trace gas of the atmosphere and widely studied due to its significant role in tropospheric chemistry (Crutzen and Zimmermann, 1991). In remote areas, CO is an important reactant of OH and thus influences the oxidation capacity of the troposphere. Also in polluted urban regions, CO is a significant OH sink (Dommen et al., 2002). However, CO reacts slower in the atmosphere compared to most other pollutants and can therefore be used as an overall indicator for the human activities related to emissions from combustion. When considering the isotope ratios of CO, this approach can be extended for the partitioning of emissions into dierent sources. It was shown in studies in a rural area (Schauinsland, Germany) (Gros et al., 2002) as well as in a remote area (Spitzbergen) (Röckmann et al., 2002) that the oxygen isotope ratio of CO is useful for the characterization of exceptional pollution events. e relatively low oxygen isotope content of CO from wood combustion (δ18O∼16‰) compared to CO from car emissions (δ18O∼20-24‰) enabled the detection of biomass burning events in these two studies. e method is not straightforward, however, because several source and sink eects have to be considered for
explaining the δ18O of CO and because the signals of the emissions are not very well known and may be variable (Tsunogai et al., 2003). Regarding CO emission from cars, diesel exhausts can have a much lower oxygen isotope composition of about 11‰ compared to the isotope ratio of atmospheric O2 of 23.88‰ (Barkan and Luz, 2005), while even lower values down to 6‰ have been observed for cold gasoline engines due to fractionation eects (Kato Published by Copernicus Publications on behalf of the European Geosciences Union. M. Saurer et al.: Traffic and wood combustion influence on CO isotopes et al., 1999b). While such differences can hamper an unambiguous source apportionment, it should be considered that a traffic mix of many cars should have a more well defined isotopic composition, determined, e.g., to be 20.7‰±0.5‰ for Mainz, Germany (Kato et al., 1999b). e main source of atmospheric CO besides the above-mentioned combustion processes is the oxidation of methane and non-methane hydrocarbons, which produces CO with a very low 18O content (Gros et al., 2002). is eect is most important for aged air-masses and background air. On the other hand, the main sink eect, the reaction with OH to produce CO2, involves a large inverse isotope eect, where the heavier 18O reacts more readily than 16O, resulting in a depletion of the remaining C18O in the atmosphere (Brenninkmeijer et al., 1999). Accordingly, a careful analysis of the isotope balance has to be made or additional tracers for constraining the isotope budget have to be measured, e.g. 14C of CO. Under conditions of heavy pollution, a simple two box-mixing model might be applicable by considering a source and background air (Kato et al., 1999a). Potentially, also δ13C of CO may provide additional source information. e carbon isotopic composition of CO from automobile exhaust is known to be close to the isotopic compositions of fuels used (Stevens et al., 1972), while isotope fractionations during biomass burning may result in deviations of the isotope ratio from the original material (Kato et al., 1999b).
