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The isotopic composition of carbon monoxide ( 13 C, 14 C, 18 O) and methane ( 13 C, D) was measured on air samples collected between Vladivostok and Moscow using the Trans‐Siberian railroad during August 1996. Apart from short term fluctuations in the direct vicinity of sources, continuous measurements of CO and CH 4 showed sustained, elevated mixing ratios over several hundreds of kilometers indicating the large scale influencing of traversed air masses by significant sources. Persistent, enhanced CH 4 levels were found over the west Siberian lowlands concurrent with significantly depleted δ 13 C and δD values. The derived isotopic signature of the CH 4 source (δ 13 C = −62.5±4.7‰ V‐PDB; δD = −311±14‰ V‐SMOW) clearly indicates the dominance of biogenic CH 4 , with the west Siberian wetlands being the most likely candidate. A second major feature in the data set is the enormous enhancement of CO (up to 1500 nmol/mol) east of Chita, extending over a 2000 km section along the river Amur. The 14 CO measurements and back trajectory analyses identify biomass burning as the origin of the highly elevated CO. This is further supported by the δ 18 O(CO) and the δ 13 C/δD signature of the accompanying moderate CH 4 enhancement.
Bergamaschi et al. (Wed,) studied this question.