The H2/CO ratio of emissions from combustion sources: comparison of top-down with bottom-up measurements in southwest Germany

The hydrogen-to-carbon monoxide (H-2/CO) emission ratio of anthropogenic combustion sources was determined from more than two years of quasi-continuous atmospheric observations in Heidelberg (49 degrees 24' N, 8 degrees 42' E), located in the polluted Rhein-Neckar region. Evaluating concurrent mixing ratio changes of H-2 and CO during morning rush hours yielded mean molar H-2/CO ratios of 0.40 +/- 0.06, while respective results inferred from synoptic pollution events gave a mean value of 0.31 +/- 0.05 mole H-2/mole CO. After correction for the influence of the H-2 soil sink on the measured ratios, mean values of 0.46 +/- 0.07 resp. 0.48 +/- 0.07 mole H-2/mole CO were obtained, which are in excellent agreement with direct source studies of traffic emissions in the Heidelberg/Mannheim region (0.448 +/- 0.003 mole H-2/mole CO). Including results from other European studies, our best estimate of the mean H-2/CO emission ratio from anthropogenic combustion sources (mainly traffic) ranges from 0.45 to 0.48 mole H-2/mole CO, which is about 20% smaller than the value of 0.59 mole H-2/mole CO which is frequently used as the basis to calculate global H-2 emissions from anthropogenic combustion sources.