CO2 emissions from cities: Direct flux measurements versus the indirect budget approach

Abstract

Many bigger cities establish climate protection plans by employing emission inventories based on indirect budget approaches. However, the emission fluxes of CO2 from cities can also be directly measured using the eddy covariance technique, which is a micrometeorological approach. For the cities of Münster (Germany), Preston (Melbourne, Australia), Essen (Germany), London (UK), and Helsinki (Finland), we compare published emission flux data collected using the eddy covariance technique with separate emission databases from the indirect budget approach as published in action plans against climate change by these cities. We also looked at comparisons from other cities - Vancouver (Canada), Beijing (China) and Copenhagen (Denmark) - for which the two approaches have previously been compared. For cities where the CO2 emissions had not been compared previously (in which the data for each city were compiled from two different sources), the two approaches agreed rather well for some cities but exhibited large deviations for other cities. Indirect budget approach emissions were 4-14 times higher than measured fluxes. After correcting fluxes for the influence of vegetation, human respiration and further potential biases of the two methods, the ratios of the indirect budget approach and direct flux measurements were between one and 25. In cities where comparisons of the approaches had previously been made and published in scientific journals (in which the data for both approaches came from the same source), the ratios of the indirect budget approach and direct flux measurements are between 1.08 and 1.11, showing that for these cities, the approaches matched to within about 10 %. We suggest that, in the future, comparisons such as the ones presented here should be performed using a double-blind procedure to contribute to the improvement of emission inventories.