Anthropogenic emissions of CO2 and CH4 in an urban environment

Abstract

Regular observations of atmospheric mixing-ratios of carbon dioxide and methane in the urban atmosphere, combined with analyses of their carbon-isotope composition ([delta]13C, [delta]14C), provide a powerful tool for assessing both the source strength and source partitioning of those gases, as well as their changes with respect to time. Intense surface fluxes of CO2 and CH4, associated with anthropogenic activities result in elevated levels of these gases in the local atmosphere as well as in modifications of their carbon-isotope compositions. Regular measurements of concentration and carbon-isotope composition of atmospheric CO2, carried out in Krakow over the past two decades, were extended to the period 1995-2000 and also to atmospheric mixing-ratios of CH4 and its carbon-isotope composition. Radiocarbon concentrations ([delta]14C) in atmospheric CO2 recorded at Krakow are systematically lower than the regional background levels. This effect stems from the addition of 14C-free CO2 into the local atmosphere, originating from the burning of fossil fuels. The fossil-fuel component in the local budget of atmospheric carbon calculated using a three-component mixing model decreased from ca. 27.5 ppm in 1989 to ca. 10 ppm in 1994. The seasonal fluctuations of this component (winter-summer) are of similar magnitude. A gradually decreasing difference between the 14CO2 content in the local atmosphere and the regional background observed after 1991 is attributed to the reduced consumption of 14C-free fuels, mostly coal, in southern Poland and the Krakow municipal area. The linear regression of [delta]13C values of methane plotted versus reciprocal concentration, performed for the data available for Krakow sampling site, yields the average [delta]13C signature of the local source of methane as being equal to -54.2[per mille sign]. This value agrees very well with the measured isotope signature of natural gas being used in Krakow (-54.4±0.6[per mille sign]) and points to leakages in the distribution network of this gas as the main anthropogenic source of CH4 in the local atmosphere.Carbon dioxide Methane Carbon isotopes Anthropogenic emissions