Anthropogenic emissions of CO_2 and CH_4 in an urban environment

摘要

Regular observations of atmospheric mixing-ratios of carbon dioxide and methane in the urban atmosphere, combined with analyses of their carbon-isotope composition (δ~(13)C, δ~(14)C), 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 CO_2 and CH_4, 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 CO_2, carried out in Krakow over the past two decades, were extended to the period 1995-2000 and also to atmospheric mixing-ratios of CH_4 and its carbon-isotope composition. Radiocarbon concentrations (δ~(14)C) in atmospheric CO_2 recorded at Krakow are systematically lower than the regional background levels. This effect stems from the addition of ~(14)C-free CO_2 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 ~(14)CO_2 content in the local atmosphere and the regional background observed after 1991 is attributed to the reduced consumption of ~(14)C-free fuels, mostly coal, in southern Poland and the Krakow municipal area. The linear regression of δ~(13)C values of methane plotted versus reciprocal concentration, performed for the data available for Krakow sampling site, yields the average δ~(13)C signature of the local source of methane as being equal to -54.2per thousand. This value agrees very well with the measured isotope signature of natural gas being used in Krakow (-54.4+-0.6per thousand) and points to leakages in the distribution network of this gas as the main anthropogenic source of CH_4 in the local atmosphere.