Influence of local emissions on concentration and isotopic composition of trace gases (CO2 and CH4) under strong anthropopression: A case study from Krakow, southern Poland

Full text: Measurements of the isotopic composition of carbon dioxide and methane together with their concentrations in the atmosphere, yield useful information on the contribution of anthropogenic sources to regional budgets of these gases and their seasonal changes. Observed correlation between isotopic composition and inverse concentration of these gases is used for estimation of mean isotopic composition of the local source. Monitoring of atmospheric CO2 has been initiated in Krakow in 1982. The sampling point is located in a polluted urban area with strong contribution of anthropogenic gases originating both from local sources (coal burning, car traffic, leakages from city gas network, landfills) and large distant emitters - industrial district located ca. 80 km to the west from Krakow (Silesia district). Quasi-continuous measurements of CO2, and CH4 concentrations in the low atmosphere are performed using gas chromatographic method. For isotope measurements, the atmospheric CO2 is continuously sampled by sorption on molecular sieve in be-weekly intervals and radiocarbon concentration is measured by liquid scintillation spectrometer, while δ13C is determined by isotope ratio mass spectrometer. Measurement error (1σ for single measurement) is in the order of 0.1 ppm for CO2 concentration, ±8 per mille for δ14C, and ± 0.1 per mille for δ13C. In 1994, a new station for regular observations of greenhouse gases in lower atmosphere was set up in the High Tatra mountains, at Kasprowy Wierch (49 deg. N, 20 deg. E, 1980 m a.s.l., 300 m above the tree line). Kasprowy Wierch, with only small influences from local sources of trace gases can be considered as a reference station for this region of Poland. The record of CO2 and CH4 concentration and their isotope composition obtained at Kasprowy Wierch is considered as a background level for Krakow observations. The presented study was aimed at better characterisation and quantification of the local fossil component coming from anthropogenic emissions of CO2. A three component mixing model was applied which allows to distinguish biospheric, and anthropogenic sources and their strength. In addition, the radiocarbon data were used. The analysed period extends from 1998 to 2000, presented in the context of the earlier obtained data. The carbon dioxide concentration together with its isotope composition at Kasprowy Wierch sampling point reveals well pronounced daily and seasonal oscillations, superimposed on the long-term trend. Much higher aplitudes are observed at Krakow. The increase of average CO2 concentration as well as methane mixing ratio is observed since 1998. Linearisation of the radiocarbon record shows steep decrease of ca. 45 per mille per year for the discussed period. Similar trend is observed for δ14C in CO2 collected in Krakow sampling point located in highly urbanised area. However, measurement values being in the same range for the both records, show fluctuations which are in counterphase. The average background methane concentration measured at Krakow reveals a systematic shift by a few percent towards higher values, when compared with the Kasprowy Wierch data. This is caused by persistent leakage of methane from the city gas network. (author)