Verification of German methane emission inventories and their recent changes based on atmospheric observations

Continuous methane concentration records and stable isotope observations measured in the suburbs of Heidelberg, Germany, are presented. While delta(13)C-CH4, shows a significant trend of -0.14 parts per thousand per year, toward more depicted values, no trend is observed in the concentration data. Comparison of the Heidelberg records with clean air observations in the North Atlantic at Izana station (Tenerife) allows the determination of the continental methane excess at Heidelberg, decreasing by 20% from 190 ppb in 1992 to 150 ppb in 1997. The isotope ratio which is associated with this continental methane pileup in the Heidelberg catchment area shows a significant trend to more depleted values from delta(13)C(scource) = -47.4 +/- 1.2 parts per thousand in 1992 to -52.9 +/- 0.4 parts per thousand in 1995/1996, pointing to a significant change in the methane source mix. Total methane emissions in the Heidelberg catchment area are estimated using the (222)radon (Rn-222) tracer method: from the correlations of half-hourly Rn-222 and CH, mixing ratios from 1995 to 1997, and the mean Rn-222 exhalation rate from typical soils in the Rhine valley, a mean methane nux of 0.24 +/- 0.5 g CH, km(-2) s(-1) is derived. For the Heidelberg catchment area with an estimated radius of approximately 150 km, Core Inventories Air 1990 (CORINAIR90) emission estimates yield a flux of 0.47 g CH, km(-2) s(-1), which is about 40% higher than the Rn-222-derived number if extrapolated to 1990. The discrepancy can be explained by overestimated emissions from waste management in the CORINAIR90 statistical assessment. The observed decrease in total emissions can be accounted fur by decreasing contributions from fossil sources (mainly coal mining) and from cattle breeding. This finding is also supported by the observed decrease in mean source Isotopic signatures.