Comparison of a spatially explicit methane inventory with fluxes measured by aircraft in an agriculturally dominated region in Switzerland

Methane is the second most important anthropogenic greenhouse gas and also included in the Kyoto protocol. Reported emissions rely on national inventories that base on a country specific bottom-up estimates. So far, only few comparisons between inventories and measurements on the regional scale exist and often a disagreement of a factor two or more was found. Therefore, additional investigations are needed to validate the currently used inventories by directly linking them to top-down flux estimates. Agriculture is the most important methane source in Switzerland contributing 83% to the anthropogenic emissions. To compare the national inventory against regional scale flux measurements, the emissions were spatially disaggregated onto a 500x500 m grid based on geostatistical data of the source locations. The Reuss Plain in Central Switzerland is located in an agriculturally dominated pre-alpine valley representing a typical Swiss rural landscape. The latter land type accounts for the most methane emissions in Switzerland. To assess the methane flux of this region, we performed aircraft measurements of methane and the 3D wind components on multiple flight legs along the main valley axis. The 15 km long legs were flown at different terrain following heights between 50 and 400 m above ground level on 16 days in 2009 and 2010. Average methane fluxes were determined with two independent methods, (1) a simple budget approach accounting for the transport of methane along the valley by the thermally induced valley winds, and (2) the eddy covariance method employing the high frequency (1 Hz) methane concentration and vertical turbulent wind measurements. Derived eddy covariance fluxes (0.30 μg CH4 m^-2 s^-1) underestimated the inventory-based fluxes (0.41 μg CH4 m^-2 s^-1) whereas a budget approach yielded higher fluxes (0.89 μg CH4 m^-2 s^-1). 85 on 1-30-2012-->