Validation of the Swiss methane emission inventory by atmospheric observations and inverse modelling

Atmospheric inverse modelling has the potential to provide observation-based esti-mates of greenhouse gas emissions at the country scale, thereby allowing for an inde-pendent validation of national emission inventories. Here, we present a regional scaleinverse modelling study to quantify the emissions of methane (CH4) from Switzerland, making use of the newly established CarboCount-CH measurement network and a highresolution Lagrangian transport model. Overall we estimate national CH4emissions tobe 196±18 Gg yr−1for the year 2013 (1σuncertainty). This result is in close agree-ment with the recently revised “bottom-up” estimate of 206±33 Gg yr−1published bythe Swiss Federal Office for the Environment as part of the Swiss Greenhouse Gas Inventory (SGHGI). Results from sensitivity inversions using alternative prior emissions,covariance settings, baseline treatments, two different inverse algorithms (Bayesianand extended Kalman Filter), and two different transport models confirms the robust-ness and independent character of our estimate. According to the latest “bottom-up”inventory the main CH4source categories in Switzerland are agriculture (78 %), waste handling (15 %) and natural gas distribution and combustion (6 %). The spatial distri-bution and seasonal variability of our posterior emissions suggest an overestimation ofagricultural CH4emissions by 10 to 20 % in the most recent national inventory, whichis likely due to an overestimation of emissions from manure handling. Urban areas donot appear as emission hotspots in our posterior results suggesting that leakages from natural gas disribution are only a minor source of CH4in Switzerland. This is consis-tent with rather low emissions of 8.4 Gg yr−1reported by the SGHGI but inconsistentwith the much higher value of 32 Gg yr−1implied by the EDGARv4.2 inventory for thissector. Increased CH4emissions (up to 30 % compared to the prior) were deduced forthe north-eastern parts of Switzerland. This feature was common to most sensitivity inversions, which rules out an artefact of the transport model and the inversion system.However, it was not possible to assign an unambiguous source process to the region.The observations of the CarboCount-CH network provided invaluable and independent information for the validation of the national bottom-up inventory. Similar systems needto be sustained to provide independent monitoring of future climate agreements.ISSN:1680-7375ISSN:1680-736