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

Abstract

Abstract. Atmospheric inverse modelling has the potential to provide observation-based estimates of greenhouse gas emissions at the country scale, thereby allowing for an independent validation of national emission inventories. Here, we present a regional scale inverse modelling study to quantify the emissions of methane (CH4) from Switzerland, making use of the newly established CarboCount-CH measurement network and a high resolution Lagrangian transport model. Overall we estimate national CH4 emissions to be 196 ± 18 Gg yr−1 for the year 2013 (1σ uncertainty). This result is in close agreement with the recently revised "bottom-up" estimate of 206 ± 33 Gg yr−1 published by the 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 (Bayesian and extended Kalman Filter), and two different transport models confirms the robustness and independent character of our estimate. According to the latest "bottom-up" inventory the main CH4 source categories in Switzerland are agriculture (78 %), waste handling (15 %) and natural gas distribution and combustion (6 %). The spatial distribution and seasonal variability of our posterior emissions suggest an overestimation of agricultural CH4 emissions by 10 to 20 % in the most recent national inventory, which is likely due to an overestimation of emissions from manure handling. Urban areas do not appear as emission hotspots in our posterior results suggesting that leakages from natural gas disribution are only a minor source of CH4 in Switzerland. This is consistent with rather low emissions of 8.4 Gg yr−1 reported by the SGHGI but inconsistent with the much higher value of 32 Gg yr−1 implied by the EDGARv4.2 inventory for this sector. Increased CH4 emissions (up to 30 % compared to the prior) were deduced for the 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 need to be sustained to provide independent monitoring of future climate agreements.