Methane budget estimates in Finland from the CarbonTracker Europe-CH4 data assimilation system

We estimated the CH 4 budget in Finland for 2004–2014 using the CTE-CH 4 data assimilation system with an extended atmospheric CH 4 observation network of seven sites from Finland to surrounding regions (Hyytiala, Kjolnes, Kumpula, Pallas, Puijo, Sodankyla, and Uto). The estimated average annual total emission for Finland is 0.6 ± 0.5 Tg CH 4 yr −1 . Sensitivity experiments show that the posterior biospheric emission estimates for Finland are between 0.3 and 0.9 Tg CH 4 yr −1 , which lies between the LPX-Bern-DYPTOP (0.2 Tg CH 4 yr −1 ) and LPJG-WHyMe (2.2 Tg CH 4 yr −1 ) process-based model estimates. For anthropogenic emissions, we found that the EDGAR v4.2 FT2010 inventory (0.4 Tg CH 4 yr −1 ) is likely to overestimate emissions in southernmost Finland, but the extent of overestimation and possible relocation of emissions are difficult to derive from the current observation network. The posterior emission estimates were especially reliant on prior information in central Finland. However, based on analysis of posterior atmospheric CH 4 , we found that the anthropogenic emission distribution based on a national inventory is more reliable than the one based on EDGAR v4.2 FT2010. The contribution of total emissions in Finland to global total emissions is only about 0.13%, and the derived total emissions in Finland showed no trend during 2004–2014. The model using optimized emissions was able to reproduce observed atmospheric CH 4 at the sites in Finland and surrounding regions fairly well (correlation > 0.75, bias 4 is also applicable for regional budget estimates, where small scale (1x1 in this case) optimization is possible with a dense observation network.