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also cannot be neglected. Daytime mean CH 4 concentrations from the Siberian tower sites were generally higher than CH 4 values reported at NOAA coastal sites in the same latitudinal zone, and the difference in concentrations between two sets of sites was reproduced with a coupled Eulerian-Lagrangian transport model. Simulations of emissions from different CH 4 sources suggested that the major contributor to variation switched from wetlands during summer to fossil fuel during winter.

[1] We collected samples of seawater, zooplankton, and sinking particles in the northwestern North Pacific to determine the source of excess CH 4 over the saturation value in equilibrium with the atmospheric CH 4 in the oxygenated open ocean, using stable carbon isotope as a tracer. We found that subsurface ($100 m depth) seawater is supersaturated (up to 12%) with 13 C-enriched CH 4 (up to À33.1%) relative to surface seawater in equilibrium with the atmosphere (À47%), suggesting that in situ addition of 13 C-enriched CH 4 must be responsible for CH 4 enrichment at depth. The d 13 C of CH 4 emitted from sinking particles (from À36.7 ± 1.2% to +5.9 ± 7.5%) is within the range of that of excess CH 4 in seawater, suggesting that the major source of subsurface excess CH 4 is sinking particles. The unusually 13 C-enriched d 13 C composition of CH 4 emitted from sinking particles suggests that active microbial CH 4 oxidation occurs within the oxic/ anoxic boundary of these particles. On the basis of the Rayleigh equation, we estimated that at least 62% of CH 4 produced within the anoxic center of sinking particles is oxidized within 100 m of the surface.

[1] Being one of the largest carbon reservoirs in the world, the Siberian carbon sink however remains poorly understood due to the limited numbers of observation. We present the first results of atmospheric CO 2 inversions utilizing measurements from a Siberian tower network (Japan-Russia Siberian Tall Tower Inland Observation Network; JR-STATION) and four aircraft sites, in addition to surface background flask measurements by the National Oceanic and Atmospheric Administration (NOAA). Our inversion with only the NOAA data yielded a boreal Eurasian CO 2 flux of À0.56 AE 0.79 GtC yr À1 , whereas we obtained a weaker uptake of À0.35 AE 0.61 GtC yr À1when the Siberian data were also included. This difference is mainly explained by a weakened summer uptake, especially in East Siberia. We also found the inclusion of the Siberian data had significant impacts on inversion results over northeastern Europe as well as boreal Eurasia. The inversion with the Siberian data reduced the regional uncertainty by 22% on average in boreal Eurasia, and further uncertainty reductions up to 80% were found in eastern and western Siberia. Larger interannual variability was clearly seen in the inversion which includes the Siberia data than the inversion without the Siberia data.In the inversion with NOAA plus Siberia data, east Siberia showed a larger interannual variability than that in west and central Siberia. Finally, we conducted forward simulations using estimated fluxes and confirmed that the fit to independent measurements over central Siberia, which were not included in inversions, was greatly improved.Citation: Saeki, T., et al. (2013), Carbon flux estimation for Siberia by inverse modeling constrained by aircraft and tower CO 2 measurements,

Abstract. Eight surface observation sites providing quasicontinuous measurements of atmospheric methane mixing ratios have been operated since the mid-2000's in Siberia. For the first time in a single work, we assimilate 1 year of these in situ observations in an atmospheric inversion. Our objective is to quantify methane surface fluxes from anthropogenic and wetland sources at the mesoscale in the Siberian lowlands for the year 2010. To do so, we first inquire about the way the inversion uses the observations and the way the fluxes are constrained by the observation sites. As atmospheric inversions at the mesoscale suffer from mis-quantified sources of uncertainties, we follow recent innovations in inversion techniques and use a new inversion approach which quantifies the uncertainties more objectively than the previous inversion systems. We find that, due to errors in the representation of the atmospheric transport and redundant pieces of information, only one observation every few days is found valuable by the inversion. The remaining high-resolution quasicontinuous signal is representative of very local emission patterns difficult to analyse with a mesoscale system. An analysis of the use of information by the inversion also reveals that the observation sites constrain methane emissions within a radius of 500 km. More observation sites than the ones currently in operation are then necessary to constrain the whole Siberian lowlands. Still, the fluxes within the constrained areas are quantified with objectified uncertainties. Finally, the tolerance intervals for posterior methane fluxes are of roughly 20 % (resp. 50 %) of the fluxes for anthropogenic (resp. wetland) sources. About 50-70 % of Siberian lowlands emissions are constrained by the inversion on average on an annual basis. Extrapolating the figures on the constrained areas to the whole Siberian lowlands, we find a regional methane budget of 5-28 TgCH 4 for the year 2010, i.e. 1-5 % of the global methane emissions. As very few in situ observations are available in the region of interest, observations of methane total columns from the Greenhouse Gas Observing SATellite (GOSAT) are tentatively used for the evaluation of the inversion results, but they exhibit only a marginal signal from the fluxes within the region of interest.

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We examined the consistency between terrestrial biosphere fluxes (terrestrial CO 2 exchanges) from data-driven top-down (GOSAT CO 2 inversion) and bottom-up (empirical eddy flux upscaling based on a support vector regression (SVR) model) approaches over 42 global terrestrial regions from June 2009 to October 2011. Seasonal variations of the biosphere fluxes by the two approaches agreed well in boreal and temperate regions across the Northern Hemisphere. Both fluxes also exhibited strong anomalous signals in response to contrasting anomalous spring temperatures observed in North America and boreal Eurasia. This indicates that the CO 2 concentration data integrated in the GOSAT inversion and the meteorological and vegetation data in the SVR models are equally effective in producing spatiotemporal variations of biosphere flux. Meanwhile, large differences in seasonality were found in subtropical and tropical South America, South Asia, and Africa. The GOSAT inversion showed seasonal variations that pivoted around CO 2 neutral, while the SVR model showed seasonal variations that tended toward CO 2 sink. Thus, a large difference in CO 2 budget was identified between the two approaches in subtropical and tropical regions across the Southern Hemisphere. Examination of the integrated data revealed that the large tropical sink of CO 2 by the SVR model was an artifact due to the underrepresented biosphere fluxes predicted by limited eddy flux data for tropical biomes. Because of the global coverage of CO 2 concentration data, the GOSAT inversion provides better estimates of continental CO 2 flux than the SVR model in the Southern Hemisphere.

[1] In situ measurements of the vertical distribution of carbon dioxide (CO 2 ) carried out with a light aircraft over a tower site (Berezorechka; 56°08′45″N, 84°19′49″E) in the taiga region of West Siberia from October 2001 to March 2012 document the detailed seasonal and vertical variation of CO 2 concentrations during daytime. The variation appears to be controlled mainly by the CO 2 flux from taiga ecosystems and the height of the planetary boundary layer (PBL). We calculated average CO 2 concentrations in the PBL and the lower free troposphere (LFT), both of which show clear seasonal cycles and an increasing long-term trend. Seasonal amplitude in the PBL had a larger value (29 ppm) than that in the LFT (14 ppm), demonstrating strong CO 2 source-sink forcing by the taiga ecosystems. Mean CO 2 concentrations during 13:00-17:00 local standard time observed at the four levels of the tower (5, 20, 40, and 80 m) showed lower CO 2 concentrations than that observed in the PBL by aircraft during June-August (growing season). This negative bias decreased with increasing inlet height such that the minimum difference appeared at the 80-m inlet (À2.4 ± 0.8 ppm). No such bias was observed during other months (dormant season). The daytime CO 2 flux, based on multiple vertical profiles obtained on a single day, ranged from À36.4 to 3.8 μmol m À2 s À1 during July-September. There was a clear difference in the fluxes between the morning and afternoon, suggesting that these data should be considered examples of fluxes during several daytime hours from the West Siberian taiga.