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This study investigates the constraint provided by greenhouse gasmeasurements from space on surface fluxes. Imperfect knowledge of the lightpath through the atmosphere, arising from scattering by clouds and aerosols,can create biases in column measurements retrieved from space. To minimizethe impact of such biases, ratios of total column retrieved CH4 and CO2(Xratio) have been used. We apply the ratio inversion methoddescribed in Pandey et al. (2015) to retrievals from the GreenhouseGases Observing SATellite (GOSAT). The ratio inversion method uses the measuredXratio as a weak constraint on CO2 fluxes. In contrast, themore common approach of inverting proxy CH4 retrievals (Frankenberg et al., 2005) prescribes atmospheric CO2 fields and optimizesonly CH4 fluxes. The TM5–4DVAR (Tracer Transport Model version 5–variational dataassimilation system) inverse modeling system is used to simultaneouslyoptimize the fluxes of CH4 and CO2 for 2009 and 2010. The resultsare compared to proxy inversions using model-derived CO2 mixing ratios(XCO2model) from CarbonTracker and the Monitoring AtmosphericComposition and Climate (MACC) Reanalysis CO2 product. The performanceof the inverse models is evaluated using measurements from three aircraftmeasurement projects. Xratio and XCO2model are compared with TCCONretrievals to quantify the relative importance of errors in these componentsof the proxy XCH4 retrieval (XCH4proxy). We find that theretrieval errors in Xratio (mean  =  0.61 %) are generally largerthan the errors in XCO2model (mean  =  0.24 and 0.01 % forCarbonTracker and MACC, respectively). On the annual timescale, the CH4fluxes from the different satellite inversions are generally in agreementwith each other, suggesting that errors in XCO2model do notlimit the overall accuracy of the CH4 flux estimates. On the seasonal timescale, however, larger differences are found due to uncertainties inXCO2model, particularly over Australia and in the tropics. Theratio method stays closer to the a priori CH4 flux in theseregions, because it is capable of simultaneously adjusting the CO2 fluxes.Over tropical South America, comparison to independent measurements showsthat CO2 fields derived from the ratio method are less realistic thanthose used in the proxy method. However, the CH4 fluxes are morerealistic, because the impact of unaccounted systematic uncertainties is moreevenly distributed between CO2 and CH4. The ratio inversion estimatesan enhanced CO2 release from tropical South America during the dry seasonof 2010, which is in accordance with the findings of Gatti et al. (2014) and Van der Laan et al. (2015). The performance of the ratio method is encouraging, because despite the addednonlinearity due to the assimilation of Xratio and thesignificant increase in the degree of freedom by optimizing CO2 fluxes,still consistent results are obtained with respect to other CH4inversions.</p

atmospheric chemistry and physics

Inverse modeling of GOSAT-retrieved ratios of total column CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; for 2009 and 2010