The ACOS CO2 retrieval algorithm - Part II: Global XCO2 data characterization

Here, we report preliminary estimates of the column averaged carbon dioxide (CO2) dry air mole fraction, X-CO2, retrieved from spectra recorded over land by the Greenhouse gases Observing Satellite, GOSAT (nicknamed 'Ibuki'), using retrieval methods originally developed for the NASA Orbiting Carbon Observatory (OCO) mission. After screening for clouds and other known error sources, these retrievals reproduce much of the expected structure in the global X-CO2 field, including its variation with latitude and season. However, low yields of retrieved X-CO2 over persistently cloudy areas and ice covered surfaces at high latitudes limit the coverage of some geographic regions, even on seasonal time scales. Comparisons of early GOSAT X-CO2 retrievals with X-CO2 estimates from the Total Carbon Column Observing Network (TCCON) revealed a global, -2% (7-8 parts per million, ppm, with respect to dry air) X-CO2 bias and 2 to 3 times more variance in the GOSAT retrievals. About half of the global X-CO2 bias is associated with a systematic, 1% overestimate in the retrieved air mass, first identified as a global +10 hPa bias in the retrieved surface pressure. This error has been attributed to errors in the O-2 A-band absorption cross sections. Much of the remaining bias and spurious variance in the GOSAT X-CO2 retrievals has been traced to uncertainties in the instrument's calibration, oversimplified methods for generating O-2 and CO2 absorption cross sections, and other subtle errors in the implementation of the retrieval algorithm. Many of these deficiencies have been addressed in the most recent version (Build 2.9) of the retrieval algorithm, which produces negligible bias in X-CO2 on global scales as well as a similar to 30% reduction in variance. Comparisons with TCCON measurements indicate that regional scale biases remain, but these could be reduced by applying empirical corrections like those described by Wunch et al. (2011b). We recommend that such corrections be applied before these data are used in source sink inversion studies to minimize spurious fluxes associated with known biases. These and other lessons learned from the analysis of GOSAT data are expected to accelerate the delivery of high quality data products from the Orbiting Carbon Observatory-2 (OCO-2), once that satellite is successfully launched and inserted into orbit.