On the consistency of methane isotopologue retrievals using TCCON and multiple spectroscopic databases

Abstract

Abstract. In this study we perform retrievals of the two main methane isotopologues 12CH4 and 13CH4 using measurements from the Total Carbon Column Observing Network (TCCON) from two sites, namely Ascension Island in the Atlantic Ocean and Tsukuba, Japan. Using the TCCON GGG2014 retrieval environment retrievals are performed using four separate spectroscopic databases and a set of spectral fit windows. Databases used include the TCCON spectroscopic database; the HITRAN2016 database; the GEISA2015 database; and the ESA SEOM-IAS database. We assess the retrievals using standard TCCON methane windows, and specific windows (in the 4190–4340 cm-1 range) based on the sensitivity of the instruments TROPOMI present on Copernicus Sentinel-5P (S5P) and the future Sentinel 5 (S5) mission present on MetOp-SG. We assess the biases in retrieving methane isotopologues using these different spectral windows and different spectroscopic databases. The sensitivity of these retrievals (across windows and databases) to errors in the a priori information, specifically pressure, temperature, methane and water vapour are also assessed. We find significant biases between retrievals calculated using differing spectroscopic databases and windows for both methane isotopologues, with up to a 3 % bias between 12CH4 retrievals and 20 % bias in 13CH4 retrievals. Retrievals using the 4190–4340 cm-1 spectral range show the results with the least variation between spectroscopic databases, and we therefore recommend that this band should be used in future TCCON methane retrievals. Results obtained with the SEOM-IAS database show the lowest fit residuals. Uncertainty on 13CH4 retrievals are relatively high (0.1–2 ppb, combination of systematic and random). The sensitivity to a priori assumptions are shown to be significant for both 12CH4 and 13CH4. Uncertainty in the pressure cross sections is shown to be the most significant, with variations across all spectroscopic databases and spectral windows.