Validation and assessment of satellite-based columnar CO₂ and CH₄ mixing-ratios from GOSAT and OCO-2 satellites over India

Abstract. The OCO-2 and GOSAT series of satellites provide near-global coverage of CO₂ and CH₄ mixing ratios. To accurately derive emission fluxes from the observed mixing ratios, it is crucial that these data meet specific precision and systematic error requirements. In this study, we report validation results for GOSAT and OCO-2 over South Asia, obtained using a portable Fourier Transform infrared Spectrometer (FTS) at a tropical rural site (Gadanki; Latitude: 13.45° N, Longitude: 79.18° E) in Southern India, from November 2015 to July 2016. Biases in CH₄ mixing ratios from GOSAT ranged from -9 to -18.5 ppb, depending on the collocation criteria, while CO₂ data from OCO-2 demonstrated better accuracy and precision, meeting the requirements of ESA’s Climate Change Initiative (CCI). Using the FLEXPART model, we also show that CH₄ emissions from regional sources accounted for only 35 % of the day-to-day observed variability. Both model-derived and observed mixing ratios exhibited the same seasonal variation, with higher values in October–November and lower values in June–July. However, the observed mixing ratios decreased by approximately 100 ppb, while the model-derived values decreased by only 20 ppb, suggesting that atmospheric chemistry and variations in background concentrations play a significant role over South India.