Validation of Long-Term Methane (CH4) Satellite Measurements from the AIRS Spectrometer and Correction of Their Temporal Drift

The study aims to develop a methodology for identifying and correcting drift in the quality of orbital data to improve the accuracy of long-term trend estimates and enhance the representativeness of orbital monitoring. We establish the presence of unidirectional, statistically significant drift in the difference between satellite instrument readings and ground-based observations. We compare the last two versions of methane (CH4) measurements from the Atmospheric InfraRed Sounder (AIRS)—specifically, Level 3 versions 6 and 7 (“IR-AIRS Only”)—with data from 18 ground-based stations of the Network for the Detection of Atmospheric Composition Change (NDACC) and 11 stations of the Total Carbon Column Observing Network (TCCON) for the 2003–2022 period. We determine that adjusting ground-based measurement data to sea level using the barometric formula, which is a necessary step for proper validation, results in significant errors, especially at high altitudes. It is proposed that such an adjustment should be based on pressure measured directly at a station. Implementing this over the examined period, we determine that the residual drift of the satellite spectrometer (Satellite Spectrometer Drift or SSD) is negative and equal to 1.64 × 1014 molecules/cm2 per day or 7.62 × 10−6 ppm per day for AIRS v6 and 7.20 × 10−6 ppm per day for AIRS v7. The correction implementation significantly improves the correspondence between AIRS v6 and v7 methane data and NDACC data, resulting in close estimates of methane trends from satellite and ground-based measurements. The robustness of the proposed correction has been demonstrated by the improvement in the consistency of station-by-station trend estimates obtained for corrected AIRS data and independent TCCON ground-based observations.