Saturated absorption spectroscopy of methane around 1667 nm

Methane is the second most important greenhouse gas after carbon dioxide, and the reduction of methane emissions is a key focus in IPCC reports. At present, atmospheric methane is monitored through ground-based observations and satellite remote sensing that depend on spectral characteristics. However, the positions of the methane lines provided by the widely utilized HITRAN database lack the precision required for accurate retrievals. In this work, we used cavity-enhanced absorption spectroscopy to record the absorption spectrum near the atmospheric detection window of methane at 1.66 mu m. From the saturated absorption peaks ("Lamb dips") observed on top of Doppler-broadened methane absorption lines, we determined the line positions with an absolute frequency accuracy of better than 2 MHz. The results also indicate the incompetence of the HITRAN database and theoretical models based on alpha b initio calculations of methane lines in this region.