High-Stability Atmospheric Methane Measurement Using Laser-Locked Cavity Ring-Down Spectroscopy

Methane is the second most important anthropogenic greenhouse gas after carbon dioxide, and its accurate measurement is essential for climate monitoring. In this work, a laser-locked cavity ring-down spectroscopy (LL-CRDS) system was developed for continuous atmospheric methane measurements. By locking the laser frequency to a longitudinal mode of the optical cavity, long-term wavelength stability was achieved, enabling highly stable methane measurements. Laboratory tests demonstrated a precision of 0.4 ppb at an integration time of 3.6 s, a minimum Allan deviation of 0.3 ppb at 15 s, and long-term stability better than 23 ppb over one month without recalibration. Field measurements with a commercial CRDS analyzer showed excellent agreement, with an average deviation of 1.4 ppb. These results demonstrate the suitability of LL-CRDS for long-term atmospheric methane monitoring requiring high precision and low maintenance.