Measurement of the ~(13)C/~(12)C of Atmospheric CH_4 Using Near-Infrared (NIR) Cavity Ring-Down Spectroscopy

摘要

A near-infrared (NIR) continuous-wave-cavity ring-down spectrometry (CW-CRDS) device was developed with the goal of measuring seasonal changes in the isotopic composition of atmospheric CH_4 on Earth and eventually on Mars. The system consisted of three distributed feedback laser diodes (DFB-LDs), two of which were tuned to the absorption line peaks of ~(12)CH_4 and ~(13)CH_4 at 6046.954 cm~(-1) and 6049.121 cm~(-1), respectively, and a third that measured the baseline at 6050.766 cm~(-1). The multiple laser design improved the long-term stability of the system and increased the data acquisition rate. The acquisition frequency was further increased by utilizing a semiconductor optical amplifier (SOA) to initiate cavity ring-down events. The high repetition rate combined with the superhigh reflectivity mirrors yielded precise isotopic measurements in this NIR region, even though the line strengths of CH_4 in this region are 200 times weaker than those of the strongest mid-IR absorption bands. The current system has a detection limit of 1.9 × 10~(-12) cm~(-1), corresponding to 10 pptv of CH_4 at 100 Torr. For ambient air samples that contained 1.9 ppmv CH_4, the δ~(13)C of the CH_4 was determined to be -48.7 ± 1.7‰ (1σ).