Validation of open-path dual-comb spectroscopy against an O₂ background

Area of Science:

• Atmospheric spectroscopy
• Environmental monitoring
• Laser technology

Background:

• Dual-comb spectroscopy offers high-precision greenhouse gas measurements over long open-air paths.
• Accuracy is limited by atmospheric variability and absorption models.
• Oxygen (O2) provides a stable, well-mixed atmospheric reference gas.

Purpose of the Study:

• To use O2 as a reference gas to validate open-path dual-comb spectroscopy accuracy.
• To assess the performance of a new dual-comb spectrometer for simultaneous O2, CO2, and CH4 measurements.
• To identify factors limiting spectroscopic accuracy in open-air conditions.

Main Methods:

• Constructed a dual-comb spectrometer operating from 1240 nm to 1700 nm.
• Measured O2 absorption features alongside CO2 and CH4 over a 560 m outdoor path.
• Collected data over seven days under varying meteorological conditions.

Main Results:

• Achieved 0.1% precision for O2 concentration measurements within 10 minutes.
• Demonstrated a mean bias of -0.07% for O2 over seven days.
• 90% of O2 measurements were within 0.4% of expected values, with excursions correlating to temperature and humidity.

Conclusions:

• Dual-comb spectroscopy, validated by O2, can improve the accuracy of open-air greenhouse gas measurements.
• Potential limitations in O2 absorption models or water vapor interference require further investigation.
• The developed spectrometer is suitable for accurate CO2 monitoring over long open-air paths and varying air densities.