Calibration experiments for dual-comb IPDA XCO2 measurements using a variable pressure absorption cell

Abstract

Measuring XCO2, the column-averaged dry-air mixing ratio of CO2 in the atmosphere, is essential for monitoring climate change and guiding mitigation efforts. Integrated Path Differential Absorption (IPDA) is a highly effective XCO2 measurement solution for space-borne lidar. However, the limited number of wavelengths limits its capabilities. IPDA, enhanced with Electro-Optical Frequency Comb (EOFC) technology, offers a robust solution. This study validates IPDA measurements using both asymmetric and symmetric Electro-Optic Dual-Comb Spectroscopy (EO-DCS) systems at 1572 nm. This indoor experimental system does not operate in a true lidar mode, but focuses on IPDA measurements of XCO2 through a transmission-style cell. We achieved atmospheric spectral transmittance by a variable pressure CO2 absorption cell, correlating pressures from 425 to 470 mb to XCO2 of 386.81–447.08 ppm. The XCO2 measurements of asymmetric/symmetric systems both exhibit excellent linearity. Their linear fittings yielded the Root Mean Square Error (RMSE) as low as 0.198/0.064 ppm, and the Mean Absolute Percentage Errors (MAPE) was as low as 0.0386%/0.0140%. This level of measurement performance has never been demonstrated in other works. Accuracy was assessed by comparing IPDA results with a pressure gauge, and precision was confirmed through repetitive measurements (1000 times over 10 min) and Allan variance analysis. Notably, our system operates effectively without frequency stabilization, showing tolerance to laser frequency drift—a significant advantage for IPDA applications. The entire experiment was conducted in a comparison between asymmetric and symmetric systems, especially in the analysis of absorption line residuals. We discuss the error transfer from phase to spectral transmittance and its implications and explained why the residuals in the asymmetric system were larger. These results underscore the superior performance of EO-DCS IPDA in measuring XCO2 and its promising potential for applications. To our knowledge, this is the only DCS IPDA calibration demonstration to date.