Simultaneous Detection of Carbon Dioxide and Its Isotope (<b>δ</b>¹³C) Based on Near Infrared Off‐Axis Integrated Cavity Output Spectroscopy

ABSTRACT Accurate monitoring of atmospheric δ 13 C isotope is critical for quantifying anthropogenic carbon emissions and ecosystem responses within global carbon cycle research. This study used off‐axis integrated cavity output spectroscopy (OA‐ICOS) combined with distributed feedback (DFB) laser and achieved an effective absorption path length of approximately 22 km, for the simultaneous detection of carbon dioxide and its isotope (δ 13 C) under atmospheric background conditions. Laboratory validation demonstrated detection limits of 104.5 ppb, 3.3 ppb, and 0.72‰ for 12 CO 2 , 13 CO 2 , and δ 13 C, respectively, with a 2000 s integration time, alongside linear calibration R 2 &gt; 0.999. While Kalman filtering reduced concentration variability by 73.5%–74.7% and improved δ 13 C precision 4.6 times. The sensor's capability to quantify isotopic ratios and molecular concentrations in real‐time addresses critical gaps in atmospheric background monitoring, offering a robust tool for climate research and carbon source–sink analysis, expanding the applicability of laser spectroscopy in environmental sensing.