Ultrahigh Resolution Portable Dual-Channel Laser Heterodyne Radiometer

An integrated portable, dual-channel all-fibered laser heterodyne radiometer (portable dc-LHR) with a high spectral resolution (0.0044 cm−1) for simultaneous remote sensing of atmospheric dry-air column-averaged mole fraction of CO2 and CH4 (XCO2 and X _{\text {CH4}} ) has been developed. Sunlight is modulated by a dual-channel fiber optical switch and mixed with two laser beams, which were step-scanned across two strong absorption features centered at 1603.59-1657.39 nm, respectively. Digital lock-in amplifiers (DLIAs) are designed to demodulate the laser heterodyne signal, while self-design interferometric devices are used to monitor laser wavelength. A forward model based on relative transmittance in a narrow band and an optimal estimation inversion algorithm are used for data inversion. The portable instrument was carried out in a field campaign in Dachaidan (Qinghai Province, China, 37.744°N, 95.342°E), together with co-located Aircore and EM27/SUN measurements, in August 2020. The retrieval uncertainties of portable dc-LHR XCO2 and XCH4 are 0.9 ppm and 3.4 ppb, respectively. Comparison in XCO2 and XCH4 observations between portable dc-LHR and EM27/SUN show a good agreement.