A Quasi-Synchronous Background Detection Laser Heterodyne Radiometer for Atmospheric Water Vapor Measurements

We present a laser heterodyne radiometer (LHR) featuring a quasi-synchronous background detection scheme for column-integrated atmospheric water vapor measurements. The proposed scheme effectively suppresses relative background drift to within 0.8%, significantly enhancing measurement stability. The instrument achieves a high spectral resolution of 0.004 cm⁻¹ with a rapid acquisition time of 25 s per heterodyne spectrum. Field experiments were conducted in August 2024 in the Ali region of the Tibetan Plateau. Continuous observations on August 14 demonstrated stable retrievals of atmospheric H₂O column concentrations with a relative uncertainty of 1.37%. Additional measurements performed on August 16 and August 21 yielded relative uncertainties of 1.16% and 2.79%, respectively, confirming the repeatability and robustness of the system under varying atmospheric conditions.  Simultaneous measurements using a Fourier transform spectrometer (Bruker EM27/SUN) showed consistent temporal variability, with a correlation coefficient of 0.77. These results indicate that the developed LHR combines fast acquisition speed, high spectral resolution, and reliable precision, making it a promising instrument for long-term and stable atmospheric water vapor monitoring, particularly in remote and high-altitude regions.