Ground-Based Remote Sensing of CO2 in the Atmospheric Column Using a Portable Laser Heterodyne Radiometer with a Balanced Photodetector

Measurement of vertical concentration profiles of atmospheric trace gases is of great interest to understand the physics, chemistry, dynamics, and radiation budget of the atmosphere as well as to validate the results provided from chemical models and satellite observations. The laser heterodyne radiometer (LHR), as a passive remote sensing technique, was introduced and developed in 1970s to meet the needs of observing O
3
hole in the atmosphere [1]–[3]. Since then, due to the lack of a suitable tunable laser source being used as a local oscillator (LO) for heterodyne measurement, LHR applications stayed almost in silence. Over the last decade, there has been a revival of the LHR technique as a result of significant advances in lasers and photonics technology [4]. Compared to the currently used Fourier transform spectrometer (FTS) for ground-based measurement of trace gases in the atmospheric column, the LHR offers unique advantages including high spectral resolution (<10
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cm
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, determined by the selected electronic filter bandwidths), high sensitivity (within a factor of ~ 2 of the quantum noise limit), high spatial resolution owing to very small coherent field of view (FoV), and cost-effective compact instrumental dimension.