Development of a Laser Heterodyne Radiometer for Atmospheric CO₂ and O₂ Measurements: Comparison with FTIR Data

There is currently a major global interest in the monitoring of greenhouse gases (GHGs), recognized as the main cause of global warming. At present, the most widely accepted and utilized technique for accurate measurements of GHGs in the atmosphere is Fourier Transform Infrared (FTIR) analysis [1]. These systems are widely used in ground-based monitoring networks, because they provide high accuracy concentration measurements of many trace gases simultaneously. However, the main disadvantage of this type of systems, besides the cost, is its size, which makes it difficult to use for characterizing hot spot GHGs sources.  This has led to a growing interest in the development of new, more portable, and compact GHGs systems. In this context, Laser Heterodyne Radiometry (LHR) is seen as a promising alternative to complement and improve current observation systems. The technique characteristics include high optical resolution, flexibility of operation and a compact instrument design. On the other hand, the optical span is restricted by the tuning range of the local oscillator, which generates certain constraints and challenges that still need to be properly studied and addressed.

In LHR systems, the incoming signal is combined with the local oscillator laser and taken to a photodetector, which provides a downshifted radiofrequency (RF) copy of the spectrum of the optical input signal. The RF signal is amplified and filtered to configure the optical resolution of the instrument and detected by a RF power meter [2]. If the laser emission frequency is swept the spectrum of the optical signal can be retrieved.

This contribution will present the development of a novel, high-performance LHR, with optical frequency comb calibration, which allows the measurement of CO₂ and O₂ atmospheric concentrations in the field. The results of a measurement campaign, in which the developed system has been compared in detail with the FTIR spectrometer of the TCCON network at the Izaña Atmospheric Observatory (Spain) [3], will be presented and discussed. We believe that the results obtained provide a clear and promising outlook on the future possibilities of using LHR systems for atmospheric composition monitoring.

[1] D. Wunch et al., “The Total Carbon Column Observing Network’s GGG2014 Data Version,” CaltechDATA, Oct. 2015, doi: 10.14291/TCCON.GGG2014.documentation.R0/1221662.

[2] A. Moreno-Oyervides, O. E. Bonilla-Manrique, O. García, and P. Martín- Mateos, “Design and evaluation of a portable frequency comb-referenced laser heterodyne radiometer,” Opt Lasers Eng, vol. 171, p. 107801, Dec. 2023, doi: 10.1016/J.OPTLASENG.2023.107801.

[3] E. Cuevas et al., “Izaña Atmospheric Research Center Activity Report 2019-2020,” State Meteorological Agency (AEMET), Madrid, Spain and World Meteorological Organization, Geneva, Switzerland, NIPO: 666-22-014-0, WMO/GAW Report No. 276, 2022, https://doi.org/10.31978/666-22-014-0.