Remote sensing of atmospheric gas plays an important role in the fields of greenhouse gas moni­toring, toxic, harmful, flammable, explosive gases, and biochemical weapons early warning. With the advent of quantum mechanics and lasers, precise spectral analysis method has developed rapidly in many funda­mental domains. Challenges exist in accurate spectrum analysis in free space, which hinder us from understanding the compo­sition of multiple gases and the chemical processes in the atmosphere. Now, a team of scientists, led by Haiyun Xia from School of Earth and Space Science, USTC, China, and School of Atmo­spheric Physics, NUIST, China, has developed a photon-counting distributed free-space spectro­scopy (PDFS).

At present, the carbon emissions peak and carbon neutrality are among of the most concerning topics of discussion worldwide. The researchers used the proposed method to obtain the spectra of carbon dioxide and semi-heavy water (HDO) over 6 kilo­meters, with a range resolution of 60 meters. “The current state-of-art techniques provide either in-situ or column-integrated spectral infor­mation. However, the molecular charac­teristics vary rapidly in time and space due to chemical reactions and physical transportation in free atmosphere. It would be beneficial to analyze the processes continuously with range-resolved spectra in open areas, especially in inacces­sible regions.” said Haiyun Xia, “our method can detect different gases simul­taneously with ranged-resolved spectra. It inherits ability from laser remote sensing and spectro­scopy.”

First, they used an optical comb to lock the probe laser frequency for precise spectra scanning. Second, they proposed a time-division multi­plexing technique to make probe and reference lasers emit alter­natively, reducing the influences of aerosol variation, laser power fluctuation, detector insta­bility, and telescope coupling effi­ciency change. Third, they developed a photon detector with a large active area, high quantum efficiency, and low dark noise to eliminate the need for a high-power laser for long-range detection. The integrated spectra of CO₂ and HDO over different ranges and the range-resolved spectra with range resolution of 60 meters were obtained. Semi-heavy water (HDO) natural abundance in the atmosphere is only about 1/3200 of H₂O, which is often ignored.

Besides the photon-counting distributed free-space spectroscopy, a coherent Doppler wind lidar (CDWL) developed by the researchers was employed to monitor local meteoro­logical conditions. The researchers measured distri­butions of the concen­trations of CO₂ and HDO over 72 hours. In the ranges of 1.2 to 2.5 kilometers, the CO₂ concen­tration shows some lower marks during the daytime, which are caused by the photo­synthesis in the parks there. The data show that the local turbulence of atmosphere is strong during the daytime. The diurnal variation of turbulence dominates the gas concen­trations near the ground.

“With further development, our method can be updated to measure the distributed spectra over the band of optical communi­cations, so that abundant gases, such as CO, CO₂, H₂O, HDO, NH₃, C₂H₂ and CH₄ can be analyzed by the system. Its appli­cation can be extended to long-range warnings of flammable, explosive and toxic substances, and monitoring of industrial pollution. Further­more, substance evolution and chemical reactions in the free atmosphere can be further inves­tigated.” said Haiyun Xia. (Source: CAS)

Reference: S. Yu et al.: Photon-counting distributed free-space spectroscopy, Light Sci. Appl. 10, 212 (2021); DOI: 10.1038/s41377-021-00650-2

Link: School of Earth and Space Science, University of Science and Technology of China, Hefei, China