Spectroscopy is a well-established nonintrusive tool that has played an important role in identifying and quantifying substances, from quantum descriptions to chemical and biomedical diagnostics. Challenges exist in accurate spectrum analysis in free space, which hinders us from understanding the composition of multiple gases and the chemical processes in the atmosphere. A photon-counting distributed free-space spectroscopy is proposed and demonstrated using lidar technique, incorporating a comb-referenced frequency-scanning laser and a superconducting nanowire single-photon detector. It is suitable for remote spectrum analysis with a range resolution over a wide band. As an example, a continuous field experiment is carried out over 72 h to obtain the spectra of carbon dioxide (CO₂) and semi-heavy water (HDO, isotopic water vapor) in 6 km, with a range resolution of 60 m and a time resolution of 10 min. Compared to the methods that obtain only column-integrated spectra over kilometer-scale, the range resolution is improved by 2–3 orders of magnitude in this work. The CO₂ and HDO concentrations are retrieved from the spectra acquired with uncertainties as low as ±1.2% and ±14.3%, respectively. This method holds much promise for increasing knowledge of atmospheric environment and chemistry researches, especially in terms of the evolution of complex molecular spectra in open areas.

光谱学是一种成熟的非侵入式工具，在从量子描述到化学和生物医学诊断的物质识别和量化方面发挥着重要作用。自由空间中精确的光谱分析存在挑战，这阻碍了我们了解多种气体的成分和大气中的化学过程。使用激光雷达技术提出并演示了光子计数分布式自由空间光谱，结合了梳状参考频率扫描激光器和超导纳米线单光子探测器。它适用于具有宽频带范围分辨率的远程频谱分析。例如，连续进行了72 h的现场实验，获得了6 km范围内的二氧化碳（CO ₂ ）和半重水（HDO，同位素水蒸气）的光谱，距离分辨率为60 m，时间分辨率为10分钟与仅获得千米尺度的柱积分光谱的方法相比，本工作的距离分辨率提高了2-3个数量级。 CO ₂和HDO 浓度是从获得的光谱中检索的，不确定性分别低至±1.2% 和±14.3%。这种方法对于增加大气环境和化学研究的知识有很大希望，特别是在开放区域复杂分子光谱的演化方面。