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Local-Scale Horizontal CO₂ Flux Estimation Incorporating Differential Absorption Lidar and Coherent Doppler Wind Lidar
Abstract:
A micro-pulse lidar system incorporating differential absorption lidar (DIAL) and coherent Doppler wind lidar (CDWL) is proposed and demonstrated. Due to the high signal-to-noise ratio (SNR) of the superconducting nanowire single-photon detector (SNSPD), the DIAL channel achieves high sensitivity in CO₂ measurement. Meanwhile, the CDWL channel is used to obtain the horizontal wind field. In the process of the optimization and calibration of the DIAL receiver, specifically, mode scrambling and temperature control of the connecting fiber between the telescope and the SNSPD enhance the stability and robustness of the system. Horizontal scanning of the CO₂ concentration and the wind field is carried out in a 6km range over a scanning span of 60° with a radial resolution of 150m and 15s. The results show that the hybrid lidar system captures the spatial distribution of CO₂ concentration and the wind field simultaneously. The horizontal net CO₂ flux in a radius of 6km is estimated by integrating the CO₂ concentration and the wind transport vector, indicating different characteristics of horizontal net CO₂ fluxes in an industrial area, a university campus, and a park. During most of the experiment, CO₂ flux remained positive in the industrial area, but balances fell to nearly zero on the campus and in the park. The horizontal net fluxes averaged over 24h in the three areas are 3.5 × 10⁵ppm·m²·s⁻¹, 0.7 × 10⁵ppm·m²·s⁻¹, and 0.1 × 10⁵ppm·m²·s⁻¹.
Fig. 1. A diagram of the hybrid lidar system: AOM, acoustic–optical modulator; EDFA, erbium- doped fiber amplifier; ADC, analog to digital converter; BD, balance detector; SNSPD, superconduct- ing nanowire single-photon detector; MCS, multi-channel scaler.
Fig. 2. Illustration of measurement areas. (A) indicates the NUIST campus, (B) is Taizishan Park, and (C) represents an industrial area.
Fig. 3. Horizontal scanning results on 2 May 2022 in Nanjing. (a1–a3) are backscatter signal PR2, CO₂ concentration, and radial velocity at 16:30. (b1–b3) are at 20:37. The blank areas in the fan charts are caused by hitting hard targets (Chimney 1 and Chimney 2 in Figure 5). The wind barbs represent the wind speed and direction.
Fig. 4. Schematic diagram of horizontal net flux. The black arrow represents the wind vector.
Photon-counting distributed free-space spectroscopy
Abstract:
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.
Fig. 1. Optical layout. a Experimental set-up. The detailed parameters of the instruments are listed in Table S2. BD balanced detector, OS optical switch, AOM acousto-optic modulator, EDFA erbium-doped iber ampliier, SNSPD superconducting nanowire single-photon detector, MCS multi- channel scaler, PM polarization-maintaining, MM multi-mode. b Light propagating in the atmosphere. The path length in the red sections represents the range resolution of ΔR, and the spectra within this range in the whole free space can be obtained. c Time sequence of the time-division multiplexing technique.
Fig. 2. Flow chart of data acquisition and processing. ① to ⑦ represent step 1 to step 7. P pressure, T temperature.
Fig. 3. Backscattering signals and spectra. a The probe signal, with 30 scanning frequencies, covers CO2 and HDO absorption lines. b The reference signal without gas absorption. c The total optical depth spectra of CO2 and HDO at different ranges. d Lorentzian fitting of the range-resolved spectra; magenta dots are the measured ΔDOD values at 4 km with ΔR = 60 m.
Fig. 4. Results of continuous observation. Range-time plots of a CO2, b HDO. Height-time plots of c CNR, d Horizontal wind speed, e Horizontal wind direction, f Turbulent kinetic energy dissipation rate (TKEDR). g CO2 concentration and Cn 2. The black line represents the CO2 concentration at 2 km measured by PDFS. The red line is Cn 2 measured by a scintillometer, with the y-coordinate reversed. h Relative humidity (RH) and temperature (tem.) The magenta dotted line represents the RH at 2 km. The blue dashed line represents the temperature measured by Vaisala WMT52, with the y-coordinate reversed.
