Figures of the Article

• (a) Schematic of a typical open-path DCS setup for kilometer-scale detection. (b) Schematic of the configuration for achieving ultra-long open-path DCS. (c) Two main application scenarios for ultra-long path DCS: horizontal open-path links over 100 km and vertical open-path links from the satellite to the ground base, which exceeds 10000 km.
• Simulated normalized transmission spectra for horizontal open-path links. Simulations were performed using concentrations of 2 ppm methane, 500 ppm carbon dioxide, and 2000 ppm water vapor at 300 K and 1 standard atmospheric pressure. (a) Transmission spectrum for a 1 km path length, covering the wavelength range from 6000−6600 cm⁻¹, which corresponds to the output range of mature fiber comb seed light sources. (b) Transmission spectrum for a 100 km path length. In this case, most of the wavelength bands suitable for measuring GHGs at kilometer-scale paths become saturated. (c) Wavelength band suitable for methane measurement in ultra-long open-path DCS. (d) Wavelength band suitable for carbon dioxide measurement in ultra-long open-path DCS. See the text for further details.
• (a) Simulated normalized transmission spectrum for a 10 km horizontal open-path link. The simulation parameters are the same as those in Fig. 2. (b) Molecular number density profiles of three GHGs (methane, carbon dioxide, and water vapor). (c) The temperature profile over Hefei, China (117°E, 32°N), on January 5, 2015, derived from MERRA-2 specified dynamics (SD) assimilation data. See the text for further details.
• Simulated normalized transmission spectra for CO₂ in the vertical open-path link (blue solid line). The black dashed line represents the transmission spectrum for a 10 km horizontal open path for comparison. (a) Transmission spectrum from 6354−6370 cm⁻¹ (approximately 1570−1574 nm), showing a strong trend of absorption saturation. (b) Enlarged view of the absorption lines from panel (a), showing narrower line widths and deeper absorption in the vertical path than the more uniformly distributed horizontal link. (c) Transmission spectrum from 6484 to 6500 cm⁻¹ (approximately 1538 to 1542 nm), showing a reasonable absorption. (d) Enlarged view of the absorption lines from panel c. See text for further details.
• Simulated normalized transmission spectra for CH₄ in a vertical open-path link (dark yellow solid line). The black dashed line represents the transmission spectrum for a 10 km horizontal open path for comparison. (a). Transmission spectrum from 6030−6160 cm⁻¹ (approximately 1623−1658 nm). (b). Transmission spectrum from 6080−6120 cm⁻¹ (approximately 1634−1644 nm), where methane, carbon dioxide, and water vapor exhibit appropriate absorption depths, making this range suitable for satellite-to-ground vertical open-path detection. (c). Enlarged view of the absorption peak in b, showing the combined absorption features of methane, carbon dioxide, and water vapor. See the text for further details.