Space-borne remote sensing of CO_2, CH_4, and N_2O by integrated path differential absorption lidar: a sensitivity analysis

摘要

CO_2, CH_4, and N_2O are recognised as the most important greenhouse gases, the concentrations of which increase rapidly through human activities. Space-borne integrated path differential absorption lidar allows global observations at day and night over land and water surfaces in all climates. In this study we investigate potential sources of measurement errors and compare them with the scientific requirements. Our simulations reveal that moderate-size instruments in terms of telescope aperture (0.5-1.5 m) and laser average power (0.4-4 W) potentially have a low random error of the greenhouse gas column which is 0.2% for CO_2 and 0.4% for CH_4 for soundings at 1.6 μm, 0.4% for CO_2 at 2.1 μm, 0.6% for CH4 at 2.3 μm, and 0.3% for N_2O at 3.9 μm. Coherent detection instruments are generally limited by speckle noise, while direct detection instruments suffer from high detector noise using current technology. The wavelength selection in the vicinity of the absorption line is critical as it controls the height region of highest sensitivity, the temperature cross-sensitivity, and the demands on frequency stability. For CO_2, an error budget of 0.08% is derived from our analysis of the sources of systematic errors. Among them, the frequency stability of ±0.3 MHz for the laser transmitter and spectral purity of 99.9% in conjunction with a narrow-band spectral filter of 1 GHz (FWHM) are identified to be challenging instrument requirements for a direct detection CO_2 system operating at 1.6 μm.