1.572μm附近CO_2吸收光谱的测量

应用自行构建的恒温差分吸收光谱探测系统,在230—320 K的温度范围内,精确探测1.572μm附近CO_2吸收谱线的变化,获得了不同温度和压强下CO_2气体的吸收截面、自增宽系数、空气增宽系数,这些参数补充和完善了现有的数据库.定量分析了温度、压强对谱线的影响,建立了光学厚度和吸收截面的数值计算模型,并已经用于我国的CO_2激光雷达,为其高精度数据反演奠定了技术基础.这些工作能够提高工作在该波段的差分吸收CO_2探测激光雷达的反演精度.

Measurement of absorption spectrum around 1.572 μm

Differential absorption lidar (DIAL) is widely accepted as a most promising remote sensing technique for measuring the atmospheric CO₂, and has been built in many countries to study the global climate change and carbon cycle. However, the imperfect information about CO₂ spectrum leads to evident errors in estimating some parameters (such as the absorption cross sections, the broadening coefficients, the optical depth, etc.) which are the critical parameters in retrieving processes of a DIAL, and will eventually result in unacceptable errors of X_CO2 retrievals. Coping with that problem, a self-built constant temperature differential absorption spectroscopy system has been set up which can be used to accurately measure the absorption spectrum of carbon dioxide in the band of 1.57 μm.#br#On that basis, the absorption spectra of the pure carbon dioxide are measured respectively at the temperatures from 230 K to 320 K and the pressures from 20 kPa to 100 kPa by the highprecision oscilloscope and wavelength meter. A series of optical depths at absorption peak is respectively calculated at different temperatures and the results show that the optical depth linearly and monotonically changes while the temperature increases from 230 K to 320 K. At the same time, the relations between the corresponding absorption cross sections and temperature are analyzed, showing that the absorption cross sections first increases and then decreases with temperature increasing. The self-broadening coefficients are inferred from the spectral data at the same temperature and different pressures, and the temperature-dependent exponent is calculated. Furthermore, the air-broadening coefficients are calculated by carbon dioxide absorption spectrum data from different mixing ratios and its temperature-dependent exponent is obtained. The temperature-dependent exponent of self-broadening coefficient is 0.644 and the temperature-dependent exponent of air-broadening coefficient is 0.764, which are almost the same as the data in the high-resolution transmission molecular absorption database (HITRAN). The numerical calculation formulae of optical depth and absorption cross section are verified through these results.#br#Those parameters supplement the widely-used HITRAN database. Moreover, quantitative analysis is conducted to explore the influences of temperature and pressure on CO₂ spectrum, thereby establishing a function for modeling the differential absorption optical depth and the absorption cross-section. The above results have already been used in China's CO₂-DIAL and lay a foundation of accurate retrieval. It is believed that other similar CO₂-DIAL of which operating wavelength is around 1.572 μm would also benefit from those newly measured parameters.