Measurement of absorption spectrum around 1.572 μm

Abstract

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 XCO2 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.