Ground-Based Hyper-Spectral Measurements Of The Solar Beam In The SWIR Band And Remote Sensing Of Total Column Of CO₂ In The Earth’s Atmosphere

The method of using ground-based hyper-spectral measurements of the solar beam to improve satellite detection ability of CO2 concentration is a new trend from the past years. However, the understanding about satellite remote sensing and ground-based verification is still limited. Based on the spectrometers from Chinese carbon satellite ground observation network, two algorithms for remote sensing of CO2 from ground-based hyper-spectral measurements of the direct solar beam in shortwave infrared (SWIR) band are developed, one as DOAS (Differential Optical Absorption Spectroscopy) method and the other as optimum estimation method, with the algorithms both applied to observation spectral. Moreover, the spatial and temporal variations of XCO2 from SCIAMACHY and GOSAT are presented. The main results are summarized as following:1. Based on LBLRTM and DISORT, a complete forward model for remote sensing of CO2 is established according to index parameters of ultra-high resolution spectrometers OSAand FTS125M.2. Based on the forward model, sensitivity tests for aerosol, surface pressure, temperature, spectral resolution, spectral offset, signal noise ratio (SNR), solar spectrum and so on, are analyzed. The results show that:The higher the spectral resolution, the larger the effect of spectral offset, the inversion errors caused by 0.005 cm-1 spectral offset are respectively about-0.5 and-1.5 ppm for spectral resolution of 0.2 and 0.02 cm-1. The signal noise ratio (SNR) is low if the spectral resolution is too high, while the factors interfering CO2 inversion are not easy to separate if the spectral resolution is too low. The effect of solar spectrum lies in Fraunhofer lines and that of water vapor lies in water vapor absorption lines, thus the inversion errors caused by solar spectrum and water vapor can be eliminated by channel selection. And the inversion errors caused by surface albedo and aerosol are less than 0.1 ppm. The instrument line shape (ILS) has significant influence on inversion, so accurate ILS is necessary. The inversion error caused by 1 hPa surface pressure is about 0.25 ppm, therefore the error of surface pressure in the inversion should be less than 1 hPa. The simulated spectrum errors caused by 1 K temperature profile are larger than that caused by 1 hPa surface pressure, but the impact of temperature can be decreased by channel selection. The inversion error caused by CO2 profile is about 1-4 ppm.3. An algorithm DOAS-like for XCO2 is developed based on channel-pair ratio. By optimizing the channel selection, Fraunhofer lines and water vapor absorption lines are avoided and impacts of surface pressure, temperature profile, SNR and spectral offset are decreased. The standard deviation between results from DOAS-like and TCOON official products is less than 0.8 ppm when TCOON observed spectra are used to retrieve.4. An optimum estimation algorithm applicable to Chinese carbon satellite ground observation network is developed. In the algorithm, the band between 1570.2-1574.2 nm which has few Fraunhofer lines and strong water vapor absorption lines is used for inversion, and the absorption line containing 1572.75 nm is used for spectral offset correction, with segmented slope correction applied to the observed spectrum. Then, XCO2 is calculated by minimizing the residual between simulated spectrum and observed spectrum.5. The spatial and temporal variations of XCO2 from SCIAMACHY and COS AT is presented. The global average annual growth of XCO2 is about 1.5ppm from 2010 to 2011 and the high XCO2 values correspond to those populated areas. The high values of XCO2 in China locate in the East and South, while the low values locate in the West and North. The spatial distribution of XCO2 has obvious seasonal variations. The averaged XCO2 concentrations in the southern and northern hemisphere grow with their own peculiar phase. And the averaged XCO2 concentration in China is higher than that in the northern hemisphere during spring, but the opposite case exists at other times, especially in July and August.