Mapping CO2 Point Source Emissions with Hyperspectral Imager Suite

The Hyperspectral Imager Suite (HISUI), which is a spaceborne hyperspectral imaging sensor developed by the Japanese Ministry of Economy, Trade, and Industry (METI), was launched in 2019 and mounted on the International Space Station. HISUI measures the spectral radiance of backscattered sunlight in 185 bands from the visible to the short-wavelength infrared (SWIR) region (400-2500 nm), and produces hyperspectral images with a spatial resolution of 31 m for the along-track and 20 m for the cross-track. SWIR spectra measured using airborne and spaceborne hyperspectral sensors have been employed for identifying and quantifying emissions from point sources of CO2 and CH4. However, the uncertainty in the full width at half maximum (FWHM) of the instrumental line shape influences the accuracy of column measurements of trace gases based on spectral radiance. In the present study, we estimated the FWHM per cross-track pixel using the absorption characteristics of atmospheric O2 at 1.27 μm, CO2 at 2.0 μm, and CH4 at 2.3 μm. We used HISUI L1R spectral data consisting of 1200 × 1000 pixels (along-track × cross-track) over a desert area in Mauritania and made 1000 spectra per cross-track pixel that were averaged over the along-track direction. The FWHM values were estimated simultaneously with surface reflectance and a coefficient that represents the wavelength shift, so that the difference between the observed and simulated spectra was minimized, assuming that O2, CO2, and CH4 mole fractions were spatially uniform. We found that the FWHM was different for the three wavelength regions, and varied depending on the cross-track pixel. Using these FWHM values, the total column abundances of CO2 over a thermal power plant in Japan were retrieved by scaling the a priori CO2 profile. We could clearly identify CO2 plumes from the two stacks of the power plant, showing an increase of more than 50 ppm in column-averaged dry-air mole fraction.