Remote sensing of CO2 and CH4 using solar absorption spectrometry with a low resolution spectrometer

Abstract

Throughout the last few years solar absorption Fourier Transform Spectrometry (FTS) has been further developed to measure the total columns of CO₂ and CH₄ . The observations are performed at high spectral resolution, typically at 0.02 cm^-1 . The precision currently achieved is generally better than 0.25%. However, these high resolution instruments are quite large and need a dedicated room or container for installation. We performed these observations using a smaller commercial interferometer at its maximum possible resolution of 0.11 cm^-1 . The measurements have been performed at Bremen and have been compared to observations using our high resolution instrument also situated at the same location. The high resolution instrument has been successfully operated as part of the Total Carbon Column Observing Network (TCCON). The precision of the low resolution instrument is 0.32% for XCO₂ and 0.46% for XCH₄ . A comparison of the measurements of both instruments yields an average deviation in the retrieved daily means of ≤q;0.2% for CO₂ . For CH₄ an average bias between the instruments of 0.47% was observed. For test cases, spectra recorded by the high resolution instrument have been truncated to the resolution of 0.11 cm^-1 . This study gives an offset of 0.03% for CO₂ and 0.26% for CH₄ . These results indicate that for CH₄ more than 50% of the difference between the instruments results from the resolution dependent retrieval. We tentatively assign the offset to an incorrect a-priori concentration profile or the effect of interfering gases, which may not be treated correctly.

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