Fourier-transform intensity measurements with 0.1% accuracy

Abstract

Remote sensing of atmospheric trace gases with climate relevance requires spectroscopic data with better than 0.1% absolute intensity accuracy. This requirement poses a big challenge, especially to Fourier-transform spectroscopy (FTS). The alternative CRDS method has the advantage of a higher sensitivity compared to FTS. CRDS is also independent from cavity length and there is no instrumental line shape. In contrast, the FTS technique has the major advantages of a broadband coverage and the entire spectral range is measured simultaneously. Both techniques are thus complementary. The demanding metrological requirements for multireflection cell absorption path determination as well as for pressure and temperature measurement to achieve 0.1% absolute intensity accuracies will be addressed. Furthermore, the instrumental line shape characterization, the treatment of offset errors, and multipassing will be discussed. For this high accuracy application, a sophisticated multispectrum fitting software is essential. Finally, highly accurate results require several measurements under different conditions to validate the overall uncertainties. As examples CO 3-0 and CO2 1.6 µm intensity measurements will be presented.