Impact of different spectroscopic datasets on CH4 retrievals from Jungfraujoch FTIR spectra

Methane (CH4) is released in the atmosphere by natural processes (e.g. wetlands, termites) as well as by anthropogenic activities (e.g. fossil fuel exploitation, rice agriculture, biomass burning, etc). Due to its high warming potential and its relatively long chemical lifetime (~9 years), atmospheric methane plays a major role in the radiative forcing responsible of the greenhouse effect. Methane also affects climate by influencing tropospheric ozone and stratospheric water. The cycle of methane is complex and to understand it requires a complete study of its emissions and its budget of sources and sinks. High quality methane data sets are needed to perform such studies.CH4 vertical distributions as well as total and partial column time series can be retrieved from high-resolution ground-based FTIR spectra, using, e.g., the SFIT-2 algorithm which implements the Optimal Estimation Method of Rodgers. A set of 5 microwindows - located in the 2 to 5.5 µm range and jointly adopted by all partners involved in the European HYMN project (www.knmi.nl/samenw/hymn/) - are fitted simultaneously during the retrieval procedure. Although this approach provides relatively high information content, CH4 retrieved profiles very often present large oscillations in the troposphere, which might result partly from inappropriate or inconsistent spectroscopic parameters. Significant improvements on retrieval quality could be reached by using more accurate CH4 spectroscopic parameters.This contribution compares 3 different sets of CH4 spectroscopic parameters (including HITRAN 2004 and 2 versions where HITRAN 2004 have been updated by recent laboratory measurements), which have been tested using one year of high resolution FTIR solar observations performed at the International Scientific Station of the Jungfraujoch (Swiss Alps, 46.5°N, 8.0 °E, 3580m a.s.l.). The impact of these different spectroscopic datasets on retrieved CH4 partial columns and vertical profiles, as well as on the fitting quality (residuals) and on the error budget characterizing our CH4 products will be evaluated and discussed.