Markus Rettinger

Strategy for high-accuracy-and-precision retrieval of atmospheric methane from the mid-infrared FTIR network

We present a strategy (MIR-GBM v1.0) for the retrieval of column-averaged dry-air mole fractions of methane (XCH4) with a precision<0.3 % (1-σ diurnal variation, 7-min integration) and a seasonal bias <0.14 % from mid-infrared ground-based solar FTIR measurements of the Network for the Detection of Atmospheric Composition Change (NDACC, comprising 22 FTIR stations). This makes NDACC methane data useful for satellite validation and for the inversion of regional-scale sources and sinks in addition to long-term trend analysis. Such retrievals complement the high accuracy and precision near-infrared observations of the younger Total Carbon Column Observing Network (TCCON) with time series dating back 15 years or so before TCCON operations began. MIR-GBM v1.0 is using HITRAN 2000 (including the 2001 update release) and 3 spectral micro windows (2613.70–2615.40 cm −1, 2835.50–2835.80 cm −1, 2921.00– 2921.60 cm−1). A first-order Tikhonov constraint is applied to the state vector given in units of per cent of volume mixing ratio. It is tuned to achieve minimum diurnal variation without damping seasonality. Final quality selection of the retrievals uses a threshold for the goodness of fit ( χ2 < 1) as well as for the ratio of root-mean-square spectral noise and information content ( <0.15 %). Column-averaged dryair mole fractions are calculated using the retrieved methane profiles and four-times-daily pressure-temperature-humidity profiles from National Center for Environmental Prediction (NCEP) interpolated to the time of measurement. MIR-GBM v1.0 is the optimum of 24 tested retrieval strategies (8 different spectral micro-window selections, 3 spectroscopic line lists: HITRAN 2000, 2004, 2008). Dominant errors of the non-optimum retrieval strategies are Correspondence to: R. Sussmann (ralf.sussmann@kit.edu) systematic HDO/H2O-CH4 interference errors leading to a seasonal bias up to ≈5 %. Therefore interference errors have been quantified at 3 test sites covering clear-sky integrated water vapor levels representative for all NDACC sites (Wollongong maximum = 44.9 mm, Garmisch mean = 14.9 mm, Zugspitze minimum = 0.2 mm). The same quality ranking of the 24 strategies was found for all 3 test sites with one optimum, i.e. MIR-GBM v1.0. Seasonality of XCH4 above the Zugspitze (47 ◦ N) shows a minus-sine shape with a minimum in March/April, a maximum in September, and an amplitude of 16.2 ± .9 ppb (0.94± 0.17 %). This agrees well with the WFM-DOAS v2.0 scientific XCH4 retrieval product. A conclusion from this paper is that improved spectroscopic parameters for CH 4, HDO, and H2O in the 2613– 2922 cm−1 spectral domain are urgently needed. If such become available with sufficient accuracy, at least two more spectral micro windows could be utilized leading to another improvement in precision. The absolute inter-calibration of NDACC MIR-GBM v1.0 XCH4 to TCCON data is subject of ongoing work.

Simultaneous retrieval of atmospheric CO2 and light path modification from space-based spectroscopic observations of greenhouse gases: methodology and application to GOSAT measurements over TCCON sites

This paper presents an improved photon path length probability density function method that permits simultaneous retrievals of column-average greenhouse gas mole fractions and light path modifications through the atmosphere when processing high-resolution radiance spectra acquired from space. We primarily describe the methodology and retrieval setup and then apply them to the processing of spectra measured by the Greenhouse gases Observing SATellite (GOSAT). We have demonstrated substantial improvements of the data processing with simultaneous carbon dioxide and light path retrievals and reasonable agreement of the satellite-based retrievals against ground-based Fourier transform spectrometer measurements provided by the Total Carbon Column Observing Network (TCCON).