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We present a strategy (MIR-GBM v1.0) for the retrieval of column-averageddry-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 theNetwork for the Detection of Atmospheric Composition Change (NDACC,comprising 22 FTIR stations). This makes NDACC methane data useful forsatellite validation and for the inversion of regional-scale sources andsinks in addition to long-term trend analysis. Such retrievals complementthe high accuracy and precision near-infrared observations of the youngerTotal Carbon Column Observing Network (TCCON) with time series dating back15 years or so before TCCON operations began. MIR-GBM v1.0 is using HITRAN 2000 (including the 2001 update release) and 3spectral micro windows (2613.70–2615.40 cm−1, 2835.50–2835.80 cm−1,2921.00–2921.60 cm−1). A first-order Tikhonov constraintis applied to the state vector given in units of per cent of volume mixingratio. It is tuned to achieve minimum diurnal variation without dampingseasonality. Final quality selection of the retrievals uses a threshold forthe goodness of fit (χ2 < 1) as well as for the ratio ofroot-mean-square spectral noise and information content (<0.15%).Column-averaged dry-air mole fractions are calculated using the retrievedmethane profiles and four-times-daily pressure-temperature-humidity profilesfrom National Center for Environmental Prediction (NCEP) interpolated to thetime of measurement. MIR-GBM v1.0 is the optimum of 24 tested retrieval strategies (8 differentspectral micro-window selections, 3 spectroscopic line lists: HITRAN 2000,2004, 2008). Dominant errors of the non-optimum retrieval strategies aresystematic HDO/H2O-CH4 interference errors leading to a seasonalbias up to ≈5%. Therefore interference errors have beenquantified at 3 test sites covering clear-sky integrated water vapor levelsrepresentative for all NDACC sites (Wollongong maximum = 44.9 mm, Garmischmean = 14.9 mm, Zugspitze minimum = 0.2 mm). The same quality ranking of the24 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-sineshape with a minimum in March/April, a maximum in September, and anamplitude of 16.2 ± 2.9 ppb (0.94 ± 0.17%). This agrees well withthe WFM-DOAS v2.0 scientific XCH4 retrieval product. A conclusion from this paper is that improved spectroscopic parameters forCH4, HDO, and H2O in the 2613–2922 cm−1 spectral domainare urgently needed. If such become available with sufficient accuracy, atleast two more spectral micro windows could be utilized leading to anotherimprovement in precision. The absolute inter-calibration of NDACC MIR-GBMv1.0 XCH4 to TCCON data is subject of ongoing work

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