Pressure broadening in the 2ν<sub>3</sub> band of methane and its implication on atmospheric retrievals

Abstract

Abstract. N2-broadened half widths and pressure shifts were obtained for transitions in the 2ν3 methane band. Laboratory measurements recorded at 0.011 cm−1 resolution with a Bruker 120 HR Fouriertransform spectrometer were analysed from 5860 to 6185 cm−1. A 140 cm gas cell was filled with methane at room temperature and N2 as foreign gas at pressures ranging from 125 to 900 hPa. A multispectrum nonlinear constrained least squares approach based on Optimal Estimation was applied to derive the spectroscopic parameters by simultaneously fitting laboratory spectra at different ambient pressures assuming a Voigt line-shape. At room temperature, the half widths ranged between 0.030 and 0.071 cm−1 atm−1, and the pressure shifts varied from –0.002 to –0.025 cm−1 atm−1 for transitions up to J"=10. Especially for higher rotational levels, we find systematically narrower lines than HITRAN predicts. The Q and R branch of the new set of spectroscopic parameters is further tested with ground based direct sun Fourier transform infrared (FTIR) measurements where systematic fit residuals reduce by about a factor of 3–4. We report the implication of those differences on atmospheric methane measurements using high-resolution ground based FTIR measurements as well as low-resolution spectra from the SCanning Imaging Absorption SpectroMeter for Atmospheric ChartographY (SCIAMACHY) instrument onboard ENVISAT. We find that for SCIAMACHY, a latitudinal and seasonally varying bias of about 1% can be introduced by erroneous broadening parameters.

Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 2: Methane

“…The methane data set used by Bergamaschi et al (2007) has been retrieved from the SCIAMACHY instrument using the Iterative Maximum A Posteriori-DOAS (IMAP-DOAS) retrieval algorithm . Very encouraging results have been obtained but also some problems had been identified, for example, an apparent time dependent latitudinal bias which was attributed recently to a certain extent to potential inaccuracies in the spectroscopic parameters of methane (Frankenberg et al, 2008a).…”

CH<sub>4</sub>, CO, and H<sub>2</sub>O spectroscopy for the Sentinel-5 Precursor mission: an assessment with the Total Carbon Column Observing Network measurements

“…For the CH 4 absorption bands in the 5996-6145 cm −1 , the pressure retrieval yielded accurate results. The CH 4 HITRAN08 database for this range relies on the line list published by Frankenberg et al (2008) who focused on pressure-broadening effects.…”

“…Currently, we do not intend such a pressure retrieval for TROPOMI spectra. To improve on the general accuracy of spectroscopic databases, we recommend a similar effort to be directed at the pressure broadening and pressure shift of absorption lines as done by Frankenberg et al (2008) for CH 4 at 5860-6185 cm −1 .…”

“…The surface pressure is measured at each TCCON station with a barometer (Wunch et al, 2011a). Dedicated effort in the past years to improve CH 4 and H 2 O spectroscopy in the 5880-6174 cm −1 range (Frankenberg et al, 2008;Tran et al, 2010;Jenouvrier et al, 2007) has enabled accurate retrieval of CH 4 columns from this spectral range. This has been confirmed by validation of GFIT retrievals with independent aircraft measurements.…”

Effects of atmospheric light scattering on spectroscopic observations of greenhouse gases from space. Part 2: Algorithm intercomparison in the GOSAT data processing for CO2 retrievals over TCCON sites

“…Aerosol and cloud extinction are retrieved as a log-value with an a priori covariance with a 1-s uncertainty of a factor of 50 at each level. The UoL-FP: 3G uses the v3.2 OCO spectroscopy and solar irradiance model as described by Crisp et al [2012] and the TCCON spectroscopy for CH 4 and H 2 O, which is based on HITRAN 2008 but includes updates to H 2 O based on Toth, 2005 andJenouvrier et al [2007] and CH 4 based on Frankenberg et al [2008].…”