Methane spectroscopy in the near infrared and its implication on atmospheric retrievals

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.