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 Q and R branches of the 2?3 methane band. Laboratory measurements were done from 5985 to 6185 cm?1 using spectra recorded at 0.011 cm?1 resolution with a Bruker 120 HR Fourier transform spectrometer. 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 new set of spectroscopic parameters is further tested with ground based direct sun FTIR measurements where 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 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.