1. CH4 laboratory measurements

Initial fits to the MkIV balloon spectra, performed using the HITRAN 2004
linelist [Rothmann et al., 2005], revealed systematic residuals near some
of the CH4 lines around 1463 cm-1. Fits to laboratory CH4 spectra showed
similar residuals, indicating a problem of spectroscopic origin. Since the
CH4 widths in this region have never been measured and therefore have large
uncertainties, we felt justified in using the laboratory spectra to improve
the spectroscopic parameters of a few of the CH4 lines.

For a better description of the methane lines in the CH3CN frequency window
we considered nine laboratory spectra. The spectra were taken at the
Kitt Peak National Solar Observatory between 1979 and 1985 and are
available online from their archive (http://diglib.nso.edu/nso_user.html).
Four spectra were taken with pure CH4 at different pressures. The
other five spectra were pressure broadened, using air as the broadening
gas. The spectral resolution was between 0.01 and 0.006 cm-1. A summary
of the measurement conditions is given in Tab. 3.


2. Modifications of CH4 line parameters

The laboratory transmittance spectra were fitted in the region
between 1462 and 1464.4 cm-1 using a nonlinear least squares algorithm
and CH4 line parameters from the HITRAN 2004 database. As some of the
spectra showed slight contaminations of water vapor, H2O was fitted
simultaneously.

The residuals resulting from the fits were analyzed and the CH4
line parameters were adjusted manually until satisfying fits were
achieved. In total the parameters of 11 lines were modified. Modified
parameters were the line frequency (5 cases), the airbroadened
halfwidth (1 case), the selfbroadened halfwidth (7 cases), the
temperature dependence of the airbroadened halfwidth (1 case), and the
pressure shift (1 case). As all laboratory spectra were taken at
comparable temperatures it has to be noted that the modification
of the temperature dependence was not performed on the basis of the
laboratory measurements but on the basis of the balloon-borne atmospheric
measurements with the MkIV instrument. The modifications of the CH4 line
parameters are summarized in Tab. 4.


3. Reference

Rothmann, L. S., et al., The HITRAN 2004 molecular spectroscopic
data base, J. Quant. Spectrosc. Radiat. Transfer, 96, 139-204, 2005.