Using high-resolution laboratory and ground-based solar spectra to assess CH4 absorption coefficient calculations

Abstract

A quadratic-speed-dependent Voigt line shape (qSDV) with line mixing (qSDV+LM), together with spectroscopic line parameters from Devi et al. [1,2] for the 2v 3 band of CH 4 , was used to retrieve total columns of CH 4 from atmospheric solar absorption spectra. The qSDV line shape [3] with line mixing [4] was implemented into the forward model of GFIT (the retrieval algorithm that is at the heart of the GGG software [5]) to calculate CH 4 absorption coefficients. High-resolution laboratory spectra of CH 4 were used to assess absorption coefficients calculated using a Voigt line shape and spectroscopic parameters from the atm line list [6]. The same laboratory spectra were used to test absorption coefficients calculated using the qSDV+LM line shape with spectroscopic line parameters from Devi et al.[1,2] for the 2v 3 band of CH 4 and a Voigt line shape for lines that don't belong to the 2v 3 band. The spectral line list for lines that don't belong to the 2v 3 band is an amalgamation of multiple spectral line lists. We found that for the P, Q, and R branches of the 2v 3 band, the qSDV+LM simulated the laboratory spectra better than the Voigt line shape. The qSDV+LM was also used in the spectral fitting of highresolution solar absorption spectra from four ground-based remote sensing sites and compared to spectra fitted with a Voigt line shape. The average root mean square (RMS) residual for 131,124 solar

Recent advances in collisional effects on spectra of molecular gases and their practical consequences

“…[436] confirmed that, by using LM and a SDVP, the CO 2 spectral fitting residuals in ground-based TCCON spectra are improved and the airmass dependences of the retrieved CO 2 total columns are substantially reduced (from a few % down to a few 0.1%). As for the case of CH 4 discussed above and in [598], LM is the main factor in these improvements. Indirect consequences of LM in CO 2 have also been pointed out in the ground-based observations of CCl 4 .…”

“…For the SCIAMACHY satellite instrument, the changes in the widths and shifts implied a latitudinal and seasonally varying bias of 1 % in the total CH 4 column when using the HITRAN 2004 line list [597]. Very recent results on CH 4 retrievals using the 2 3 band can be found in [598] where LM and SDVP derived from laboratory spectra are used, showing that the former is the major player in the observed improvements. Note that atmospheric CH 4 measurements were also made using a balloon-borne diode-laser spectrometer centered on the R(6) manifold of the  3 band [500].…”

“…However, because many of the observed CO 2 and O 2 lines are saturated, the non-Voigt line shape does change the equivalent widths of the lines, which is reflected in the retrieved column amounts. In contrast, the official TCCON retrievals [594] are performed with a VP and no LM, despite having a spectral resolution of 0.02 cm -1 and a SNR of 600 sufficient to discern the subsequent degradation in the residuals [436,598]. The resulting airmass dependence of the column amounts is corrected post-retrieval by exploiting the fact that the retrieval error in the TCCON windows caused by the neglect of LM (along with other factors that also cause airmassdependent artifacts) is linear for small airmasses and consistent between summer and winter, as illustrated in Fig.…”

Retrieval of atmospheric CO<sub>2</sub> vertical profiles from ground-based near-infrared spectra

“…The linelist used by GGG is a compilation of different versions of the HITRAN linelists (Gordon et al, 2017;Rothman et al, 2005Rothman et al, , 2009Rothman et al, , 2013Toon, 2015;Toon et al, 2016). GGG2020 has the option to use either the qSDV+LM line shape or the Voigt line shape for some windows and gases (Mendonca et al, 2016(Mendonca et al, , 2017(Mendonca et al, , 2019. The reference linelists and the uncertainties on air-and self-broadened Lorentz half-width coefficients, and their temperature dependence, are summarized in Table 4.…”

“…The Total Carbon Column Observing Network (TCCON) is a ground -based network of high-resolution (0.02 cm -1 ) groundbased Fourier transform Infrared (FTIR) spectrometers that record Short Wave IR (SWIR) solar absorption spectra (Wunch et al, 2011b). TCCON produces retrievals of XCO2 is retrieved and widely used to validate satellite observations and to study the carbon cycle (Wunch et al, 2010a(Wunch et al, , 2017Keppel-Aleks et al, 2012, 2013. New versions of the TCCON retrieval algorithm (GGG) are released every few years, and each new version is designed to improve the quality of the data.…”