Multi-laboratory measurements of 12CH4 2nu3 -band line parameters. Part II: Spectrum analysis based on the speed dependent Voigt profile with first-order line mixing

Abstract

Methane (CH4), the second most important greenhouse gas, playing a critical role in atmospheric monitoring for global climate change research. The 1.64 µm spectral region (6050–6250 cm−1), corresponding to the 2nu3 vibrational band of methane, is particularly advantageous for remote sensing due to its strong absorption features and minimal interference from water vapor. As a result, wavelengths within this band correspond to a primary detection region for satellite missions such as MERLIN and GOSAT-2. However, current uncertainties in the HITRAN database — particularly in the line-shape parameters of strong lines within the 2nu3 band where discrepancies of 5%–10% persist, limit the precision of atmospheric concentration retrievals. To address this challenge, we employ optical-frequency-comb-referenced cavity ring-down spectroscopy measurements of N2-broadened methane samples analyzed with the speed-dependent Voigt profile and first-order line mixing. This study provides line-shape parameters line mixing coefficients for the 2nu3 band transitions from R(0) to R(10) at pressures between 10 and 50 kPa. Air-broadened values for these quantities based on scaling rules are also presented for atmospheric applications. Our measurements achieve relative uncertainties of less than 1 %, thereby significantly improving upon prior data archived in the HITRAN2020 database. These results provide essential experimental benchmarks for refining methane spectral databases, thereby enhancing the accuracy of satellite remote sensing and ground-based observations.