Trace detection of methane using continuous wave cavity ring-down spectroscopy at 1.65 μm

Abstract

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) is demonstrated for the detection of tropospheric methane. Spectra of the 2ν₃ overtone band of methane at wavelengths near 1.65 μm have been recorded in both laboratory air and under reduced pressures, and show quantitative agreement with absorption intensities and pressure-broadened line widths reported in the HITRAN 96 database. Experiments demonstrate a minimum detectable absorption coefficient that is currently α_min = 1.5 × 10⁻⁸ cm⁻¹, equivalent to a number density of CH₄ at ambient temperature of 2.9 × 10¹¹ molecules cm⁻³ in a low pressure environment where Doppler broadening dominates the spectral lineshapes. The effects of pressure broadening reduce the detection limit of CH₄ in 1 atm of air to 1.3 × 10¹² molecules cm⁻³, corresponding to 52 parts per billion by volume. This is well below the 1.72 parts per million by volume average methane concentration in the lower troposphere.