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

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) is demonstrated for the detection of tropospheric methane. Spectra of the 2nu(3) overtone band of methane at wavelengths near 1.65 mum 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 alpha(min) = 1.5 x 10(-8) cm(-1), equivalent to a number density of CH4 at ambient temperature of 2.9 x 10(11) 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 x 10(12) 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.