Infrared QC laser applications to field measurements of atmospheric trace gas sources and sinks in environmental research: enhanced capabilities using continuous wave QCLs

Abstract

The advent of continuous wave quantum cascade lasers operating at near room temperature has greatly expanded the capability of spectroscopic detection of atmospheric trace gases using infrared absorption at wavelengths from 4 to 12 μm. The high optical power, narrow line width, and high degree of single mode purity result in minimal fractional absorptions of 5x10^-6 Hz^-1/2 detectable in direct absorption with path lengths up to 210 meters. The Allan plot minima correspond to a fractional absorbance of 1x10^-6 or a minimum absorption per unit path length 5x10^-11 cm^-1 in 50s. This allows trace gas mixing ratio detection limits in the low part-per-trillion (1 ppt = 10^-12) range for many trace gases of atmospheric interest. We present ambient measurements of NO₂ with detection precision of 10 ppt Hz^-1/2. The detection precision for the methane isotopologue ¹³CH₄ is 25 ppt Hz^-1/2 which allows direct measurements of ambient ratios of ¹³CH₄/¹²CH₄ with a precision of 0.5‰ in 100 s without pre-concentration. Projections are given for detection limits for other gases including COS, HONO and HCHO as CWRT lasers become available at appropriate wavelengths.