High precision measurements of atmospheric nitrous oxide and methane using thermoelectrically cooled mid-infrared quantum cascade lasers and detectors

A compact, fast response, mid-infrared absorption spectrometer using thermoelectrically (TE) cooled pulsed quantum cascade (QC) lasers and TE detectors has been developed to demonstrate the applicability of QC lasers for high precision measurements of nitrous oxide and methane in the earth's atmosphere. Reduced pressure extractive sampling with a 56 m path length, 0.51 volume, multiple pass absorption cell allows a time response of <0.1 s which is suitable for eddy correlation flux measurements for these gases. Precision of 0.3 ppb (rms, 1 s averaging time) or 0.1% of the ambient concentration for N2O (4 ppb or 0.2% of ambient for CH4), has been demonstrated using QC lasers at 4.5 mum (7.9 mum for CH4), corresponding to an absorbance precision of 4 x 10(-5) Hz(-1/2) (8 x 10(-5) Hz(-1/2) for CH4). Stabilization of the temperature of the optical bench and the pulse electronics results in a minimum Allan variance corresponding to 0.06 ppb for N2O with an averaging time of 100 s (0.7 ppb with an averaging time of 200 s for CH4). The instrument is capable of long-term, unattended, continuous operation without cryogenic cooling of either laser or detector. (C) 2004 Elsevier B.V. All rights reserved.