Atmospheric trace gas measurements using a dual quantum-cascade laser n-dd-infrared absorption spectrometer

We present an overview of the dual QC laser spectrometer developed at Aerodyne Research and various examples of its application for atmospheric trace gas detection. The instrument incorporates two pulsed QC lasers, a compact 76-m (or 56-m) multipass absorption cell, a dual HgCdTe detector, and a sophisticated signal generation, data acquisition and processing system. Recent findings and hardware innovations are highlighted. Our results show that the precision and minimal detectable absorbance obtainable with pulsed QC lasers are comparable to those achieved with cryogenically cooled CW Pb-salt lasers in spite of the broader laser linewidths inherent to pulsed operation. This is demonstrated through in situ measurements of several trace gases, including methane, nitrous oxide, carbon monoxide, formaldehyde, formic acid, nitrous acid and ethylene. Recent measurements of HCHO and HCOOH on board a NOAA aircraft are presented. The precision, stability and intrinsic accuracy of the instrument were assessed through inter-comparisons measuring CH4 and CO. These measurements were made either comparing two QC lasers sweeping over different transitions or comparing the dual QCL spectrometer and a standard instrument (NDIR CO). The absorbance precision achieved is typically 2 x 10(-5) Hz(-1/2). For long-lived species, such as CH4 and N2O, this implies 1-Hz fractional precisions of 0.1% or better, which fulfill the requirement for meaningful measurements from aircraft platforms. Spectroscopically derived mixing ratios are accurate within 5% or better. The spectrometer is equipped to perform automatic, periodic calibrations with zero and span gases whenever higher accuracy is required.