Remote Sensing of Long Path-Averaged Ambient Methane and Nitrous Oxide Greenhouse Gas Concentrations with a Mono-Static Quantum Cascade Laser Based System

We report on a field deployable open-path quantum cascade laser (QCL) based system for simultaneous detection of ambient greenhouse atmospheric trace gases methane and nitrous oxide gases with accuracies less than 1 % and ranges up to 1 km. The transmitter utilizes the "intra-pulse" thermally chirped tuning technique of a single distributed-feedback (DFB) laser. Quantum Cascade Lasers have been successfully used to monitor atmospheric pollutants in the mid-infrared (mid-IR) region. However, their use for multiple gases in ambient conditions is less prevalent. We focused on CH4 and N2O because they are long-lived greenhouse gases in the atmosphere with significant global warming impacts.;Gas spectra were acquired by a pulsed QC laser using a thermal down chirp technique over 1298--1300 cm-1 optimal spectral window with 0.008 cm-1 spectral resolution. Based on careful optimization of the spectral window for absorption features of CH4 and N2O, a dual-species, cost-effective, robust and fast response open-path laser based monitor has been developed and demonstrated for ambient trace gas monitoring.