Quantum cascade lasers for open and closed path measurement of atmospheric trace gases

We report the application of quantum cascade (QC) lasers to measurement of atmospheric trace gases in both closed path and open path configurations. The QC laser, a recently available commercial device, is Peltier cooled and pulsed, with emission near 965 cm(-1). We use direct absorption with a rapid sweep integration and spectral fits to derive absolute concentrations from tabulated line parameters without calibration. In the closed path configuration, with absorption in a long pathlength multipass cell (210 in. 50 Torr), we examined laser line widths and sensitivity limits. We measured ammonia with a precision of 0.05 nmole/mole (0.05 ppbv) RMS at 1 Hz, limited by detector noise. The laser linewith was 0.007 cm(-1) HWHM, based on measurements of ethylene absorption line shapes with a current pulse width of similar to14 ns. In the open path configuration, we measured ammonia in the exhaust of automobiles driving through a probe beam. Atmospheric pressure line broadening and turbulence limit the sensitivity, giving a column density noise level at 20 Hz of 1.4 ppm-m. We observed ammonia column densities up to 40 ppm-m in the exhaust plumes. In future systems we will include a CO2, channel, allowing normalization to fuel use rate.