Development of a tunable diode laser instrument for airborne in situ measurements of carbon monoxide (CO) and nitrous oxide (N₂O)

We have developed an instrument to fly on the NASA WB-57F aircraft which measures simultaneously two atmospheric tracers, CO and N₂O. These gases have lifetimes which make them appropriate dynamical tracers in the upper troposphere and lower stratosphere. To measure these species for studies of current interest we need a continuous record with a sensitivity of about 1 ppbv, and an absolute accuracy of about 1 percent. To trace the effect of small scale features requires a response time around 1 second. We describe the details of a tunable diode laser-based instrument designed for automatic operation aboard the WB-57F aircraft at altitudes between the surface and 65,000 feet. A single diode laser, cooled by liquid nitrogen, is used to provide radiation at absorption lines of CO (2190.018 cm^-1) and N₂O (2190.350 cm^-1). Fully reflective optics directs the infrared beam through a long pass cell (36 m path length) of small volume (0.3 l) operated at relatively low pressure (40 mb). The instrument operates autonomously for up to 8 hours. Calibration is achieved by the introduction of standard, calibrated mixtures into the inlet sample flow.