Airborne tunable diode laser system for rapid CO and CH4 measurement

Abstract

A tunable diode laser (TDL) instrument, differential absorption CO measurement, (DACOM) has accumulated over 300 h of fast response CO data on the NASA CV990 and Electra aircraft during four major field expeditions of the NASA-sponsored Global Tropospheric Experiment program. New capabilities are being incorporated into DACOM including the ability to make simultaneous CO and CH4 measurements at a measurement rate approaching 50/s. This enables very high spatial resolution gas concentration measurements from aircraft platforms and offers the potential to measure directly vertical gas flux using the eddy correlation technique. In this technique, flux values are derived by correlating fast response measurements of vertical wind velocity and gas concentration. An analysis of error sources affecting eddy flux measurements is presented. To achieve fast response times, a small but high-performance Venturi jet vacuum pump, driven by an aircraft engine air compressor, creates the low-pressure (≈0.12 atm) high-volume flow (≈1800 liters/min) needed in the White cell. Simultaneous detection of CO and CH4 is accomplished by a beam combining the radiation of two TDLs (7.6 μm for CH4 and 4.7 μm for CO) prior to entering the White cell. Two color detectors (InSb/HgCdTe) are used to detect the respective differential absorption signals.