Airborne measurements of tropospheric formaldehyde by tunable diode laser absorption spectroscopy

A ground based tunable diode laser absorption spectrometer (TDLAS) developed at NCAR for the measurement of formaldehyde (HCHO) has been modified for use aboard tropospheric aircraft. Measurements of HCHO are essential to comprehensive investigations of atmospheric oxidation processes, and aircraft platforms provide the advantage of vertically and spatially resolved measured. Initial deployment of the aircraft system occurred during the spring and summer of 1996 as part of the NARE and STERAO campaigns. Data coverage exceeded 95 percent out of a sum total of 175 flight hours. Sensitivities achieved during STERAO were approximately 40-60 pptv for 4.5 min of measurement and 80- 120 pptv for 55s; NARE sensitivities were slightly worse. For both campaigns, post-flight fitting of background spectra indicated periodic outgassing and contamination of the background matrix air. Analysis of data collected during the May 1995 SOS intercomparison suggests that background subtraction largely nullifies any outgassing effect. Background matrix gas HCHO concentrations were determined by fitting background spectra and were then used to correct the associated ambient data sets. Finally, fits of the difference of successive backgrounds appear to approximate measurement replicate precisions and are more informative than calculated fit precisions.