FAAM Core instrument measuring Carbon monoxide (CO). Aerolaser AL5002 Used pre May 2011
AL 5002 VUV Fast Fluorescence CO Analyser
The AL5002 (s/n 127) is a commercial gas analyser produced by Aerolaser GmbH.
The analyser employs the measurement of the fluorescence of CO when exposed to UV light at a wavelength of 150nm, which is proportional to the concentration of CO.
Ambient air is drawn into the cabin from the starboard air sample pipe (port #11) along several meters of 1/8" od PFA line at a flowrate of approximately 180 ml/min, by means of a vacuum pump (Vaccubrand model MV2) . The air is then dried using a Perma Pure Nafion gas dryer (model MD-110-24P) prior to the detector. The fluorescence cell is maintained at a low pressure, typically around 7.5 Torr, and constant temperature, typically 40°C.
The monochromator optical filters, providing the collimated 150nm source from a plasma lamp, are flushed with mass flow controlled and filtered (via Entegris GateKeeper N2 purifier) high-purity (N6.0) nitrogen to prevent ingress of highly absorbing molecular oxygen or/and contaminating species such as CO. The plasma lamp itself requires a constant supply of 0.25% CO2/AR (N6.0), also mass flow controlled, in order to produce emissions of the required intensity.
The AL5002 exhibits a very fast response to changes in ambient CO concentrations. The manufacturer quotes 0.1s for a 10-90% response time (albeit with a 2s signal delay). FAAM data are output at 1 Hz from the analyser and presented similarly at 1hz in the Core data.
The residence time of sample in the inlet line is not negligible. Calculation of the flow rates and length of tubing in the sample inlet suggest a residence time of over 10 seconds. CO data (eg icon CO Time Lag (21.77 kB 2009-12-14 13:22:02) certainly show a time lag in flight, typically of 15 seconds, the measured lag is additionally associated with a small amount of smoothing in the data. FAAM are working to correct this.
Over the course of a flight, the CO analyser sensitivity (Hz/ppbv) and background (Hz) both tend to change (where Hz is the analyser's photomultiplier raw photon count rate, per sec). FAAM's standard practice is to calibrate these quantities periodically through a flight by means of an on-board CO/Air standard and Sofnocat 514 CO scrubber (internal to analyser) . These calibrations take around 150s, and are usually performed where flight levels change significantly. No CO measurement data are available during these times and as such the precise location of calibrations is usually a matter for consultation with the Mission Scientist of the day. Instrument calibrations are automatically applied to the data as they are performed; unfortunately this can result in apparent step changes either side of a calibration, where the sensitivity/background has changed significantly. FAAM are seeking to improve this method.
The analogue output from the instrument is logged by the aircraft data recording system and scaled accordingly to produce a concentration measurement (ppbv).
The AL5002 is operated throughout the range 0-2000 ppbv, and its linearity has been demonstrated to 100,000 ppbv (see Publications). The system is therefore calibrated at nominally 500 ppbv, above the level of typical ambient measurements in the free troposphere.
The manufacturer estimates the instrument precision to be ±1.5 ppbv at 100 ppbv measured CO, and a lowest detection limit of 3 ppbv for 1 Hz data has been documented (see Publications). A 1σ precision of ±2.8 ppbv can be derived from Poisson Statistics of the mean background count rate (~34000 Hz) and nominal sensitivity of 65 Hz/ppbv.
FAAM on-board CO/Air calibration standards are traceable to a NOAA-GMD / WMO 2000 scale CO/Air standard, and are compared several times a year. As a result, overall measurement uncertainties for the FAAM AL5002 CO measurements are estimated to be around ±5%.
The principle and operation of the AL5002 instrument was documented in the following journal paper:
An improved fast-response vacuum-UV resonance fluorescence CO instrument, Gerbig et al.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D1, PAGES 1699-1704, JANUARY 20, 1999
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|FAAM Instrument description|