IMPORTANT NOTICE: the system described below has been permanently withdrawn from the FAAM core chemistry measurements capability following upgrade to an Air Quality Design Inc. dual channel chemiluminescence NOx analyser.  The technique described below only applies to NOx data collected prior to flight B595 (19/05/2011). 
 
FAAM CORE TE42C Chemiluminescence NO-NO2-NOx Analyser
 
The model TE42C is a commercial gas analyser produced by Thermo Scientific. The analyser employs the characteristic chemiluminescence produced by the reaction between Nitric Oxide (NO) and Ozone, the intensity of which is proportional to the NO concentration. NO2 measurements are approximated using its thermal reduction to NO by a heated (350°C) molybdenum converter. 
 
Note that the TE42C NO2 conversion technique is highly non-selective (eg by comparison to photolytic conversion), and that other NOy compounds (PAN and PAN-like nitrates, alkyl nitrates, nitric acid...) do interfer with this NO2 measurement (positive bias).  The model TE42C however is a well-recognised instrument, which provides a relatively accurate estimate of NOx particularly close to the boundary layer, and in polluted atmospheres where further NOx oxidation into other NOy reservoir species is minimal (close to NOx sources, in air masses of low photochemical lifetime). 
 
The TE42C also carries out a dynamic zero using a pre-reactor as part of its measurement cycle.
 

System Components

Ambient air is drawn into the cabin from the starboard air sample pipe (port #12) along several meters of 1/4" od PFA line using an external sample pump. Air passes through this Full Teflon diaphragm pump (KNF model N726.1.2 FT.18), and through a PTFE buffer volume where excess flow is vented to the cabin. At low altitudes the residence time of this system is negligible because of the very high flow from this pump (>20LPM). At the highest altitudes the residence time is estimated to approach 10s.
 
The sample air is drawn through the TE42C monitor with a nominal capillary controlled flowrate of ~1 LPM, via a particulate filter PFTE membrane (5 μm pore size) using a second external diaphragm pump (KNF model MPU485-N026.3), the exhaust of which is connected to one of the aircraft vent pipes due to the large excess of ozone needed for the measurement.
Cabin air, dried with Molecular Sieve, is used for the TE42C ozonator, with a nominal capillary controlled flowrate of ~0.065 LPM).
 

Images



TE49C in Rack
Air Sample Pipe Inlet (open)
Internal Sample Lines

Documentation

The TE42C user manual is available here: icon Model 42C Instruction Manual (5.74 MB 2009-11-17 14:55:43)
The Core Chemistry Technical Specification is available here: pdf icon CoreChemistry_TSSE_v7_Nov2006 (188.15 kB 2012-01-30 14:34:08) (188 KB)
 

Data

The 10s duty cycle of the TE42C rotates between measuring NO, NOx and the pre-reactor (zero) values. A full cycle takes 30s, and Thermo Scientific give the response time of the instrument as 60s (ie two complete cycles). Data are reported at 1  Hz, although it will be observed that readings for NO, NOx and NO2 are only updated every 10s. In this case NO2 is simply treated as NOx - NO.
 
The analogue output from the instrument is logged by the aircraft data recording system and scaled accordingly to produce a concentration measurements for NO, NO2 and NOx in ppbv. Instrument calibrations are referenced to this analogue output.
 
Periodic calibrations are carried out using dilution of a calibrated gas mixture (500 ppbv nominal NO in Nitrogen), enabling the generation of calibration points across the 0-50 ppbv operating range of the analyser. Overall measurement uncertainty is considered by FAAM to be ± (6% + 2) ppbv.
 

Publications

Here's a recent paper describing the operation of the TE42C.
Stewart, D. J., Taylor, C. M., Reeves, C. E., McQuaid, J. B., Biogenic nitrogen oxide emissions from soils: impact on NOx and ozone over west Africa during AMMA (African Monsoon Multidisciplinary Analysis: observational study, Atmos. Chem. Phys., Vol 8, 8, 2285-2297, 2008. [link]
 
  

Further Details

Contact Stéphane Bauguitte at FAAM