Instrument: Dual
GC/ECD for the ambient measurement of PAN, PPN and MPAN.
Principal investigators: Frank
Flocke and Andrew J. Weinheimer, National Center for Atmospheric
Research, Atmospheric Chemistry Division, Boulder, Colorado
PAN
type compounds play an important role in the odd nitrogen chemistry
of the atmosphere. Owing to their thermal lifetime characteristics
they can be transported over long distances at cold temperatures
in the upper troposphere and act as an important source of NOx in
remote areas such as the Pacific or Mid-Atlantic Oceans through
thermal decomposition during subsidence. PAN is formed from the
oxidation of many shorter chain hydrocarbons, both anthropogenic
and biogenic, in the presence of NOx. PPN is primarily
formed from longer-chain anthropogenic hydrocarbons and MPAN mainly
during isoprene- and terpene-oxidation processes. Because of their
formation process being closely linked to the mechanism for photochemical
ozone production in the atmosphere, PANs can be used as excellent
tracers for these processes. In addition, the interrelationships
between the different PAN type compounds can provide interesting
insight into the photochemical processes in polluted air, by allowing
the extraction of information about the relative roles of anthropogenic
and biogenic hydrocarbons involved in ozone production e.g. in
continental outflow.
The
instrument is a compact dual gas chromatograph that is equipped
with a common sampling loop and a common ECD detector. Ambient
air is drawn through the sampling loop through a pressure- and
temperature controlled inlet and alternately injected onto each
column oven. The instrument can be configured for the measurement
of PAN, PPN and MPAN, or PAN and PPN only, providing a measurement
every 3 or 2 minutes, respectively. One measurement of PAN, PPN
and MPAN combined with one of PAN and PPN only is a third option,
providing two measurements every 5 minutes. Part of the instrument
package is a newly developed in-situ photolytic PAN source
that provides in-flight calibration for PAN. The system weighs
about 220 lbs (including gases and pumps) and occupies one side
of a standard aircraft rack (48” high by 25” deep). The instrument
requires one operator on the aircraft.
The
instrument was successfully deployed on board the NCAR C-130 during
the TOPSE mission conducted February through May, 2000. The in-flight
limit of detection for PAN and PPN is 5 pptv or better, the measurement
precision is 5%, the accuracy better than 10%.
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