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 NO_x 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 NO_x. 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%.