An optimized method for airborne peroxyacetyl nitrate (PAN) measureme

In this paper we describe a gas-chromatographic method for PAN measurements in the background atmosphere, which has been adapted to the special requirements of aircraft based campaigns. The instrument is installed in a 1.21 m high, 19 inch rack which has a total weight of 70 kg and a power consumption of 750 VA. The gas chromatograph is equipped with a commercial liquid injector and a valve system for injection of gaseous samples. The gas-inlet system allows automatic injection of samples with defined and constant mass, independent from ambient pressure variations. Two different methods are used for calibration: Liquid PAN calibration samples and a diffusion source for gas-phase calibrations. Both methods have reproducibilities better than 90% and agree with each other to better than 85%. An optimum selectivity of the gas-chromatographic separation is obtained by a combination of two short megabore capillary columns of different polarity. The flow rates are 15 cm3/min, the column temperature is 26°C. For detection an electron-capture detector, operated at 30°C, is used. To allow a reliable control of these relatively low temperatures the instrument is equipped with peltier cooling. To avoid baseline or signal drifts caused by pressure variations in the aircraft cabin an electronic control of the system pressure is integrated into the instrument. The lower limit of detection is better than 15 ppt (3 ), the time needed for one measurement is less than 4 min. Preliminary results from a flight campaign conducted in June 1994 demonstrate the suitability of the instrument for airborne PAN measurements.