AN OPTIMIZED METHOD FOR AIRBORNE PEROXYACETYL NITRATE (PAN) MEASUREMENTS

In this paper we describe a gas-chromatographic method for PAN measure ments in the background atmosphere, which has been adapted to the spec ial requirements of aircraft based campaigns. The instrument is instal led in a 1.21 m high, 19 inch rack which has a total weight of 70 kg a nd a power consumption of 750 VA, The gas chromatograph is equipped wi th a commercial liquid injector and a valve system for injection of ga seous samples. The gas-inlet system allows automatic injection of samp les 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 e ach other to better than 85%. An optimum selectivity of the gas-chroma tographic separation is obtained by a combination of two short megabor e capillary columns of different polarity. The flow rates are 15 cm(3) /min, the column temperature is 26 degrees C. For detection an electro n-capture detector, operated at 30 degrees C, is used. To allow a reli able control of these relatively low temperatures the instrument is eq uipped 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 limi t of detection is better than 15 ppt (3 sigma), the time needed for on e measurement is less than 4 min. Preliminary results from a flight ca mpaign conducted in June 1994 demonstrate the suitability of the instr ument for airborne PAN measurements.