USE OF BACKWARD TRAJECTORIES TO INTERPRET THE 5-YEAR RECORD OF PAN AND O-3 AMBIENT AIR CONCENTRATIONS AT KEJIMKUJIK NATIONAL-PARK, NOVA-SCOTIA

Air parcel trajectory data are used in two ways to elucidate the tempo ral trend analysis of 5 years of peroxyacetyl nitrate (PAN) and O-3 Ob servations at a rural site in eastern Canada. In the first method, ''p robability of residence'' contours are constructed to determine the mo st probable origin of air parcels containing the highest and lowest 10 % of PAN and O-3 mixing ratios. High PAN is found to emanate always fr om areas of high anthropogenic activity, except when the transport pat h is to a large extent over the ocean, especially in the summer. High O-3 originates from the same regions except in the winter when because of low photochemical activity, O-3 is actually titrated and air from less populated areas is richer in O-3. The trajectories indicate that transport at higher altitude leads to higher mixing ratios; this is es pecially the case for O-3 in winter and spring. The complementary meth od of clustering the trajectories has allowed qualitative derivation o f seasonal cycles for background and polluted air masses of different origin. Background air from the north shows a distinct PAN maximum in March; it is discussed that this could be either due to enhanced photo chemistry or to the import of polluted air from the Arctic. Polluted a ir masses show the same March peak, but a second peak in late summer/e arly fall. Oceanic air from the south has a January maximum in PAN but otherwise is consistently low in PAN. O-3 also has a spring maximum, but in polluted air it is broader stretching into the summer. It is po stulated that this is due to additional O-3 formation in the summer, w hile in the winter, actual O-3 loss is indicated for polluted air. By inference it is deduced that the second PAN peak in polluted air is al so due to additional formation in comparison with background air, whil e in the summer, extra PAN loss mechanisms operate that are less impor tant for O-3. Indications for both dry deposition and thermal decompos ition have been found.