Long-term measurements of atmospheric peroxyacetylnitrate (PAN) at rural sites in Ontario and Nova Scotia; seasonal variations and long-range transport

Abstract

The atmospheric behaviour of PAN at locations well-removed from major urban areas has been studied over extended periods using highly sensitive, automated gas chromatographic techniques at two sites in eastern Canada; Longwoods in southern Ontario (Februaryactober 1983), and Kejimkujik in Nova Scotia (May 1984 onwards). In addition, data have also been obtained from more limited measurement programs in northern Ontario at Chalk River (November-December 1983) and North Bay (January-February 1984). The variations in PAN mixing ratios observed at these sites have been examined with reference to available data for other atmospheric components, such as O₃, HNO₃, particulate nitrate and sulphate. Air mass back-trajectory calculations have been performed for Kejimkujik to relate observed periods of elevated PAN mixing ratio to specific source regions for precursor NMHC and NO_x. For the sites studied, periods of enhanced PAN mixing ratio are observed to be possible throughout the yearly cycle, indicating that significant photochemical activity can occur even during wintertime. This contrasts with the available data for ozone, where photochemical formation in polluted air masses appears to be suppressed in winter. Seasonal patterns of PAN behaviour are probably sitedependent, but the observations suggest that the highest PAN maxima are favoured during fall and late winter/early spring. The long-term data from Kejimkujik also reveal an underlying seasonal variation in the typical background value of PAN observed at this site; this background value maximizes in late winter/early spring and minimizes in summer. In general, the magnitude of the observed PAN maxima appears to decrease with distance from the source region of influence. PAN accounts for a significant fraction of the observed oxidized nitrogen species at all the sites studied, and appears to make an increasing contribution as the transport time from the source region increases. In background tropospheric air, the contribution of PAN may be as high as 80-90%. DOI: 10.1111/j.1600-0889.1988.tb00113.x