Atmospheric measurements of peroxyacetylnitrate (PAN) in rural, south-east England: Seasonal variations winter photochemistry and long-range transport

Abstract

Peroxyacetylnitrate (PAN) is an important secondary pollutant formed in the ambient atmosphere during photochemical episodes; it constitutes a significant component of the atmospheric NO x cycle. The behaviour of PAN in the atmosphere has been studied at Harwell, a rural site in south-east England, using a sensitive technique with a detection limit of < 30 ppt(v). Even during winter months, daytime levels of PAN above the detection limit were always present, demonstrating the ubiquity of this molecule in the atmosphere. Results for the periods November 1974–October 1975 and January 1980–June 1981 show the existence of a pronounced seasonal variation. Maximum levels were observed during summer months, and minimum values during winter. There is a large difference between these maxima and minima, in contrast to typical patterns for ozone, and it is suggested that PAN is a better indicator of overall chemical reactivity in the lower atmosphere. Simultaneous measurement of CFCl 3, was found to be a good indicator of air mass quality and anthropogenic influence; increased levels of this tracer were always associated with elevated PAN concentrations. Unexpectedly high levels of PAN during two episodes in February 1981 showed the possibility of significant photochemical activity during winter months. Back-trajectory analysis for these periods, indicated the influence of mainland European emissions and subsequent transport of photochemically-aged air masses. Similar observations of high PAN made during February 1975 suggest that the occurrence of late-winter photochemical episodes may not be uncommon.