Influence of continental sources on oceanic air composition at the easternedge of the North Atlantic Ocean, TOR 1992-1995

The atmospheric concentrations of O-3, NMHCs, PAN and NOx were measured at the TOR station in Porspoder (Brittany, France) from 1992 to 1995. The resu lts from April 1992 to December 1994 are reported here. In order to study t he behaviour of trace gases in the air masses coming from the Atlantic Ocea n, the origin of these air masses was characterised by the analysis of the five-day back-trajectories reaching 950 mbar at Porspoder. Three kinds of o ceanic air masses are considered, depending on their origin: North Atlantic northern and southern latitudes and North American continent. It appears t hat the oceanic high-latitude troposphere was strongly influenced by contin ental emissions, which in turn impacted the median latitude troposphere dur ing polar outflow events. A greater accumulation of anthropogenic compounds in the Arctic sector was calculated and may be at the origin of the high o xidant levels measured in spring in this same sector. The influence of nort h American emissions on the European coast was also studied and seems to re ach a maximum in spring. Long-range transport of long-lived species was obs erved throughout the year, but evidence for winter transport of more reacti ve species, such as ethene, is also shown. Meteorological parameters and lo cal photochemistry appear to play an important role in the distribution of the pollutants over the North Atlantic Ocean throughout the year. Further i nvestigations are needed to give more complete information about the origin , formation and destruction of the secondary pollutants.