Long-Range Transport of Ozone from the North American Boundary Layer to Europe: Observations and Model Results

Conclusions

It was shown that the “textbook example” of intercontinental ozone transport from North America to Europe given by Stohl and Trickl (1999) indeed seems to be a very typical transport episode. All other episodes observed so far (two more were found in the years 1998 and 1999) followed the same pattern, namely lifting within a WCB, followed by transport towards Europe. WCBs frequently originate close to the North American east coast and have a high probability to draw polluted air from the continental rim into the upper troposphere. Emissions from Europe, which is in the main corridor of the “North American” WCBs, on the other hand have little chances of reaching the upper troposphere directly.

A closer examination of the maximum WCB inflow region at the North American seaboard shows that it shifts closer to the main emission regions in late spring. Export of ozone from North America to Europe may,thus, strongly increase from a winter minimum during the spring. In fact, all of our observations of intercontinental ozone transport episodes so far took place in spring. It is, therefore, a tempting hypothesis that transport from North America is a major ingredient to the often-observed spring-time maximum in European background ozone concentrations.