A textbook example of long-range transport: Simultaneous observation of ozone maxima of stratospheric and North American origin in the free troposphere over Europe

We describe an episode of a stratospheric intrusion into the free troposphere over Europe followed by a long-range transport of ozone from the North American boundary layer. Observational data showed the presence of a thin tongue of stratospheric air in the free troposphere for at least 36 hours. This filament was found in data from two ozone soundings and was recorded continuously for 26 hours by a high-resolution ozone lidar. The stratospheric air also intercepted two high Alpine summits, causing elevated ozone and beryllium 7 concentrations. Trajectory, particle dispersion model, and potential vorticity analyses confirmed the stratospheric nature of the tongue. In the lidar data, following the intrusion, pockets of elevated ozone concentrations (80-100 ppb) were found in the free troposphere close to the tropopause. The low potential vorticity values and high water vapor content in these ozone-rich pockets and trajectory analyses suggest that the ozone was photochemically produced in the boundary layer over eastern North America, followed by rapid uplifting in a warm conveyor belt over the Atlantic Ocean ahead of a frontal system. This was confirmed by ozone and water vapor measurements aboard commercial airliners crossing the warm conveyor belt. The air mass trajectories in both the stratospheric intrusion and the warm conveyor belt were tightly bundled, emphasizing the importance of the coherency of airstreams for long-range ozone transport.