Long‐range transport and tropospheric ozone variability in the western Mediterranean region during the Intercontinental Transport of Ozone and Precursors (ITOP‐2004) campaign

Abstract

Two ground‐based ozone lidars have been operated at Observatoire de Haute Provence (OHP) for 11 days during the Intercontinental Transport of Ozone and Precursors (ITOP‐2004) measurement campaign. Ozone and scattering ratio vertical profiles have been measured from the boundary layer up to the tropopause. At first order, tropospheric ozone temporal variability is due to local pollution within the planetary boundary layer and to stratosphere‐troposphere exchange. Remaining ozone rich layers within the free troposphere are related to long‐range transport processes. Transport pathways are discussed combining Lagrangian particle dispersion modeling analysis and upstream airborne in situ measurements of the chemical composition of these air masses. High ozone and CO mixing ratios measured within polluted plumes aboard the aircraft correspond to ozone and aerosol layers seen by the lidar. Most of these layers have their origin in North America where they are uplifted either by forest fires or by warm conveyor belts in the vicinity of frontal regions. During the campaign, these polluted and thin (<1 km) layers remain coherent and are transported in a Lagrangian manner over the Atlantic Ocean. The layers observed above OHP in the lower free troposphere exhibit ozone mixing ratio 50% larger than background values. With an ozone content of 3 to 6 Dobson Units, these layers increase by 5 to 10% the background tropospheric ozone column.