1Service d'Aéronomie, 3ème étage, Tour 45-46, 4 Place Jussieu, 75005 Paris, France
2Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Institut für Physik der Atmosphäre, 82230 Wessling, Germany
3Departement of Meteorology, University of Reading, PO Box 243, Earley Gate, Reading, RG6 6BB, UK
4NOAA ESRL / CIRES, University of Colorado at Boulder, Boulder CO 80309, USA
5NOAA ESRL, 325 Brodway, Boulder, CO 80305, USA
6Atmospheric Chemistry Division, NCAR, 1850 Table Mesa Drive Boulder, CO 80305, USA
7RSMAS/MAC University of Miami, Miami, FL 33149 USA
8Departement of Chemistry, Fort Hays State University, Hays KS 67601, USA
*now at: CEREA, a Joint Laboratory ENPC/EDF R&D, 6–8 avenue Blaise Pascal, Cité Descartes Champs-sur-Marne, 77455 Marne la Vallée, France
Abstract. The photochemical evolution of an anthropogenic plume from the New-York/Boston region during its transport at low altitudes over the North Atlantic to the European west coast has been studied using a Lagrangian framework. This plume, originally strongly polluted, was sampled by research aircraft just off the North American east coast on 3 successive days, and 3 days downwind off the west coast of Ireland where another aircraft re-sampled a weakly polluted plume. Changes in trace gas concentrations during transport were reproduced using a photochemical trajectory model including deposition and mixing effects.
Chemical and wet deposition processing dominated the evolution of all pollutants in the plume. The mean net O3 production was evaluated to be -5 ppbv/day leading to low values of O3 by the time the plume reached Europe. Wet deposition of nitric acid was responsible for an 80% reduction in this O3 production. If the plume had not encountered precipitation, it would have reached the Europe with O3 levels up to 80-90 ppbv, and CO levels between 120 and 140 ppbv. Photochemical destruction also played a more important role than mixing in the evolution of plume CO due to high levels of both O3 and water vapour showing that CO cannot always be used as a tracer for polluted air masses, especially for plumes transported at low altitudes. The results also show that, in this case, an important increase in the O3/CO slope can be attributed to chemical destruction of CO and not to photochemical O3 production as is often assumed.