| Summary: | 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 then 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 are 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 photochemical O 3 production is estimated to be −5 ppbv/day leading to low O 3 by the time the plume reached Europe. Model runs with no wet deposition of HNO 3 predicted much lower average net destruction of −1 ppbv/day O 3 , arising from increased levels of NO x via photolysis of HNO 3 . This indicates that wet deposition of HNO 3 is indirectly responsible for 80% of the net destruction of ozone during plume transport. If the plume had not encountered precipitation, it would have reached Europe with O 3 concentrations of up to 80 to 90 ppbv and CO 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 O 3 and water vapour showing that CO cannot always be used as a tracer for polluted air masses, especially in plumes transported at low altitudes. The results also show that, in this case, an increase in O 3 /CO slopes can be attributed to photochemical destruction of CO and not to photochemical O 3 production as is often assumed.
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