Lagrangian analysis of low altitude anthropogenic plume processing across the North Atlantic

[摘要] The photochemical evolution of an anthropogenic plume from theNew-York/Boston region during its transport at low altitudes over theNorth Atlantic to the European west coast has been studied using aLagrangian framework. This plume, originally strongly polluted, wassampled by research aircraft just off the North American east coast on 3successive days, and then 3 days downwind off the west coast of Ireland whereanother aircraft re-sampled a weakly polluted plume.Changes in trace gas concentrations during transport are reproducedusing a photochemical trajectory model including deposition and mixingeffects.Chemical and wet deposition processing dominated the evolutionof all pollutants in the plume. The mean net photochemical O₃ production isestimated to be −5 ppbv/day leading to low O₃ by the timethe plume reached Europe.Model runs with no wet deposition of HNO₃ predicted much lower averagenet destruction of −1 ppbv/day O₃, arising from increased levels of NO_xvia photolysis of HNO₃. This indicates that wet deposition of HNO₃ isindirectly responsible for 80% of the net destruction of ozone during plume transport.If the plume had notencountered precipitation, it would have reached Europe with O₃ concentrationsof up to 80 to 90 ppbv and CO between 120 and 140 ppbv. Photochemical destruction also played a more important role thanmixing in the evolution of plume CO due to high levels of O₃ andwater vapour showing that CO cannot always be used as a tracer forpolluted air masses, especially in plumes transported at lowaltitudes. The results also show that, in this case, anincrease in O₃/CO slopes can be attributed to photochemical destructionof CO and not to photochemical O₃ production as is often assumed.

[学科分类] 大气科学