CHEMICAL SIGNATURES OF AGED PACIFIC MARINE AIR - MIXED-LAYER AND FREETROPOSPHERE AS MEASURED DURING PEM-WEST-A

The Pacific Ocean is one of the few remaining regions of the northern hemisphere that is relatively free of direct anthropogenic emissions. However, long-range transport of air pollutants is beginning to have a significant impact on the atmosphere over the Pacific. In September a nd October 1991, NASA conducted the Pacific Exploratory Mission-West A expedition to study the atmospheric chemistry and background budgets of key atmospheric trace species, Aircraft sampling centered on the no rthern Pacific, 0 degrees to 40 degrees N and 115 degrees to 180 degre es E. The paper summarizes the chemical signature of relatively well-a ged Pacific marine air(residence time greater than or equal to 10 days over the ocean), The chemical signatures show that marine air is not always devoid of continental influences. Aged marine air which circula tes around the semipermanent subtropical anticyclone located off the A sian continent is influenced by infusion of continental air with anthr opogenic emissions. The infusion occurs as the result of Asian outflow swept off the continent behind eastward moving cold fronts. When comp ared to aged marine air with a more southerly pathway, this infusion r esults in enhancements in the mixing ratio of many anthropogenic/conti nental species and typically those with lifetimes of weeks in the free troposphere. Less enhancement is seen for the short-lived species wit h lifetimes of a few days as infused continental emissions are deplete d during transport (about a week) around the semipermanent subtropical high.