Continental sources, transoceanic transport, and interhemispheric exchange of carbon monoxide over the Pacific

The Pacific Exploratory Mission to the Tropics phase B (PEM-Tropics B) aircraft campaign in March-April 1999 surveyed the chemical composition of the Pacific atmosphere from 35 deg N to 35 deg S and up to 12 km altitude. We use these observations in combination with a global 3D model driven by assimilated meteorological observations to investigate the transport of northern hemispheric pollutants over the Pacific. We focus on carbon monoxide (CO) and tag it in the model by its region of origin. The model reproduces the observed large-scale latitudinal, longitudinal, and vertical gradients of CO concentrations over the Pacific. Biomass burning in Southeast Asia, which was particularly intense in spring 1999, contributed most of the CO enhancements observed in the free troposphere over the northern tropical Pacific but played only a minor role in the boundary layer. Fossil fuel combustion in Europe and Asia contributed most of the observed CO enhancements in the boundary layer over the North Pacific; the European influence dominated over Asian influence north of 35 deg N. European influence over the Pacific is particularly strong in spring because of wintertime accumulation of CO at high latitudes. North American pollution made little contribution to CO anywhere over the Pacific. Circulation of Eurasian industrial pollution around the Pacific High and into the trade winds produced a tropical "river of pollution" flowing in the lower troposphere from the northeastern to the western equatorial Pacific and in the vicinity of the South Pacific Convergence Zone. This pathway, however, made little contribution to interhemispheric exchange. (Author)