Global source attribution of tropospheric ozone: Long‐range transport from various source regions

Abstract

We examine contributions from various source regions to global distributions and budgets of tropospheric ozone (O 3 ) in the context of intercontinental transport, using tagged tracer simulation with a global chemical transport model. For tagging O 3 , we consider regional separation of the model domain on the basis of the distributions of O 3 chemical production. We define 14 polluted source regions (14 tracers) in the boundary layer (North America, Europe, China, etc.) and 8 regions (8 tracers) in the free troposphere; O 3 production in the remaining (remote) tropospheric region and O 3 transport from the stratosphere are also tagged as separate tracers. O 3 transport from the polluted source regions like North America, Europe, and Asia generally accounts for more than 40% of ozone abundances even in remote locations. O 3 exports from boundary layer in China and Asian free troposphere are discerned through much of the Northern Hemisphere, suggesting significant and extensive impacts of eastern Asian pollution. In particular, O 3 from Asian free troposphere plays the most important roles in distribution and seasonal variation of O 3 in the middle‐upper troposphere almost globally. In June–September, the model calculates a large O 3 contribution (5–10 ppbv) from Asian free troposphere in the upper troposphere over the South Pacific associated with long‐range interhemispheric transport from Asia to the southern midlatitudes (via the western Indian Ocean, Africa, and Atlantic) in the upper troposphere. O 3 transported from biomass burning regions such as South America, Africa, and Australia widely distributes in the Southern Hemisphere. Our simulation demonstrates that there is a significant interhemispheric O 3 transport from South America to the northern midlatitudes in the upper troposphere which reaches Japan, North Pacific, and the United States in conjunction with O 3 export from North Africa. Our tagged O 3 simulation estimates that the annual mean global tropospheric O 3 burden, as calculated to be 344 Tg in this study, comes from chemical production in the source regions (48%) and in the remote regions (29%) and from stratosphere‐troposphere exchange (23%).