Asian chemical outflow to the Pacific in spring: Origins, pathways, and budgets

We analyze the Asian outflow of CO, ozone, and nitrogen oxides (NOx) to the Pacific in spring by using the GEOS-CHEM global three-dimensional model of tropospheric chemistry and simulating the Pacific Exploratory Mission-West (PEM-West B) aircraft mission in February-March 1994. The GEOS-CHEM model uses assimilated meteorological fields from the NASA Goddard Earth Observin g System (GEOS). It reproduces relatively well the main features of troposp heric ozone, CO, and reactive nitrogen species observed in PEM-West B, incl uding latitudinal and vertical gradients of the Asian pollution outflow ove r the western Pacific although simulated concentrations of CO tend to be to o low (possibly because biogenic sources are underestimated). We use CO as a long-lived tracer to diagnose the processes contributing to the outflow. The highest concentrations in the outflow are in the boundary layer (0-2 km ), but the strongest outflow fluxes are in the lower free troposphere (2-5 km) and reflect episodic lifting of pollution over central and eastern Chin a ahead of eastward moving cold fronts. This frontal lifting, followed by w esterly transport in the lower free troposphere, is the principal process r esponsible for export of both anthropogenic and biomass burning pollution f rom Asia. Anthropogenic emissions from Europe and biomass burning emissions from Africa make also major contributions to the Asian outflow over the we stern Pacific; European sources dominate in the lower troposphere north of 40 degreesN, while African sources are important in the upper troposphere a t low latitudes. For the period of PEM-West B (February-March) we estimate that fossil fuel combustion and biomass burning make comparable contributio ns to the budgets of CO, ozone, and NOx in the Asian outflow. We find that 13% of NOx emitted in Asia is exported as NOx or PAN, a smaller fraction th an for the United States because of higher aerosol concentrations that prom ote heterogeneous conversion of NOx to HNO3. Production and export of ozone from Asia in spring is much greater than from the United States because of the higher photochemical activity.