Asian Outtow to the Paciic Ocean in Springtime: a 3d Simulation of the Pem-west (b) Mission with Assimilated Meteorology

We present an analysis of the Asian outtow of CO, ozone, and nitrogen oxides (NO x) over the Paciic by using the GEOS-CHEM global 3-D model of tropospheric chemistry to simulate observations from the PEM-West B aircraft mission in February-March 1994. The GEOS-CHEM model uses assimilated meteorological elds from the NASA Goddard Earth Observing System (GEOS). It reproduces well the main features of tropospheric ozone and related species, including the latitudinal and vertical gradients of the Asian pollution outtow over the western Paciic observed in PEM-West B. We use CO as a long-lived tracer to diagnose the processes contributing to this outtow. The highest concentrations in the outtow are in the boundary layer (0-2 km), but the strongest outtow uxes are in the lower free troposphere (2-5 km). The latter reeects the episodic lifting of pollution over central and eastern China ahead of eastward-moving cold fronts. This frontal lifting, followed by westerly transport in the lower free troposphere, is the principal process responsible for export of both anthropogenic and biomass burning pollution from East Asia. For the period of PEM-West B (February-March), we estimate that fossil fuel combustion and biomass burning make comparable contributions to the budgets of CO, ozone, and NO x in the Asian outtow. We nd that 13% of NO x emitted in eastern Asia is exported as NO x or PAN, a smaller fraction than for the United States because of higher aerosol concentrations that promote heterogeneous conversion of NO x to HNO 3. Production and export of ozone from East Asia is much greater than from the United States because of the higher photochemical activity. Examination of intercontinental contributions to the Asian outtow in the model indicates that anthropogenic emissions from Europe and biomass burning emissions from Africa make major contributions. European sources dominate the outtow in the lower troposphere at high latitudes while African sources are important in the upper troposphere at low latitudes. 3