| Origins and pathways for the Asian outflow of ozone (O3) and carbon monoxide (CO) to the Pacific are examined using observations from the Pacific Exploratory Mission phase A (PEM-West A, 1991), phase B (PEM-West B, 1994), and Transport and Chemical Evolution over the Pacific (TRACE-P, 2001) and from the ozonesonde soundings along the Asian coast. The analysis uses a global three-dimensional chemical tracer model driven by assimilated meteorological observations.; A wet deposition scheme for use in the model is developed and tested by simulating the aerosol tracers 210Pb and 7Be, with the goal of analyzing the constraints offered by 210Pb-7Be-O3 relationships on the sources of tropospheric O3 over the western Pacific and the role of Asian outflow. It is found that the combination of 210 Pb and 7Be provides a sensitive test of wet deposition and vertical transport in the model. Aircraft observations of Asian outflow along the Asian coast show strong 210Pb-O3 correlations in Sep–Oct but such correlations are only seen at low latitudes in Feb-Mar. Observations further downwind over the Pacific show stronger 210Pb-O 3 correlations in Feb–Mar than Sep–Oct. The model reproduces these results and attributes the seasonal contrast to strong O3 production and vertical mixing in Sep–Oct, seasonal shift of convection from China in Sep–Oct to Southeast Asia in Feb–Mar, and slow but sustained net O3 production in Asian outflow over the western Pacific in Feb–Mar. The model reproduces the observed absence of 7Be-O 3 correlations over the western Pacific during Sep–Oct, implying strong convective and weak stratospheric influence on O3. Overall the observed 210Pb-7Be-O3 relationships provide an important constraint on model simulations of tropospheric O 3.; The sources contributing to tropospheric O3 over the Asian Pacific Rim in different seasons are quantified. In the middle/upper troposphere (MT/UT), maximum Asian pollution influence occurs in summer. In the lower troposphere (LT), the season of maximum Asian pollution influence shifts to summer at midlatitudes from fall at low latitudes. Asian biomass burning makes a major contribution to ozone at <32°N in spring. Maximum European pollution influence (<5ppbv) occurs in spring in the LT, and North American pollution influence exceeds European influence in the UT/MT. Lightning influence over the Pacific Rim is minimum in summer due to westward UT transport at low latitudes associated with the Tibetan anticyclone. The Asian outflow flux of ozone to the Pacific is maximum in spring and fall, and includes a major contribution from Asian anthropogenic sources year-round.; The major processes driving Asian CO-pollution outflow during TRACE-P (spring) are frontal lifting ahead of southeastward-moving cold fronts and transport in the boundary layer behind the cold fronts, and convective transport. Outflow of seasonal biomass burning in Southeast Asia during spring takes place mostly by deep convection but also by northeastward transport and frontal lifting, mixing with the anthropogenic outflow. Boundary layer outflow over the western Pacific is largely devoid of biomass burning influence. European and African plumes in Asian outflow during TRACE-P were weak. Interannual variability of diagnosed transport pathways appears to be related to El Nino-Southern Oscillation. The Asian outflow flux of CO to the Pacific varies seasonally by a factor of 3–4 (maximum in March and minimum in summer). |
