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Simultaneous ground-based measurements of ozone (O<sub>3</sub>) and carbon monoxide (CO) were conducted in March 2005 as part of the East Asian Regional Experiment (EAREX) 2005 under the umbrella of the Atmospheric Brown Clouds (ABC) project. Multiple air quality monitoring networks were integrated by performing intercomparison of individual calibration standards and measurement techniques to ensure comparability of ambient measurements, along with providing consistently high time-resolution measurements of O<sub>3</sub> and CO at the surface sites in East Asia. Ambient data collected from eight surface stations were compared with simulation results obtained by a regional chemistry transport model to infer recent changes in CO emissions from East Asia. Our inverse estimates of the CO emissions from China up to 2005 suggested an increase of 16% since 2001, in good agreement with the recent MOPITT satellite observations and the bottom-up estimates up to 2006. The O<sub>3</sub> enhancement relative to CO in continental pollution plumes traversed in the boundary layer were examined as a function of transport time from the Asian continent to the western Pacific Ocean. The observed ΔO<sub>3</sub>/ΔCO ratios show increasing tendency during eastward transport events due likely to en-route photochemical O<sub>3</sub> formation, suggesting that East Asia is an important O<sub>3</sub> source region during spring.