Sensitivity of air parcel movements near abundant emission areas to the background ozone level in the subtropical northwestern Pacific

Pollutants emission out of East Asia has been increasing steadily in the last 20 years. However, background ozone measurements in the subtropical northwestern Pacific at Yonagunijima (24°28'N and longitude 123°01'E) and Lanyu (22°02eN,121°33'E) show no meaningful increasing or decreasing trend. Rather, a moderate increasing and a decreasing trend before and after 2002 can be distinguished. Statistical analysis of trajectories of air parcels arriving at Lanyu suggests that more than half of air parcels would pass by major emission sources in China, Korea and Japan. Cluster analysis indicates four major routes. Studying trajectories of each route, we note that those moving closer to land area tend to carry higher level of ozone to Lanyu. Through a numerical modeling experiment, we confirm the importance of upstream pollutants and distinguish their effects on the evolution of ozone level along the route. Further analysis suggests that the change of the occurring frequency of air-parcels moving closer to the land area affects the observed change of the ozone trend downstream.