Mechanisms that influence the formation of high‐ozone regions in the boundary layer downwind of the Asian continent in winter and spring

The seasonal variation of ozone (O 3 ) in the boundary layer (BL) over the western Pacific is investigated using a chemistry‐transport model. The model results for January and April–May 2002 were evaluated by comparison with PEACE aircraft observations. In January, strong northwesterlies efficiently transported NO x from the continent, leading to an O 3 increase of approximately 5–10 ppbv over a distance of about 3000 km. In April, southwesterlies dominated due to anticyclone development over the western Pacific. Along this flow, O 3 continued to be produced by NO x emitted from East Asia. This resulted in the formation of a high‐O 3 (> 50 ppbv) region extending along the coastal areas of East Asia. This seasonal change in O 3 was driven in part by a change in the net O 3 production rate due to increases in solar UV and H 2 O. Its exact response depended on the NO x values in the BL. The net O 3 production rate increased between winter and spring over the Asian continent and decreased over the remote western Pacific. Model simulations show that about 25% of the total O 3 (of 10–20 ppbv) increase over the coastal region of Northeast Asia was due to local production from NO x emissions from China, and the rest was due to changes in background levels as well as emissions from Korea, Japan, and east Siberia. Uplift of BL air over Asia, horizontal transport in the free troposphere, and subsidence were the principal mechanisms of transporting Asian O 3 to the central and eastern North Pacific.