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. Tropospheric ozone in East Asia is influenced by the transport of ozone fromforeign regions around the world. However, the magnitudes and variations insuch influences remain unclear. This study was performed to investigate theinfluences using a global chemical transport model, GEOS-Chem, through thetagged ozone and emission perturbation simulations. The results show thatforeign ozone is transported to East Asia (20–60∘ N, 95–150∘ E) mainly through the middle and uppertroposphere. In East Asia, the influence of foreign ozone increases rapidlywith altitude. In the middle and upper troposphere, the regional meanconcentrations of foreign ozone range from 32 to 65 ppbv, being 0.8–4.8 times higherthan its native counterpart (11–18 ppbv). Annually, ∼60 %of foreign ozone in the East Asian middle and upper troposphere comes fromNorth America (5–13 ppbv) and Europe (5–7 ppbv), as well as from foreignoceanic regions (9–21 ppbv). Over the East Asian tropospheric columns,foreign ozone appears most in spring when ozone concentrations in theforeign regions are high and the westerlies are strong and least in summerwhen the South Asian High blocks eastward foreign ozone from reaching EastAsia south of 35∘ N. At the East Asian surface, the annual mean offoreign ozone concentrations is ∼22.2 ppbv, which iscomparable to its native counterpart of ∼20.4 ppbv. In themeantime, the annual mean of anthropogenic ozone concentrations from foreignregions is ∼4.7 ppbv, half of which comes from NorthAmerica (1.3 ppbv) and Europe (1.0 ppbv). Seasonally, foreign ozoneconcentrations at the East Asian surface are highest in winter (27.1 ppbv)and lowest in summer (16.5 ppbv). This strong seasonality is largelymodulated by the East Asian monsoon (EAM) via its influence on verticalmotion. The large-scale subsidence prevailing during the East Asian wintermonsoon (EAWM) favours the downdraft of foreign ozone to the surface, whilewidespread convection in the East Asian summer monsoon (EASM) blocks suchtransport. Interannually, the variation in foreign ozone at the East Asiansurface is found to be closely related to the intensity of the EAM.Specifically, the stronger the EAWM is in a winter, the more ozone fromNorth America and Europe reaches the East Asian surface because of thestronger subsidence behind the East Asian trough. In summer, ozone fromSouth and South-east Asia is reduced in strong EASM years due to weakenedsouth-westerly monsoon winds. This study suggests substantial foreigninfluences on ozone at the East Asian surface and in its troposphericcolumns. It also underscores the importance of the EAM in the seasonal andinterannual variations in foreign influences on surface ozone in East Asia.

Foreign influences on tropospheric ozone over East Asia through global atmospheric transport