Abstract. Tropospheric ozone in East Asia is influenced by the transport of ozone from
foreign regions around the world. However, the magnitudes and variations in
such influences remain unclear. This study was performed to investigate the
influences using a global chemical transport model, GEOS-Chem, through the
tagged ozone and emission perturbation simulations. The results show that
foreign ozone is transported to East Asia (20–60∘ N, 95–150∘ E) mainly through the middle and upper
troposphere. In East Asia, the influence of foreign ozone increases rapidly
with altitude. In the middle and upper troposphere, the regional mean
concentrations of foreign ozone range from 32 to 65 ppbv, being 0.8–4.8 times higher
than its native counterpart (11–18 ppbv). Annually, ∼60 %
of foreign ozone in the East Asian middle and upper troposphere comes from
North America (5–13 ppbv) and Europe (5–7 ppbv), as well as from foreign
oceanic regions (9–21 ppbv). Over the East Asian tropospheric columns,
foreign ozone appears most in spring when ozone concentrations in the
foreign regions are high and the westerlies are strong and least in summer
when the South Asian High blocks eastward foreign ozone from reaching East
Asia south of 35∘ N. At the East Asian surface, the annual mean of
foreign ozone concentrations is ∼22.2 ppbv, which is
comparable to its native counterpart of ∼20.4 ppbv. In the
meantime, the annual mean of anthropogenic ozone concentrations from foreign
regions is ∼4.7 ppbv, half of which comes from North
America (1.3 ppbv) and Europe (1.0 ppbv). Seasonally, foreign ozone
concentrations at the East Asian surface are highest in winter (27.1 ppbv)
and lowest in summer (16.5 ppbv). This strong seasonality is largely
modulated by the East Asian monsoon (EAM) via its influence on vertical
motion. The large-scale subsidence prevailing during the East Asian winter
monsoon (EAWM) favours the downdraft of foreign ozone to the surface, while
widespread convection in the East Asian summer monsoon (EASM) blocks such
transport. Interannually, the variation in foreign ozone at the East Asian
surface is found to be closely related to the intensity of the EAM.
Specifically, the stronger the EAWM is in a winter, the more ozone from
North America and Europe reaches the East Asian surface because of the
stronger subsidence behind the East Asian trough. In summer, ozone from
South and South-east Asia is reduced in strong EASM years due to weakened
south-westerly monsoon winds. This study suggests substantial foreign
influences on ozone at the East Asian surface and in its tropospheric
columns. It also underscores the importance of the EAM in the seasonal and
interannual variations in foreign influences on surface ozone in East Asia.
The Climatology and Interannual Variability of the East Asian Winter Monsoon in CMIP5 Models
