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

Abstract

<p><strong>Abstract.</strong> Tropospheric ozone in East Asia is influenced by the transport of ozone from foreign regions around the world. However, the magnitudes and variations of such influences remain unclear. This study was performed to investigate this influence and its variations with space and time using a global chemical transport model, GEOS-Chem, for emission zero-out and tagged ozone simulations. The results show that foreign ozone varies significantly with latitude, altitude, and season in the East Asian troposphere. The transport of foreign ozone to East Asia occurs primarily through the middle and upper troposphere, where the concentration of foreign ozone (32–65 ppbv) in East Asia is 0.5–6 times higher than that of native ozone (11–18 ppbv) and has strong seasonality, being largest in spring and lowest in winter. Foreign ozone in East Asia increases rapidly with altitude. At the surface, the annual average foreign ozone concentration is ~ 22.2 ppbv, which is comparable to its native counterpart of ~ 20.4 ppbv. The annual mean concentration of anthropogenic ozone from foreign regions is ~ 4.7 ppbv at the East Asian surface, and half of it comes from North America (1.3 ppbv) and Europe (1.0 ppbv). The presence of foreign ozone at the East Asian surface is 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. In summer, the South Asian High facilitates the build-up of South Asian ozone in the East Asian upper troposphere and constrains North American, European, and African ozone to the regions north of 35° N. The interannual variations of foreign ozone at the East Asian surface have been found to be closely related to the EAM. When the EAWM is strong, North American and European ozone are enhanced at the East Asian surface, as the subsidence behind the East Asian trough becomes stronger. In strong EASM years, South and Southeast Asian ozone is reduced at the East Asian surface due to weakened south-westerly monsoon wind. This study suggests substantial foreign influences on tropospheric ozone in East Asia and underscores the importance of the EAM in the seasonal and interannual variations of foreign influences on surface ozone in East Asia.</p>