Large and increasing stratospheric contribution to tropospheric ozone over East Asia

Severe surface ozone pollution in South Korea and China in May–June is due in part to an elevated background subsiding from the free troposphere (750–350 hPa). Using IAGOS commercial aircraft observations and the GEOS-Chem model, we show that free tropospheric ozone over East Asia in May–June is the highest in the world and has increased from 68 ± 3 ppb (mean and interannual standard deviation) in 2000–2004 to 78 ± 4 ppb in 2015–2019. Free tropospheric ozone over East Asia is highest when carbon monoxide (CO) is low, both in the observations and GEOS-Chem, implying a large stratospheric influence on ozone. We find from GEOS-Chem that East Asia is a global hotspot for stratospheric downwelling of ozone and that this makes a major contribution to the free tropospheric ozone over the region in May–June. Stratospheric downwelling of ozone over East Asia in GEOS-Chem increased by 40 % from 2000–2004 to 2015–2019, which can explain the observed free tropospheric ozone increase over this period. Increased stratospheric downwelling over East Asia appears to be driven by a strengthening of the jet stream. The large and increasing stratospheric contribution to the surface ozone background over East Asia is a major impediment to meeting ozone air quality standards.