EGUsphere EGUsphere EGUsphere EGUsphere Copernicus Publications Göttingen, Germany 10.5194/egusphere-2025-1799 Large and increasing stratospheric contribution to tropospheric ozone over East Asia Colombi Nadia K. https://orcid.org/0009-0004-5052-0095 1 Jacob Daniel J. 1 2 Ye Xingpei https://orcid.org/0000-0002-7899-5031 3 Yantosca Robert M. https://orcid.org/0000-0003-3781-1870 2 Bates Kelvin H. https://orcid.org/0000-0001-7544-9580 4 Pendergrass Drew C. https://orcid.org/0000-0002-8210-4983 2 Yang Laura Hyesung https://orcid.org/0000-0002-0057-7120 2 Li Ke https://orcid.org/0000-0002-9181-3562 5 Liao Hong 5 Harvard University, Department of Earth and Planetary Science, Cambridge, MA 02138, USA Harvard University, John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA 02138, USA Peking University, Department of Atmospheric and Oceanic Science, Beijing 100084, China University of Colorado, Department of Mechanical Engineering, Boulder, CO 80309, USA Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China 07 05 2025 2025 1 17 Copyright: © 2025 Nadia K. Colombi et al. 2025 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1799/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1799/egusphere-2025-1799.pdf

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.

 Nanjing University of Information Science and Technology n/a