20 Mar 2024
 | 20 Mar 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Causes of growing middle-upper tropospheric ozone over the Northwest Pacific region

Xiaodan Ma, Jianping Huang, Michaela Hegglin, Patrick Joeckel, and Tianliang Zhao

Abstract. Long-term ozone (O3) changes in the middle to upper troposphere are critical to climate radiative forcing and tropospheric O3 pollution. Yet, these changes remain poorly quantified through observations in East Asia. Concerns also persist regarding the data quality of the ozonesondes available at the World Ozone and Ultraviolet Data Center (WOUDC) for this region. This study aims to address these gaps by analyzing O3 soundings at four sites along the northwestern Pacific coastal region over the past three decades, and assessing their consistency with an atmospheric chemistry-climate model simulation. Utilizing the European Centre for Medium-Range Weather Forecasts (ECMWF) – Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) nudged simulations, it is demonstrated that trends between model and ozonesonde measurements are overall consistent, thereby gaining confidence in the model’s ability to simulate ozone trends and confirming the utility of potentially imperfect observational data. A notable increase in O3 mixing ratio around 0.29–0.82 ppb a-1 extending from the middle to upper troposphere is observed in both observations and model simulations between 1990 and 2020, primarily during spring and summer. The timing of these O3 hotspots is delayed when moving from south to north along the measurement sites, transitioning from late spring to summer. Investigation into the drivers of these trends using tagged model tracers reveals that ozone of stratospheric origin (O3S) dominates the absolute O3 mixing ratios over the middle-to-upper troposphere in the subtropics, contributing to the observed O3 increases by up to 96 % (40 %) during winter (summer), whereas ozone of tropospheric origin (O3T) governs the absolute value throughout the tropical troposphere and contributes generally much more than 60 % to the positive O3 changes, especially during summer and autumn. During winter and spring, a decrease of O3S is partly counterbalanced by an increase of O3T in the tropical troposphere. This study highlights that the enhanced downward transport of stratospheric O3 into the troposphere in the subtropics and a surge of tropospheric source O3 in the tropics are the two key factors driving the enhancement of O3 in the middle-upper troposphere along the Northwest Pacific region.

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Xiaodan Ma, Jianping Huang, Michaela Hegglin, Patrick Joeckel, and Tianliang Zhao

Status: open (extended)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2411', Anonymous Referee #1, 25 Apr 2024 reply
  • CC1: 'Comment on egusphere-2023-2411', Kaihui Zhao, 13 May 2024 reply
Xiaodan Ma, Jianping Huang, Michaela Hegglin, Patrick Joeckel, and Tianliang Zhao
Xiaodan Ma, Jianping Huang, Michaela Hegglin, Patrick Joeckel, and Tianliang Zhao


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Short summary
Our study examines 30 years of tropospheric ozone changes in the Northwest Pacific region. We found a significant increase in ozone levels during spring and summer in the middle-upper troposphere. This change is driven by a complex interplay between stratospheric and tropospheric ozone, with implications for climate and air quality in East Asia. Further research into these mechanisms is needed.