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

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.