June 18th, 2026

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Key Findings

• Ozone air quality in the North China Plain worsened in the past decade despite precursor emission controls.
• Multiple environmental factors acted together to influence ozone variability and extreme events.
• Lower aerosol concentrations reduced removal of radicals and increased the amount of sunlight reaching the surface, promoting chemical reactions that form ozone.
• Vegetation changes during hot and dry weather conditions contributed to periods of elevated ozone.
• Increased biogenic emissions associated with afforestation, along with reduced ozone uptake by plants due to drought stress, worsen high-ozone events.

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Meiyun Lin, Larry W. Horowitz, Yuanyu Xie, Isabelle De Smedt. JGR Atmospheres. DOI: 10.1029/2025JD044764

Surface ozone is an important air pollutant that affects air quality and human health. It forms through chemical reactions involving sunlight, temperature, and precursor gases in the atmosphere. Interactions with atmospheric particles and vegetation can also influence how ozone forms and accumulates. This study examined changes in ozone extremes across the North China Plain between 2013 and 2023 using observations and a suite of GFDL model simulations. The researchers evaluated several factors that can influence ozone formation, including aerosol concentrations, meteorological conditions, and vegetation-related processes.

The results show that reductions in aerosol pollution reduced radical removal and allowed more sunlight to reach the surface, both of which enhanced photochemical reactions that produce ozone. Furthermore, vegetation changes during periods of hot and dry weather created conditions that favored higher ozone concentrations. Afforestation boosts emissions of natural ozone‐forming gases like isoprene, particularly during heat waves, while water‐stressed vegetation becomes less effective at removing ozone from the air.

Together, these factors influenced the frequency and intensity of high-ozone extremes observed in the region over the past decade. The analysis highlights how changes in air pollution, climate conditions, and vegetation processes can interact to affect surface ozone levels. The insights carry broader relevance for rapidly urbanizing and climate‐vulnerable regions, including parts of India, and large metropolitan areas in North America and Europe.

(A) Probability density distribution of daily maximum 8-hour average ozone (MDA8 O₃) in the North China Plain during May, June, and September from 2017 to 2023, based on observations and GFDL-AM4V simulations using Base and HighAero_HighNOx settings. (b) As in (a), but comparing model simulations using Base, Greening, and FIXDEPV settings. The FIXDEPV experiment is identical to Greening, except that dry deposition velocity of reactive gases is fixed at 2018 monthly values instead of being interactive. Statistics shown are the median (μ), the 95th percentile (q95), and the percentages of site-days with MDA8 O₃ below 50 ppbv (B50), above 100 ppbv (D100), and above 110 ppbv (D110).