Tropospheric ozone assessment report: Missions, developments, and outlook

The Tropospheric Ozone Assessment Report (TOAR), an activity of the International Global Atmospheric Chemistry Project (IGAC), has become a major international framework for assessing tropospheric ozone as an air pollutant, greenhouse gas, and ecosystem stressor from the surface to the tropopause. This contribution summarizes the evolution, scientific achievements, and remaining challenges of TOAR-I and TOAR-II, and outlines priorities for a possible TOAR-III. TOAR-I established the foundation for global ozone assessment by compiling a harmonized surface ozone database and producing standardized metrics for evaluating ozone distributions, trends, and exposure risks relevant to human health, vegetation, and climate. TOAR-II has expanded this foundation through 16 Focus Working Groups and a broad Community Special Issue, with stronger integration of surface observations, satellites, ozonesondes, aircraft, reanalysis products, and models. Its scientific advances include improved characterization of ozone variability and trends, greater emphasis on attribution to emissions, meteorology, transport, and stratosphere-troposphere exchange, and more explicit assessment of climate, health, crop, and ecosystem impacts. Remaining challenges include sparse long-term observations in the Global South, tropics, oceans, polar regions, and rapidly changing megacities; limited vertical profiling and precursor measurements; uncertainties in ozone deposition, radical chemistry, wildfire influences, and hemispheric transport; and inconsistent methods for trend detection, source attribution, and model evaluation. Future TOAR activities should therefore strengthen observational capacity, harmonize multi-platform data products, improve process-level understanding and predictive capability, and better connect ozone exposure with health burdens, crop yield and nutritional quality, forest carbon sequestration, ecosystem responses, and climate forcing.