Tropospheric Ozone Assessment Report II: Overview

Abstract The International Global Atmospheric Chemistry (IGAC) Tropospheric Ozone Assessment Report Phase II (TOAR-II) provides an updated, comprehensive assessment of the global distribution, burden and trends of tropospheric ozone, building on the foundation established by TOAR-I, 2014–2019. Accurate knowledge of tropospheric ozone concentrations remains essential for understanding atmospheric chemistry with far reaching implications for climate radiative forcing, vegetation damage and human health. Here, we present the process and summarize the main conclusions of TOAR-II, 2020–2026. Following TOAR-I, TOAR-II provides updates to trends in ozone concentrations and other metrics and addresses trends in ozone precursor emissions. This was accomplished as a community-driven process by forming Focus Working Groups to continue and expand on TOAR-I topics. Focus Working Group findings were published in a TOAR-II Community Special Issue with 58 review and original research papers. From this Community Special Issue, as well as other relevant literature, seven assessment papers were synthesized and submitted to this issue, along with this overview paper. These papers update the primary themes of TOAR-I: health, vegetation and climate; address unresolved uncertainties and inconsistencies in TOAR-I satellite ozone observations; provide an overdue update on stratosphere–troposphere exchange (STE) and present critical regional assessments for ozone over oceanic and polar regions, and South America. The TOAR-II assessment would not be possible without an extensive data collection and data harmonization effort. TOAR-II data are curated under findable, accessible, interoperable and reusable principles, and the TOAR data repository has been certified by the Core Trust Seal. In addition, TOAR-II developed common statistical analysis guidelines for the assessment papers. Although TOAR-II provides an extensive assessment of the current state of tropospheric ozone, many gaps and scientific challenges remain for future assessments. In particular, there is under-sampling of ozone across Southeast Asia and the Southern Hemisphere, especially Africa and Australia/Oceania. Low-cost sensor data and geostationary satellite data for tropospheric ozone, which still require evaluation, should be included. Ozone produced from wildfires and exacerbated by extreme weather events, and attribution of changes in ozone to pollution regulations on precursor emissions require greater attention in future analyses. These topics will be prioritized in the IGAC TOAR-III effort along with incorporating advances in artificial intelligence (AI) and strengthening collaboration with policy-relevant initiatives. This article is part of the theme issue ‘Tropospheric Ozone Assessment Report II’.