New TOAR Theme Issue provides a comprehensive assessment of global tropospheric ozone

A new Theme Issue in Philosophical Transactions A brings together the main scientific outcomes of the Tropospheric Ozone Assessment Report Phase II (TOAR-II), providing an updated picture of how ozone is distributed around the world, how it has changed over time, and how it affects people, ecosystems, and the climate.

For the Earth System Data Exploration (ESDE) research group at the Jülich Supercomputing Centre (JSC), this publication marks an important milestone after several years of work supporting the TOAR initiative. The ESDE group has initiated and led the development and operation of the data infrastructure that supported the assessment, helping to create an easy entry point to observations and make the tools needed for global tropospheric ozone research accessible to the scientific community.

Understanding tropospheric ozone

Tropospheric ozone is found in the lowest part of Earth’s atmosphere. Unlike the protective ozone layer in the stratosphere, ozone near the Earth’s surface is considered an air pollutant and greenhouse gas. This pollutant can harm human health and vegetation, and also plays an important role in the climate system.

TOAR is an international scientific activity initiated by the International Global Atmospheric Chemistry (IGAC) project. Its goal is to provide an up-to-date assessment of the global distribution and trends of tropospheric ozone, from the Earth’s surface to the tropopause, and to make the underlying data accessible to the research community. More than 200 experts from over 35 countries have contributed to this activity. TOAR receives financial and logistical support from IGAC, Forschungszentrum Jülich, the US National Oceanic and Atmospheric Administration (NOAA), and the World Meteorological Organization (WMO).

The first phase of the assessment, TOAR-I (2014–2019), created the first comprehensive global assessment of tropospheric ozone, which brought together observations and analyses from around the world to improve our understanding of ozone’s distribution in the atmosphere and its impact on climate, health and vegetation.

Building on this foundation, TOAR-II expanded the available information, while also extending the geographical and scientific scope of the assessment. The project generated a Community Special Issue of 58 peer-reviewed publications, bringing together contributions from the international TOAR-II research community on a wide range of topics related to tropospheric ozone.

ESDE and JSC: supporting global tropospheric ozone research

The JSC, and in particular the ESDE research group, has played a central role in developing and maintaining the data infrastructure supporting TOAR.

At JSC, the TOAR Data Centre provides information and access to global air quality observations used by the International Tropospheric Ozone Assessment Report initiative. It brings together harmonised and verified surface ozone observations and related data, and provides a range of services and tools for exploring and working with the data, including an interactive dashboard, chatbot, data services and APIs, and software for integrating TOAR data into research workflows.

The TOAR Data Portal provides a central access point to a wide range of ozone-related datasets and their descriptions, including surface, satellite, aircraft, ozone sonde, and LiDAR observations, as well as model data.

These resources provide an important foundation for analysing changes in ozone and understanding its effects on health, vegetation and climate. They also demonstrate the role of data infrastructure in enabling large-scale international scientific assessments such as TOAR.

Furthermore, Martin Schultz, the ESDE group leader, served for over 10 years as co-chair of TOAR-I and TOAR-II, helping shape the initiative and coordinate the work and publications arising from TOAR.

Eight papers bring together the TOAR-II assessment

The new Theme Issue was assembled by eight different publications. The central TOAR-II Overview paper by H. M. Worden et. al. provides a comprehensive assessment of the global distribution, burden, and trends of tropospheric ozone, building on the scientific foundation established during TOAR-I.

Seven additional publications take a closer look at specific aspects of the global ozone landscape:

• Changing Health Impacts of Global Ozone Exposure by Gaige Hunter Kerr, et. al.: characterizes health impacts attributable to long-term surface-level ozone exposure.
• Analysis of global ozone distribution and trends to assess vegetation impacts by Emberson, L.D., et. al.: explores the risks that surface ozone poses to vegetation, including crops such as wheat, rice, and soybean, as well as 3 different forest species.
• Ozone distribution and variability in the oceanic and polar regions by Roberto Sommariva, et. al.: reports the most comprehensive dataset yet on lower atmosphere ozone concentrations over open oceans and polar regions.
• South America regional analysis by Rodrigo J. Seguel, et. al.: presents the first regional assessment of tropospheric ozone over South America for the early twenty-first century.
• Past and present tropospheric ozone using satellite observations by Daan Hubert, et. al.: leverages longer and improved satellite records, additional datasets, and new harmonisation approaches to provide a second, more confident, TOAR assessment of present-day and long-term changes.
• Ozone interactions with climate change: drivers and long-term trends by Sophie Szopa, et. al.: assesses the current state of knowledge regarding ozone’s important role as a short-lived climate forcer, together with the influence that climate change has on global tropospheric ozone.
• Stratosphere-Troposphere Exchange of Ozone by James Keeble, et. al.: synthesises recent advances in the understanding of magnitude, mechanism, variability, and future evolution of ozone transport across the tropopause.

Together, these different assessments give a broad overview of tropospheric ozone–from its distribution and atmospheric transport to its impact on human health, ecosystems and climate change.

A foundation for future ozone research

The TOAR-II assessment represents the culmination of several years of international collaboration and builds on the first global assessment completed in 2019 (TOAR-I). Its results, together with the data and tools developed through TOAR, offer a valuable resource for scientists investigating air quality and atmospheric change, while also improving our understanding of the role of tropospheric ozone in a changing world.

However, the work is not yet done and will continue with TOAR-III, the next phase of the international assessment.

Forschungszentrum Jülich will continue to maintain and develop the TOAR data infrastructure, ensuring that the growing body of ozone observations remains accessible to the scientific community and supports TOAR-III through the involvement of Anke Nölscher from the Institute of Climate and Energy Systems – Troposphere (ICE-3), who is acting as a new steering committee member. Martin Schultz from the Earth System Data Exploration – Jülich Supercomputing Centre (ESDE–JSC), Germany and Helen Worden from the National Center for Atmospheric Research (NCAR), U.S., have stepped down as TOAR co-chairs and handed over the responsibilities to Rodrigo Seguel from the Pontificia Universidad Católica (PUC), Chile, and Sophie Szopa from the Laboratoire des Sciences du Climat et de l’Environnement – Institut Pierre-Simon Laplace (LSCE-IPSL), France.

Read more about the TOAR-II assessment and the data infrastructure supporting it on the TOAR Data website.