Third ESA Ozone CCI User Workshop

10 Nov 2026, 08:00 → 11 Nov 2026, 20:15 Europe/Brussels

Microsoft Teams

J. Moritz Menken (DLR), Marta Abalos (Universidad Complutense de Madrid), Daan Hubert (BIRA-IASB)

Background

The European Space Agency’s Climate Change Initiative (CCI) aims to realise the full potential of the long-term Earth Observation archives collected by ESA and Third Party satellite missions. Since 2010, the Ozone CCI team (https://climate.esa.int/en/projects/ozone) has been developing, maturing, generating and sustaining multiple complementary multi-decadal satellite ozone Climate Data Records (CDRs) tailored for climate monitoring, climate research and climate modelling applications. The CCI portfolio consists of total and tropospheric ozone column data products as well as vertically resolved ozone products by nadir and limb-viewing sensors.

Workshop objectives

This third user workshop will be an online event aimed at

• bringing together scientists involved in the generation of ozone Climate Data Records, as well as (potential) users of such multi-decadal ozone time series, and the broader ozone and climate communities;
• presenting the state of the art in ozone Climate Data Record production;
• discussing results from major Climate Data Record users, e.g., stratospheric ozone assessments for WMO/UNEP, tropospheric ozone assessments for TOAR, studies focusing on UT/LS, evaluation of climate modelling, data assimilation and reanalysis;
• collecting and updating user requirements for ozone Climate Data Records;
• discussing remaining challenges for the generation of ozone Climate Data Records.

Attend the workshop

An MS Teams meeting link will be circulated to all registered participants on Monday 9 November. If you want to attend the workshop, please register before 10:00 UTC Monday 9 November. We will unfortunately not be able to handle last minute access requests.

Tuesday 10 November

08:00 → 12:00 Tropospheric ozone

Convener: J. Moritz Menken (DLR)

08:00 Welcome & introduction

Speaker: J. Moritz Menken (DLR)

08:10 Overview of tropospheric ozone Climate Data Records by ESA CCI

The Ozone Essential Climate Variable has been an essential part of ESA’s Climate Change Initiative (CCI, https://climate.esa.int/en/projects/ozone). This initiative strives to develop, mature, generate, and sustain harmonised, consistent, and validated multi-decadal satellite ozone time series specifically for climate monitoring, climate modelling, and climate research applications.

We summarise the Climate Data Records currently being developed by the CCI team and tailored to study processes and monitor the state of ozone across the troposphere. Different measurement techniques are employed which result in CDRs that encode different information about tropospheric ozone.

Speaker: Daan Hubert (BIRA-IASB)

08:30 [Keynote] Tropospheric ozone reanalysis: satellite assimilation advances and multi-reanalysis evaluation

Satellite observations have provided long-term global pictures of tropospheric ozone, which is strongly linked to climate due to its influence on Earth’s radiative budget by absorbing infrared radiation. Satellite measurements also play essential roles in observing the global ozone precursor gases, such as nitrogen oxides (NOx) and carbon monoxide (CO). However, integrating the observational information on multiple species remains challenging in a consistent manner. To provide integrated information on tropospheric ozone and related species, we developed the Tropospheric Chemistry Reanalysis by assimilating multi-constituent satellite observations (OMI, SCIAMACHY and GOME-2 tropospheric NO2, MOPITT CO, TES ozone, MLS ozone, CO, HNO3, and OMI SO2 and HCHO, TROPOMI NO2, CO, SO2, and HCHO) for 2005-2024. The TCR products were used for several scientific applications. For example, the capability to track changes in tropospheric ozone from regional to global scales by the emission reductions during the COVID-19 lockdown was demonstrated (Miyazaki et al., 2021). We also assessed the impact of TROPOMI tropospheric NO2 on the performance of ozone analyses (Sekiya et al., 2022). To more broadly evaluate the current capability of tropospheric ozone reanalysis, we assess the regional and inter-annual tropospheric ozone variations and the impacts of assimilating individual satellite observations with multiple reanalysis products (Jones et al., 2026; Sekiya et al., 2025). These assessments highlight that tropospheric ozone reanalysis provide valuable information on variations in ozone in the free troposphere, while better exploitation of assimilated observations is required to enhance the utility of the surface ozone analyses.

Speaker: Takashi Sekiya (Japan Agency for Marine-Earth Science and Technology)

08:55 GEMS Satellite Ozone Evaluation Using Ozonesonde Measurements During ACCLIP Campaign

This study examines how well the Geostationary Environment Monitoring Spectrometer (GEMS) tracks day-to-day changes in O3 during the Asian summer monsoon. We used 62 daily ozonesonde measurements from Anmyeon, South Korea, during the pre-ACCLIP (2021) and ACCLIP (2022) campaigns. In this season, O3 from the surface to the lower stratosphere changes rapidly with the monsoon circulation, Rossby wave breaking, and storm activities. Compared with the ozonesonde observations, the latest GEMS product (v3) performs much better than the previous version (v2). It follows daily total O3 more closely, with the largest gain in the lower troposphere, where version 2 overestimated O3. Version 3 also captures events that version 2 missed: a high-O3 episode in 2021, when stratospheric air descended into the troposphere, and low-O3 episodes caused by a typhoon in 2022. The horizontal structure of O3 is also more realistic and less biased. These results highlight ozonesonde measurements as a useful cross-validation dataset for satellite O3 products, as they capture circulation-driven ozone variability.

Speaker: Joowan Kim (Kongju National University)

09:15 Height-resolved ozone products from the geostationary sounders: Sentinel-4, GEMS and TEMPO

The geostationary uv nadir sounder Sentinel-4 was successfully launched in July 2025, adding coverage of Europe to that of the existing geostationary sounders GEMS (over East Asia, Nov 2020-) and TEMPO (over North America, Aug 2023-). The three sensors form a constellation offering hourly height resolved ozone measurements, including sensitivity to tropospheric ozone. Within the ESA S4 L2 Operational Processor Development project, RAL has been responsible for developing the operational height-resolved ozone processor for Sentinel-4, building on the heritage of the scheme used in Ozone CCI for the polar orbiting uv nadir sounders (including GOME-2, OMI and S5P). More recently within Ozone CCI, the Sentinel-4 processor has been adapted and optimized to process also GEMS and TEMPO data, offering the prospect of consistent retrievals from all three sensors. In this presentation, we will summarize the current status of the geostationary ozone products from the RAL scheme applied to all three platforms, including comparison to global retrievals from S5P and CAMS model fields.

Speaker: Richard Siddans (NCEO / RAL)

09:35 [Discussion] Tropospheric data: User needs & requirements

Speakers: J. Moritz Menken (DLR), Marta Abalos (Universidad Complutense de Madrid)

10:05 Coffee & tea

10:25 TOAR II : using satellite observations to study past and present tropospheric ozone

Satellite observations enable global mapping and monitoring of ozone in the free troposphere. The first Tropospheric Ozone Assessment Report (TOAR-I) faced uncertainties in earlier records which were limited the first confidence in regional distributions and, especially, decadal trends. The second Tropospheric Ozone Assessment Report (TOAR-II) focusing on satellites leverages longer and improved satellite records, additional datasets and new harmonization approaches, and provides a more confident assessment of present-day and longer-term changes. We quantify the 2018–2022 climatology and seasonal cycle at 1°x1° resolution. The present-day global tropospheric ozone burden is 336  6 Tg and summertime maxima larger than 50 DU are observed over the Middle East. We evaluate trends in a combined tropospheric column record since 2005 with the difficulty of ozone changes mostly nonlinear. Increases of 2–4.5% per decade during 2005–2019 occur over 20% of the globe (high confidence), most notably across Asia, followed by widespread declines after 2020. Dynamical and episodic drivers, such as El Niño–Southern Oscillation, extreme wildfires, and the COVID-19 pandemic, strongly influenced variability, underscoring the sensitivity of tropospheric ozone to both natural and anthropogenic influences. A public release of the satellite data record used in the TOAR-II assessment dedicated to satellite observation is available for users.

Speaker: Gaëlle Dufour

10:45 TOAR II Climate and TOAR III

Speaker: Sophie Szopa (Laboratoire des Sciences du Climat et de l'Environnement (LSCE-IPSL), CEA, CNRS)

11:05 [Keynote] Current challenges in modelling tropospheric ozone

Our understanding of the processes that control ozone in the troposphere has undergone steady change since the topic was discovered in the mid-twentieth century. The role of VOCs, NOx and aerosols is now well understood at a process level, yet significant disagreement exists when model simulations are compared with observations. Model-model uncertainty remains large. In this talk I will review some of the history in the development of process understanding and highlight new processes that are missing from many models. I will also discuss how observations can be used to help improve confidence in the models we use.

Speaker: Alex Archibald (University of Cambridge)

11:30 Current Status and Evaluation of the TEMPO Ozone Profile (O3PROF) Product

The Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument provides hourly observations of atmospheric pollutants over North America from geostationary Earth orbit at nearly neighborhood-scale spatial resolution. This presentation focuses on evaluating and validating the operational Version 4 (V04) TEMPO ozone profile (O3PROF) product using independent ground-based and satellite observations.

TEMPO tropospheric column ozone and 0–2 km column ozone are evaluated against measurements from the Tropospheric Ozone Lidar Network (TOLNet) and ozonesondes, while stratospheric column ozone is compared with Microwave Limb Sounder (MLS) observations. The tropospheric and 0–2 km column ozone products generally meet the precision requirements and capture the observed variability. Although systematic positive biases are found in the tropospheric and 0–2 km columns relative to TOLNet and ozonesondes, and a negative bias is observed in stratospheric column ozone relative to MLS, all three ozone column quantities show good correlations with independent observations.

Finally, known limitations of the operational V04 O3PROF product and planned improvements for future versions are discussed.

Speaker: Junsung Park (Center for Astrophysics | Harvard and Smithsonian)

11:50 Concluding remarks

Speaker: J. Moritz Menken (DLR)

Wednesday 11 November

16:00 → 20:15 Stratospheric / total ozone

Convener: Marta Abalos (Universidad Complutense de Madrid)

16:00 Welcome & introduction

Speaker: Marta Abalos (Universidad Complutense de Madrid)

16:10 [Keynote] Polar Ozone in the 2026 WMO/UNEP Assessment

World Meteorological Organization / United Nations Environment Programme Scientific Assessments of Ozone Depletion (Ozone Assessments) are comprehensive reports that summarize the state of knowledge about stratospheric ozone and its recovery. Ozone Assessments are produced by the scientific community every four years as mandated by the Montreal Protocol of 1987.

In this talk I will give a brief overview of selected findings reported in the polar ozone chapter of the 2026 Ozone Assessment. I will focus on two topics: observed changes Antarctic and Arctic ozone to date, particularly in recent years, and future projections of ozone recovery. I will emphasize the importance of continuing space-borne observations of stratospheric ozone and related trace gases.

Speaker: Krzysztof Wargan (Global Modeling and Assimilation Office, NASA Goddard Space Flight Center)

16:35 The CMIP7 Ozone Forcing

In order to be able to attribute observed climate change to greenhouse gases and separate these from the effects of changes in ozone, it is of great importance to represent the spatial and temporal ozone changes in climate simulations, preferably in models with interactive chemistry that accurately reproduce the two-way chemistry-climate feedbacks and which resolve the troposphere and stratosphere at the same time. In fact, neglecting chemistry-climate feedbacks can lead to substantially different climate impacts (Dietmüller et al., 2013). However, since this is not always possible due to time and hardware limitations of running these computationally expensive chemistry-climate models for long-term climate simulations, a cheaper (although less accurate method) is to prescribe the ozone changes in the radiation schemes of the Earth system models used. We here provide an overview of the third-generation ozone forcing produced for CMIP7 model simulations in support of AR7, including merging methodology and philosophy as well as first evaluations of the database.

Speaker: Michaela I. Hegglin (FZ Julich, University of Reading)

16:55 Overview of stratospheric / total ozone Climate Data Records by ESA CCI

The Ozone Essential Climate Variable has been an essential part of ESA’s Climate Change Initiative (CCI, https://climate.esa.int/en/projects/ozone). This initiative strives to develop, mature, generate, and sustain harmonised, consistent, and validated multi-decadal multi-satellite ozone time series specifically for climate monitoring, climate modelling, and climate research applications.

We summarise the Climate Data Records currently being developed by the CCI team and tailored to study processes and monitor the state of ozone across the stratosphere. Different measurement techniques are employed which result in CDRs that encode different information about ozone.

Speaker: Daan Hubert (BIRA-IASB)

17:15 Stratospheric transport in Chemistry-Climate Models

The representation of stratospheric transport in Chemistry-Climate Models (CCMs) is key for accurately reproducing and projecting the evolution of the ozone layer. We evaluate stratospheric transport in CCMs that have participated in three model intercomparison initiatives (CCMVal-2, CCMI-1, and CCMI-2022) over the last ∼ 15 years using modern satellite datasets and reanalyses. Key long-standing model biases persist across generations, with some worsening in recent simulations. Transport remains overly fast in the models, with a global mean age of air young bias of ∼ 1 year for the CCMI-2022 median. In the springtime southern polar stratosphere, the final warming is delayed (∼ 3 weeks), downwelling is underestimated (∼ 25 %), and the depth of the ozone minimum is overestimated (∼ 10 DU) on average in the most recent models. Long-term trends in transport over 1980–1999 are consistent across model generations and highlight the crucial role of ozone depletion in contributing to accelerate the Brewer-Dobson circulation and delaying the southern polar vortex breakdown.

Speaker: Marta Abalos (Universidad Complutense de Madrid)

17:35 Role of Natural Halogen Chemistry on the Evolution of Global Stratospheric Ozone Depletion

The stratospheric ozone layer acts as Earth’s primary shield against harmful ultraviolet radiation. Anthropogenic emissions of chlorinated and brominated compounds caused substantial ozone depletion during the 20th century, including the formation of the Antarctic ozone hole. While the role played by these anthropogenic compounds has been well established, the contribution of naturally emitted halogens, particularly bromine and iodine, remains poorly quantified. Here, using chemistry-climate model simulations spanning 1910 to 2100, we show that natural halogen chemistry makes a major contribution to lower-stratospheric ozone loss. Between 1990 and 2020, when anthropogenic halogen loading reached its peak, natural halogens accounted for up to half of the total halogen-induced ozone loss in the extrapolar lower-stratosphere and substantially enhanced ozone destruction along the Antarctic periphery by ~35%. Their relative influence is projected to increase by the end of this century, exceeding 80% of total halogen-induced ozone loss in the extrapolar lower-stratosphere and delaying the recovery of the natural Antarctic seasonal ozone cycle by nearly five decades. These findings reveal that natural halogens play a critical but underrepresented role in shaping the past and future changes in the ozone layer and should be accurately represented in ozone projections.

Speaker: Julian Villamayor (Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Blas Cabrera, CSIC, Madrid, Spain)

17:55 Coffee & tea

18:15 [Discussion] Stratospheric data: User needs & requirements

Speakers: Marta Abalos (Universidad Complutense de Madrid), J. Moritz Menken (DLR)

18:45 [Keynote] What satellite measurements reveal about past, present, and future ozone changes

Since 1978, satellites have provided continuous, comprehensive daily global mapping of ozone, allowing us to observe the depletion and early stages of recovery of the stratospheric ozone layer. Since the early 1990s, they have also provided a comprehensive picture of trace gases involved in the transport and chemical destruction of ozone, allowing us to determine not only how ozone is changing, but why. These measurements have revealed not only the expected drawdown of halogens in response to the Montreal Protocol and its Amendments and Adjustments and the associated turnaround of ozone abundances, but also the impact of events such as volcanic eruptions and large wildfires that perturb both the transport and chemistry of the stratosphere. Satellite measurements also allow us to diagnose deficiencies in chemistry-climate models and narrow the range of uncertainty in the expected return of ozone to a benchmark value. In this presentation, I will review the role of satellite data in current and past Scientific Assessments of Ozone Depletion and discuss the future of satellite data and what it means for our understanding of ozone changes.

Speaker: Jessica Neu (Jet Propulsion Laboratory, California Institute of Technology)

19:10 A regional Bayesian composite of ground-based ozone partial columns for trend estimation.

The ability to identify significant trends in stratospheric ozone from individual ground-based records may be constrained by the significant uncertainties and heterogeneity of these records. Merging is likely to reduce the uncertainty of the combined data record, and thus of its trend estimates, by taking the most reliable information from all the included data records.
We present regional composites of homogenized ground-based NDACC and HEGIFTOM records (ozonesondes, FTIR, Dobson Umkehr, lidar and microwave radiometers) for 2000–2024. The monthly mean profiles are integrated into two sets of four partial columns, with uncertainties propagated from individual measurements. The ozone partial columns provide the common basis between instruments of different vertical resolution.
The definition of spatially coherent regions is based on CAMS reanalysis and the data records are merged with the BAyeSian Integrated and Consolidated (BASIC) algorithm (Ball et al, 2017), which combines a Gaussian-mixture likelihood, a climatological prior and PCA-based uncertainty scaling allowing for the down-weighting of outliers. BASIC therefore generates the most reliable dataset for the region. Trends are estimated by weighted multi-linear regression model with the LOTUS model. In comparison with the weighted mean, BASIC produces more representative uncertainties when inter-instrument consensus is low and reduces trend uncertainties by 9.4 % on average for Central Europe, Hawaii and Lauder stations data records. The composites show positive trends in the upper stratosphere, predominantly negative trends in the middle stratosphere, and non-significant trends in the lower stratosphere, qualitatively consistent with satellite-based estimates, with some discrepancies in the middle stratosphere.
The method could be extended to gridded satellite data, entering as additional members of each regional group on common partial column vertical resolution and regional boxes, their regional-mean sampling error would need to be considered in the uncertainty budget. The composite will then seek to combine the long-term stability of ground-based records with the spatial coverage of satellites.

Speaker: Eliane Maillard Barras (MeteoSwiss, Payerne, Switzerland)

19:30 Consistency Between Zonal Mean Stratospheric and Total Column Ozone Trends (2000-2024)

We present an updated assessment of stratospheric and total column ozone trends over the 2000-2024 period using six merged limb-profile datasets and six merged total ozone datasets. Long-term changes were quantified using a multiple linear regression (MLR) framework that accounts for dynamical and chemical variability. In addition to standard regressors (solar cycle, QBO, ENSO, stratospheric aerosol optical depth (SAOD)), we include Arctic and Antarctic Oscillation indices and the eddy heat flux in each hemisphere as proxies for dynamic variability. Volcanic (and wildfire) aerosol forcing is represented by separate proxies for three periods dominated by the major volcanic events of El Chichón, Pinatubo, and post-2000 volcanic eruptions, including Hunga-Tonga. These period-specific proxies are employed to better account for varying dynamical responses that largely depend on the season and location of the eruptions. All profile datasets consistently show positive trends in the upper stratosphere, with the strongest ozone recovery in southern mid-latitudes, in agreement with other studies. In the lower stratosphere, trends remain weak, spatially heterogeneous, and predominantly negative. A comparison of stratospheric column trends derived from profile data with total ozone trends shows close agreement across latitude bands. Within the trend uncertainties, total column trends since 2000 are largely driven by stratospheric ozone changes, while tropospheric contributions to zonal-mean total ozone trends (the difference between total and stratospheric column trends) appear negligible.

Speaker: Brian Auffarth (Universität Bremen)

19:50 Overview of Limb Ozone Climate Data Records from ESA Ozone_cci and Their Application to Ozone Trend Assessments

The ESA Ozone Climate Change Initiative (Ozone_cci) has developed a suite of long-term ozone climate data records derived from satellite limb observations, providing essential information for studies of ozone variability and trends. This presentation will provide an overview of the limb-based ozone datasets produced within the project, including the SAGE-CCI-OMPS+ and MEGRIDOP datasets, and other new developments. Particular attention will be given to the principles of their construction, which aimed at providing stable, homogeneous records suitable for climate applications.
The main focus of the presentation will be on the use of these datasets for the evaluation of long-term ozone changes and trends in the stratosphere. We will show the analyses of global, regional and seasonal ozone trends in the stratosphere. These analyses contributed to recent publications and WMO Ozone Assessment.

Speaker: Viktoria Sofieva (Finnish Meteorological Institute)

20:10 Concluding remarks

Speaker: Marta Abalos (Universidad Complutense de Madrid)