2026 Annual Meeting of the Task Force on Hemispheric Transport of Air Pollution
April 8-10
Hamburg, Germany and online
The annual meeting of TF-HTAP, a task force under the UNECE Convention on Long-Range Transboundary Air Pollution, was hosted by the Helmholtz Centre Hereon and held at the Bucerius Law School in Hamburg, Germany. Approximately 20 in-person participants and 120 online participants met over two and a half days from April 8 to10 2026, to discuss progress in the four current HTAP activities. MCHgMAP; PFAS; OPNS; and Fires.
Session 1: MCHgMAP
Chair: Oleg Travnikow, Jožef Stefan Institute, Slovenia.
The mercury research component of the current TF HTAP activities relies on the ongoing Multi-Compartment Mercury Modelling and Analysis Project (MCHgMAP). MCHgMAP is an international scientific initiative developed under TF HTAP to inform the effectiveness evaluation of both the Air Convention and the Minamata Convention on Mercury through a multi-model assessment of spatial and temporal trends in mercury pollution. A distinctive feature of the project is the involvement of different types of models – atmospheric, oceanic, terrestrial, and mass balance – for multi-compartment simulation of Hg cycling in the environment.
The MCHgMAP session focused on the discussion of results from the first phase of the project, which included compiling a variety of emissions and monitoring data and conducting a series of multi-model experiments. These experiments were aimed at evaluating changes in Hg concentrations and deposition levels in different regions, as well as attributing these changes to anthropogenic and natural drivers, using both modelling results and observations.
Presentations and discussions in this session included a general overview of the project setup and the progress achieved, followed by an analysis of simulated spatial patterns and seasonal variations in Hg levels on a global scale. Further discussions addressed temporal trends in atmospheric Hg concentrations and deposition, analysing environmental and anthropogenic drivers of long-term changes. It was shown that, in the base case, the model ensemble is unable to reproduce the observed decline in Hg concentrations in the Northern Hemisphere, which was largely attributed to inconsistencies in available inventories of primary and secondary Hg emissions. Mercury trends in the Southern Hemisphere were uncertain and influenced by emissions from artisanal and small-scale gold mining.
Discussions of Hg distribution and trends in the global ocean included a detailed analysis of both observational data and modelling results, revealing regions of high concentrations and an absence of significant long-term changes. In addition, the impact of past and future emissions on the global mercury cycle was examined based on mass balance simulations. Finally, key findings reported to the effectiveness evaluation of the Minamata Convention were presented, along with proposals for future work of the project within the TF HTAP activities and of relevance to the Convention.
Session 2: PFAS
Chair: Johannes Bieser, Helmholtz Centre Hereon, Germany
The PFAS session focused on two main objectives: updating the community on project progress and planning the next steps. Writing teams established during previous meetings presented the progress of their respective chapters.
The Observations group provided an overview of available data across various environmental compartments and presented global transport patterns as well as temporal trends across different matrices.
The Models group delivered a state-of-the-art overview of atmospheric PFAS modelling and the processes currently implemented. The presented models range from local-scale models with complex chemistry to global-scale models with simplified or no chemical mechanisms.
The Emissions group presented the currently available emission inventories and datasets. Several inventories estimating PFAS production and release were discussed. For modelling purposes, the gridded emission inventory POPE was introduced as a ready-to-use dataset. An updated version of the PFAS emission inventory will be made available following the meeting.
The second part of the meeting focused on discussing the next steps. The HTAP PFAS group will continue its work on the white paper/report. The groups that presented will proceed with finalising their respective chapters.
The next step will be to establish writing teams for the Model Scenarios and Policy Interaction chapters. These chapters will aim to identify the needs of policymakers and define relevant and impactful model scenarios, both for policy engagement and scientific research.
An extended group discussion, held both in person and online, was conducted to gather ideas for these chapters. The outcomes of this discussion will be summarised and presented at the next HTAP PFAS group meeting, which will be held online.
Session 3: HTAP3-OPNS
Chair: Tim Butler, GFZ Helmholtz Centre for Geosciences, Germany
The session opened with a recap of the motivation and goals of the HTAP3-OPNS exercise: multi-model simulations of long-range air pollution transport (Ozone, Particles, and Nitrogen and Sulphur deposition) using future scenarios from the GAINS integrated assessment model, in support of the revision of the CLRTAP Gothenburg Protocol. The revision is currently expected to be concluded by the end of 2028, implying that a scientific assessment of future air quality and source–receptor relationships should be delivered by mid-2027 to inform policy negotiations.
The exercise has been running since Q4 2025, and many groups have already presented preliminary results in previous online meetings or uploaded partial results to the AeroCom server.
Four groups presented updates in this session. All have focused on the four main global scenario runs (CLE2015, CLE2040, MTFR2040, and HILO2040), along with a range of global perturbation runs. The results presented were broadly consistent across models.
Several groups also plan to perform additional regional perturbation runs with both global and regional models. A further presentation outlined plans to develop Gaussian Process emulators based on the full set of perturbation runs, with encouragement for all groups to submit as many such runs as possible.
A brief update was given on the availability of boundary condition data and technical guidance for regional model simulations.
It was noted that none of the submitted datasets fully follow the prescribed file-naming convention; all groups are encouraged to consult the technical guidance for data submission.
Discussion also highlighted the large scope of the data request and the need for improved coordination of planned analyses, particularly model evaluation. The HTAP leadership team acknowledged these points and will provide further guidance on the data request and support the formation of analysis groups.
Session 4: Fires
Chair: Cyndi Whaley, Environment and Climate Change Canada
Presentations included an overview of the HTAP Fires project, timeline, and progress; as well as 3 presentations on preliminary analysis of HTAP Fires data, or related data. Those analysis presentations included intercomparison of biomass burning emission datasets, data assimilation experiments, source attribution, regional analyses, and case studies for North America and the Arctic. There were also 2 presentations related to the biomass burning emissions for the HTAP Fires project; historical and future, including regional analyses, and plans to release a 2nd future scenario in May.
There were 2 presentations related to the toxics’ part of HTAP Fires, including a presentation on the analysis of two anthropogenic PCDD/Fs emission inventories, and another on linking to the Arctic Monitoring and Assessment PAC research, which emphasized flat trends of these toxic compounds in the Arctic despite decreasing trends in global industrial emissions, although some Arctic sites may be impacted by the proximity of increasing natural emissions (e.g., wildfires).
Finally, the HTAP Fires session concluded with discussions. There were no additional suggestions on toxics observations that could be used for model evaluation. However, the discussion around coordination of analysis of HTAP Fires model results was productive with about 5 more people indicating their interest in contributing to the analysis (added to ~14 names that were already gathered).
A highlight of the discussion was regarding bolstering analysis coordination with the HTAP OPNS analysis team, given the consistency between the two projects – particularly as the baseline and future transient simulations for both projects are the same.
Finally, we decided that some refinement to the recommended future emissions is needed for HTAP Fires and OPNS because some participants want to extend the future scenarios to 2100 (from 2050). It was decided that an analysis comparing CMIP6, CMIP7, and IIASA GAINS future anthropogenic and agricultural burning emissions is needed before that recommendation can be finalised.