Similarly to the ozone_CCI, there are multiple links across the interface between SPIN and the ESA aerosol_CCI and stratospheric activities within these two projects are highly complementary. Both projects bring together the relevant research groups developing aerosol products from ongoing satellite experiments. The high level of expertise made available through these instruments specific research groups is essential in light of the ever increasing complexity of the retrieval techniques. Beyond the need to adequately define their respective work plans, the coexistence of both projects in the slightly different frameworks of the aerosol_CCI and SPIN offers an exciting opportunity to share skill, expertise, and information. Cross-fertilization between these projects is expected to lead to a better understanding of the specific issues related to the different sensors, and to an overall improvement in the quality and reliability of the aerosol products to be delivered to both projects. To foster the links between the aerosol_CCI and aerosol activities in SPIN, and to suitably coordinate all related activities and thus avoid duplication of work, it has been decided to create a stratospheric aerosol working group comprising all aerosol_CCI and SPIN aerosol partners. The working group will constitute a strong and formal link between both projects, with a view to:

• exchange information concerning the advances in the different work packages of interest for the working group and the scientific issues in both projects
• collaborate on topics related to their own domain of expertise
• report on the activities of the working group and on the status of the partner project in progress meetings of their own project

The working group will work on a best effort basis, without additional deliverables to be provided other than the reporting activities specified above. The working group will communicate using the information channels that are the most efficient for the purpose at a given time (work meetings, teleconferences, mail ex-changes, etc.). The organization of work meetings will be foreseen to minimize travels. The principle of the creation of such a formal link between SPIN and aerosol_CCI has already been presented to the aerosol_CCI consortium and approved during its fourth Progress Meeting. According to the objectives defined above, a new task has been assigned to the partners in charge of the stratospheric issues to report on activities of the working group after each common meeting.