Atmospheric chemistry plays an essential role in the Earth system. Whether from the perspective of a changing radiation balance, the impact of pollutants on vegetation, the size distribution of cloud droplets or the effect of desert dust on ocean bio-geochemistry, understanding the composition and chemistry of the atmosphere is vital. The UK has a long history in atmospheric chemistry research, from the early days of sulphur smogs, through stratospheric ozone depletion to ongoing research on air quality and climate. The UK community is diverse and plays a central role in global efforts to advance understanding through laboratory, field and process modelling studies.

Much of this world-leading atmospheric chemistry research focuses on developing detailed understanding of the various processes controlling atmospheric composition. However, addressing many of society's greatest environmental problems now requires a wider 'Earth System' approach which considers the ensemble of processes occurring on the planet, from the biosphere, to the oceans, cryosphere and the atmosphere, rather than focusing on a single domain in isolation. Earth System Models represent our understanding of processes and interactions across these domains and creating such models is one of the grand challenges for science. Achieving an effective integration of state-of-the-science understanding derived from process studies into Earth System Models is a challenge. This network will encourage this integration in order to better develop the UK's capabilities in atmospheric chemistry and Earth System Modelling.

The network will achieve this exchange in two main ways. The first will be through a series of meetings, the second will be through the community undertaking two science projects.

The first activity will be a comprehensive series of meetings bring together the various processes focused groups with Earth System Modelling focused groups. These will include groups from universities, NERC Centres and Meteorological Services. These will include both large, multi-day inclusive meetings to bring together the whole community and smaller, more focused meetings. The three large meetings will discuss science topics and future directions. They will identify important science topics which will be followed up with smaller focused meetings (~12). Although the topics of the majority of these meetings will be chosen at the large meetings, three have been chosen in advance. One of these involves reactive carbon emissions from the biosphere and two involve the atmospheric chemistry schemes used within models of different complexities.

The network will also undertake two challenging science projects supported by two PDRA posts. These focus on developing observationally constrained metrics of two key processes for atmospheric chemistry: transport and deposition. Both of these processes need to be 'fit for purpose' for ESMs to be suitable for atmospheric chemistry studies. Groups across the country, including those running ESMs, will provide relevant model simulations. Observationally based metrics will be constructed and the models compared to assess their suitability.

Other activities will include workshops to allow process focused scientists to engage with the current generation of NERC/MO ESM; meetings to discuss technical/software/governance issues with ESMs; support for the network to engage with similar international efforts; and the creation of an Emerging Scientists network to foster the next generation of scientists.

Potential Impact:
The impact of the network outside the immediate academic community will occur through two mechanisms. Engagement with agencies such as the Met Office and the ECMWF (letters of support are enclosed) are central to the proposal. The Met Office especially forms a close partnership with the NERC in the development of future ESM capabilities. This network has representatives of the Met Office on its steering group and on one of the science projects. The Met Office has been overwhelmingly positive in its response to the network. Similarly the ECMWF has expressed significant interest in these activities as it links strongly with their MACC project. The costs for engagement with the Met Office and ECMWF have been included.

In this regard the impact of the network will be felt in future closer working relationships between the operational centres, NERC Centres and HEIs. This will in turn produce better models with enhanced scientific capabilities. Improved understanding of the science will lead to improved policy advice to governments. Although much of the atmospheric chemistry emphasis of an ESM is on climate and air quality / public health related issues, the development of increasingly complete representations of atmospheric chemistry will allow for enhanced research and policy advice on a range of topics such as food security, visibility reduction, ocean health, and ecosystem degradation.

Direct engagement with policy is also possible. DEFRA offer the most obvious overlap with the networks activities and are leading the effort here (see letter of support). DEFRA is in the process of appointing a 'NERC Policy Placement Fellow'. The remit of this person is to support the science to policy processes between Government Departments and NERC. They will spend ~30% of their time physically located with the Atmosphere and Local Environment Programme (ALE) offices in London. Once that person has been appointed they will be invited to attend the network meetings, engage with activities and suggest meeting topics. It is expected that at least one of the meetings organized by the network would aim at providing specific insights into how policy is made and how science can inform policy.

The network will engage widely with a variety of international efforts in the area of Earth System Modelling, via overarching programs such as HTAP, CCMVal, etc. These programs include important links to policymakers and closely engage with high-level policy-relevant organisations such as CLRTAP, WMO, and IPCC. The network includes several individuals with roles in these programs and organisations, and this will help ensure its work has impact.

Through these activities the network will provide a substantive connection between the science and policy communities.