The ACITES network was funded by the Natural Environment Research Council (NERC) under the grant references - NE/K001280/1, NE/K001272/1 and NE/K001329/1. These grants were led by Professor Mathew Evans (University of York), Dr Oliver Wild (Lancaster University) and Professor David Stevenson (University of Edinburgh).

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 encouraged this integration in order to better develop the UK's capabilities in atmospheric chemistry and Earth System Modelling.

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

The first activity comprised of a comprehensive series of meetings that brought together the various processes focused groups with Earth System Modelling focused groups. These included groups from universities, NERC Centres and Meteorological Services. Both large, multi-day inclusive meetings to bring together the whole community and smaller, more focused meetings were undertaken. The three large meetings discussed science topics and future directions. They identified important science topics which were then followed up with smaller focused meetings (~12). Three topics were chosen in advance of the meetings; One involved reactive carbon emissions from the biosphere and two involved the atmospheric chemistry schemes used within models of different complexities.

The network also undertook two challenging science projects supported by two PDRA posts. These focussed 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, provided relevant model simulations. Observationally based metrics were constructed and the models compared to assess their suitability.

Other activities included 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.

The primary objective of the network was to enhance the UK's capability in modelling atmospheric chemistry within an Earth System perspective. This was achieved mainly through a series of workshops and meetings, and through the community working together on two challenging atmospheric chemistry focused projects.

The aims of the meetings were to identify and prioritise potential scientific projects, questions and developments in atmospheric chemistry for Earth System Modelling; to investigate synergies and intellectual pull-through across different scales and approaches to modelling; to identify links between atmospheric chemistry and other aspects of the Earth System; to bring together players from HEIs, NERC Centres and the wider science community (Met Office, JWCRP, ECMWF) to facilitate collaborative research; to link with international efforts in atmospheric chemistry; to engage with policy stakeholders and to develop the UK atmospheric chemistry community in general.

The atmospheric chemistry projects investigated two specific topics vital for representing atmospheric composition within the Earth system. The first of these addresses the requirements for the transport of atmospheric constituents for accurate representation of atmospheric chemistry and the second addresses the representation of deposition processes which govern both the composition of the lower atmosphere and the impacts it has on the biosphere at the Earth's surface.

The physics developed for Earth System Models typically targets the transport of water vapour, heat, and momentum. However mass-conservative transport of both long- and short-lived chemically-transforming tracers is less completely considered. This leads to errors in our ability to represent atmospheric composition on these larger, longer scales. This sub-project (with a funded PDRA) aimed to take advantage of the community's knowledge and experience to create observationally-based metrics of model transport to assess whether the representation of transport in models is fit for the purposes of atmospheric chemistry.

The deposition of atmospheric components to land and oceans is the ultimate fate of all compounds emitted into the atmosphere. These deposition processes can significantly impact the chemical composition of the atmosphere and an accurate representation of this deposition is a key, but under explored, component of the atmospheric chemistry system. This sub-project (again with a funded PDRA) developed observationally constrained metrics to assess the deposition schemes used in a range of the community's models and assess their suitability for atmospheric chemistry.

Other activities included running workshops to introduce the process-based/observational community to the current generation of NERC/Met Office ESMs; meetings to discuss the technical/software/governance issues with ESMs and the creation of an emerging scientists network to foster the next generation of atmospheric chemists specializing in Earth System projects.

The network engaged with science partners such as the Met Office and the ECMWF, with agencies such as DEFRA and with the wider international community. Overall this network aimed to bring together the wide and diverse range of atmospheric chemistry occurring in the UK so as to support and focus atmospheric science in the UK, help define future research plans, and to encourage engagement with the wider NERC strategy.