Clouds formed by aircraft (contrails) are the most easily visible human forcing of the climate system. Trapping energy in the Earth system, they contribute more than half of the total climate impact of aviation. This makes reducing contrails an important goal to achieve the UK's climate commitments. Theoretical considerations indicate two pathways for reducing contrails. First, improving engine design to emit fewer particulates may reduce contrail lifetimes and so their climate impact. Second, rerouting aircraft to avoid contrail forming regions. Assessing these pathways requires accurate models of contrail formation, evaluated at the level of an individual aircraft. This evaluation requires observations of contrails across their lifetime, coupled to details of the generating aircraft. Even where they are matched to specific aircraft, existing observations typically view a contrail once, (limiting their use for measuring contrail lifecycles) or cannot provide the detail on the contrail microphysical properties (such as ice crystal number or shape) necessary to assess the efficacy of different pathways to contrail reduction. Improving confidence in our contrail models urgently requires novel observations of contrail properties and lifecycles from individual aircraft. The impact of aircraft on clouds is not limited to contrails forming in clear air. Over half of contrails form embedded in existing clouds and the particulates emitted by aircraft can affect cloud formation several days after they were released. These effects produce a cooling, potentially large enough to offset all other warming effects of aviation, but are not represented in aircraft-level models used for planning contrail avoidance strategies. There are few observational constraints of these effects, targeted observations of the impact of individual aircraft on cloud microphysics are required to assess them and to improve future model simulations. To address these uncertainties and around contrail formation, persistance and climate impact as well as aerosol-cloud interactions, COBALT has three core components: 1. A measurement campaign in the southern UK, combining an array of ground-based cameras with a steerable cloud radar, to make high resolution observations of contrail formation from individual aircraft. Guided by aircraft transponder information, these observations will be focused on contrails and clouds modified by aircraft, characterising contrail formation and perturbed cloud properties within the first few hours of their lifecycle. 2. Counterpart satellite observations, using novel techniques to characterise contrail and cloud development from an hour to several days behind the aircraft. Building on techniques for studying natural cirrus, this will produce a complete characterisation of the contrail lifecycle, along with the first estimate of the aviation aerosol impact on existing cirrus clouds at a global scale. 3. The complete lifecycle characterisation will be combined with flight data from aircraft operators to produce a unique dataset designed specifically for the evaluation of aircraft-level models of contrail formation. An initial focus will be placed on evaluating aircraft-scale models, as these are currently being used to plan aircraft diversions. A comparison of climate model parametrisations of contrail formation will assess the ability of the parametrisations to reproduce the wide-area (>1000km2) contrail observations taken by the camera array. Led by an inter-disciplinary team of scientists and engineers, with partners in key international research centres and industry groups, COBALT will provide the tools necessary to evaluate our current models and ability to avoid contrails, guiding future modelling and operational trials of sustainable fuels and contrail avoidance.

The EPSRC CDT in Net Zero Aviation in partnership with Industry will collaboratively train the innovators and researchers needed to find the novel, disruptive solutions to decarbonise aviation and deliver the UK's Jet Zero and ATI's Destination Zero strategies. The CDT will also establish the UK as an international hub for technology, innovation and education for Net Zero Aviation, attracting foreign and domestic investment as well as strengthening the position of existing UK companies. The CDT in Net Zero Aviation is fully aligned with and will directly contribute to EPSRC's "Frontiers in Engineering and Technology" and "Engineering Net Zero" priority areas. The resulting skills, knowledge, methods and tools will be decisive in selecting, integrating, evaluating, maturing and de-risking the technologies required to decarbonise aviation. A systems engineering approach will be developed and delivered in close collaboration with industry to successfully integrate theoretical, computational and experimental methods while forging cross theme collaborations that combine science, technology and engineering solutions with environmental and socio-economic aspects. Decarbonising aviation can bring major opportunities for new business models and services that also requires a new policy and legislative frameworks. A tailored, aviation focused training programme addressing commercialisation and route to market for the Net Zero technologies, operations and infrastructure will be delivered increasing transport and employment sustainability and accessibility while improving transport connectivity and resilience. Over the next decade innovative solutions are needed to tackle the decarbonisation challenges. This can be only achieved by training doctoral Innovation and Research Leaders in Net Zero Aviation, able to grasp the technology from scientific fundamentals through to applied engineering while understanding the associated science, economics and social factors as well as aviation's unique operational realities, business practices and needs. Capturing the interdependencies and interactions of these disciplines a transdisciplinary programme is offered. These ambitious targets can only be realised through a cohort-based approach and a consortium involving the most suitable partners. Under the guidance of the consortium's leadership team, students will develop the required ethos and skills to bridge traditional disciplinary boundaries and provide innovative and collaborative solutions. Peer to peer learning and exposure to an appropriate mix of disciplines and specialities will provide the opportunity for individuals and interdisciplinary teams to collaborate with each other and ensure that the graduates of the CDT will be able to continually explore and further develop opportunities within, as well as outside, their selected area of research. Societal aspects that include public engagement, awareness, acceptance and influencing consumer behaviour will be at the heart of the training, research and outreach activities of the CDT. Integration of such multidisciplinary topics requires long term thinking and awareness of "global" issues that go beyond discipline and application specific solutions. As such the following transdisciplinary Training and Research Themes will be covered: 1. Aviation Zero emission technologies: sustainable aviation fuels, hydrogen and electrification 2. Ultra-efficient future aircraft, propulsion systems, aerodynamic and structural synergies 3. Aerospace materials & manufacturing, circular economy and sustainable life cycle 4. Green Aviation Operations and Infrastructure 5. Cross cutting disciplines: Commercialisation, Social, Economic and Environmental aspects 75 students across the UK, from diverse backgrounds and communities will be recruited.

Hydrogen and alternative liquid fuels (HALF) have an essential role in the net-zero transition by providing connectivity and flexibility across the energy system. Despite advancements in the field of hydrogen research both in the physical sciences and engineering, significant barriers remain to the scalable adoption of hydrogen and alternative liquid fuel technologies, and energy services, into the UK's local and national whole system infrastructure. These are technical barriers, organisational barriers, regulatory and societal barriers, and financial barriers. There are, therefore, significant gaps between current levels of hydrogen production, transportation, storage, conversion, and usage, and the estimated requirement for achieving net-zero by 2050. To address this, our proposed research programme has four interlinked work packages. WP1 will develop forward-thinking HALF technology roadmaps. We will assess supply chain availability and security. Selected representative HALF use cases will be used to identify and quantify any opportunities, risks and dependencies within a whole systems analysis. We will also develop an overarching roadmap for HALF system integration in order to inform technology advancement, industry and business development, as well as policy making and social interventions. WP2 will improve HALF characterisation and explore urgent new perspectives on the energy transition, including those related to ensuring resilience and security while also achieving net-zero. We will contrast the energy transition delivered by real incentives/behaviour versus those projected by widely-used optimisation models. The WP provides the whole systems modelling engine of the HI-ACT Hub, with a diverse array of state-of-the-art tools to explore HALF integration. WP 3 will explore the vital coupling of data and information relating to whole system planning and operational decision support, through the creation of a cyber physical architecture (CPA). This will generate new learning on current and future opportunities and risks, from a data and information perspective, which will lead to a whole system ontology for accelerated integration of hydrogen technologies. WP 4 considers options for a future energy system with HALF from a number of perspectives. The first is to consider expert views on HALF energy futures, and the public perceptions of those views. The second perspective considers place-based options for social benefit in HALF energy futures. The third perspective is to consider regulatory and policy options which would better enable HALF futures. Embedded across the research programme is the intent to create robust tools which are investment-oriented in their analysis. A Whole Systems and Energy Systems Integration approach is needed here, in order to better understand the interconnected and interdependent nature of complex energy systems from a technical, social, environmental and economic perspective. The Hub is led by Prof Sara Walker, Director of the EPSRC National Centre for Energy Systems Integration, supported by a team of 16 academics at a range of career stages. The team have extensive experience of large energy research projects and strong networks of stakeholders across England, Wales, Scotland and Northern Ireland. They bring to the Hub major hydrogen demonstrators through support from partners involved in InTEGReL in Gateshead, ReFLEX in Orkney, and FLEXIS Demonstration in South Wales for example. We shall engage to create a vibrant, diverse, and open community that has a deeper understanding of whole systems approaches and the role of hydrogen and alternative liquid fuels within that. We shall do so in a way which embeds Equality, Diversity and Inclusion in the approach. We shall do so in a way which is a hybrid of virtual and in-person field work consultation and develop appropriate digital tools for engagement.

Hydrogen and alternative liquid fuels (HALF) have an essential role in the net-zero transition by providing connectivity and flexibility across the energy system. Despite advancements in the field of hydrogen research both in the physical sciences and engineering, significant barriers remain to the scalable adoption of hydrogen and alternative liquid fuel technologies, and energy services, into the UK's local and national whole system infrastructure. These are technical barriers, organisational barriers, regulatory and societal barriers, and financial barriers. There are, therefore, significant gaps between current levels of hydrogen production, transportation, storage, conversion, and usage, and the estimated requirement for achieving net-zero by 2050. To address this, our proposed research programme has four interlinked work packages. WP1 will develop forward-thinking HALF technology roadmaps. We will assess supply chain availability and security. Selected representative HALF use cases will be used to identify and quantify any opportunities, risks and dependencies within a whole systems analysis. We will also develop an overarching roadmap for HALF system integration in order to inform technology advancement, industry and business development, as well as policy making and social interventions. WP2 will improve HALF characterisation and explore urgent new perspectives on the energy transition, including those related to ensuring resilience and security while also achieving net-zero. We will contrast the energy transition delivered by real incentives/behaviour versus those projected by widely-used optimisation models. The WP provides the whole systems modelling engine of the HI-ACT Hub, with a diverse array of state-of-the-art tools to explore HALF integration. WP 3 will explore the vital coupling of data and information relating to whole system planning and operational decision support, through the creation of a cyber physical architecture (CPA). This will generate new learning on current and future opportunities and risks, from a data and information perspective, which will lead to a whole system ontology for accelerated integration of hydrogen technologies. WP 4 considers options for a future energy system with HALF from a number of perspectives. The first is to consider expert views on HALF energy futures, and the public perceptions of those views. The second perspective considers place-based options for social benefit in HALF energy futures. The third perspective is to consider regulatory and policy options which would better enable HALF futures. Embedded across the research programme is the intent to create robust tools which are investment-oriented in their analysis. A Whole Systems and Energy Systems Integration approach is needed here, in order to better understand the interconnected and interdependent nature of complex energy systems from a technical, social, environmental and economic perspective. The Hub is led by Prof Sara Walker, Director of the EPSRC National Centre for Energy Systems Integration, supported by a team of 16 academics at a range of career stages. The team have extensive experience of large energy research projects and strong networks of stakeholders across England, Wales, Scotland and Northern Ireland. They bring to the Hub major hydrogen demonstrators through support from partners involved in InTEGReL in Gateshead, ReFLEX in Orkney, and FLEXIS Demonstration in South Wales for example. We shall engage to create a vibrant, diverse, and open community that has a deeper understanding of whole systems approaches and the role of hydrogen and alternative liquid fuels within that. We shall do so in a way which embeds Equality, Diversity and Inclusion in the approach. We shall do so in a way which is a hybrid of virtual and in-person field work consultation and develop appropriate digital tools for engagement.