Aerosol science, the study of airborne particles from the nanometre to the millimetre scale, has been increasingly in the public consciousness in recent years, particularly due to the role played by aerosols in the transmission of COVID-19. Vaccines and medications for treating lung and systemic diseases can be delivered by aerosol inhalation, and aerosols are widely used in agricultural and consumer products. Aerosols are a key mediator of poor air quality and respiratory and cardiac health outcomes. Improving human health depends on insights from aerosol science on emission sources and transport, supported by standardised metrology. Similar challenges exist for understanding climate, with aerosol radiative forcing remaining uncertain. Furthermore, aerosol routes to the engineering and manufacture of new materials can provide greener, more sustainable alternatives to conventional approaches and offer routes to new high-performance materials that can sequester carbon dioxide. The physical science underpinning the diverse areas in which aerosols play a role is rarely taught at undergraduate level and the training of postgraduate research students (PGRs) has been fragmentary. This is a consequence of the challenges of fostering the intellectual agility demanded of a multidisciplinary subject in the context of any single academic discipline. To begin to address these challenges, we established the EPSRC Centre for Doctoral Training in Aerosol Science in 2019 (CDT2019). CDT2019 has trained 92 PGRs with 40% undertaking industry co-funded research projects, leveraged £7.9M from partners and universities based on an EPSRC investment of £6.9M, and broadened access to our unique training environment to over 400 partner employees and aligned students. CDT2019 revealed strong industrial and governmental demand for researchers in aerosol science. Our vision for CDT2024 is to deliver a CDT that 'meets user needs' and expands the reach and impact of our training and research in the cross-cutting EPSRC theme of Physical and Mathematical Sciences, specifically in areas where aerosol science is key. The Centre brings together an academic team from the Universities of Bristol (the hub), Bath, Birmingham, Cambridge, Hertfordshire, Manchester, Surrey and Imperial College London spanning science, engineering, medical, and health faculties. We will assemble a multidisciplinary team of supervisors with expertise in chemistry, physics, chemical and mechanical engineering, life and medical sciences, and environmental sciences, providing the broad perspective necessary to equip PGRs to address the challenges in aerosol science that fall at the boundaries between these disciplines. To meet user needs, we will devise and adopt an innovative Open CDT model. We will build on our collaboration of institutions and 80 industrial, public and third sector partners, working with affiliated academics and learned societies to widen global access to our training and catalyse transformative research, establishing the CDT as the leading global centre for excellence in aerosol science. Broadly, we will: (1) Train over 90 PGRs in the physical science of aerosols equipping 5 cohorts of graduates with the professional agility to tackle the technical challenges our partners are addressing; (2) Provide opportunities for Continuing Professional Development for partner employees, including a PhD by work-based, part-time study; (3) Deliver research for end-users through partner-funded PhDs with collaborating academics, accelerating knowledge exchange through PGR placements in partner workplaces; (4) Support the growth of an international network of partners working in aerosol science through focus meetings, conferences and training. Partners and academics will work together to deliver training to our cohorts, including in the areas of responsible innovation, entrepreneurship, policy, regulation, environmental sustainability and equality, diversity and inclusion.

Aerosols consist of liquid droplets or solid particles dispersed within a gas phase (typically air). Such droplets and particles can range in size from nanometres to millimetres. Aerosols are widely used to treat asthma via inhalation of therapeutic drugs and, in principle, enable the treatment of systemic diseases and the delivery of vaccines. They also find widespread application in consumer and agrochemical products, are prevalent in the atmosphere as particulate matter (PM) affecting air quality and human health, and are vehicles for the transmission of respiratory pathogens such as SARS-CoV-2, the virus responsible for COVID-19, and the bacterium responsible for tuberculosis. In all cases, the dispersed phase is dynamic, changing rapidly in moisture content and particle/droplet size during transport in the atmosphere, and often interchanging phase. Further complexity arises in most real-world systems: the droplets/particles can be multiphase consisting, for example, of dispersed solid nanoparticles within a liquid host droplet. Understanding such complex multiphase systems is crucial for designing pharmaceutical formulations to deliver drugs to the lungs, controlling the drying kinetics and engineered final particle structure in industrial processes such as spray-drying, and rationalising the airborne survival of viruses and bacteria in exhaled respiratory aerosol. Despite the importance of this broad range of problems, there are very few relevant studies of the dynamic transformation of aerosol droplets containing dispersed nanoparticles. We will integrate complementary expertise at the Universities of Bristol, Manchester and Sheffield to investigate the many physicochemical parameters that control the stability and structure of dried microparticles formed from solution aerosol droplets containing nanoparticles. The Bristol team has developed an array of state-of-the-art experimental methods to study the evaporation and drying of aerosol droplets in real time by monitoring their evolving size, composition, phase state and structure, while also capturing the final dried microparticles for post-mortem analysis. At Manchester, the team has extensive modelling capabilities to simulate the drying kinetics of evaporating aerosol droplets to account for changes in fluid viscosity, composition and temperature. The Sheffield team has developed synthetic routes to produce tailored polymer nanoparticles of varying size, shape, and surface chemistry in water, polar solvents or non-polar solvents, including the bio-inspired synthesis of several virus mimics. This combined expertise will enable us to examine a wide range of nanoparticles of selected size and character at known concentrations within host liquid droplets. Such nanoparticle-loaded droplets will be generated with reproducible size in a controlled environment of known temperature and gas phase composition, and their evaporation will be studied in real time (on timescales ranging from milliseconds to hours) through to the point of solidification. The structure of the final dried microparticles will be examined by scanning electron microscopy. These experiments will be compared with model predictions of evolving particle size and composition, and the structure and moisture stability of the microparticles will be evaluated. Ultimately, these observations will enable us to develop a framework for predicting how the various microphysical processes that occur during drying and the character of the nanoparticles within the host droplets affect the final microparticles. Working closely with industrial partners with expertise in the pharmaceutical, consumer product and aerobiology sectors, we will establish robust physical principles for understanding the dynamics occurring in aerosols of complex composition and phase in domains extending from drug delivery to the lungs to spray-drying of commercial products to mechanisms of disease transmission.