Microscale dynamics and light scattering characteristics of ice crystals in contrails

Context
Up until the COVID-19 pandemic, global aviation was forecasted to grow at a rate of 3 to 4% per annum. The industry has suffered more than most due to global travel restrictions and government financial support is being tied to environmental conditions, which is being called the 'green restart.' In 2019, CO2 emissions from the global aviation industry represented 3% of the total anthropogenic radiative forcing. However, aviation emissions including nitrogen oxide (NOx), sulphates and particulate matter (PM) also contribute to climate forcing. PM emitted by aircraft engines facilitate the formation of contrails, which are visible line shaped clouds that form behind aircraft you see in the sky. Contrails are made up of ice crystals and are formed when water vapour in the exhaust plume condenses onto soot particles (approximately 100 nm in diameter) to form liquid water droplets which then freeze as the plume cools. Contrails affect the earth's radiation balance and have a warming effect that is more significant than that of aviation's CO2 emissions, yet regulatory frameworks for reducing contrail impacts are held back by significant scientific uncertainties. Notwithstanding these uncertainties, tackling contrails presents an opportunity to significantly and rapidly reduce aviation's environmental footprint.
Significant uncertainties in the climate effect of contrails remain, in part due to a lack of physical understanding of the microphysics of ice nucleation and its consequences on optical properties. The growth and shape of ice nucleates depends on the dynamics of PM surface properties: (i) PM tends to be hydrophobic, reducing ice nucleation, but sunlight and ozone oxidise PM surface that becomes hydrophilic, assisting ice formation; (ii) other atmospheric substances (hydrocarbons, SOx, NOx) may condense on PM and modify surface properties before water accumulates; (iii) PM porosity may be important, since water enters in pores and forms ice, which then assists ice growth; (iv) contrail ice particles could further absorb available water vapour in the atmosphere, thereby reducing the occurrence, coverage area and optical properties of natural cirrus, which could offset the net warming effect of contrails. The above behaviour may modify significantly the optical properties of ice crystals, which must be included in models to reduce uncertainties of estimates of contrail climate impact. Few studies have experimentally investigated the microphysics of ice nucleation on soot particles from aircraft engines, which are typically less than 100 nm in diameter.
Aims and objectives
The aim is to experimentally investigate the temporal evolution of the shape, size and light scattering properties of ice crystals that form on soot particles in conditions representative of cooling aircraft engine exhaust plumes. The student will perform controlled experiments by using suspended PM of variable composition and porosity in an ambient of different temperatures and gas compositions with variable residence times. The suspended particles will be illuminated by different spectral light characteristics to quantify the optical characteristics. The findings will be incorporated in models that evaluate the climate impact of aircraft contrails. The indirect forcing of contrails due to the effect on the optical properties of natural cirrus will be quantified by coupling an existing high-resolution contrail model with a general atmospheric circulation model, including aerosol-cloud interactions. This contributions of the project are that it will: (i) incorporate the interactions and feedbacks between contrails and natural cirrus for the first time; and (ii) more accurately quantify the impact of contrails on Earth's surface temperature with the inclusion of accurate optical properties, atmospheric processes and feedbacks. These modelling tools will be used to evaluate the potential for strategies to reduce the warming effect of contrails.

Aerosol science has a significant impact on a broad range of disciplines, extending from inhaled drug delivery, to combustion science and its health impacts, aerosol assisted routes to materials, climate change, and the delivery of agricultural and consumer products. Estimates of the global aerosol market size suggest it will reach $84 billion/year by 2024 with products in the personal care, household, automotive, food, paints and medical sectors. Air pollution leads to an estimated 30-40,000 premature deaths each year in the UK, and aerosols transmit human and animal infections. More than 12 million people in the UK live with lung disease such as asthma, and the NHS spends ~£5 billion/year on respiratory therapies. Many of the technological, societal and health challenges central to these areas rely on core skills and knowledge of aerosol science. Despite this, an Industrial Workshop and online survey (held in preparation for this bid) highlighted the current doctoral skills gap in aerosol science in the UK. Participating industries reported that only 15% of their employees working with aerosol science at doctoral-level having received any formal training. A CDT in aerosol science, CAS, will fill this skills gap, impacting on all areas of science where core training in aerosol science is crucial.

Impact on the UK aerosol community: Aerosol scientists work across governmental policy, industrial research and innovation, and in academia. Despite the considerable overlap in training needs for researchers working in these diverse sectors, current doctoral training in aerosol science is fragmentary and ad hoc (e.g. the annual Fundamentals of Aerosol Science course delivered by the Aerosol Society). In addition, training occurs within the context of individual disciplines, reinforcing artificial subject boundaries. CAS will bring coherence to training in the core physical and engineering science of aerosols, catalysing new synergies in research, and providing a focal point for training a multidisciplinary community of researchers. Working with the Aerosol Society, we will establish a legacy by providing training resources for future researchers through an online training portal.

Impact on industry and public-sector partners: 45 organisations have indicated they will act as CAS partners with interests in respiratory therapies, public health, materials manufacturing, consumer and agricultural products, instrumentation, emissions and environment. Establishing CAS will deliver researchers with the necessary skills to ensure the UK establishes and sustains a scientific and technical lead in their sectors. Further, it will provide an ideal mechanism for delivering Continuing Professional Development for the existing workforce practitioners. The activity of CAS is aligned to the Industrial Strategy Challenge Fund (e.g. through developing new healthcare technologies and new materials) and the EPSRC Prosperity Outcomes of a productive, healthy (e.g. novel treatments for respiratory disease) and resilient (e.g. adaptations to climate change, air quality) nation, with both the skilled researchers and their science naturally translating to long-lasting impact. Additionally, rigorous training in responsible innovation and ethical standards will lead to aerosol researchers able to contribute to developing: regulatory standards for medicines; policy on air quality and climate geoengineering; and regulations on manufactured nano-materials.

Public engagement: CAS will provide a focal point for engaging the public on topics in aerosol science that affect our daily lives (consumer products, materials) through to our health (inhalation therapeutics, disease transmission and impacts of pollution) and the future of our planet (geoengineering). Supported by a rigorous doctoral level training in aerosol science, this next generation of researchers will be ideally positioned to lead debates on all of these societal and technological challenges.