Optical Properties of Venusian Clouds

Abstract: Despite many years of study, the atmosphere of Venus is not well understood. When the spectrum of light from Venus's atmosphere is examined, there is an as yet unexplained region of absorption at approximately320 -500nm (Pérez-Hoyos et al., 2018). This corresponds to the blue to ultraviolet region of the spectrum, or "near-UV", although the exact range is quoted differently by different authors, (see for example Pollack et al., 1980;Frandsen, Wennberg, & Kjaergaard, 2016;Ekonomov et al., 1984;Limaye et al., 2019).The absorption is believed to occur in the upper cloud layer of Venus, which consists of droplets of anH2SO4/H2O mixture. The absorption cannot be explained by the droplets alone, or by the inclusion of SO2 gas, and so an additional particulate or aerosol absorber mixed in with the droplets or gas has been proposed (Ekonomov et al., 1984; Pollack et al., 1980).Although several candidates have been suggested, no chemical species has been conclusively proven to fit the observed absorption pattern and atmospheric chemistry of Venus. This project will examine the optical properties of sulphuric acid droplets containing candidate absorbers. This will be done initially in bulk liquid or aerosol distributions, and then in single droplets using optical tweezers. The project will also consider the physical and more general optical properties of the absorber candidates in droplets, including real refractive index and ability to maintain a spherical shape. Glory patterns -a series of concentric coloured rings of light produced in spherical droplets when light is internally reflected (Laven, 2005)-have been observed on Venus. The observed glories suggest the presence of absorber particles in the droplets. The particles must be entirely within the droplets as attaching particles to the droplet surface would result in distortion of the glory, which is not observed. Some candidates are impossible to insert into droplets and so, regardless of their agreement with the optical properties and absorption detected, cannot be the absorber (Petrova, 2018).The LMD (Le Laboratoire de Météorologie Dynamique) Venus atmosphere model comprehensively models the clouds of Venus. The physical, chemical, and optical properties of promising candidates from the laboratory experiments can be included in the model to simulate their behaviour in the Venusian atmosphere and therefore to check their suitability as candidates. As well as helping narrow down the search or strengthening the evidence for the identification of the absorber, the inclusion of a confirmed absorber in the model will improve its ability to model realistic behaviour to solve other problems.
Computer modelling will be particularly useful if no single candidate seems sufficient to provide all of the absorption detected. Modelling would then be used to combine candidate absorbers in variable amounts and pairings to see if any are promising. If promising candidate mixtures are predicted, experimental tests of the mixtures could begin, and the process could proceed iteratively to identify the absorber ratios.

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.