Core Aerosol Science

The dynamics of aerosols dictate the motion of particles in the suspending medium, and dictates phenomena such as pollution dispersion, lung deposition and nanomaterial collection. Aerosol particles obey Newtonian laws of motion and allow for a description of aerosol motion in relation to the fluid dynamics of the system. The size of the suspended particles dictates whether continuum mechanics or molecular dynamics best describes the particle motion, and in many cases the particles lie within a transition regime between the bounding length scales. The course will introduce the aerosol general dynamic equation, which is the basis of aerosol mechanics and dynamics. Transport due to diffusion, external forces and advection will be described in relation to practical applications such as inertial impaction and particle size selection.

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The focus of this module is to enable students to develop an understanding of the importance of aerosol filtering and capture systems, whether this is to reduce disease transmission, minimise exposure to harmful allergens or to capture airborne material for further analysis. The engineering approaches to system design require a comprehensive understanding of the natural deposition processes and deposition modes, methods of capture (filtration, cyclonic separation, deposition) and subsequent analysis techniques. Furthermore, the principles of inertial impaction sampling as applied to aerosol capture are applicable to a variety of transport phenomena, and analytical and aerosol sizing.  The cohort will explore several prototype systems developed by the academic team for real-world environmental challenges including food security (protecting crops from airborne disease), environmental stewardship (detection of the fungal spores in forests), infection control (protect patients from airborne hospital acquired infection) and the challenges of biowarfare detection (for both military and civilian applications). In addition, the cohort will investigate the operating principles of inertial impaction samplers and subsequent mass analysis. Covering a broad range of inter-disciplinary factors, students will be equipped with an understanding of the challenges facing aerosol scientists when sampling airborne material both in the laboratory and the field.

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Nucleation involves the formation of particles from gas phase precursors. New particle formation is a two step processes first involving nucleation of new particles followed by the growth of those particles to larger sizes. Nucleation and new particle formation are relevant across a wide range of industrial and environmental contexts. For example, synthesis of nano-materials can involve approaches like floating catalyst chemical vapour deposition, a gas-to-particle conversion process. In the atmosphere, new particle formation is known to be a ubiquitous but poorly understood source of ambient particles that ultimately constitute up to half of all cloud droplet seeds. Planetary dust and meteoric smoke also form through nucleation and growth. Classical nucleation theory is often applied to a range of nucleation and growth contexts. However, classical nucleation theory has important shortcomings that limit its applicability to many real-world systems. In recent years, advancements in instrumental approaches have enabled significant new insights into nucleation and growth mechanisms. We will discuss contexts where nucleation and growth are important, theoretical approaches to describe them, and instrumentation to study newly formed particles.

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Aerosol particles may be produced charged, for example in combustion or sprays, or they may acquire charge due to the attachment of ions naturally present in the air. Aerosol charging affects particle-particle collisions (coagulation), particle-surface interactions (deposition), particle collection by droplets (scavenging) and droplet evaporation. For example, the presence of charge on inhaled pharmaceutical aerosol may increase deposition within the lung due to the generation of image charges in the airway epithelial layer. Additionally, the scavenging of ions by aerosol particles forms the basis of the operating principle of smoke alarms, with aerosol laden air having a higher charge concentration than clean air due to the preservation of charge through ion-aerosol attachment rather than its loss through ion-ion recombination. The presence of charge on aerosols can also be used as a diagnostic tool, with measurements of electrical mobility used to determine particle physical properties such as in the size distribution recorded by a Scanning Mobility Particle Sizer (SMPS). We will explore particle charging mechanisms and the different ways in which particle charge can be quantified and controlled, looking at the real-world consequences of particle charging and applications in which it can be exploited.

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As with all dispersion colloids, aerosols are thermodynamically unstable. However, they adopt a metastable composition with partitioning of components between the gas and condensed particle phases that can be treated using conventional equilibrium thermodynamics. Most commonly, the equilibrium response of solid and liquid particles to changes in gas phase humidity is used as an exemplar. Crystalline particles under dry conditions dissolve (deliquesce) and become solution droplets at high relative humidity. Water is removed reversibly on drying to low relative humidity, although a hysteresis in phase behaviour is often observed with particles crystallising only at a high degree of solute supersaturation. Other volatile and semi-volatile components similarly partition in equilibrium proportions between the gas and condensed phases, changing aerosol particle size and composition. For small particles, the surface curvature of the particle surface can be sufficient that the vapour pressures of volatile components, including water, are enhanced. Understanding all of these principles is crucial to predicting cloud droplet and ice particle formation, the mass concentrations of particulate matter in polluted urban centres, and the response of particles on inhalation to the respiratory tract. Complex phase behaviours can also be observed such as liquid crystals and glasses.

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The interaction of light with aerosol particles is central in models of climate and regional air quality. Measurements of aerosol optical properties using optical spectroscopy facilitate non-contact studies of chemical reactions in aerosols and at their surfaces, while aerosol-light interactions are exploited in optical trapping techniques. This module provides a broad overview of the key parameters that govern aerosol-light interactions, including the theories that allow predictions of optical behaviour from aerosol microphysical properties. Key ideas include light scattering and absorption, aerosol optical cross sections, angular scattering and how these depend on the aerosol microphysical properties of particle size, composition, shape and phase. Then, the unit develops how the important intrinsic aerosol optical property of complex refractive index relates to molecular parameters and intermolecular interactions, providing an overview of mixing rule theories applied commonly to predicting refractive indices for internally-mixed aerosol systems. A broad overview will be presented of spectroscopic techniques applied commonly in both the laboratory and in the field to the measurement of optical properties including non-contact in situ instruments, filter-based methods and remote sensing principles. This module provides the fundamental ideas and theories that will underpin subsequent Aerosol Science modules and research projects involving optical and spectroscopic methods.

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Measurements of the aerosols around us, in the atmosphere and in industrial processes, have been fundamental to the advancement of the field. The earl-iest measurements of aerosols and the principles by which they manipulated particles using electrical and aerodynamic forces are still widely used today. Aerosol measurements must deal with particle sizes that span more than four orders of magnitude: from nucleated clusters of a few molecules (~10-9 m, ~1 nm) to cloud droplets and dust particles that are up to tens of microns in size (~10-5 m, 10 μm). Particle composition and shape can vary significantly with size and time, reflecting the diverse origins of particles. Particles can be non-volatile such as salt, soot, and metals, and also contain semi-volatile compounds such as nitrates and many organic compounds. The distribution of semi-volatile compounds between the gas-phase, their nucleation as new particles and condensation onto non-volatile particles is dependent on the sampling and measurement conditions and affects the number, size distribution and shape of particles that we observe. We will see how aerosol measurements can be made of individual particles and of the collection of particles, how they can provide limited information integrated over time, and progress towards detailed resolution with respect to size and time.

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31 Jan- 1 Feb 2022 

Aerosols are dynamic with the evaporation or condensation of volatile and semi-volatile components changing the phase, composition and size distributions of dispersed particles. Ultimately the aerosol heads towards a state of equilibrium, with thermodynamic factors governing the distribution of components between the gaseous and condensed phases. Water condensation leads to the activation of aerosol particles to form cloud droplets in the atmosphere. Propellants evaporate from pharmaceutical aerosol generated from metered dose inhalers. The rapid evaporation of solvent can lead to the fabrication of micro-structured particles in spray dryers. Mechanistically, the mass and heat transfer can be intimately coupled, with changes in particle temperature leading to unsteady evaporation. Rates of particle compositional and size change can also be limited by gas transport, interfacial exchange or slow transport within the particle bulk, dependent on particle size, composition and viscosity, gas composition, temperature and pressure. Developing inhomogeneities in composition can lead to surface enrichment of solutes and the formation of core-shell or hollow particles. We will explore the experimental tools used to benchmark models and the factors influencing the mechanisms of aerosol evaporation and condensation.

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14 – 15 February 2022

The course will cover the physics, biology and behaviour of inhaled aerosols, either intentional for delivery of drugs or unintentional caused by atmospheric aerosols, e.g. air pollution particles. The approach is multidisciplinary and covers the consequences on human health, including scientific and clinical aspects, the physics of the flow of aerosols, including the fluid mechanics of the human airways, the relevant characteristics of the aerosols that determine the interaction and bioreactivity with the human airways. The characterisation of the aerosols, including sizing and physicochemical characteristics, is an important component in optimising, for example, drug delivery, or in predicting deposition of airborne aerosols. A mathematical description of the fluid mechanics and the particle motion will be delivered to ensure understanding of the approaches used for these detailed predictions as well as simplified evaluation of design parameters that determine particle deposition and delivery efficiency of drugs. The description of the characterisation methods of aerosols will include intrusive (probe based) and non-intrusive (optical) methods and provide the principle of operation, measurement accuracy and limitations. 

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21 – 22 February 2022

Aerosol synthesis of materials is a vibrant field of particle technology and chemical reaction engineering. The aim of this unit is to give students an understanding of, and to critically apprise the key concepts involved in the use and production of aerosolised materials, specifically aerosolised chemicals, and their application in the manufacture of high end ceramics, metal oxides, as well as other materials that impact transportation, construction, pharmaceuticals, energy and communications. Students will be exposed to state of the art literature and research underpinning fundamental concepts and physiochemical phenomena involved in generating aerosols (e.g. gas to particle conversions, spray processes, droplet generation and size distribution) and in the use of aerosols in the synthesis of materials (e.g. aerosol assisted chemical vapour deposition and flame aerosol synthesis) and fabrication of devices (e.g. aerosol jet printing). Topics such as the application of rapid expansion of supercritical solutions will also be explored in the context of current literature. Emphasis will also be placed on scale up of aerosol processing methods to larger levels of production manufacturing.

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28 February – 1 March 2022

Aerosol analysis is central to identifying the mechanisms by which aerosols form and are transformed in various environments, including in ambient air and during nanomaterials synthesis. Chemical composition analysis provides information about the elements and molecules present in an aerosol sample, which may provide information about the sample’s source and impacts with respect to health or, in the case of nanomaterials synthesis, purity. The phase of aerosol (e.g. solid, liquid, viscous/glassy) can crucially affect both composition and reactivity, for example by inhibiting diffusion of reactive molecules within the particle thereby limiting reactivity to only portions of the particle (e.g. on the surface). The volatility of organic molecules has a large impact on the fraction of molecules partitioned to the particle phase, and this partitioned fraction can change based on ambient conditions like particulate mass concentrations. A wide array of approaches have been developed to address all of these key properties of aerosols. We will explore these approaches, including conventional ones that involve collecting the aerosol onto filters for off-line sample analysis, as well as the diverse array of online approaches for aerosol analysis, including ones involving mass spectrometry, volatility and equilibrium partitioning, and phase state.

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14 – 15 March 2022

Bioaerosols, suspensions of airborne particles that contain or are derived from living organisms, are ubiquitous in the environment. They include viruses, bacteria, fungi, pollen, plant or animal debris, as well as fragments and products of these organisms. There are currently a wide variety of bioaerosol sampling methods available, but a major challenge in bioaerosol research is that no standard protocols have been established. Bioaerosols are an important transmission route for infectious and sensitization agents, inducing infectious and non-infectious diseases in both animals and humans. In this module we will discuss the pros- and cons- of the various sampling and analysis methods, explore bioaerosol sources in indoor and outdoor environments including considering their impact on cloud microphysics. The training will provide a detailed understanding of infection transmission and control in indoor environments including healthcare settings. We will also introduce the concept of the microbiome, exploring the influence aerosols have on our own microbiome and how this can further impact our health.

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Spring 2022 

Aerosols provide a reactive surface area upon which heterogeneous reactions can occur which change gas-phase composition, enable the synthesis and manufacture of novel materials, and modify the properties of the aerosol itself, for example their toxicity and ability to scatter and absorb radiation. A famous example is the formation of the stratospheric ozone hole over Antarctica, which is the result of chemical reactions on the surface of polar stratospheric clouds that activate chlorine compounds. The rates at which these processes occur depend critically on the chemical nature of the adsorbed gaseous molecule, the aerosol size, its physical properties (solid or liquid, bulk viscosity, morphology) and its chemical composition (e.g. salt, dust, inorganic or organic). The fundamental principles of the physical and chemical processes which control the accommodation of reactive gases at aerosol surfaces and their subsequent fate (at the surface or within the bulk of the aerosol) will be discussed together with important applications of heterogeneous chemical processing. Experimental techniques to probe the details of heterogeneous chemical or photochemical reactions involving aerosols, for example to determine reactive uptake coefficients and reaction mechanisms, as well as the use of electronic structure calculations to complement understanding, will also be discussed.

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28 – 29 March 2022

We will start from the conservation equations for single particles, including, mass, momentum and energy for dilute, non-interacting single particles, balanced by a range of source terms including evaporation, condensation, coalescence and reaction. Additional (often coupled) terms appear in the momentum equation for the gas and condensed phase due to drag, gravitational and electrostatic forces. We will introduce pdf approaches for the population balance for the conservation equations, and transformation of the conservation equations into pdf equations. We will also discuss strategies for solution of population balance equations, implications for solutions for large numbers of particles, and the trade-offs between Lagrangian and population balance methods. 

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