Aerosols are important in a wide range of scientific disciplines, from the delivery of drugs to the lungs, to their impact on the earth's climate and their role in climate change, through to their application in the delivery of fuels for combustion, and their processing in plasmas to prepare functionalised materials. Defined as a dispersion of solid or liquid particles within the gas phase, aerosol properties are governed by the chemical composition and size of the individual particles. It is also widely recognised that the chemical composition of the surface of a particle can play a critical role in governing the properties of the aerosol. This is primarily because aerosols can present a large surface area to the surrounding gas phase. Any chemistry that occurs must be mediated through transfer of molecules from the gas phase into the bulk of the particle across the surface. The chemical make-up of the surface can significantly influence this transfer. Further, it is recognised that particles are generally not uniform in composition throughout their volume. For example, a single particle may consist of organic and water phases that are not mixed, but are phase separated. This can have a profound influence on the properties of a particle when compared with the properties expected for a particle characterised by uniform mixing.In this research we will investigate the relationship between the chemical, physical and optical properties of aerosol particles and their chemical composition and uniformity in composition. We will develop new techniques to examine the internal structure within a single particle, to explore how different chemicals mix or separate in a single particle, and to investigate the ease with which molecules are taken up at the surface of the particle. In addition, we will develop a new instrument to measure how efficiently a particle absorbs light. In the atmosphere, aerosol particles can scatter sunlight back into space, counteracting the heat trapping properties of the greenhouse gases. However, some pollutant particles, such as black carbon produced in combustion, strongly absorb sunlight enhancing the warming of the atmosphere. The impact of aerosols remains poorly quantified and new techniques are required to study their light absorption properties.The novel experiments described above are based around two new powerful techniques. Using a tightly focussed laser beam, we can hold onto a single particle indefinitely. Known as optical tweezers, this approach has been widely used for holding particles in liquids. However, we have shown that the same approach can be used to hold onto aerosol particles. Further, light can become trapped in spherical aerosol droplets in much the same way as light undergoes total internal reflection in the formation of a rainbow. The light can travel a distance of metres around the edge of the droplet before escaping. By measuring the wavelength of the light, we can determine how far the light must travel to make one complete circuit of the droplet circumference. Not only can this provide a very accurate way of determining the size of the droplet, but it can enable us to make sensitive measurements of the composition of the droplet near the droplet surface. It is anticipated that the development and application of these new techniques will yield important new information on the properties of aerosols and their behaviour in many of the complex scientific problems highlighted above.

The greenhouse gases carbon dioxide (CO2) and methane (CH4) are by far the biggest contributors to recent and ongoing climate change. Of all the known greenhouse gases (excluding water vapour), CO2 and CH4 have the highest concentrations in the atmosphere and they are rising rapidly. CO2 is particularly problematic because there is so much of it (about 200 times more than CH4) and because once emitted to the atmosphere, much of it will stay there for several hundred years. Whereas, by comparison, CH4 has a lifetime in the atmosphere of about a decade, but it is a much more potent greenhouse gas than CO2 - that is, for equal amounts of CO2 and CH4 in the atmosphere, CH4 will trap heat radiation about 70 times more effectively than CO2 (over a 20-year time period). With the ratification of the Paris Agreement, the world has committed to avoiding dangerous climate change and the most obvious way to do this is by reducing emissions of CO2 and CH4. How will we know if emission mitigation policies are effective? Which nations or regions are meeting their emissions reduction targets? How will natural CO2 and CH4 fluxes respond to extreme weather events? And which aspects of the carbon cycle remain unsolved? For example, despite decades of study, scientists are still not sure why CH4 emissions are currently rising. To answer these questions we need to be able to measure and quantify CO2 and CH4 emissions and concentrations, and have the ability to separately quantify natural and manmade sources. Our current abilities to do so are severely limited, especially for CH4, which has a diverse array of natural and manmade sources. If we cannot determine the effectiveness of mitigation policies, then our ability to predict climate change impacts will be compromised by large uncertainties. 'Polyisotopologues' are one very promising new tool for distinguishing between different source emissions. The chemical elements that make up CO2 and CH4 molecules (carbon (C), oxygen (O) and hydrogen (H)) can have different masses, called isotopes. Different sources can have different isotopic 'fingerprints' or 'signatures' (because source reaction processes may favour a lighter or heavier molecule), thus measuring isotopic signatures is a useful way to gain insight into sources. Isotopic measurements have been made routinely for several decades; whereas the state-of-the-art technology developed in this project would allow us to measure molecules with more than one rare isotope. For example, most C has a relative atomic mass of 12 and H a mass of 1. The rarer isotopes of C and H have masses of 13 and 2, respectively. Isotopologues of CH4, which are measured routinely, include 12CH4, 13CH4 and 12CH3D (where 'D' represents the heavy H atom with mass 2). Whereas polyisotopologues of CH4 include 13CH3D and 12CH2D2 - these are far more challenging to measure, yet could provide invaluable insight into source emissions and sinks. POLYGRAM (POLYisotopologues of GReenhouse gases: Analysis and Modelling) will push the frontiers for both CO2 and CH4 polyisotopologue measurement capability using the latest advances in laser spectroscopic analysis and very high-resolution isotope ratio mass spectrometry. In addition to these challenging technological developments, we will establish a small global atmospheric sampling network to examine latitudinal and longitudinal variations in polyisotopologues, which will help us to constrain overall global budgets of CO2 and CH4. We will carry out field campaigns to determine polyisotopologue source signatures, for example, of CH4 from wetlands, cattle and landfills, and of CO2 from plant photosynthesis and respiration, and from fossil fuel burning. We will conduct laboratory experiments to estimate the reaction rates for CH4 isotopologues when they are oxidised and destroyed in the atmosphere. Finally, we will carry out atmospheric transport modelling for both gases to better interpret and understand the measurements.