Aviation has the capacity to drive changes in atmospheric composition, and therefore climate and air quality, increasing human mortality through increases in cases in cardiopulmonary disease. Non-CO2 aviation emissions are estimated to have a considerable effect on the climate, and with rapid growth in the aviation sector their associated impacts could increase. There is much uncertainty surrounding the climatic impact of aviation-induced ozone and aerosols, in part due to broad range of emissions species emitted, which are not always reported in aviation emissions inventories. This thesis assesses the impact of aviation on atmospheric trace gas and aerosol concentrations, climate, air quality and human health effects for year 2000 civil aviation. These impacts are estimated through: (i) the development of an extended aviation emissions inventory, inclusive of speciated hydrocarbons; (ii) assessing the atmospheric and climatic impact from aviation based on an extend aviation emissions inventory, a comparison of these impacts with a reduced emissions aviation emissions inventory, along with a sensitivity study for emissions species included; (iii) assessing the impact of aviation on human health effects when variations in fuel sulfur content (FSC) are applied along with resulting impacts on radiative effects, and; (iv) the atmospheric, climatic, air quality and human health impacts of the use of alternative fuels in aviation. An aviation emissions inventory was developed to represent aviation-borne non-CO2 emissions: nitrogen oxides, carbon oxide, speciated hydrocarbons, sulfur dioxide, black carbon and organic carbon emissions while taking in to account the geometric mean diameter of carbonaceous particles released. Aviation non-CO2 emissions are assessed to result in a radiative effect of –13.29 mW m-2 [assessed from the ozone (O3DRE) and aerosol (aDRE) direct radiative effects, and aerosol cloud albedo effect (aCAE)], primarily driven by a cooling aCAE. In comparison an emissions inventory which only considers aviation nitrogen dioxide and black carbon emissions results in a radiative effect of –8.19 mW m-2 primarily driven by reductions in the cooling aCAE assessed. It is found that air pollution from aviation reaches ground level, as such modifying surface PM2.5 (particulate matter within the 2.5 μm size range) which results in increased human exposure. Standard aviation is estimated to result in 3597 mortalities a-1. Variations in FSC from 0–6000 ppm aviation’s human health effects range from 2950–9057 mortalities a-1. These variations in FSC result in an aviation non-CO2 radiative effect ranging from –6.08 mW m-2 to –75.48 mW m-2. It is found that variations in aviation FSC elicit a near-linear relationship between aviation-induced mortality and non-CO2 radiative effect. Additional investigations in the vertical release of aviation-borne sulfur dioxide emissions show that it possible to reduce aviation-induced mortality and increase aviation-induced cooling by adjusting the FSC of fuel used at different altitudes. An investigation of the use of Fischer-Tropsch (FT) and fatty acid methyl ester (FAME) fuels (FT50, FT100, FAME20 and FAME40 fuel blends) within aviation found that aviation-induced nitrogen dioxide and ozone concentrations were reduced in tandem with associated ozone radiative effects. Additionally due relative reductions between sulfur dioxide and carbonaceous aerosol emissions FT fuel blends were estimated to produce negative aDREs, while FAME fuel blends gave a positive aDRE. In all cases FT and FAME fuel blends decreased the aCAE induced cooling effect from aviation. FT50 is the only fuel blend currently specified for use in today’s civil aviation fleet. This fuel blend is simulated to reduce aviation’s non-CO2 emissions cooling radiative effect to –10.89 mW m-2 and reduce aviation-induced mortality by 460 mortalities a-1. Through the sustainable development of FT fuels from bio-sourced feedstocks this fuel blend has the potential to reduce aviation’s climatic impact and human health effects (when reductions in aviation’s net CO2 emissions are considered in tandem).

Atmospheric aerosol particles influence our planet's climate and contribute to poor air quality, increased mortality and degraded visibility. Central to these issues is how atmospheric aerosol particles interact with gas species to affect chemistry and cloud formation. Recent research shows that some aqueous solutions relevant to atmospheric aerosol (notably secondary organic material, which constitutes a large mass fraction of atmospheric aerosol particles) can be highly viscous and can behave mechanically like a solid. This has led to suggestions that these particles exist out of equilibrium with the gas phase in the atmosphere, with implications for heterogeneous chemistry and ice nucleation. In order to quantify any kinetic limitations, it is vital to have quantitative data about the diffusion of various relevant species within these materials. This thesis describes the direct measurement and application of water diffusion coefficients in aqueous solutions relevant to atmospheric aerosol, including sucrose and secondary organic material. A water diffusion model is developed, validated and used with a new parameterisation of the water diffusion coefficient in secondary organic material to quantify the rate of uptake and loss of water from aerosol particles. It is shown that, although this material can behave mechanically like a solid, at 280 K water diffusion is not kinetically limited on timescales of 1 s for atmospheric-sized particles. This is not the case, however, for colder conditions: modelling of 100 nm particles predicts that under mid to upper tropospheric conditions radial inhomogeneities in water content produce a low viscosity surface region and more solid interior. This may significantly affect aerosol chemistry and the ability of particles to nucleate ice. Also reported are the diffusion coefficients of sucrose in aqueous sucrose at higher concentrations than have been previously investigated. These measurements provide insights into the role of organic molecules in aerosol evaporation and chemistry. Together with the diffusion coefficients of water measured in this material, they will also offer a valuable means to study the fundamental nature of diffusion in a simple but widely used material, and specifically the breakdown of the Stokes-Einstein relationship.

Dust plays multiple important roles in the Earth system with emissions from northern Africa contributing on the order of 60% to the global total. Current model estimates of annual dust production from this crucial region vary by a factor of up to 5. This low agreement between models is to a great extent due to differences in the representation of near-surface winds. One barrier to better understanding of wind processes is the sparse observation network in northern Africa combined with regionally varying, but not necessarily documented, reporting procedures that lead to uncertainties and biases. Previous studies have utilised long-term station observations of visibility over this region to investigate dust climatology, but this work is the first to focus specifically on emission, based on quality-controlled reports from station observers and measurements of 10 m wind-speed. The interannual, seasonal and diurnal cycles of dust emission frequency (FDE), as well as trends, are investigated using existing and new analysis methods, such as the estimation of emission thresholds. Spatially, it is shown that threshold wind-speeds for dust emission are highest in northern Algeria and lowest in Sudan and around the latitude band 16◦N - 21◦N. FDE peaks in spring at most stations, while in the Sahel seasonal cycles vary between stations depending on their proximity to the Saharan Heat Low, and as a result of seasonal exposure to both the summer monsoon and winter Harmattan. Seasonally, FDE is largely controlled by changes in strong winds, rather than changes in emission thresholds. The relative contribution of different wind-speeds to dust uplift are investigated using the observed winds and calculated thresholds. Case studies and field campaign data are analysed to determine the plausibility of SYNOP high-wind reports. In northern regions, 50% of uplift is associated with high winds which occur only 0.3% - 0.5% of the time. This contrasts with an occurrence range of 0.7% - 2.5% for southern regions. Winds of 12 – 15 ms−1 contribute the most to northern total DUP, while in the south the range is lower at 7-11 ms−1. A percentage occurrence of 0.3% equates to only 5.5 events per year. Previous studies have documented changes in the dust output from northern Africa on interannual to decadal time scales, though the reasons for this variability are still debated. This study shows that the likely contributors to an observed decreasing trend in FDE are changes in circulation patterns, changes to the Bowen ratio and, most significantly, the effect of a change in roughness on wind-speed as a result of a greening of the Sahel. This work forms a base for further investigations into mechanisms for dust emission in northern Africa and their relative importance, as well as providing reference material for model and reanalysis evaluation.

The coupling between stratosphere and troposphere (ST) has been studied extensively using simple circulation models. It is known that the ozone-rich stratosphere interact with the troposphere through both radiative and dynamical processes. However, many of the models used in these studies only assume a slab ocean with a fixed sea surface temperature (SST) profile. To investigate the role of the ocean in the stratosphere-troposphere coupling, a fully coupled atmosphere-ocean model, FORTE (Fast Ocean Rapid Troposphere Experiment) is used in this study. In this project the Earth is modelled as a perfect sphere with its surface covered with water. In the first set of our experiments we introduce a perturbation to the stratosphere by increasing ozone concentration by a factor of five. In the second experiment we repeat the ozone perturbation experiment with a fixed SST profile such that the atmosphere-ocean coupling is shut off. Our results demonstrate that by including a dynamical ocean, the strength of the jet streams is less sensitive to stratospheric ozone perturbations whereas the extent of their latitudinal displacements is greater. Both of these are found to be a consequence of SST anomalies induced by ocean dynamics. On the other hand, our results show that in the presence of an interactive ocean, there is a general increase in tropospheric air temperature except for polar regions, while lacking the banded anomaly pattern observed in our fixed SST experiment and other ST coupling studies.

The contribution of aerosol to the radiative forcing of Earth's atmosphere remains the largest uncertainty in climate models. Atmospheric aerosol particles scatter and absorb both solar and terrestrial radiation, in addition to perturbing the albedo and lifetime of clouds. The key property governing the amount of light that interacts with an aerosol particle is the particle extinction cross section (δext), commonly measured by probing an ensemble of particles using spectroscopic techniques. However, the uncertainties in particle size, distribution and number density associated with aerosol ensembles leads to imprecise characterisations of δext. By probing a single particle at a time, δext will be measured with higher precision. This thesis develops a new single particle cavity ring-down spectrometer (SP-CRDS) for measuring δext at λ = 405 nm, optically confining a single particle using a Bessel laser beam trap and probing this particle using CRDS. Moreover, a similar instrument performing CRDS at λ = 532 nm is developed. Using these SP-CRDS instruments, coupled with elastic light scattering measurements, the variations in δext with particle size and composition are determined for a variety of evaporating organic species or aqueous droplets containing atmospherically relevant, hygroscopic inorganic solutes. Particle refractive indices (RIs) are retrieved by fitting the elastic light scattering and δext measurements to light scattering models, such as Mie theory. The variations in RI are determined as a function of relative humidity for wavelengths across the visible spectrum. The RI retrieval precision from (Text data, or using light elastically scattered from a Gaussian beam, is shown to be better than 0.2%. However, the RI retrieval precision from elastic light scattering measurements using a Bessel beam, instead of a Gaussian beam, is degraded significantly to 0.5%. The accuracy in RI retrievals from (Text data is found to be ~0.07% for particle radii>1 μm, i.e. an accuracy exceeding the requirements for calculating aerosol radiative forcing to better than 1 %. Therefore, SP-CRDS measurements of δext will be important in reducing the uncertainties associated with aerosol radiative forcing.

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Aerosols have a significant influence on both human health and global climate, and are associated with a potentially important role in future climate change. The microphysical and optical properties of aerosols need to be well quantified to understand the radiative impact, important in weather and climate models. This work focuses on two globally significant species from two different aircraft campaigns, mineral dust and biomass burning aerosol. An analysis is presented of mineral dust properties from the Saharan region during the Fennec campaign in June 2011 and 2012. In making observations it is crucial to understand the associated measurement limitations. The Low Turbulence Inlet (LTI) was designed to improve inlet efficiency and is shown to quantifiably measure coarse mode mineral dust up to 10 μm. This has allowed for Rosemount inlet characterisation with enhancements relative to the LTI determined to be 8 times for particles up to 3.0 μm. The physical and optical properties and vertical structure of aged Rondônian regional haze from the SAMBBA campaign in Rondônia, Brazil in September 2012 are characterised. Good agreement is shown between in-situ extinction coefficients and those retrieved using lidar remote sensing. Characterisation of airborne thermo-optical techniques for measuring black carbon content is achieved through comparisons with SP2 measurements. Total number concentration was reduced by 77 % compared with 96 % for the SP2, suggesting not all volatile material was removed with the thermooptical technique. The imaginary part of the aerosol refractive index was 40 % lower using the Maxwell-Garnett mixing rule compared with the volume-mixing rule for Rondônian regional haze. This difference is small compared with the uncertainty derived from the assumed refractive index of the sub-components, articularly for organic carbon; the singlescattering albedo at 550 nm is shown to range from 0.88 to 0.95. Humidification effects on the optical properties of biomass burning aerosol were found to be negligible due to modest ambient relative humidities. Comparisons with retrieved properties from the SCAR-B campaign in 1995 suggest consistency in biomass burning and agricultural practices over time.

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The magnitude of aerosol effective radiative forcing is the dominant source of uncertainty in net anthropogenic forcing over the industrial period. Aerosol effective radiative forcing is also one of the largest sources of uncertainty in recent decadal and near-future anthropogenic forcing. Knowledge about the main sources of aerosol forcing uncertainty can be used to guide the development of models and ultimately reduce forcing uncertainty. The research in this thesis identifies important parametric sources of aerosol radiative forcing uncertainty in global models using perturbed parameter ensembles, statistical emulation and variance-based sensitivity analyses. Industrial and recent decadal anthropogenic emission periods are used to quantify the sources of aerosol forcing uncertainty over different timescales. Natural aerosol parameters dominate the uncertainty in aerosol forcing over the century-scale industrial period. However, anthropogenic and model process parameters are dominant over recent decades. In each case specific parameters have been identified as priorities for model development that targets aerosol forcing uncertainty reduction. At the regional scale changes in climatic effects over recent decades may be partly attributable to anthropogenic aerosol forcing. The credible ranges of aerosol radiative forcing, quantified in 11 climatically important regions, support some hypotheses about the role of aerosols in regional climate forcing and call others into question. Reducing uncertainty in the identified parameters would further clarify the role of anthropogenic aerosols in influencing large-scale climate effects. Physical atmosphere model parameters are found to be far more important than aerosol parameters as sources of top-of-the-atmosphere radiative flux uncertainty. However, aerosols are the dominant source of uncertainty in how the radiative flux changes in response to aerosol emissions (the aerosol radiative forcing). Observations of present-day radiative fluxes provide only a weak constraint of aerosol radiative forcing. These results provide insight into, and motivation for, model development that focusses on uncertainty reduction rather than quantification.