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High resolution measurements of aerosol concentrations and fluxes have been made in an urban street canyon with busy traffic in the city centre of Manchester, UK. Measurements of aerosol concentrations have been made in 150 size bins covering the range 4.6 nm < Dp < 32 μm. Vertical turbulent fluxes have been measured in the size range 0.1 J.Lm< Dp < 3 μm using the eddy covariance technique. By placing the instruments on a platform lift, some information about vertical profiles has been gained. These measurements have been supplemented by measurements of flow and turbulence using up to four ultrasonic anemometers. Measurements of turbulence were made over three week-long campaigns in February, April and May 2001, whereas turbulence and aerosol measurements were combined for a two-week campaign in October 2001.

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This thesis investigates the conservation properties of a 3D global tracer advection model and how the model output can be used in the development of a tracer-relative chemical transport model (CTM). The global tracer advection scheme is based on the finite volume NIRVANA scheme. Shape preservation is improved with the inclusion of a reduced grid at the polar regions. When solid body rotation acts on uniform mass and tracer fields then mass conservation is observed. If a cosine bell tracer distribution is used then mass non- conservation is observed, with the magnitude depending on the angle of rotation relative to the grid (-1.9 x 10-3C%< x <6.6 x 1O-4C% per revolution, where C is the maximum Courant number). Advection of a passive ozone tracer by analysed winds results in a numerical mass change that is increased when using a coarser temporal and spatial resolution. The global non- conservation arises from the difference in treatment of the mass continuity equation by the offline tracer model and the NWP model producing the analyses. Due to the effects of spatial truncation it is better to use re-analyses produced at low resolution than to take high resolution operational analyses and truncate them. As a result of temporal truncation of the winds any offline tracer scheme is expected to violate global conservation. A comparison with the chemical change in the global ozone burden obtained using an ozone photochemistry parameterisation quantifies the relative importance of this numerical change. This comparison also reveals the importance of a global diagnostic (e.g. global burden) over a local diagnostic (e.g. ozone hole minimum) in determining the most accurate numerical scheme. The numerical mixing rate in the model was also determined by representing the volcanic eruption of Sarychev Peak in 2009. This was then compared to a mixing rate determined from observations and was found to be too fast in the upper troposphere lower stratosphere region of the atmosphere. However, the numerical diffusivity is within the range of those determined in the atmosphere. The output from the global tracer advection model is then used to investigate the feasibility of a tracer-relative CTM. This CTM uses a reference tracer that is almost inert, such as potential vorticity, and describes the evolution of all other tracer species relative to it, under the assumption that all tracer contours are parallel. The dominant source of error in this method is the assumption that the 3D tracer distribution can be described using the background state (determined from the reference tracer) and the parallel contour approximation. A secondary error is introduced by the assumption that the heating field also aligns with the background state, but greatly simplifies the steps required for this new model.

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Atmospheric aerosols play a key role in affecting climate, pollution and human health by in- fluencing the properties and lifetimes of clouds and precipitation. Despite this, the molecular mechanisms involved in the nucleation of nanoscale liquid droplets are not yet fully under- stood. In this thesis, we attempt to gain a better understanding of the surface properties and small-scale thermodynamics of nanoscopic clusters of water using molecular dynam- ics simulation to complement the recent surge of scientific interest in atmospheric aerosol nucleation. Existing literature is reviewed on simulation techniques to obtain the surface tension of planar and curved interfaces, as well as studies on vapour-liquid nucleation. This is followed by several in-depth computational investigations into the surface properties of water. In the first, we apply a free-energy perturbation technique, namely the test-area (TA) method, to compute the surface tension of planar, cylindrical and spherical geometries of water. In addition, a mathematical analysis of the TA method is carried out, revealing further insight into the underlying features of perturbation methods as well as their limi- tations. The results are compared to previous literature and the differences discussed. In the second and third, the excess free energy is obtained for small clusters of water using an extension of a non-equilibrium disassembly method originally developed for studies on argon clusters. The results are used to construct free-energy curves that are subsequently fed into classical rate expressions and compared to experimental values. In the final part of the thesis, the formalism generalising the cluster disassembly method to multi-component systems is presented.

As the earth’s climate continues to change, it is becoming increasingly clear that the mitigation of anthropogenically released greenhouse gases, such as those emitted by the growing aviation industry, is a high priority. Through the emission of short–lived and long–lived climate–forcing chemical species, there are several ways in which the aviation industry affects the climate. The long–lived greenhouse gas CO2 has been well–studied, but the effects of the short–lived climate forcer NOx , which perturbs ambient O3 and CH4 in the atmosphere, is less well understood. Through changes in aircraft engine design and behaviour, the relative emissions of these two climate forcers can be tuned to address specific mitigation targets. However, a trade-off exists between aviation NOx and CO2 emissions as reducing one results in an increase in the other, and vice versa. Here, the trade–off between CO2 and NOx is investigated using the MOZART–3 chemistry transport model (CTM) and a simple climate model (SCM), LinClim. LinClim, which is much less computationally intensive, assumes a linear relationship between aviation NOx emissions and associated O3 burden and CH4 lifetime change. By using the more sophisticated MOZART–3, it was found that both these NOx – O3 and NOx – CH4 relationships are linear while aviation emissions are below 3 Tg N Yr-1 but thereafter, become non–linear. A new non-linear net NOx RF parameterisation is developed from the results of the CTM runs and used to investigate this trade-off. Experiments showed that a small CO2 increase (+2 percent) raised the overall forcing from aviation, despite a larger reduction in NOx emissions (-20 percent). When background NOx levels were high the experiments showed that a 43 percent reduction in NOx emissions was required to counteract the radiative forcing of the additional CO2 emissions, and when NOx was reduced by 20 percent, only a 0.5 percent CO2 penalty could be allowed before an additional forcing was incurred. When background NOx emissions were low, the results were more complex as net NOx forcing became negative. Therefore, any reduction in aviation NOx emissions actually increased the net forcing. In this case additional NOx emissions were necessary to reduce overall forcing from aviation emissions. These results indicate that the most important emission to mitigate in the aviation industry is CO2 , its long–lived cumulative nature causes it to contribute substantially more to global climate change than aviation NOx emissions, which, depending on the state of the background atmosphere, contribute either a small positive or small negative, net forcing.

One of Earth’s most powerful natural resources is wind; harnessing its power in order to generate electricity is an ever expanding market throughout the world. In order to best utilise this resource it is important to be able to make accurate predictions of future wind speeds. This thesis focuses on accurately forecasting future wind speeds using statistical methods, in particular utilising past wind speed data and numerical weather prediction (NWP) data. Wind speed data are non-stationary time series, and the relationship between future wind speeds and NWP data may change over time. With this in mind, we look at models that can capture smooth variation in the series, specifically looking at dynamic linear models. In practice, many different forms of model can be used for predicting future wind speeds. We also look at methods for choosing between models, or for combining the predictions that different models make. We offer novel approaches that allow for abrupt changes in the choice of model based on the accuracy of recent predictions, with these approaches offering improvements in forecast accuracies. The final focus of this thesis is on developing novel methods for predicting ramp events: sudden, sharp changes in wind speeds. Predicting these events accurately is important, as ramp events can lead to substantial changes in the amount of wind energy produced. However, such events are currently not predicted well by standard forecasting models.