Evaluating Emissions of Nitrogen Oxides in Megacities

It is estimated that 7 million deaths globally are due to poor air quality. Understanding the sources, transport, transformation and eventual fate of pollutants in the atmosphere is important in informing interventions to reduce this number. This thesis focuses on measuring emissions of Nitrogen Oxides in two megacities - London, UK (spring 2017) and Delhi, India (pre- and post-monsoon 2018) - and using them to evaluate emissions inventories. A good understanding of urban emissions is important, as the proportion of the global population living in them is ever increasing and with it total exposure to poor air quality. Nitrogen oxides are of interest due to their role in tropospheric ozone formation and the detrimental health effects of nitrogen dioxide. Here, eddy covariance has been used to measure \NOx emission in both cities, and developments have been made in mass balance methods towards an alternative measurement of \NOx emission in London. The measurements made in London, in agreement with previous measurements, found that there is an underestimation of NOx emission by the UK's National Atmospheric Emissions Inventory by an average of 1.4 x and that this became more pronounced at the weekends due to differences between diurnal traffic flow and the diurnal profile used to scale road traffic emissions. In Delhi, NOx emissions were measured at two sites, and a local inventory was shown to overestimate the emissions significantly, however, the spatial variation in emissions was well captured. Finally, work on the mass balance method highlighted key areas for continued development for the method to be applicable for reactive gases and large targets such as London. These include improving the treatment of chemical transformation between emission and measurement, and more robust treatment of the air mass being sampled to connect it with emissions inventories.