The impact of emissions of nitrogen oxides from aviation on tropospheric chemistry – the counterbalancing roles of ozone and methane

Skowron, Agnieszka Maria (2013) The impact of emissions of nitrogen oxides from aviation on tropospheric chemistry – the counterbalancing roles of ozone and methane. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Aviation is a unique anthropogenic emission source in that it is the only man-made source of emissions injected directly to the remote and uncontaminated regions of the atmosphere. It constitutes a relatively small fraction of total anthropogenic climate impact. However, with the potential increase in growth of air traffic and potential reductions of emissions in other sector, the importance of aircraft contribution to anthropogenic climate change may increase in the future. Aviation NOx emissions result in a short-term increase in tropospheric ozone (warming) and the long-term destruction of a small amount of ambient methane (cooling), positive and negative radiative forcing responses. In addition, the methane reduction results in a long-term reduction in tropospheric ozone (cooling) and a longterm reduction in water vapour in the stratosphere (cooling) from reduced oxidation of methane, both negative radiative forcing effects. The aircraft net NOx response (the sum of all these components) is thought to result in a positive (warming) radiative forcing under constant emissions assumptions. The quantification of aircraft NOx effects requires spatio-temporal analyses of its dependencies, which is challenging and result in significant variations in aircraft NOx ‘impact’. An investigation of a series of different factors influencing the effect of aircraft NOx emission on climate was performed using the global chemistry transport model (MOZART-3) in combination with a radiative transfer model (Edwards–Slingo), and rationale of the existing uncertainties associated with aircraft NOx estimates was conducted. A wide spectrum in the magnitude and balance of chemical responses from aircraft NOx perturbations, affecting Global Warming Potential estimates, was observed. The derived aircraft net NOx radiative forcing was found to be as low as -0.4 and as high as 19.9 mWm-2/Tg(N)yr-1. The balance between aircraft ozone and methane changes proved to be experiment-specific, and it was observed to depend on background conditions due to surface emissions or region of emission, aircraft NOx emission inventory, or size of aircraft NOx emission rate. Thus, it was shown that it is impossible to define a unique GWP for aviation NOx. Moreover, an aviation NOx GWP increases with reduction of aircraft NOx emissions and decreases with increased aircraft NOx emissions.