Examining the effects of aviation NOx emissions as a short-lived climate-forcer

Freeman, Sarah, Joanne (2017) Examining the effects of aviation NOx emissions as a short-lived climate-forcer. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

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.