Aviation Induced Tropospheric Ozone

Abstract

In recent years, mitigation of the anthropogenic climate impact has become increasingly more important. Over the past decades, the aviation industry has grown significantly, making it a large contributor to greenhouse gases in the atmosphere. One such greenhouse gas is Ozone (O3) in the troposphere, which has a warming effect on the global climate. Aviation Nitrogen Oxides (NOx) emissions play a role in the formation and loss of O3 in the troposphere, however, the chemistry is highly non-linear which makes mitigation more complex. To gain a better understanding of the contribution of aviation to tropospheric O3, this thesis looks at how aviation’s tropospheric O3 contribution varies and how the Ozone Burden Efficiency (OBE) metric can be used to better explain these global variations. The OBE is a metric which quantifies the efficiency of the background chemistry and the global transportation phenomena to promote the net formation of O3. To conduct this research, chemistry climate simulation results of the ECHAM/MESSy Atmospheric Chemistry (EMAC) model were used for a period of 2003 to 2018.

The analysis showed that across the simulated time period aviation’s contribution to tropospheric O3 increased from about 1.6% to approximately 1.9%. Looking at the vertical spread of the aviation induced O3 showed that downwards transport of O3 drives global distributions, creating large O3 mixing ratios in the free troposphere under the flight altitudes. Furthermore, the analysis of the vertical spread showed that the percentage contribution of aviation to local O3 mixing ratios is larger at ground level than at flight altitudes, showing that aviation induced O3 also contributes to air quality.

Analysing the OBE, showed that the OBE of aviation for the whole troposphere is about 4.5 Tg (O3) · Tg−1 (NOx). Further analysis showed that the OBE in the planetary boundary layer is about 2.5 Tg (O3)· Tg−1 (NOx), whereas the OBE at flight altitudes is about 1.3 Tg (O3) · Tg−1 (NOx). The larger OBE at ground level is caused by the large scale downward transport of the formed O3 and the low amount of local NOx emissions, whereas at flight altitudes, the local NOx emissions are not as small compared to the local O3 burden. Furthermore, analysis of the OBE metric also shows that the OBE of aviation is largest in the free troposphere below the flight altitudes, where it is estimated to be around 8 Tg (O3) · Tg−1 (NOx). This result highlights the large amount of O3 which is transported downwards to lower altitudes from the flight levels. Overall, the analysis using the OBE metric shows that it is most useful for emission sectors where the emissions are dominated by NOx emissions rather than carbon species which also contribute towards the production of O3. However, this attribute of the OBE metric makes it difficult to use OBE values to compare different emissions sources.