Abstract
Abstract. Aviation-attributed climate impact depends on a combination of composition changes in trace gases due to emissions of carbon dioxide (CO2) and non-CO2 species. Nitrogen oxides (NOx = NO + NO2) emissions induce an increase in ozone (O3) and a depletion of methane (CH4), leading to a climate warming and a cooling, respectively. In contrast to CO2, non-CO2 contributions to the atmospheric composition are short lived and are thus characterised by a high spatial and temporal variability. In this study, we investigate the influence of weather patterns and their related transport processes on composition changes caused by aviation-attributed NOx emissions. This is achieved by using the atmospheric chemistry model EMAC (ECHAM/MESSy). Representative weather situations were simulated in which unit NOx emissions are initialised in specific air parcels at typical flight altitudes over the North Atlantic flight sector. By explicitly calculating contributions to the O3 and CH4 concentrations induced by these emissions, interactions between trace gas composition changes and weather conditions along the trajectory of each air parcel are investigated. Previous studies showed a clear correlation between the prevailing weather situation at the time when the NOx emission occurs and the climate impact of the NOx emission. Here, we show that the aviation NOx contribution to ozone is characterised by the time and magnitude of its maximum and demonstrate that a high O3 maximum is only possible if the maximum occurs early after the emission. Early maxima occur only if the air parcel, in which the NOx emission occurred, is transported to lower altitudes, where the chemical activity is high. This downward transport is caused by subsidence in high-pressure systems. A high ozone magnitude only occurs if the air parcel is transported downward into a region in which the ozone production is efficient. This efficiency is limited by atmospheric NOx and HOx concentrations during summer and winter, respectively. We show that a large CH4 depletion is only possible if a strong formation of O3 occurs due to the NOx emission and if high atmospheric H2O concentrations are present along the air parcel's trajectory. Only air parcels, which are transported into tropical areas due to high-pressure systems, experience high concentrations of H2O and thus a large CH4 depletion. Avoiding climate-sensitive areas by rerouting aircraft flight tracks is currently computationally not feasible due to the long chemical simulations needed. The findings of this study form a basis of a better understanding of NOx climate-sensitive areas and through this will allow us to propose an alternative approach to estimate aviation's climate impact on a day-to-day basis, based on computationally cheaper meteorological simulations without computationally expensive chemistry. This comprises a step towards a climate impact assessment of individual flights, here with the contribution of aviation NOx emissions to climate change, ultimately enabling routings with a lower climate impact by avoiding climate-sensitive regions.
Highlights
The importance of anthropogenic climate change has been well established for years (Shine et al, 1990), and it is well known that air traffic contributes substantially to the total anthropogenic climate change (Lee et al, 2009; Brasseur et al, 2016; Grewe et al, 2017a)
We investigate the impact of weather situations on changes in O3 and CH4 concentrations induced by NOx emissions in the upper troposphere of the North Atlantic flight sector
We demonstrated the importance of transport processes on locally induced aviation-attributed NOx emission on O3 and CH4 concentrations over the North Atlantic flight sector
Summary
The importance of anthropogenic climate change has been well established for years (Shine et al, 1990), and it is well known that air traffic contributes substantially to the total anthropogenic climate change (Lee et al, 2009; Brasseur et al, 2016; Grewe et al, 2017a). A major fraction of its contribution comes from non-CO2 emissions, which lead to changes in greenhouse gas concentrations as well as contrail and contrail–cirrus formation in the atmosphere (Kärcher, 2018). The climate impact of CO2 is mainly characterised by the emission strength, due to its long lifetime. NonCO2 effects are known to be characterised by a high spatial and temporal variability. This implies that the total contribution to concentrations of non-CO2 emissions is influenced by the emissions strength and by the time and location of the emission itself. NO2 forms O(3P) leading to the formation of O3
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