Abstract
Aviation emissions of nitrogen oxides (NOx) alter the composition of the atmosphere, perturbing the greenhouse gases ozone and methane, resulting in positive and negative radiative forcing effects, respectively. In 1981, the International Civil Aviation Organization adopted a first certification standard for the regulation of aircraft engine NOx emissions with subsequent increases in stringency in 1992, 1998, 2004 and 2010 to offset the growth of the environmental impact of air transport, the main motivation being to improve local air quality with the assumed co-benefit of reducing NOx emissions at altitude and therefore their climate impacts. Increased stringency is an ongoing topic of discussion and more stringent standards are usually associated with their beneficial environmental impact. Here we show that this is not necessarily the right direction with respect to reducing the climate impacts of aviation (as opposed to local air quality impacts) because of the tradeoff effects between reducing NOx emissions and increased fuel usage, along with a revised understanding of the radiative forcing effects of methane. Moreover, the predicted lower surface air pollution levels in the future will be beneficial for reducing the climate impact of aviation NOx emissions. Thus, further efforts leading to greater fuel efficiency, and therefore lower CO2 emissions, may be preferable to reducing NOx emissions in terms of aviation’s climate impacts.
Highlights
Base −30% nitrogen oxides (NOx) −30% CO −30% NMVOC −30% all together (ALL) Representative Concentration Pathways (RCP) 8.5 Representative Concentration Pathways52 (RCPs) 4.5 RCP 2.6 RCP 8.5 RCP 4.5 RCP 2.6 Aircraft REACT4CLow NOx High Tech High NOx Low Tech radiative forcing (RF), mW m−2 sO3 CH4 −6.9 −9.5 −6.7 −8.5 −23.4
Using a suitable three-dimensional chemistry transport model (CTM) of the global atmosphere (MOZART3)[16,17], we examine the changes in the tropospheric composition and the net RF from aviation NOx emissions for 30% reductions in the most recent present-day inventories available (2006) of O3 precursor emissions (NOx, carbon monoxide - CO, non-methane volatile organic compounds - NMVOC) and for a future (2050) range of Representative Concentration Pathways (RCP) scenarios together with ICAO-CAEP aviation emission projections
The largest aviation net NOx RF is observed under RCP 8.5 and the smallest for RCP 2.6
Summary
Aviation net NOx radiative forcing in 2050. The resulting RFs (Table 1) highlight that an aviation net NOx RF can vary greatly depending on the background condition, and both anthropogenic surface and aircraft emissions affect the aviation net NOx RF. If the anthropogenic surface emissions were kept constant at present levels, the 2050 aircraft net NOx RF of the high air-traffic growth would be 17.5 mW m−2, that is 48% greater than under the 2050 RCP 2.6 and 28% greater than under 2050 RCP 8.5 background conditions. This observed increase of aviation net NOx RFs in the future is in agreement with other studies that have explored the climate impact from aviation NOx emissions in 2050. Unger et al.[19] calculated that both the positive short-term O3 and negative CH4 RFs in 2050 increased by ~80% for the AEDT Base scenario (4.0 Tg(N) year−1), whereas Khodayari et al.[20] estimated
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