The International Civil Aviation Organization (ICAO) has implemented increasingly stringent standards for aircraft NOx emissions since 1981, primarily to improve local air quality. It was assumed that this would have a co-benefit of reducing climate impact. This paper investigates the validity of this assumption, given the complex interplay between NOx emissions, fuel efficiency, and radiative forcing. Aircraft NOx emissions contribute to both short-term warming (via ozone formation) and long-term cooling (via methane destruction). The net effect depends on various atmospheric conditions and the balance between these competing factors. Furthermore, the interaction between aircraft NOx and surface NOx emissions also needs to be considered, as the climate effect of aviation NOx is dependent on background atmospheric conditions. The study aims to analyze the impact of future aviation emission scenarios and surface emission changes on the climate impact of aviation NOx, questioning the prevailing approach of prioritizing NOx reduction over fuel efficiency.

Previous research has generally advised reducing both NOx and CO2 emissions from aviation. However, this presents a technological challenge due to the trade-off between NOx and CO2 emissions from aircraft engines. Studies have shown that reducing NOx might lead to a fuel penalty, potentially resulting in a net climate disbenefit. There's limited research exploring the impact of changing surface emissions on the climate effect of aviation NOx. While existing models explored the impact of future aviation emissions on atmospheric composition and radiative forcing, inconsistencies and uncertainties remain, largely due to differences in emission projections, model chemistry schemes, and the treatment of physical processes. This study aims to address this gap by analyzing the impact of varying background emission scenarios on the effectiveness of reducing aviation NOx emissions.

The study uses the MOZART-3 three-dimensional chemistry transport model (CTM) to simulate changes in tropospheric composition and radiative forcing from aviation NOx emissions. The model considers both aviation and surface NOx emissions with varying degrees of mitigation. Aviation NOx emissions are projected using ICAO-CAEP data for both low and high air-traffic growth scenarios in 2050, coupled with technology development scenarios. Surface emissions are based on the IPCC AR5 data for 2006 and the RCP scenarios (2.6, 4.5, and 8.5) for 2050. To assess the effect of reducing individual precursors, several sensitivity experiments are performed, which involve reducing NOx, CO, NMVOC emissions, and all three simultaneously by 30%. The Edwards-Slingo radiative transfer model (RTM) is employed to calculate the radiative forcing associated with ozone changes. The study utilizes both the original and a revised simplified expression for the calculation of methane radiative forcing from Etminan et al. (2016), allowing for comparison of the results. The temperature responses from unit aircraft CO2 versus NOx emissions are analyzed using a simple climate model (LinClim) and by calculating Absolute Global Temperature change Potentials (AGTP) based on steady-state CTM/RTM results.

The results show a significant increase in aviation NOx radiative forcing from 2006 to 2050 across all RCP scenarios, primarily driven by increased aviation NOx emissions. However, the reduction in surface NOx emissions under the RCP scenarios partially mitigates this increase. The study demonstrates that the net aviation NOx radiative forcing varies significantly depending on background conditions. Reducing surface NOx emissions proves to be more effective in reducing the net aviation NOx radiative forcing than reducing aircraft NOx emissions, as a 1% change in surface NOx emissions changes the aviation net NOx RF by ~1.5%. The analysis of individual precursor reductions indicates that decreasing surface NOx has the most significant impact, while reducing CO has the least impact on the net radiative forcing. Using the revised methane radiative forcing expression, the net aviation NOx radiative forcing becomes negative, further reducing the effectiveness of solely targeting aviation NOx emissions. This is because decreased surface NOx emissions increase the CH4 lifetime, increasing the negative forcing from the reduction in methane lifetime caused by aviation NOx. The study also compares the temperature response from a unit emission of aircraft CO2 versus NOx, highlighting the much more substantial and long-lasting effect of CO2 emissions. It shows that a smaller change in CO2 emissions has a much larger impact on total forcing than an equivalent change in NOx emissions.

The findings challenge the assumption that stricter NOx regulations automatically translate to significant climate benefits for aviation. The strong influence of background NOx levels on the net radiative forcing from aviation NOx suggests that focusing solely on reducing aircraft NOx emissions might not be the most effective climate strategy. The results indicate that mitigating CO2 emissions through improved fuel efficiency is likely more impactful. The study's sensitivity experiments clearly demonstrate the greater efficacy of reducing surface NOx emissions compared to aviation NOx emissions in reducing the climate impact. The revised methane radiative forcing calculation further supports this conclusion, highlighting the substantial reduction in net NOx forcing when considering the long-term effects of methane. While acknowledging the need to reduce aircraft NOx for local air quality reasons, the study strongly suggests a re-evaluation of prioritization for climate-change mitigation.

This study demonstrates that the climate impact of aviation NOx emissions is highly sensitive to background atmospheric conditions, particularly surface NOx levels. Reducing surface NOx emissions is more effective at mitigating the climate impact than reducing aviation NOx emissions. Moreover, the inclusion of the updated methane radiative forcing emphasizes that focusing on improving fuel efficiency and lowering CO2 emissions is likely more beneficial for the climate than solely focusing on reducing aviation NOx. Further research should prioritize integrated assessments that consider both climate and air quality impacts under changing background conditions.

The study utilizes a specific chemistry transport model (MOZART-3) and radiative transfer model (Edwards-Slingo). The results might vary depending on the chosen models and their parameterizations. While the study addresses several emission scenarios, uncertainties remain regarding future emission projections and technological developments in aviation. Additionally, the analysis does not account for all potential effects of NOx emissions, such as the effects on aerosol formation.