Introduction

China is a populous country, with a population of 1.412 billion in 20221. At the same time, China has one of the fastest-growing economies and aviation industries globally. According to the World Bank, in the decade (2010–2019), China’s GDP growth averaged 7.68%. China’s average air passenger traffic growth rate was 11.18%1. And air transportation between China and foreign countries has developed dramatically in recent years. In 2019, the total turnover of China’s foreign routes was 46.37 billion ton-kilometers, an increase of 93.13% over 2014; China’s foreign routes completed 74.25 million passengers, a rise of 135.34% over 2014; China’s foreign routes met 2.42 million tons of goods and mail, an increase of 44.0% over 20142. Although affected by the epidemic, the traffic volume during 2020–2022 has decreased; China-Foreign routes are still one of the fastest growing and busiest routes in the world. However, the aviation industry’s rapid growth also emits more pollutants, and environmental damage is increasingly severe3. Therefore, the global aviation industry must achieve the goal of carbon neutrality to study the impact of pollutant emissions of China’s external routes on the climate.

Carbon dioxide (CO2) accounts for most aircraft emissions, dramatically impacting the greenhouse gas effect and many studies have focused on calculating CO2 emissions4,5,6. In addition to CO2, aircraft activities also generate other emissions, including carbon monoxide (CO), hydrocarbon (HC), nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter (PM2.5). CO can cause significant damage to the body’s physical and mental systems near airports, and even high concentrations of CO can even cause death7. HC and NOx may form photochemical smog, damaging human and animal health and affecting plant growth around airports8. SO2 is an integral part of acid rain, which can cause severe damage to soil, water sources, forests, etc9. PM2.5 affects human lungs and shortens human life span10. The primary calculation methods of non-CO2 emissions are the ICAO method (Boeing Fuel Flow Method 2 (BFFM2) method and First Order Approximation (FOA) method11,12, the American EPA method13 and the European EMEP method14, etc. Some studies have applied them to calculate non-CO2 emissions15,16,17,18,19. However, these methods are more suitable for accounting for aircraft emissions during the Landing and Take-off (LTO) stage and cannot be combined with CO2 calculations4. The primary purpose of the EPA method is to calculate the emission inventory during the LTO cycle. EMEP calculation method mainly analyzes the emission characteristics of an aero-engine from the fuel perspective and ignores the differences between engine types. The ICAO method is more general, but the distance difference is not enough, and there is no difference between specific aircraft4. Therefore, Cui et al. propose a Modified BFFM2-FOA-FPM method based on the Fuel Percentage Method (FPM). This method can calculate the CO2 and non-CO2 emissions. The error rate between the results and official data is relatively low3,4,20,21.

A range of different climate pollutants contributes to anthropogenic climate change22. Emissions of different climate pollutants are often described using a standard metric, generally scaled relative to CO2; hence non-CO2 emissions are typically communicated (and aggregated) as carbon dioxide equivalent (CO2-e) quantities. Since the 1970s, scholars worldwide have begun to study the climate effects of greenhouse gases. The ozone depletion potential (ODP) parameter was put forward in consideration of the destruction of the ozone layer by the increasing number of man-made greenhouse gases in the atmosphere, which represents the ability of a substance to deplete stratospheric ozone relative to halocarbon CFC-1123. CFC-11 was used as the reference gas to define the halocarbon global warming potential (HGWP) and calculate the HGWP of some gases, excluding CO2, CH4, and other gases24. The above research mainly aims at man-made greenhouse gases such as CFCs, HFCs, and PFCs. Until 1990, Rodhe put forward the integrated greenhouse effect of greenhouse gases AG (x) (Accumulated Greenhouse Effect)25, and the research scope expanded from halocarbons to CH4, N2O, and other major greenhouse gases. This index is equivalent to the Absolute Global Warming Potential (AGWP) defined by IPCC26. It converts the emissions of greenhouse gases into “equivalent CO2 emission”. Therefore, although AG (x) is an absolute measure, it can still compare the relative size of different greenhouse gases based on CO2.

At present, the most widely used CO2 equivalence metric is the Global Warming Potential (GWP), defined as the integrated change in radiative forcing (the perturbation of the Earth’s atmospheric energy balance, which leads to warming) over a specified period following an emission pulse of a given climate pollutant, relative to the same quantity of CO2. Emissions of a given climate pollutant (E) can thus be converted to a CO2-e emission quantity by multiplying by the appropriate GWP conversion factor for the specified time horizon27. The widely used GWP concept now contains the atmospheric chemical and carbon cycle processes of greenhouse gases due to the addition of the concentration-response function. It is a comprehensive indicator to measure the warming capacity of greenhouse gases. Applying this standard, any greenhouse gas can be converted into equivalent CO2, and the relative magnitude of its climate effect is apparent at a glance. Among them, the 100-year variation of the Global Warming Potential value (GWP100) has officially adopted the international climate policy (currently established in the Kyoto Protocol and the draft Paris Agreement28) and the standardized life cycle assessment (LCA)/carbon footprint method29). Subsequently, GWP100 has become the objective standard for expressing emissions in scientific literature and general media and has become the abbreviation of the relative climate impact of specific products or activities30,31. However, the GWP does not consider the environmental absorption of greenhouse gases that may reduce the climate effect of greenhouse gases.

According to the European Union’s official data, the aviation industry’s greenhouse gas emissions accounted for almost 3% of the total greenhouse gas emissions in 2006. Because the environmental costs were not considered, people chose to travel by air, which made the greenhouse gas emissions of the aviation industry show a rapid growth trend. Since 1990, EU aviation carbon emissions have increased by 87%32. These negative impacts are mainly due to burning fossil kerosene during flight operations33, in which about 2.5 kg CO2 is emitted per liter of kerosene34,35. Due to the negative impact on the environment and increasing social pressure, political regulations have been formulated to reduce these emissions. In 2005, the EU planned to levy an aviation carbon tax. In December 2006, the European Commission put forward a legislative proposal to bring greenhouse gas emissions from civil aviation into the EU emissions trading scheme for the first time. In November 2008, the European Parliament adopted a new bill to include the international aviation industry in the EU carbon emissions trading scheme, which came into force on January 1, 201232. The aviation carbon tax has become an economical means for the EU to reduce carbon dioxide in the aviation industry33. However, EU ETS mainly provides economic compensation for the environmental impact of air travel.

A more far-reaching strategic concept in the aviation sector is critical to enhancing sustainable aviation that does not rely on fossil fuels. Therefore, research on sustainable aviation fuel has gradually increased in recent years. Synthetic biomass fuel, green hydrogen, atmospheric CO2 synthetic fuel, and direct liquid green hydrogen can achieve climate neutrality in the aviation industry36. For the aviation industry, SAFs used in commercial aircraft provide a long-term and environment-friendly solution, while battery technology needs further development to improve safety and reliability37. The sustainability of sustainable aviation fuel use was evaluated based on life cycle assessment and cost calculation methods and alternative fuels were compared with fossil kerosene regarding environmental and economic impacts38.

To sum up, there is no systematic research on the two main ways of aviation emission reduction (SAFs and ETS), and the existing research only focuses on one of them; In addition, there is no analysis of specific regions and specific emissions, and the conclusions are not targeted. To solve the above problems, this paper first applies the Modified BFFM2-FOA-FPM method3,4 to calculate the emissions of China-Foreign routes during 2014–2021. Then it builds an Aviation FAIR-GWP concentration method to calculate the equivalent concentration of non-carbon dioxide pollutants in the aviation field. At the same time, it considers the role of SAFs and ETS separately and combines them to study their specific impact on control emissions. Finally, we take the China-Foreign routes, the fastest-growing routes in the world, as an example to calculate the actual emissions and make accurate analyses to provide more systematic results and suggestions for the effective emission reduction of the aviation industry.

Results

Taking China-Foreign routes as examples, this paper focuses on the role of Sustainable Aviation Fuels (SAFs) and Emission Trading Scheme (ETS) on aviation emission reduction and compares the effects of the two approaches. Based on the Aviation FAIR-GWP concentration method (see details in Method), considering the absorption of greenhouse gases by the environment, and taking the Carbon Dioxide Equivalent Concentration (CDEC) as the measurement index, we compare which path (SAFs or ETS) plays a more significant role. This paper sets four scenarios: the baseline scenario, the scenario with only ETS (CNG2020 strategy), the scenario with only SAFs, and the scenario with ETS and SAFs.

The CDEC under the baseline scenario

Under the baseline scenario, we assume that the annual emissions (until 2100) equal the average from 2014 to 2021. It can be seen from the emissions that from 2014 to 2019, the emissions of China-foreign routes have been growing, but due to the impact of the COVID-19, the emissions in 2020 and 2021 have declined significantly. Because the period considered in this paper is 2023–2100, the emissions of domestic and foreign routes will show a trend of rising first and then falling, and the period 2014–2021 is also in line with this trend, so this paper takes the average emissions of this period as the baseline emissions, that is, the baseline scenario.

The number of routes has increased from 367 in 2014 to 521 in 2019, but with the outbreak of COVID-19, the number of routes declined to 217 in 2020, and the impact continued in 2021, with 203 routes. The routes from 2014 to 2021 cover about 80 countries in total. We calculated the emissions of six pollutants (CO2, CO, HC, NOx, SO2, and PM2.5) from China-Foreign routes (see the Supplementary tables for details). As shown in Fig. 1, from 2014 to 2016, the overall emissions of six pollutants from China-foreign routes showed a trend of gradual increase. Taking the most significant proportion of CO2 as an example, the overall emissions increased from 28,147,816.42 tons in 2014 to 45,928,507.74 tons in 2016. Later, with the proposed emission reduction measures such as the CNG2020 strategy, the overall emissions of air pollutants were generally controlled3, and the growth rate of the six pollutants gradually declined or even became negative. As a result, the average growth rates of the six pollutants (CO2, HC, NOx, SO2, PM2.5, and CO) in 2017–2019 were 1.19%, −1.43%, 2.51%, 1.19%, 4.34%, and −0.23%, respectively. At the beginning of 2020, COVID-19 broke out, and the number of China-foreign routes decreased sharply. Therefore, the emissions of six pollutants in 2020 and 2021 were less. Taking CO2, which accounted for the most significant proportion, as an example, the emissions in 2020–2021 were 7,602,624.891 tons and 7,557,405.133 tons, respectively.

Fig. 1
figure 1

Overall emissions of six pollutants in China-foreign routes from 2014 to 2021.

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We selected several emissions of CO2, N2O (part of NOx), and CH4 (par of HC) that have a significant impact on climate and calculated their concentration changes from 2023 to 2100 to analyze the impact of pollutant emissions from domestic and foreign routes on climate, as shown in Fig. 2. As mentioned above, climate impact under baseline scenario refers to the impact of pollutant emissions from China-foreign routes on the climate without regulating the carbon trading scheme and using sustainable aviation fuel. That is, the pollutant emissions after 2023 are the average from 2014–2021. Figure 2 shows the concentration change of CO2, N2O, and CH4 and the change of CDEC from 2023 to 2100 (The left axis represents CDEC and CO2 concentrations, while the right axis represents N2O concentration and CH4 concentration). Among them, the concentration change of CO2 and CDEC show the growth value rather than the actual value. The CO2 and N2O show a cumulative upward trend, and the concentration increase of CO2 will reach 7.52E-07 ppm. And the concentration of N2O will reach 5.65E-10 ppb, by 2100. The concentration of CH4 increased from 1.46E-11 ppb in 2023 to 5.71E-10 ppb in 2032, then decreased to 5.65E-10 ppb in 2033 after reaching the peak and then remained unchanged. The concentrations of the three pollutants have increased without restriction, with CO2 accounting for the most significant proportion. The concentration of aerosol and tail will not be accumulated, so it is not considered. The growth value of CDEC also will show a gradual upward trend, from 9.65E-09 ppm in 2023 to 7.53E-07 ppm in 2100. In addition to CO2, N2O plays a significant role in climate change. In addition, it can be seen from Fig. 3 that there is little difference between the change of CDEC and the change of CO2 concentration, and the CDEC will not be shown separately in the following text.

Fig. 2
figure 2

Concentration of CO2, N2O, CH4, and CDEC under the baseline scenario.

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Fig. 3
figure 3

Concentration of CO2 under the six sub-scenarios with only ETS and those under baseline scenario.

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The CDEC under the scenario with only ETS

We calculated the concentration change of CO2, N2O, and CH4 and the change of CDEC during 2023–2100 under the constraint of only a carbon emission trading scheme and compared it with the baseline situation. As China-Foreign routes belong to international routes and the goal of the CNG2020 strategy is to achieve aviation carbon neutrality of international routes by 203539,40, China-Foreign routes fall the scope of the CNG2020 strategy. If the carbon neutrality of China-Foreign routes can be achieved by 2035, there will be three sub-scenarios: 2023–2026–2035 (recovery to the level of 2019 in 2023, reach carbon emissions peak in 2026, and reach carbon neutrality in 2035), 2024–2027–2035 (recovery to the level of 2019 in 2024, reach carbon emissions peak in 2027, and reach carbon neutrality in 2035), 2025–2028–2035 (recovery to the level of 2019 in 2025, reach carbon emissions peak in 2028, and reach carbon neutrality in 2035). Suppose aviation carbon neutrality cannot be achieved in 2035 and will be achieved by 2040. In that case, there are also three sub-scenarios: 2023–2030–2040 (recovery to the level of 2019 in 2023, reach carbon emissions peak in 2030, and reach carbon neutrality in 2040), 2024–2030–2040 (recovery to the level of 2019 in 2024, reach carbon emissions peak in 2030, and reach carbon neutrality in 2040), 2025–2030–2040 (recovery to the level of 2019 in 2025, reach carbon emissions peak in 2030, and reach carbon neutrality in 2040). Figure 3 shows the concentration of CO2 of China-foreign routes in 2023–2100 under six sub-scenarios with carbon emission trading scheme constraints and in 2023–2100 under baseline scenario (The left axis represents CDEC and CO2 concentrations, while the right axis represents N2O concentration and CH4 concentration). The concentration changes of CH4 and N2O are not obvious, so no comparison is made.

Under the constraint of the carbon emission trading scheme, the CO2 concentration decreased significantly. As shown in Fig. 3, under baseline scenario, the growth value of CO2 concentration will increase from 9.65E-09 ppm in 2023 to 7.52E-07 ppm in 2100. If aviation carbon neutralization can be achieved in 2035, the growth value of CO2 concentration under the three sub-scenarios of 2023–2026–2035, 2024–2027–2035, 2025–2028–2035 will be 1.35E-07 ppm, 1.38E-07 ppm, and 1.40E-07 ppm respectively by 2100, which is about 17.96%-18.61% of that under no constraints. However, under the three sub-scenarios of 2023–2030–2040, 2024–2030–2040, and 2025–2030–2040, the CO2 concentration will be 2.78E-07 ppm, 2.52E-07 ppm, and 2.27E-07 ppm, respectively, by 2100, which is about 30.27%-37.10% of that under general sub-scenarios. For CDEC, there is also a noticeable decline. Under baseline scenario, the CO2 concentration-equivalent increased from 9.65E-09 ppm in 2023 to 7.53E-07 ppm in 2100. If aviation carbon neutralization can be achieved in 2035, the CDEC in 2023–2026–2035, 2024–2027–2035, 2025–2028–2035 will be 1.36E-07 ppm, 1.39E-07 ppm, and 1.41E-07 ppm respectively in 2100, which is about 18.06%-18.70% in the baseline condition. However, in the three cases of 2023–2030–2040, 2024–2030–2040, and 2025–2030–2040, the CDEC is forced to be 2.80E-07 ppm, 2.53E-07 ppm, and 2.29E-07 ppm, respectively, by 2100, which is about 30.37%-37.18% in the baseline case. It is worth noting that the concentration of CH4 under the six baseline sub-scenarios are all 3.21E-09, which is higher than the baseline scenario (3.13E-09 ppm). And N2O under the six baseline sub-scenarios has not changed, so the reduction of CO2 concentration mainly causes the significant reduction of CDEC. Under the constraint of the carbon emission trading scheme, the primary control is the emission of CO2.

The CDEC under the scenario with only SAFs

According to the characteristics of sustainable aviation fuel, the NOx and SO2 in the pollutants generated are 0, PM2.5 can be reduced by 30%, and CO2 and CH4 can be reduced by 50%-90% (the average is 70%)37. Based on this, we calculated the changes in the concentrations of CO2, CH4, and N2O during 2023–2100 under the constraint of only sustainable aviation fuel use and compared them with the situation of no constraint and only carbon emission trading scheme. By 2021, the maximum mixing proportion of sustainable aviation fuel will be only 50%. Still, Airbus expects to achieve 50% SAFs mixed commercial use in 2025 and has tested 100% of SAFs on mainstream aircraft and is expected to achieve this in 203041,42. Therefore, we have assumed three scenarios: 2025–50% (50% sustainable aviation fuel mix proportion will be achieved in 2025, and the later mix proportion will not change), 2025–50% + 2030–100% (50% sustainable aviation fuel mix proportion will be achieved in 2025, and 100% by 2030), and 2025–50% + 2035–100% (50% sustainable aviation fuel mix proportion will be achieved in 2025, and 100% by 2035). Figure 4 shows the concentration of CO2, N2O, and CH4 of China-foreign routes in 2023–2100 under three scenario with sustainable aviation fuel constraints and in 2023–2100 under baseline scenario.

Fig. 4: Concentration of CO2, N2O, CH4 under the three sub-scenarios with only SAFs and those without constraints.
figure 4

a Concentration of CO2. b Concentration of CH4. c Concentration of N2O.

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Under the constraint of sustainable aviation fuel use, CO2 concentration, CH4 concentration, and N2O concentration have decreased significantly, among which CH4 decreased most significantly, followed by N2O and CO2. As shown in Fig. 4a−c, the CO2 concentration in the three cases of 2025–50%, 2025–50% + 2030–100%, 2025–50% + 2035–100% will be 4.99E-07 ppm, 2.62E-07 ppm, and 2.80E-07 ppm respectively by 2100, that is, 66.94%, 34.94% and 37.18% in the case of no constraints (7.52E-07 ppm). Under baseline scenario, the CH4 concentration will be stabilized to 5.65E-10 ppb from 2033, while the CH4 concentration in the three sub-scenarios of 2025–50%, 2025–50% + 2030–100%, 2025–50% + 2035–100% will be stabilized to 4.59E-11 ppb, 2.12E-11 ppb, 2.12E-11 ppb respectively by 2100, which is about 8.13%, 3.75%, and 3.75% of that in the no constraints condition. Similarly, by 2100, the N2O concentration under three sub-scenarios of 2025–50%, 2025–50% + 2030–100%, 2025–50% + 2035–100% will be 1.67E-09 ppb, 3.87E-10 ppb, and 5.50E-10 ppb respectively, which is about 53.43%, 12.35% and 17.56% of the no constraints (3.13E-09 ppb). The CDEC under the three sub-scenarios of 2025–50%, 2025–50% + 2030–100%, and 2025–50% + 2035–100% will be 4.99E-07 ppm, 2.63E-07 ppm and 2.80E-07 ppm respectively, by 2100.

Compared with the case of only carbon trading scheme constraints, it can be found that the carbon trading scheme constraints are relatively more effective for the concentration of CO2, but the overall difference is not significant. By comparison, under the three sub-scenarios of achieving carbon neutrality in 2035, the control effect of CO2 is more prominent. Under the sub-scenarios of 2023–2026–2035, the concentration of CO2 can be reduced to 1.35E-07 ppm by 2100. The use of sustainable aviation fuel can have a significant impact on N2O concentration and CH4 concentration, especially in the case of 2025–50% + 2030–100%. In addition, with sustainable aviation fuel, the minimum CDEC can reach 2.63E-07 ppm by 2100, which is less than the case that only the carbon trading scheme is constrained. Therefore, in general, using sustainable aviation fuel is more effective in controlling the pollutant emissions of the route.

The CDEC under the scenario with ETS and SAFs

We calculated the changes in the concentrations of CO2, CH4, and N2O during 2023–2100 under the common constraints of the carbon emission trading scheme and sustainable aviation fuel use, and compared them with those under baseline scenario, only with carbon emission trading scheme and only with sustainable aviation fuel use. According to the above, nine scenarios can be combined: 2023–2026–2035 + 2025–50% (recovery to the level of 2019 in 2023, achieve carbon emission peak in 2026, achieve carbon neutrality in 2035, and achieve 50% sustainable aviation fuel mixing ratio in 2025), 2023–2026–2035 + 2025–50% + 2030–100% (recovery to the level of 2019 in 2023, achieve carbon emission peak in 2026, achieve carbon neutrality in 2035, and achieve 50% sustainable aviation fuel mixing ratio in 2025, and achieve 100% mix proportion in 2030), 2024–2027–2035 + 2025–50% (recovery to the level of 2019 in 2024, achieve carbon emission peak in 2027, achieve carbon neutrality in 2035, and achieve 50% sustainable aviation fuel mixing ratio in 2025), 2024–2027–2035 + 2025–50% + 2030–100%(recovery to the level of 2019 in 2024, achieve carbon emission peak in 2027, achieve carbon neutrality in 2035, and achieve 50% sustainable aviation fuel mixing ratio in 2025, and achieve 100% mix proportion in 2030), 2025–2028–2035 + 2025–50% (recovery to the level of 2019 in 2025, achieve carbon emission peak in 2028, achieve carbon neutrality in 2035, and achieve 50% sustainable aviation fuel mixing ratio in 2025), 2025–2028–2035 + 2025–50% + 2030–100% (recovery to the level of 2019 in 2025, achieve carbon emission peak in 2028, achieve carbon neutrality in 2035, and achieve 50% sustainable aviation fuel mix proportion in 2025, and achieve 100% mix proportion in 2030), 2023–2030–2040 + 2025–50% + 2035–100% (recovery to the level of 2019 in 2023, achieve carbon emission peak in 2030, achieve carbon neutrality in 2040, and achieve 50% sustainable aviation fuel mix proportion in 2025, and achieve 100% mix proportion in 2035), 2024–2030–2040 + 2025–50% + 2035–100% (recovery to the level of 2019 in 2024, achieve carbon emission peak in 2030, achieve carbon neutrality in 2040, and achieve 50% sustainable aviation fuel mix proportion in 2025, and achieve 100% mix proportion in 2035), 2025–2030–2040 + 2025–50% + 2035–100% (recovery to the level of 2019 in 2025, achieve carbon emission peak in 2030, achieve carbon neutrality in 2040, and achieve 50% sustainable aviation fuel mix proportion in 2025, and achieve 100% mix proportion in 2035).

Figure 5 compares the emission concentration of China-foreign routes in 2023–2100 under nine sub-scenarios with the carbon emissions trading scheme and sustainable aviation fuel constraints, and the pollutants in 2023–2100 under baseline scenario. Among them, the changing trend of CH4 concentration and N2O concentration is the same as that of the three scenarios with only sustainable aviation fuel constraints. Under the joint constraints of the carbon emissions trading scheme and the sustainable use of aviation fuel, the reduction of CO2 concentration is more pronounced, as shown in Fig. 5 a, 2023–2026–2035 + 2025–50%, 2023–2026–2035 + 2025–50% + 2030–100%, 2024–2027–2035 + 2025–50%, 2024–2027–2035 + 2025–50% + 2030–100%, 2025–2028–2035 + 2025–50%, 2025–2028–2035 + 2025–50% + 2030–100%, 2023–2030–2040 + 2025–50% + 2035–100%, 2024–2030–040 + 2025–50% + 2035–100%, The CO2 concentration under the nine sub-scenarios of 2025–2030–2040 + 2025–50% + 2035–100% is 1.10E-07 ppm, 9.91E-08 ppm, 1.09E-07 ppm, 7.97E-08 ppm, 1.09E-07 ppm, 7.74E-08 ppm, 1.94E-07 ppm, 1.75E-07 ppm, 1.62E-07 ppm, which is about 14.59%, 13.18%, 14.56%, 10.60%, 14.47%, 10.30%, 25.76%, 23.22% and 21.49% of the baseline condition (30,628.35ppm). Similarly, the CDEC in 2100 is about 14.63%, 13.18%, 14.61%, 20.61%, 14.52%, 10.30%, 25.75%, 23.21% and 21.49% of that in the baseline case.

Fig. 5: Concentration of CO2, N2O, CH4 under the nine sub-scenarios with ETS and SAFs and those under baseline scenario.
figure 5

a Concentration of CO2. b Concentration of CH4. c Concentration of N2O.

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Compared with the case where only the carbon trading scheme is constrained, and only the sustainable aviation fuel is constrained, it can be found that for the concentration of CO2 and the CDEC, the joint constraint of the two is more effective, especially in the six cases where carbon neutrality will be achieved in 2035. In the case of 2025–2028–2035 + 2025–50% + 2030–100%, the concentration and equivalent concentration of CO2 can be reduced to 7.74E-08 ppm and 7.76E-08 ppm by 2100. Moreover, the joint constraint of the two also has an impact on N2O concentration and CH4 concentration. Still, the impact effect is the same as that of the three scenarios under the constraint of only sustainable aviation fuel. Therefore, the joint constraints of the two control the emissions of several aviation pollutants. In contrast, the most significant impact is the concentration of CO2 and the CDEC. Especially in 2035, it is more effective than using the carbon trading scheme alone or sustainable aviation fuel alone.

Discussion

This study focuses on the changes in air pollutant emission concentration and CDEC of China-foreign routes. First, we collect the flight information of all international routes (including aircraft type, flight frequency, airline, flight distance, flight time, etc.) of China-foreign routes and then calculate the CO2, CO, HC, NOx, PM2.5, and SO2 of each route. The calculation of emissions can better summarize the impact of aircraft activities on the environment and provide data and method references for proposing corresponding countermeasures. Then, the pollutant concentrations of China-foreign routes are calculated under different scenarios: the baseline scenario, the scenario with only ETS (CNG2020 strategy), the scenario with only SAFs, and the scenario with ETS and SAFs. Finally, we discuss the change in greenhouse gas concentration (carbon dioxide, methane, and nitrous oxide) from 2023 to 2100 and compare the more powerful approach to control emissions.

The main contributions of this paper to the literature are reflected in the following aspects. First, this paper attempts to systematically compare the two main ways of aviation emission reduction-carbon emissions trading scheme and sustainable aviation fuel. Some papers focus on the research of carbon emissions trading schemes or sustainable aviation fuel, but there is still no research to compare these two. In addition, this paper uses China-foreign routes, the fastest-growing region in the world, as an example to calculate the actual emissions and carry out an accurate analysis. The research framework can also be used in other countries or regions to provide more systematic results and suggestions for the effective emission reduction of the global aviation industry. Second, this study focuses on the non-carbon dioxide pollutants in aircraft emissions. For aviation industry, carbon dioxide is the most important greenhouse gas, but the warming effect of other gases (such as CH4 and N2O) is also substantial. Currently, the research on aircraft air pollutants and their climate effects focuses on carbon dioxide. In contrast, research on aircraft non-carbon dioxide pollutants is relatively scarce, and our research can fill this gap.

The main conclusions and policy recommendations are as follows: First, the effect of carbon dioxide on greenhouse gases is still the largest under the scenario under baseline scenario. The second highest concentration is N2O, indicating that carbon dioxide and nitrous oxide play a significant role in climate change. Secondly, under the constraint of a carbon trading scheme, CO2 concentration and equivalent concentration decreased significantly. Compared with the baseline case, if aviation carbon neutralization can be achieved in 2035, the CO2 concentration in three cases (2023–2026–2035, 2024–2027–2035, 2025–2028–2035) will be about 17.96–18.61% and 18.06–18.70% respectively in the baseline case by 2100, and 30.27–37.10% and 30.37–37.18% respectively in the three cases of 2023–2030–2040, 2024–2030–2040, and 2025–2030–2040. In addition, since the concentrations of methane and nitrous oxide under the six sub-scenarios and the baseline scenario have not changed, the reduction in the concentration of CO2 mainly causes a significant reduction in the equivalent concentration of carbon dioxide. Under the constraint of the carbon emission trading scheme, the primary control is the emission of CO2. Third, under the constraint of only sustainable aviation fuel use, the CO2 concentration, CH4 concentration, and N2O concentration have significantly decreased, with the most apparent decrease in CH4, followed by N2O and N2O. Fourth, by comparing the constraint of only sustainable aviation fuel use with that of only the carbon trading scheme, we can find that the constraint of the carbon trading scheme is relatively more effective for the concentration of CO2. By comparison, under the three sub-scenarios of achieving carbon neutrality in 2035, the control effect of CO2 is more prominent. The use of sustainable aviation fuel can affect both N2O concentration and CH4 concentration, especially in the case of 2025–50% + 2030–100%. Fifth, under the joint constraints of the carbon trading scheme and sustainable aviation fuel, compared with the case where only the carbon trading scheme is constrained and only the sustainable aviation fuel is constrained, the joint constraints of the two are more effective for the concentration of CO2 and the equivalent concentration of CO2, especially in the six cases of achieving carbon neutrality in 2035. In the case of 2025–2028–2035 + 2025–50% + 2030–100%, the concentration and equivalent concentration of CO2 can be reduced to 7.74E-08 ppm and 7.76E-08 ppm by 2100. And the joint constraint of the two also has an impact on N2O concentration and CH4 concentration. Therefore, the joint constraints of the two control the emissions of several aviation pollutants. In contrast, the most significant impact is the concentration of CO2 and the equivalent concentration of CO2. Especially in 2035, it is more effective than using the ETS alone or sustainable aviation fuel alone.

Several limitations exist in this study. First, we have not considered the change in aircraft transformation cost and maintenance cost caused by using sustainable aviation fuel, the carbon trading price, and the price of sustainable aviation fuel, which will impact the scenario set in this paper. Second, this paper does not consider the differences in emission reduction properties of sustainable aviation fuel with different raw materials and processing technologies. Uncertainties are not considered in this study43 and it will be an interesting research direction to incorporate uncertainties in the research. Therefore, further investigation should consider these factors.

Methods

Aviation FAIR-GWP concentration method

Aircraft emissions are CO2, CO, HC, NOx, SO2, and PM2.53,4. The primary greenhouse gases are CO2, CH4 (part of HC), and N2O (part of NOx)44. Based on the concentration calculation of the FAIR method, this paper considers the role of environmental absorption and calculates the concentrations of carbon dioxide, nitrogen oxide, and methane in a specific period. Then it uses the GWP method to convert the concentration to the equivalent concentration of carbon dioxide to avoid the calculation confusion caused by only the GWP method. The Finite Amplitude Pulse Response (FAIR) model (version 1.0) tracks the time integral fraction of carbon in the air and uses this model to determine the efficiency of carbon sinks, thereby calculating carbon dioxide concentration, radiative forcing, and temperature changes in the atmosphere. Compared with other climate change research models, FAIR 1.0 has well calibrated the temperature and carbon cycle response of the Earth system model45. FAIR 1.3 extends to calculating non-CO2 greenhouse gas concentrations. Based on the FAIR 1.3 model44 and the GWP method, we build a general framework for the Aviation FAIR-GWP concentration method, as shown in Fig. 6.

Fig. 6
figure 6

Framework of Aviation FAIR-GWP concentration method.

Full size image

According to the FAIR method43, CO2 would be partitioned into four boxes: geological processes (\({{\rm{\tau }}}_{0}\)), the deep ocean (\({{\rm{\tau }}}_{1}\)), the biosphere (\({{\rm{\tau }}}_{2}\)), and the ocean mixed layer (\({{\rm{\tau }}}_{3}\)). The partition fractions are \({{\rm{\alpha }}}_{i}\) and \(\mathop{\sum }\nolimits_{i=0}^{3}{\alpha }_{i}=1\). The concentration of \({{\rm{CO}}}_{2}\) is

$${{\rm{C}}}_{{{CO}}_{2}}=278+\mathop{\sum }\limits_{i=0}^{3}\frac{{R}_{i}}{{{\rm{M}}}_{a}}\frac{{\omega }_{{{CO}}_{2}}}{\omega }$$
(1)

\({{\rm{M}}}_{a}=5.1352* {10}^{18}\) \({\rm{kg}}\) is the dry mass of the atmosphere, \({\omega }_{{{CO}}_{2}}=44.01\) is the molecular weight of CO2, and \(\omega =28.966\) is the molecular weight of dry air.

In Eq. (1), the formula of \({R}_{i}\) is

$$\frac{{\rm{d}}{{\rm{R}}}_{{\rm{i}}}}{{\rm{dt}}}={{\rm{\alpha }}}_{{\rm{i}}}{{\rm{E}}}_{{{\rm{CO}}}_{2}}-\frac{{{\rm{R}}}_{{\rm{i}}}}{\partial {{\rm{\tau }}}_{{\rm{i}}}},{\rm{i}}=0,1,2,3$$
(2)

\({E}_{{{CO}}_{2}}\) is the CO2 emission.

The variable \(\partial\) is gotten by

$$\mathop{\sum }\limits_{i=0}^{3}\partial {{\rm{\alpha }}}_{i}{{\rm{\tau }}}_{{\rm{i}}}\left[1-{{\exp }}\left(\frac{-100}{{\partial {\rm{\tau }}}_{{\rm{i}}}}\right)\right]={r}_{0}+{r}_{C}\left[\sum _{t}{E}_{{{CO}}_{2},t}-{{\rm{C}}}_{{{CO}}_{2}}+278\right]+{r}_{T}\Delta T$$
(3)

\({r}_{c}=\)0.019 yt GtC−1, \({r}_{T}=\)4.165 yr K−1, and \({r}_{0}=35\). \(\Delta T\) is the temperature change.

The formulas (1)–(3) form a cycle, and we use One hundred thousand Monte Carlo simulations to solve them.

For CH4 and N2O, based on existing studies46,47, we assume that the proportion of CH4 in aviation HC is 0.4 and that of N2O in aviation NOx is 0.29. This ratio can be verified by the CO2 e-emissions by GWP100. Referring to exiting paper48, the ratio of CO2 e-emissions divided by the CO2 emissions in GWP100 is 1.7. In the GWP100 method27, the CO2 e-emissions are

$${{\rm{E}}}_{{{CO}}_{2}}={{CO}}_{2}+25* {{CH}}_{4}+298* {N}_{2}O.$$
(4)

The ratio \({{\rm{E}}}_{{{CO}}_{2}}/{{CO}}_{2}\) of Eq. (7) is 1.7 too.

For CH4 and N2O, their concentrations are

$${{\rm{C}}}_{t}={{\rm{C}}}_{t-1}+\frac{1}{2}\left(\delta {{\rm{C}}}_{t-1}+\delta {{\rm{C}}}_{t}\right)-{{\rm{C}}}_{t-1}\left(1-{{\exp }}\left(-\frac{1}{\tau }\right)\right)$$
(5)

\(\tau\) the atmospheric lifetime. For CH4, \(\tau =9.3\); For N2O, \(\tau =121\).

\(\delta {{\rm{C}}}_{t}\) can be gotten by

$$\delta {{\rm{C}}}_{t}=\frac{{E}_{t}}{{{\rm{M}}}_{a}}\frac{\omega }{{\omega }_{f}}.$$
(6)

\({{\rm{\omega }}}_{{\rm{f}}}\) is the molecular mass. For CH4, \({{\rm{\omega }}}_{{\rm{f}}}=16.04\); For N2O, \({{\rm{\omega }}}_{{\rm{f}}}=44.01\). \({E}_{t}\) is CH4 or N2O emission of year \({\rm{t}}\).

According to the GWP method27, the Carbon Dioxide Equivalent Concentration (CDEC) is

$${{\rm{C}}}_{E}={{\rm{C}}}_{{{CO}}_{2}}+{\varphi }_{1}* {{\rm{C}}}_{{{CH}}_{4}}+{\varphi }_{2}* {{\rm{C}}}_{{N}_{2}O}.$$
(7)

Because we consider the period until 2100 (larger than 20 years), so for CH4, \({\varphi }_{1}=25\); For N2O, \({\varphi }_{2}=298\).

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.