Substantial aircraft contrail formation at low soot emission levels | Nature

## Summary
Here we show that lean-burn combustion reduces soot particle number emissions by three orders of magnitude compared with conventional rich–quench–lean engines 4 , 5 —but does not significantly decrease volatile particles or contrail ice crystal numbers—both can exceed 10 15 particles per kg of burned fuel. Lean-burn engines are designed to improve engine emission performance through a fuel injection system and an airflow distribution that expands regions with low fuel-to-air ratios in the combustor 28 , with the objective of lowering nitrogen oxides and soot particle emissions. By contrast, in the low-soot regime ( ≪ 10 14 soot particles emitted per kg of fuel burned) anticipated for lean-burn engines, microphysical contrail models 3 , 31 , 34 predict a much wider range of possible ice crystal numbers, spanning four orders of magnitude, but these predictions lack experimental validation. Particle emissions by lean-burn engines The particle number emission index (EI x ) measures the number of non-volatile or total particles emitted per kilogram of burned fuel, assuming that the fuel carbon content is completely converted to CO 2 ( Methods ).

## Article Content
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Subjects
Atmospheric science
Environmental impact
Abstract
Contrail cirrus clouds are a main contributor to the climate forcing from aviation
1
. Yet, the number of contrail ice crystals forming behind aircraft with modern lean-burn engines is unknown. Theory spans a four orders of magnitude range in ice crystal numbers
2
,
3
—rendering related climate effects unpredictable. Here we show that lean-burn combustion reduces soot particle number emissions by three orders of magnitude compared with conventional rich–quench–lean engines
4
,
5
—but does not significantly decrease volatile particles or contrail ice crystal numbers—both can exceed 10
15
particles per kg of burned fuel. Our findings arise from in-flight observations behind an A321neo aircraft with lean-burn engines, thus providing real-world confirmation of some laboratory work
6
and narrowing the range of theoretical expectations. Our results indicate that the tested lean-burn engine configurations alone are unlikely to reduce the warming effect of contrails, suggesting that modifications of fuel composition and lubrication oil venting architecture may be required. We show that contrail ice particle numbers in the low-soot regime can be reduced by using low-sulfur fuels and that organic fuel constituents and lubrication oil vapours can increase contrail ice particle numbers. Future research should explore how reductions in volatile particles, apart from soot, affect contrail ice formation.
Main
Aviation plays a vital part for mankind, industry and economy and the transport of goods and people. Aircraft also contribute to climate change mainly by carbon dioxide (CO
2
) emissions and by formation of contrail cirrus. Notably, the annual mean effective radiative forcing from contrails is almost on par with that from the carbon dioxide emissions of aviation since the historical start of air traffic
1
,
7
. Meanwhile, global air traffic has recovered from the 2020 pandemic
8
and is expected to increase by a factor of 2–3 by 2050 (ref.
9
). Hence, there is an urgent need for an international aviation strategy that reflects the essential role of aviation for economy and global competitiveness—and that also curbs aircraft emissions, contrails and related climate effects
10
. This is also expressed in environmental efforts by the International Civil Aviation Organization in their commitment to fly net-zero carbon emissions by 2050, signed by 16 main players in the aviation sector
11
. Regulators have reacted to this aviation challenge, and the European Union has released the Destination 2050 Roadmap
12
,
13
, which sets limits on aircraft CO
2
emissions and may demand the monitoring and reporting of the non-CO
2
effects of aviation
14
. Parallel to a debate on uncertainty or the best-suited metric
1
, academia and industry have made considerable progress to better understand the non-CO
2
effects of aviation. Recent studies
15
,
16
developed probability distributions of aircraft CO
2
and non-CO
2
climate impacts to assess the risk of mitigation measures with opposing effects on climate. The analysis in these studies favours the reduction of aircraft non-CO
2
effects for measures with CO
2
:non-CO
2
trade off ratios smaller than 1:5. Unlike well-mixed CO
2
emissions with atmospheric lifetimes of many decades
17
, contrail cirrus may persist at cold and humid cruise conditions for only several hours
18
,
19
,
20
. Hence, in contrast to CO
2
, measures to reduce warming contrail cirrus would have an immediate effect on the climate, which is one of the levers required to meet the international climate targets.
Current developments of advanced solutions to reduce the total climate effect from aviation include alternative bio-based or synthetic fuels
4
,
21
, engine and aircraft technology
9
as well as operational measures
22
. The latter includes avoiding the formation of contrails by flying above or below ice-supersaturated regions
23
, in which warming contrails would form. However, these operational measures may come at the cost of slightly increased fuel consumption
22
.
Another promising strategy to reduce the contrail climate effect is bio-based or synthetic aviation fuel (SAF)
24
produced with renewable energies, which can also have a reduced CO
2
footprint compared with conventional Jet A-1 fuel. Owing to their lower aromatic fuel content, SAFs lead to a reduction in soot particle emissions
4
,
5
,
24
. For conventional rich–quench–lean (RQL) engine technologies, the soot particles around 30 nm in size serve as one of the primary nuclei sources for contrail ice crystals
25
, and a substantial reduction in soot and contrail ice crystals has been observed when burning low-aromatic SAFs
21
,
26
,
27
. This can reduce the lifetime of contrails
20
and their radiative forcing
7
.
Although the effects of SAF on contrails and climate have been investigated on engines fitted with RQL combustor technologies, in-flight emissions and contrail data from mod

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## Expert Analysis

### Merits
- Notably, the annual mean effective radiative forcing from contrails is almost on par with that from the carbon dioxide emissions of aviation since the historical start of air traffic 1 , 7 .
- Parallel to a debate on uncertainty or the best-suited metric 1 , academia and industry have made considerable progress to better understand the non-CO 2 effects of aviation.
- Another promising strategy to reduce the contrail climate effect is bio-based or synthetic aviation fuel (SAF) 24 produced with renewable energies, which can also have a reduced CO 2 footprint compared with conventional Jet A-1 fuel.
- Table 1 Composition of probed fuels and world average Jet A-1 Full size table The DLR research aircraft Falcon 20E 38 was equipped with a comprehensive set of instruments to measure trace gases, in particular, CO 2 , water vapour and nitrogen oxides, as well as properties of aerosol and contrail ice particles, and meteorological data, as described in detail in the Methods .

### Areas for Consideration
- Regulators have reacted to this aviation challenge, and the European Union has released the Destination 2050 Roadmap 12 , 13 , which sets limits on aircraft CO 2 emissions and may demand the monitoring and reporting of the non-CO 2 effects of aviation 14 .
- Recent studies 15 , 16 developed probability distributions of aircraft CO 2 and non-CO 2 climate impacts to assess the risk of mitigation measures with opposing effects on climate.
- In particular, there are no published particle number emission or contrail data for lean-burn engines at cruise, creating a substantial gap in data required to calculate and assess the related climate effects.

### Implications
- Download PDF Subjects Atmospheric science Environmental impact Abstract Contrail cirrus clouds are a main contributor to the climate forcing from aviation 1 .
- Our results indicate that the tested lean-burn engine configurations alone are unlikely to reduce the warming effect of contrails, suggesting that modifications of fuel composition and lubrication oil venting architecture may be required.
- Future research should explore how reductions in volatile particles, apart from soot, affect contrail ice formation.
- Aircraft also contribute to climate change mainly by carbon dioxide (CO 2 ) emissions and by formation of contrail cirrus.

### Expert Commentary
This article covers burn, contrail, ice topics. Notable strengths include discussion of burn. Areas of concern are also raised. Readability: Flesch-Kincaid grade 0.0. Word count: 2381.