Reduced ice number concentrations in contrails from low aromatic biofuel blends

. Sustainable aviation fuels can reduce contrail ice numbers and radiative forcing by contrail cirrus. We measured apparent ice emission indices for fuels with varying aromatic content at altitude ranges of 9.1 – 9.8 km and 11.4 – 11.6 km. Measurement data were collected during the ECLIF II/NDMAX ﬂight experiment in January 2018. The fuels varied in both aromatic quantity and type. Between a sustainable aviation fuel blend and a reference fuel Jet A-1, a maximum reduction in apparent ice emission indices of 40% was found. We show vertical ice number and extinction distributions for three different 5 fuels and calculate representative contrail optical depths. Optical depths of contrails (0.5 – 3 minutes in age) were reduced by 40 to 52% for a sustainable aviation fuel compared to the reference fuel. Our measurements suggest that sustainable aviation fuels result in reduced ice particle numbers, extinction coefﬁcients, optical depth and climate impact from contrails. Here, we give a more comprehensive overview of the ECLIF II/NDMAX contrail ice measurements and extend the study to a larger altitude range between 9.1 and 9.8 km. We also add observations for higher altitudes between 11.4 and 11.6 km. The inﬂuence of ambient parameters on microphysical contrail properties is discussed by Bräuer et al. (2021). In the following, we analyse apparent ice emission indices formed from burning fuels of varying composition and also show vertical proﬁles of apparent ice emission indices and extinction coefﬁcients in the young contrails. We further contribute to investigations of the contrail climate impact by deriving the individual optical depths of young contrails resulting from burning different fuels. with respect to 1-2 minutes of the observations, might explain lower COD in the global model contrails compared to the observations.

plume and increases the relative humidity with respect to liquid water (Kaufmann et al., 2014). If the conditions in the exhaust exceed water saturation, the non-volatile, ultra-fine soot particles emitted by the engines serve as condensation nuclei for water droplets. The droplets immediately freeze into ice particles (Heymsfield et al., 2010;Kärcher, 2018). The development of a 25 line-shaped contrail is governed by the superposition of dynamic and microphysical processes and the particle and trace gas concentrations are inhomogeneously distributed. A fraction of the ice crystals follow a downward movement and form the lower primary wake. At the same time the ice crystals in the upper part of the contrail, near the flight level, grow by uptake of water and form the secondary wake. Schumann et al. (2013) showed that the ice particle concentrations are larger in the secondary wake of the contrail. Vertical profiles of one to four minutes old contrails were also analysed by Gayet et al. (2012) 30 and Jeßberger et al. (2013) with similar results. Kleine et al. (2018) assessed the sublimation effects with data of the ECLIF I experiment and showed that both soot and ice particle number concentrations are a function of the position behind and below the contrailing aircraft. An overview of contrail observations has been compiled by  and microphysical data on aged contrail cirrus have been analysed by Voigt et al. (2017) and Chauvigné et al. (2018).
The use of alternative jet fuels and their effect on soot emissions has been researched during ground and flight experiments 35 before (Beyersdorf et al., 2014;Zschoke et al., 2012)(Zschoke et al., 2012Moore et al., 2015Moore et al., , 2017Schripp et al., 2018;Tran et al., 2020), proving that the use of jet fuels with varying properties is possible in the modern aircraft fleet. Moore et al. (2017) for example used in situ data to show that biofuel blending reduces soot particle number and mass emissions by 50 to 70%. A reduction in contrail ice particle numbers of similar magnitude was first reported by Voigt et al. (2021) for semisynthetic and biofuel blends observed during the ECLIF I and ECLIF II/NDMAX experiments for limited conditions near 10 km altitude.

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Here, we give a more comprehensive overview of the ECLIF II/NDMAX contrail ice measurements and extend the study to a larger altitude range between 9.1 and 9.8 km. We also add observations for higher altitudes between 11.4 and 11.6 km. The influence of ambient parameters on microphysical contrail properties is discussed by Bräuer et al. (2021).As ambient conditions have a large impact on microphysical contrail properties (Bräuer et al., 2021), a more comprehensive overview of the contrail ice measurements during ECLIF II/NDMAX is needed to assess the impact of biofuel blends on aviations climate impact. With this publication, we extend the study by Voigt et al. (2021) 45 to a larger altitude range between 9.1 and 9.8 km and we also add observations for higher altitudes between 11.4 and 11.6 km.
We also describe our results with respect to a different reference fuel than the one used by Voigt et al. (2021). In the following, we analyse apparent ice emission indices formed from burning fuels of varying composition and also show vertical profiles of apparent ice emission indices and extinction coefficients in the young contrails. We further contribute to investigations of the contrail climate impact by deriving the individual optical depths of young contrails resulting from burning different fuels.In the following, we analyse medium values and vertical profiles of apparent ice 50 emission indices formed from burning fuels of varying composition. Burkhardt et al. (2018) showed with a global simulation that the climate impact of contrails is non-linearly dependent on apparent ice emission indices. Therefore, we contribute to the assessment of the contrail climate impact by deriving optical parameters like the extinction coefficients and contrail optical depths in addition to apparent ice emission indices.

ECLIF II/NDMAX
The ECLIF II/NDMAX flight experiment was part of the DLR project Emission and Climate Impact of Alternative Fuels (ECLIF) and the NASA DLR Multidisciplinary Airborne Experiment (NDMAX). It aimed to quantify the impact of jet fuel aromatic content and molecular structure on soot emissions, ice crystals formation and contrail properties (Bräuer et al., 2021;Voigt et al., 2021). The experiment took place in January 2018 over northern Germany. As emissions source aircraft, the DLR 60 A320 Advanced Technology Research Aircraft with two V2527-A5 engines was used. The aircraft is shown in the photograph of Figure 1. The NASA DC-8 Airborne Science Laboratory followed the A320 in a distance between 4 and 30 km (far field) and measured non-volatile particle number and mass, ice crystal number size distributions, carbon dioxide (CO 2 ) and other emissions. The distances correspond to a contrail age between 30 seconds and three minutes. These distances are necessary to avoid saturation of the optical particle counters. The aircraft followed each other on an elongated, oval flight track at altitudes 65 between 7.8 and 11.6 km. Distributions of the temperature and the relative humidity with respect to ice over the altitude are shown in Figure 1. During the ECLIF II/NDMAX airborne measurements, three different jet fuels were studied: a reference fuel Jet A-1 (Ref 3) and two blends of reference fuels and HEFA produced from camelina oil (SAF 1 and SAF 2). Relevant fuel properties are described in Table 1. By varying blending ratios, different aromatic contents were obtained in the fuels. Aromatics are 70 cyclic hydrocarbons, characterized by conjugated double bonds. Incomplete combustion of hydrocarbons in the fuels leads to the generation of soot particles. One type of aromatics, the stable, bicyclic naphthalene molecules, are thought to increase the sooting behaviour during fuel combustion (Chin and Lefebvre, 1990;Brem et al., 2015). Therefore, SAF 1 and SAF 2 are designed to vary in their naphthalene content, while their total aromatic content is in the same range. Results of the ground measurements during ECLIF II/NDMAX are published by Schripp et al. (2021).

Particle and trace gas measurements
Ice number concentrations were measured with the Fast Forward Scattering Spectrometer Probe (FFSSP) in a particle size range between 1 and 25 µm (Baumgardner and Gandrud, 1998). The instrument was mounted next to the CO 2 inlet on the upper side of the DC-8 fuselage. The probe has been previously used for contrail measurements (Voigt et al., 2010Gayet et al., 2012;Chauvigné et al., 2018), and its electronics received an update in 2017, such that the recording of single 80 particle data is possible. The sampling area of 0.19 mm 2 was determined by laboratory calibrations and the instrument was size-calibrated on the basis of a T-Matrix calculation for an ice particle aspect ratio of 0.5 (Borrmann et al., 2000;Luo et al., 2003;Rosenberg et al., 2012). The FFSSP particle size distributions were corrected for small particles, following Bräuer et al.
(2021), so that particle concentration between 0.5 and 1 µm can also be estimated. The correction is based on the Cloud and Aerosol Spectrometer (CAS), which was also part of the ECLIF II/NDMAX instrumentation and measures ice particles with 85 diameters between 0.5 and 50 µm. A function was fitted to the ratio between the total CAS number concentration and the CAS number concentration for particles larger than 1 µm. The correction function increases exponentially with decreasing contrail effective diameter (Francis et al., 1994). The FFSSP ice number concentrations are corrected by multiplying them with the sizedependent correction function. The error of the correction increases with decreasing effective diameter (Bräuer et al., 2021).
CO 2 was measured with a commercial Picarro G1301-m greenhouse gas analyser based on wavelength-scanned cavity ring-90 down spectroscopy (Crosson, 2008). Air from outside the aircraft was sampled by a backward facing inlet. Several calibrations were performed with commercial gas standards. The accuracy depends on the cell pressure of the instrument and its temperature during operation. Data are corrected for water vapour content following Rella et al. (2013). The time delay of the gas flow on the way from the inlet to the measurement cell was estimated to be 3.3 s.
3 Calculation of hydrogen to carbon ratio, emission index and extinction coefficient 95 Kerosene contains mainly carbon, hydrogen and sulphur. It can be assumed that the sulphur content is negligible (in general less than 0.07 mass%) and therefore the hydrogen to carbon (H:C) mole fraction ratio can be calculated for the known mass fraction of hydrogen w H as followed: x Environmental conditions, instabilities in the trailing vortices and dilution lead to spatial inhomogeneities in the exhaust (Un-100 terstrasser, 2016; Schumann and Heymsfield, 2017). Therefore, ice particle concentrations in an aircraft plume are normalized by CO 2 as a proxy for fuel burn in order to calculate apparent emission indices (AEI). The ice number concentrations are related to the mass of fuel burnt by scaling the measurements to the fuel-dependent CO 2 emission index. For this calculation, we assume that the combustion system has 100% fuel conversion efficiency. As ice particles are not directly emitted by the engines, the term apparent ice particle emission index is used. Emission indices are calculated following Moore et al. (2017).

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The individual impact of a contrail on radiation through the atmosphere depends on the extinction properties of the ice crystals. The extinction coefficient b ext depends on the extinction efficiency Q ext , the projected area of the ice particles and the ice number concentration N ice : The extinction efficiencies were calculated for an aspect ratio of 1.0 and a wavelength of 550 nm and approach a value of 2 110 for large ice particles.
4 Results and discussion

Fuel-dependent apparent ice emission indices
As shown in Bräuer et al. (2021), temperatures near the contrail formation threshold temperature prevail when contrails are formed at altitudes below 9 km, leading to an incomplete activation of soot particles into water (Kärcher and Voigt, 2017). At 115 these low altitudes, even small temperature variations under 1 K significantly change the particle activation fraction and the ice number concentrations in the contrails. Therefore, we concentrate our study on altitudes above 9 km. Voigt et al. (2021) showed a subset of the data at 9.1 to 9.8 km altitude, restricted to fuel flows of 1100±100 kg h -1 and relative humidity with respect to ice larger than 108%. Here, we use the complete data set for the fuel intercomparison and discuss the resulting impact and limitations. To present ice crystal number in contrails independent of contrail age, thrust level and dilution, we calculate apparent ice emission indices (AEI). In Figure 2, AEI are compared on the basis of the hydrogen to carbon (H:C) ratio of the varying fuels.
For the contrails measured during ECLIF II/NDMAX (age between 30 seconds and three minutes), sublimation effects can affect the ice numbers in the vortex phase and have to be excluded to receive a climate-relevant value for AEI. Therefore, we follow Bräuer et al. (2021) and calculate the mean of the upper 15% AEI. To ensure only contrails with full soot activation 125 are considered, relative humidity with respect to ice is restricted to larger than 100% for altitudes between 9.1 and 9.8 km and larger than 120% for altitudes between 11.4 and 11.6 km.
For high altitudes, we report a 23% reduction of AEI when burning SAF 1 compared to Ref 3 and a reduction of 40% when burning SAF 2. For altitudes between 9.1 and 9.8 km, a 6% reduction is achieved when burning SAF 2 compared to SAF 1.
These values are in general agreement with previous observations. Voigt et al. (2021) found AEI reductions in the range of 50 130 to 70%, when comparing to a different Jet A-1 reference fuel with lower hydrogen content. It can be stated, that the number of ice crystals is reduced through the reduction of fuel aromatic content, which is also monitored by an increase in fuel hydrogen content. Changes in fuel polycyclic aromatic composition can further increase the reductions. at flight altitudes between 9.1 and 9.8 km. The number of plume encounters per altitude and fuel type can be found in Table 1.

Vertical profiles of contrail properties
The vertical profiles of AEI in Figure 3a  conditions, the physical depth is constant for varying fuels, even though in theory, different particle sizes lead to variations in sedimentation and sublimation processes (Unterstrasser and Görsch, 2014;Kleine et al., 2018).
Sublimation effects, which lead to a decrease of the ice crystal numbers in vertical direction below the A320, depend on the relative humidity over ice, temperature and atmospheric stability. Figure 4 shows two image recordings of the DC-8 forward camera during ECLIF II/NDMAX. Figure 4a shows a contrail unaffected by sublimation and Figure 4b shows a contrail 150 strongly affected by sublimation with a secondary wake forming above the descending contrail vortices. In Figure 3a and b, sublimation effects of different emphasis can be observed in the vertical profiles of AEI. For both altitudes, AEI are increased at the level of the secondary wake near the initial emission level at 0 m. For altitudes between 9.1 and 9.8 km, AEI are also slightly increased in the lower primary wake and sublimation effects are reduced at these altitudes. However, the highest AEI are always found in the upper secondary wake as also shown by Kleine et al. (2018). Mean values of AEI are depicted by dashed, vertical 155 lines and in contrast to mean AEI in Figure 2, these values consider all sublimation effects. At altitudes between 9.1 and 9.8 km the mean AEI is 1.3·10 15 kg -1 for SAF 1 and 7.4·10 14 kg -1 for SAF 2. The mean AEI at altitudes between 11.4 and 11.6 km is 5.9·10 14 kg -1 for SAF 1, 5.8·10 14 kg -1 for SAF 2 and 1.2·10 15 kg -1 for Ref 3.
The global climate impact of contrails is non-linearly dependent on the reduction of initial ice crystal numbers . The dependence of contrail microphysical and radiative properties on initial ice crystal numbers then remains over 160 the contrail cirrus life cycle. We calculate the extinction coefficients of the contrails to present the relation between contrail ice crystals and radiation. Unterstrasser and Gierens (2010) show that extinction is a suitable variable for comparing similar aged contrails. The contrail life cycle further depends on meteorological parameters like temperature and humidity, vertical wind shear, atmospheric stability, the depth of the supersaturated layer in which the contrails are formed and the radiation budget Unterstrasser et al., 2017). 165 The vertical profiles of the extinction coefficients are shown in Figure 3c and d. At altitudes between 9.1 and 9.8 km the mean extinction coefficients are 2.5 km -1 for SAF 1 and 1.5 km -1 for SAF 2. The mean extinction coefficients at altitudes between 11.4 and 11.6 km are 0.4 km -1 for SAF 1 and 0.6 km -1 for SAF 2. The mean Ref 3 extinction coefficient at the altitude between 11.4 and 11.6 km is 1.5 km -1 and hence a factor of 2.5 higher than the mean extinction coefficients of both biofuels at the same altitude. In a next stepIn the following section the extinction coefficients are used to calculate the fuel-dependent contrail optical 170 depths in the following section.

Fuel-dependent contrail optical depth
The contrail optical depth (COD) is a dimensionless measure of the degradation that a beam of radiation directed straight downwards experiences when passing through a contrail (Wallace and Hobbs, 2006). It is derived by integrating the extinction with respect to the vertical physical contrail depth. The ECLIF II/NDMAX CODs for the measured fuels are calculated by For high altitudes, a COD reduction of 40 to 52% can be calculated when comparing the biofuel blends to the reference 180 fuel. Due to atmospheric variability, it is not possible to evaluate the tendencies of the contrail optical depth that result from the sustainable aviation fuels. SAF 2 with reduced naphthalene content produces reduced AEI compared to SAF 1. But when calculating the climate relevant parameter of optical depth during this early contrail age, the differences are reduced or even reversed. The reason is, that reduced particle numbers under similar contrail formation conditions, will lead to larger particles, as there is more water vapour available for particle growth. The ice particle sizes are in the further contrail evolution strongly 185 influenced by atmospheric conditions and therefore, they are highly variable. The total contrail extinction (Unterstrasser and Gierens, 2010;Unterstrasser and Görsch, 2014) and the radiative forcing of contrail cirrus  are strongly dependent on initial ice crystal numbers. The optical depth varies strongly during the life cycle of a contrail (Unterstrasser and Gierens, 2010;Vázquez-Navarro et al., 2015).
In Figure 5, the calculated COD are compared with in situ and satellite observations (dashed and dotted lines) and model- have been changed in subsequent studies Bock and Burkhardt, 2019). The variation of values in Figure 5 shows the variability in individual contrail radiative impact. Figure 5 shows the variability and the range of contrail optical depths as a quantification of the individual contrail radiative impact. Different averaging volumes and a larger contrail age of 7.5 minutes in the simulation, with respect to 1-2 minutes of the observations, might explain lower COD in the global model contrails compared to the observations.Varying averaging volumes and contrail ages lead to differences between COD derived from simulations and observations.

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For contrail optical depths (COD) smaller than 1, the contrail radiative forcing is proportional to COD (Meerkötter et al., 1999). Lee et al. (2021) give a consolidated estimate of contrail cirrus effective radiative forcing of 57.4 mW m -2 . The estimations are based on several global climate models (Burkhardt and Kärcher, 2011;Chen and Gettelman, 2013;Schumann et al., 2015;Bock and Burkhardt, 2016;Bickel et al., 2020). A study by Gettelman et al. (2021) calculates a similar effective contrail radiative forcing of 62 mW m -2 . Uncertainties for these values are high, inter alia, because the COD remains highly uncertain 210 (Schumann et al., 2021a, b). Sanz-Morère et al. (2020) state that global estimations of average COD can vary from 0.065 to 0.3. The individual COD of ECLIF II/NDMAX are slightly higher than the underlying COD values of the current best effective radiative forcing estimate in Lee et al. (2021), which can be explained by the early development stage of the ECLIF II/NDMAX contrails.

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For ECLIF II/NDMAX, up to 40% reduction in apparent ice emission indices was measured. SAF 1 and 2 had similar aromatic content but varied in aromatic type. An additional reduction in AEI of up to 20% was measured for the SAF with reduced naphthalene content. The individual contrail optical depth was reduced between 40-52% for a sustainable aviation fuel compared to the reference fuel. For the future, a drastic reorientation of fuel compositions could provide strong benefits for climate, which comes without the cost of enhanced CO 2 emissions when rerouting air traffic. Significant reduction of aviation climate forcing 220 can be achieved by the widespread implementation of SAF blends in airport fuelling systems and by the use of unblended sustainable aviation fuels or even hydrogen fuels.
Data availability. The data are collected at the NASA data repository at https://science-data.larc.nasa.gov/aero-fp/projects/.

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performed the contrail ice data evaluation and wrote the paper. All authors contributed to the manuscript.
Competing interests. The authors declare that they have no conflict of interest.