# Direct effect of aircraft soot emission: a laboratory view

Olga B. Popovicheva, Natalia M. Persiantseva, Natalia K. Shonija, Benjamin Demirdjian, Daniel Ferry, Jean Suzanne

► To cite this version:

Olga B. Popovicheva, Natalia M. Persiantseva, Natalia K. Shonija, Benjamin Demirdjian, Daniel Ferry, et al.. Direct effect of aircraft soot emission: a laboratory view. Roy G.D.; Frolov S.M.; Starik. A.M. Non Equilibrium Processes: Vol. 2: Plasma, Aerosols, and Atmospheric Phenomena, Torus Press, pp.215-224, 2005. [hal-00070979](https://hal.science/hal-00070979v1)

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DIRECT EFFECT OF AIRCRAFT SOOT EMISSION:
A LABORATORY VIEW

O. B. Popovicheva*, N. M. Persiantseva*, N. K. Shonija*,
B. Demirdjian†, D. Ferry†, and J. Suzanne†

*M. V. Lomonosov Moscow State University
Moscow, Russia

†CRMC-N/CNRS
Marseille, France

Direct effect of aviation on the atmosphere is related to the impact
of aircraft soot emission on the contrail formation. A new laboratory
approach is elaborated to obtain a more complete knowledge on the
potential of aircraft-generated particulates to act as contrail nuclei. A
comprehensive examination of the morphology, microstructure, and sur-
face chemistry of engine soot and its laboratory-made surrogate is per-
formed using various experimental techniques. It is found that engine
soot particles may be separated in two components: a main fraction
and a fraction of impurities. The high concentration of soluble inor-
ganics and organics stimulate a significant water uptake by soot parti-
cles at low temperatures. Possible pathways of the cloud condensation
model (CCN)/ice nuclei (IN) formation in the plume are suggested.
In the cooling plume, the hydrophilic fraction of impurities of aircraft-
generated soot would nucleate ice in the condensation-freezing mode,
whereas the hydrophobic main fraction allows a direct ice deposition at
low temperatures. Insoluble aerosols of the aircraft exhaust would in-
crease the background level of ice nuclei in the upper troposphere (UT)
causing the cirrus cloud formation.

**Introduction**

The major source of uncertainty in assessing the aircraft impact on climate change is the emission of aerosols which may induce significant
perturbations in the Earth's radiation balance through the direct and indirect effects. It was recently concluded [1] that soot appears to be the most important aviation aerosol impacting contrail and cirrus formation. The quantification of this impact has been advanced but the state of scientific understanding is still poor mainly because of insufficient studies of the ice nucleating ability of aircraft engine soot aerosols forming the contrails and cirrus clouds.

The phenomenon of aircraft contrail formation attracts much attention as a visible and direct anthropogenic impact on the atmosphere. Analysis of ice residuals in contrails [2] and aerosols from aircraft plumes [3] proves that soot aerosols facilitate contrail ice nucleation. Nevertheless, sparse documentation exists on the ice nucleating properties of soot particles measured at contrail levels. Laboratory studies can provide relevant information, but the current laboratory data on surrogate soot from different sources demonstrate a great variety of their nucleation properties. The lack of experimental data on actual soot from aircraft engines led some investigators [4] to assume that exhaust soot is hydrophobic. Such assumptions did not allow to answer several general questions arising, namely, (i) why supersaturation with respect to water is needed for visible contrail formation while modest ice supersaturation is sufficient for cirrus formation, (ii) why only a small portion of exhaust soot particles act as CCN in contrails, and (iii) what is the effect of the fuel sulfur content on the ice-forming ability of exhaust particles.

The authors' previous works [5–7] successfully helped to remove the lack of some experimental data concerning the physico-chemical properties of original soot sampled at the outlet of aircraft engine operating at cruise conditions. The present study is a continuation of this work with the purpose to improve the knowledge about the direct effect of aircraft engine soot on ice nucleation in contrails.

# Characterization of Physico-Chemical Properties of Engine Soot and Laboratory Surrogate

## Sampling of Engine Soot

One of the principal points in the laboratory studies is the sampling of original engine soot for subsequent comprehensive laboratory anal-
ysis. It gives a great advantage in perspective assessments of the air-
craft impact on the atmosphere. The jet engine combustor D30-KU of
Russian aircraft Tu-134 and Tu-154 was used at the ground test facili-
ties in the Central Institute of Aviation Motors (Moscow). The engine
was operated to simulate the cruise conditions with an average air-to-
fuel equivalence ratio of 4, pressure $p \approx 7$ atm, and inlet temperature
$T_i = 300$ K. The composition of the aviation kerosene was controlled
with respect to the fuel sulfur content (FSC) and metal impurities. A
specially designed sampling system was elaborated to collect soot in the
hot combustor exhaust during the controlled combustion regime.

Aircraft Engine Soot: Key Physico-Chemical Properties

More than ten experimental techniques were applied to characterize the
physico-chemical properties of the sampled original engine soot. The
size, shape, and microstructure of the soot particles were character-
ized by atomic force microscopy (AFM) and transmission electron mi-
croscopy (TEM) techniques coupled to electron diffraction. Soot surface
area and porosity were determined, respectively, by a single-point BET
technique and adsorption/structure analysis. The chemical composition
and concentration of impurities were determined by atomic emission
spectroscopy (AES) and X-ray energy dispersive spectroscopy. Fourier
transform infrared spectroscopy was used to investigate the chemical na-
ture of the soot surface. The total water soluble mass fraction of soot
was determined after an ultrasonic treatment in distillated water, fil-
tration, and evaporation. The composition of water soluble compounds
was measured by ion chromatography that allows measuring both neg-
ative and positive ion mass in water extracted from the soot surface.
A heating treatment at 300 °C for 0.5 h allowed estimating the volatile
fraction of soot. The contact angles for water/ice droplets on the soot
surfaces were obtained by the sessile drop technique. Finally, a new
method was elaborated for a direct measure of soot hygroscopicity via
water adsorption in a dessicator without any sample pretreatment to
save its original properties.

High temperature/pressure combustion conditions within an air-
craft engine have a dramatic influence on engine soot. It is character-
ized by low surface area, high density, and absence of microporosity.
The composition and structure heterogeneities separate the engine soot
particles in two components: the main fraction of normally amorphous particles of diameter of 30–100 nm and the fraction of impurities characterized by a variety of structures and a large amount of chemical impurities. The TEM picture of typical particles from the main fraction is shown in Fig. 1. These particles contain mainly carbon with small amounts of oxygen and sulfur. The particles of the fraction of impurities are usually well-crystallized and contain high

concentrations of Fe, O, K, and Mn. In addition, they are always associated with relatively high concentrations of sulfur. Figure 2 shows an impurity in engine soot, it is a large crystal of $Fe_2O_3$ contaminated by C, S, and Mn. The significant difference between the two fractions of engine soot is qualitatively consistent with the observations of fine and intermediate black carbon particles in contrail ice residuals [2].

The surface chemistry analysis of engine soot shows the features of hydrophilic carbonyl $C=O$ groups in aliphatic and aromatic structures as well as hydrophobic $C^1-H$ groups in substituted aromatics. The most distinctive feature of engine soot is a set of bands connected to $HSO_4^-$ ions and organic sulfates. It indicates that surface sulfur appears as derivatives of sulfuric acid and may produce highly soluble areas of the soot surface. But the other part of the surface remains relatively hydrophobic. Therefore, the assumption about engine soot being composed of a carbon core covered by sulfuric acid [8] may be only taken as a simplified model of the original soot.

In engine soot, $13.5 \pm 0.5\%$ (wt.) of water soluble fraction (WSF) was found. Near $3.5\%$ (wt.) of anions in the extracted water is dominated by sulfates $SO_4^{2-}$ and $1.5\%$ (wt.) by organic ions such as acetate $CH_3COO^-$, formate $HCOO^-$, and oxalate $C_2O_4^{2-}$, as well as by inorganic ions $Cl^-$ and $NO_3^-$.

Cations of $K^+$, $Na^+$, and $NH_4^+$ are present at 0.4, 0.1, and $0.08\%$ (wt.), respectively. But the main water soluble

<figure><img src="image_2.png" /><figcaption>**Figure 1** Main fraction of engine soot particles</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2** Fraction of impurities of engine soot</figcaption></figure>

compounds, around 8% (wt.), were not identified in this study and one
may only surely assume that they are nonsulfate containing species and
probably related to volatile organic species which compose ~ 17% (wt.)
of engine soot. Engine soot demonstrates a surprisingly high level of hy-
drophilicity: it is covered statistically by a water monolayer already at
1% RH. It is suggested, that the high concentration of chemical impuri-
ties in the WSF explains the ability of engine soot to interact with water.
The presence of a high concentration of soot surface solutes allows in-
terpreting the absorption in terms of the dissolution and water cluster
formation over heterogeneous areas of the soot surface. At the highest
RH, the amount of water adsorbed on engine soot reaches roughly eight
statistical water monolayers at 295 K. This conclusion is one of the most
important results of engine soot characterization as it may increase the
level of understanding of the ice nucleation phenomena.

**Laboratory-Made Surrogate of Engine Soot**

At present, the majority ● of the data concerning the hygroscopicity and
reactivity of black carbon particles in the atmosphere are based on
laboratory-made soots which show a large variation in their proper-
ties depending on origin. It raises the main question: What kind of
laboratory surrogates may mimic aircraft engine soot?

To resolve this problem, several laboratory soots which are popu-
lar between researchers were examined. Kerosene flame soots [9] pro-
duced in usual oil burners by combustion of different kinds of aviation
kerosenes were studied. The most detailed analysis was done for labora-
tory surrogate soot produced by burning the same TC1 kerosene which
was used for engine soot production [7]. The analysis of TC1 kerosene
soot showed primary amorphous particles of diameter near 40 nm and
high surface area in correlation with a significant microporosity. A small
WSF of 0.3% (wt.) with negligible amount of SO₄⁻ ions was found in
water extracted from TC1 soot. This is why this soot will be referred
to as “insoluble.”

In contrast to engine soot, laboratory-made kerosene flame soots
appeared to be really hydrophobic and able to adsorb water only at RH
above 30%. The mechanism of water adsorption on such soot surfaces
is highly related to the existence of some polar active sites and compli-
cated by the presence of nonoxidized organics which are displaced by
water molecules during the adsorption process. At the highest RH, the
adsorption is connected with the capillary condensation in the meso-
and macropores which takes place in the soot interparticle cavities.
Soot particles of TC1 soot adsorb six times less water than engine soot.

Finally, one concludes that
combustion conditions in the
laboratory oil burner, in con-
trast to aircraft engine, do not
produce a highly hygroscopic
soot. Rather, kerosene flame
soot appears to represent only
the insoluble fraction of engine
soot.

All kerosene soots of the
current study demonstrate a
low wettability: contact angles
were found in the 70°–80°
range. The low wettability cor-
relates with the behavior of

<figure><img src="image_4.png" /><figcaption>**Figure 3** From left to right: pure water, TC1 soot in water, and engine soot in water</figcaption></figure>
kerosene flame soot in water that is typical for hydrophobic black carbons. Figure 3 shows the view of TC1 soot in water: ● one sees that it prefers to stay on the water surface meaning that it is hardly wetted and does not go down after many hours in water. The behavior ● of hygroscopic engine soot in water is strongly different (see Fig. 3). Engine soot immersed into water immediately sinks and accumulates at the bottom of the glass. But one may notice in Fig. 3 that there is some part in engine soot which behaves like TC1 soot, i.e., it accumulates on the water surface. Therefore, engine soot may be considered as a mixture of two components in terms of hygroscopicity: a relatively hydrophobic insoluble main fraction almost free of impurities and a more hydrophilic fraction containing soluble impurities.

# Soot-Induced Processes in Young Plume

## Hydration of Soot Particles

<figure><img src="image_5.png" /><figcaption>**Figure 4** Isotherms of water adsorption on engine soot: 1 — 233 K; 2 — 240; 3 — 258; and 4 — 295 K</figcaption></figure>

Hydration properties of aircraft-generated aerosols are of high importance because they provide direct information about the possible amount of water on the soot surface and the ability of soot particles to act as cloud condensation nuclei in the cooling plume. Figure 4 plots the amount of water molecules adsorbed per surface unit on engine soot vs. the relative humidity. Water adsorption isotherms measured at low temperatures on engine soot

show that the water adsorption increases when the temperature decreases. Such a behavior may be explained by the high heat of water adsorption on the soluble surface compounds. At the plume saturation conditions, T ≈ 233 K, engine soot particles adsorb near 40 ml of water. Obviously, low temperatures facilitate water nucleation on engine soot in the cooling plume.
In contrast, the increase of water adsorption on TC1 soot with the temperature decrease is not so much significant. At the plume saturation conditions, insoluble hydrophobic soot may adsorb not more than 1-3 statistical monolayers of water.

## Ice Nucleation on Aircraft-Generated Soot Particles: Contrail Formation

Numerous field observations and theoretical studies assume aircraft-generated soot to produce the heterogeneous ice nuclei for contrail formation but the mechanism is undetermined. The main conclusion of the present study allows one to answer the main question concerning a possible pathway for contrail formation.

It is assumed that when the exhaust plume reaches the water supersaturation, the most hydrophilic fraction of soot may easily nucleate water because its wettable surface allows the growth of water droplets large enough to overcome the Kelvin barrier. Water supersaturations ($S_w$) needed for the cloud condensation nuclei formation may be estimated using the standard Koehler theory for partially soluble particles. At $T = 235$ K, one finds $S_w \approx 0.08\%$ for particles of 80 nm in diameter with the WSF $\approx 13.5\%$ (wt.). Such a small value for $S_w$ allows one definitely to conclude about the CCN formation on hydrophilic fraction of engine-soot particles. For particles acting as CCN, ice nucleation proceeds by condensation-freezing at threshold contrail-formation conditions and by immersion-freezing nucleation upon further plume cooling. Because of the high portion of such CCN fraction in the exhaust soot, the strong condition for the supersaturation with respect to the liquid phase is required for the contrail formation.

Since the strong evidence is found in in situ observations [10] that $\sim 1/3$ of soot particles must be involved in the contrail formation, it is believed that they should be first of all from the hydrophilic fraction of impurities which, originally, contains large amounts of soluble components. Finally, the field observations that there is no visible difference in appearance in contrails at low and normal FSC [11] may be explained by high WSF in engine soot which is mostly water soluble, nonsulfate containing organics and inorganics. It may be accumulated on the soot surface independently of the sulfate species and impact to CCN formation even at zero FSC. Since the case with high FSC showed an earlier
onset of the contrail formation [11], it means that the surface sulfates start to play some role in the CCN formation because its amount is proportional to water soluble organics. Moreover, the heterogeneity in the soluble compound distribution over the surface of engine-generated soot indicates that even at low FSC, some fraction of particles will accumulate a relevantly high amount of soluble components and participate in the contrail formation by the way similar to the case of normal FSC.

For hydrophobic soot with a small WSF, low water adsorption would allow direct deposition of ice at low temperatures. This process has been observed in the literature for kerosene-exhaust particles [12]. It is more likely that the main fraction of engine exhaust will participate in the ice contrail particle growth only if some chemical modification of its surface takes place due to the plume processing. As a support to this mechanism of ice nucleation in a contrail, it is worth noting that only a part of exhaust soot needs to be activated in order to generate a visible contrail [12]. It is only at lower temperatures in the ice-supersaturated regions of the UT that aged insoluble aircraft-generated engine-soot particles from the main fraction may be activated to produce heterogeneous ice nuclei of cirrus clouds.

## Acknowledgments

The Russian scientists acknowledge the financial support of the grant of the President of Russian Federation, SSNo. 11713.200.2. The authors gratefully thank Prof. A. M. Starik for fruitful cooperation in this work.

## References

1. Penner, J. E., D. H. Lester, D. J. Griggs, D. J. Dokken, and H. McFarland, Eds. 1999. IPCC Special Report. Aviation and the global atmosphere. Cambridge: University Press.

2. Petzold, A., J. Strom, S. Ohlsson, and F. P. Schroder. 1998. Elemental composition and morphology of ice-crystal residual particles in cirrus clouds and contrails. *Atmos. Research* 49:21-34.

3. Chen, Y., S. M. Kreidenweis, L. M. McInnes, D. C. Rogers, and P. J. DeMott. 1998. Single particle analysis of ice nucleating aerosols in the upper troposphere and lower stratosphere. *Geophys. Res. Lett.* 25:1391-94.
4. Karcher, B., Th. Peter, U. M. Biermann, and U. Schumann. 1996. The initial composition of jet condensation trails. *J. Atmos. Sci.* 53:3066-82.

5. Popovitcheva, O. B., N. M. Persiantseva, M. E. Trukhin, N. K. Shonija, Yu. Ya. Buriko, A. M. Starik, B. Demirdjian, D. Ferry, and J. Suzanne. 2000. Experimental characterization of aircraft combustor soot: Microstructure, surface area, porosity, and water adsorption. *Physical Chemistry Chemical Physics* 2:4421-26.

6. Popovitcheva, O. B., N. M. Persiantseva, N. K. Shonija, N. A. Zubareva, A. M. Starik, E. E. Loukhovitskaya, A. N. Secundov, D. A. Usenko, V. M. Zakharov, J. Suzanne, D. Ferry, and B. Demirdjian. 2003. Aircraft engine soot: Characteristic properties as cloud condensation nuclei in upper troposphere. In: *Combustion and atmospheric pollution*. Eds. G. Roy, S. Frolov, and A. Starik. Moscow: TORUS PRESS. 444-49.

7. Popovicheva, O. B., N. M. Persiantseva, E. E. Lukhovitskaya, N. K. Shonija, N. A. Zubareva, B. Demirdjian, D. Ferry, and J. Suzanne. 2004. Aircraft engine soot as contrail nuclei. *Geophys. Res. Lett.* 31:L11104.

8. Gysel, M., S. Nyeki, E. Weingartner, U. Baltensperger, H. Giebl, R. Hitzenberger, A. Petzold, and C. W. Wilson. 2003. Properties of jet engine combustion particles during the PartEmis experiment: Hygroscop-icity at subsaturated conditions. *Geophys. Res. Lett.* 30(11):1566.

9. Persiantseva, N. M., O. B. Popovicheva, and N. K. Shonija. 2004. Wetting and hydration of insoluble soot particles in the upper troposphere. *J. Environ. Monit.* 2:939-45.

10. Schroder, F. P., B. Karcher, A. Petzold, R. Baumann, R. Busen, C. Hoell, and U. Schumann. 1998. Ultrafine aerosol particles in aircraft plumes: *In situ* observations. *Geophys. Res. Lett.* 25:2789-93.

11. Schumann, U., J. Strom, R. Busen, R. Baumann, K. Gierens, M. Kraut-strunk, F. P. Schoder, and J. Stingl. 1996. *In situ* observations of particles in jet aircraft exhausts and contrails for different sulfur-containing fuels. *J. Geophys. Res.* 101:6853-61.

12. Diehl, K., and S. K. Mitra. 1998. A laboratory study of the effects of a kerosene-burner exhaust on ice nucleation and the evaporation rate of ice crystals. *Atmosph. Environ.* 32:3145-51.