The interaction of water with laboratory soots possessing a range of properties relevant for atmospheric studies is examined by two complementary methods: gravimetrical measurement of water uptake coupled with chemical composition and porosity analysis and HTDMA (humidified tandem differential mobility analyzer) inference of water uptake accompanied by separate TEM (transmission electron microscopy) analysis of single particles. The first method clarifies the mechanism of water uptake for bulk soot and allows the classification of soot with respect to its hygroscopicity. The second method highlights the dependence of the soot aerosol growth factor on relative humidity (RH) for quasi-monodisperse particles. Hydrophobic and hydrophilic soot are qualitatively defined by their water uptake and surface polarity: laboratory soot particles are thus classified from very hydrophobic to very hydrophilic. Thermal soot particles produced from natural gas combustion are classified as hydrophobic with a surface of low polarity since water is found to cover only half of the surface. Graphitized thermal soot particles are proposed for comparison as extremely hydrophobic and of very low surface polarity. Soot particles produced from laboratory flame of TC1 aviation kerosene are less hydrophobic, with their entire surface being available for statistical monolayer water coverage at RH approximately 10%. Porosity measurements suggest that, initially, much of this surface water resides within micropores. Consequently, the growth factor increase of these particles to 1.07 at RH > 80% is attributed to irreversible swelling that accompanies water uptake. Hysteresis of adsorption/desorption cycles strongly supports this conclusion. In contrast, aircraft engine soot, produced from burning TC1 kerosene in a gas turbine engine combustor, has an extremely hydrophilic surface of high polarity. Due to the presence of water soluble organic and inorganic material it can be covered by many water layers even below water saturation conditions. This soot demonstrates a gradual diameter growth factor (D(wet)/D(dry)) increase up to 1.22 at 93% relative humidity, most likely due to the presence of single particles with water soluble material heterogeneously distributed over their surface.

The laboratory combustion technique operating on a typical combustor of a gas turbine engine is used for soot sampling. Soot particles are derived by combustion of a hydrocarbon mixture at typical cruise C 3 H 8 Èn-C 4 H 10 conditions. Size, morphology, microstructure, surface area, porosity, and the chemical nature of the soot surface particles are studied by transmission electron microscopy (TEM), Raman and Auger electron spectroscopies (AES), volumetry and gravimetry. Structural irregularities such as micropores determine the speciÐc adsorbability of non-polar gases such as Kr, and With respect to water adsorption, CH 4 C 6 H 6. aircraft combustor soot is far from being hydrophobic. Initial water adsorption on polar heterogeneities leads to pore Ðlling at increasing pressures. The microstructure of soot particles is easily transformed under the inÑuence of adsorbates, giving rise to swelling e †ects. Due to its speciÐc physico-chemical properties aircraft combustor soot may act as contrail condensation nuclei at low sulfur content in the jet fuel.

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Quantification of atmospheric processes including the water uptake by soot particles of various origin, emitted from different sources, requires identification of hydrophobic and hydrophilic soot. Water uptake measurements are performed on well-characterized laboratory soots available for atmospheric studies. Comparative analysis of water adsorption isotherms on soots of various compositions allows us to suggest a concept of quantification. Systematic analysis demonstrates two mechanisms of water/soot interaction, namely, bulk dissolution into soot-water-soluble coverage (absorption mechanism) and water molecule adsorption on surface active sites (adsorption mechanism). The formation of water film extended over the surface is suggested as a quantification measure which separates hygroscopic from non-hygroscopic soot. Water uptake on hygroscopic soot takes place by the absorption mechanism: it significantly exceeds the formation of many surface layers. If soot particles are made mostly from elemental carbon and/or are covered by a water-insoluble organic layer, they are classified as non-hygroscopic. Low water adsorption on some active sites following cluster formation is a typical mechanism of water interaction with hydrophobic soot. If a water film extended over the surface is formed due to the cluster confluence it is suggested that soot is hydrophilic. A few classical models are applied for parameterization of water interactions on hydrophilic and hydrophobic soots.

[1] The physico-chemical properties of aircraft engine soot are characterized with respect to their ability to act as CCN.Comparison with laboratory-generated kerosene soot shows a significant influence of combustion conditions on the morphology, microstructure, chemical composition, surface nature, and hygroscopicity of soot. Engine soot particles separate into two components based on composition and structural heterogeneities: a main soot fraction and a fraction of impurities containing an appreciable amount of metal and sulfur. The high concentration of soluble sulfates, of inorganics and of organics in the fraction that contains impurities, explains the engine soot hygroscopicity and its ability to act as CCN at threshold conditions for contrail formation. Laboratory -generated kerosene soot is not able to reproduce the hygroscopicity of engine soot, but we show that it is a good surrogate for the insoluble black carbon fraction of aircraft soot in the upper troposphere.