No abstract

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.