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.

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We use a laboratory approach for the quantification of the water uptake by particles with varying amounts of sulfates to simulate the aging of fossil fuel combustion particles in the atmosphere. Diesel fuel and aviation kerosene laboratory-made soots are chosen as representative of the hydrophobic fraction of diesel and aircraft engine-emitted particulates and commercial carbon blacks are used as surrogates for industry emissions. The transformation of soot hydration properties from hydrophobic, through hydrophilic, to hygroscopic is demonstrated based on the amount of water uptake by the laboratory-produced EC soot covered by sulfates as the amount of sulfate increases. The mechanism of water/soot interaction changes from an adsorption on active sites to a water film formation and to the dissolution of water into the water-soluble sulfate, while the amount of sulfate increases. Laboratory simulations show that coverage of EC soot by organics diminishes the effect of sulfuric acid deposition. We demonstrate that the surface chemical properties and the size of primary particles affect the amount of water taken up by the soot particles and their aging in the atmosphere.

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