Quantification of water uptake by soot particles

Abstract

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.

Mapping the Operation of the Miniature Combustion Aerosol Standard (Mini-CAST) Soot Generator

“…This results in the soot acting as poor CCN with activated fractions of 0%-30% and κ ∼ 10 −3 . We note that this differs from the findings of Popovicheva et al (2008), who found aircraft engine soot to be hygroscopic in the sense Kerminen et al (1997) Kroll et al (2012), and u Petzold et al (2005).…”

“…Popovicheva et al (2008) used a gravimetric method to measure sub-saturated water uptake on CAST burner soot at two conditions: flame C/O ratio of 0.29 (4% OC) and 0.4 (27% OC), which are the same conditions examined by Möhler et al (2005). They find that more monolayers of water adsorb on the surface of the higher-OC soot than the lower-OC soot, which leads them to classify the former soot as hydrophilic and the latter soot as hydrophobic.…”

“…Soot from an aircraft engine combustor was found to adsorb many more water monolayers at RH < 90%, and this soot was classified as hygroscopic. Fourier transform infrared spectroscopy (FTIR) analysis of the lower-OC CAST soot revealed only carbonyl functional groups, while polyaromatics, carbonyl, carboxyl, and hydroxyl groups were found on the higher-OC CAST soot and on the combustor soot (Popovicheva et al 2008). Thus, the increased water uptake was attributed to the presence of these more-polar, functionalized compounds.…”

Detailed heterogeneous chemistry in an urban plume box model: reversible co-adsorption of O&lt;sub&gt;3&lt;/sub&gt;, NO&lt;sub&gt;2&lt;/sub&gt;, and H&lt;sub&gt;2&lt;/sub&gt;O on soot coated with benzo[a]pyrene

“…Multiplying the BET value by the soot concentration yields a total surface concentration of [PS] g =5×10 −5 cm 2 cm −3 . However, BET values for soot can range from 6 m 2 g −1 for aircraft engine combustor soot (Popovicheva et al, 2008) to approximately 500 m 2 g −1 for the post-treated black carbon Degussa FW2 (Dymarska et al, 2006), which can lead to large differences in [PS] g . Thus, the implemented BET-value of 500 m 2 g −1 may result in an upper limit for the concentration of surface reaction sites.…”

Small-Scale Study of Siberian Biomass Burning: II. Smoke Hygroscopicity

“…Two mechanisms of water/soot interaction have been proposed: bulk dissolution into water-soluble coverage (hygroscopic soot) and the water molecule adsorption onto surface oxygen-containing functionalities (non-hygroscopic soot) (Popovicheva et al, 2008b). The formation of a water film extended over the surface separates hydrophobic from hydrophilic soot.…”