Adsorption of nitrogen and water vapor by insoluble particles and the implication on cloud condensation nuclei activity

The adsorption parameters of three insoluble species, black carbon, kaolinite and montmorillonite particles were measured using an accelerated surface area and porosimetry system with nitrogen and water molecules as adsorbates to estimate the ability of insoluble species to act as cloud condensation nuclei (CCN). The surface area of adsorbents was determined using the Brunauer-Emmett-Teller (BET) isotherm analysis and further incorporated in Frenkel-Halsey-Hill (FHH) isotherm analysis to illustrate the multilayer adsorption process at high saturation conditions. The derived BET adsorption parameters suggest a higher multilayer adsorption at surface coverage (θ)=1 for water adsorption than N2 adsorption, i.e., 24±2% vs 8±3%. The water adsorption on carbon surface showed no consistent trend for the BET isotherm possibly due to the hydrophobic properties. However, for water saturation (S) in the range of 0.4-0.9, all three components followed a good FHH isotherm trend. The required supersaturation (SSc) to form CCN using the FHH-adsorption activation theory showed a lower criterion for montmorillonite by 0.05-0.3% than that for black carbon and kaolinite. The equivalent hygroscopicity parameters, κ, was estimated as ~0.002 for montmorillonite and less than 0.001 for both black carbon and kaolinite at a dry diameter of 200 nm. The results suggested that montmorillonite is a better CCN than the other two species. In addition to the adsorption parameter determination, the surface coverage was revised using the original definition and showed a higher required supersaturation than the simplified formula which was applied in earlier studies. Such deviation is more significant for composition having higher hygroscopicity and suggests the possible overestimation of CCN activation using the simplified equation, commonly applied in most studies.