Modeling of Joint Capillary Condensation of Trace Chemicals and Water on Fractal Soot Aggregates

ELLA IVANOVA, Ali Hasani, Egor Demidov, Gennady Gor, Alexei Khalizov, New Jersey Institute of Technology

     Abstract Number: 71
     Working Group: Carbonaceous Aerosol

Abstract
Soot from combustion is an effective light absorber that contributes significantly to direct climate forcing. It is also an air pollutant. Soot particles are branched fractal aggregates of spherical primary carbon particles (monomers). The negative impacts of soot on climate and human health depend on the morphology of soot aggregates. Experiments show that the condensation of vapors on fractal aggregates often causes their restructuring into compact globules. Furthermore, it was shown that even a small amount of condensate could induce restructuring if the coating material is located in the gap between monomers [1].

This work uses a numerical model to predict the joint capillary condensation of trace gas atmospheric chemicals and water on the fractal soot surface, as observed in the experiments, where a less than 3% mass fraction of water-soluble chemicals promoted aggregate collapse after humidification to 85% relative humidity In the model, as in an experiment, we use a two-step approach. First, we start by exposing the aggregate to the sub-saturated vapors of several individual chemicals using a kinetic condensation model [1, 2]. Then, we consider condensation of water vapor as a function of increasing humidity based on a thermodynamic model. Initially, the soot surface is hydrophobic; however, after exposure to hydrophilic chemicals, capillary water condensation becomes possible. The outcome depends strongly on the contact angle and shape of the meniscus of the condensate. The model results are in agreement with the experiments and confirm that even a small amount of water-soluble chemicals promote the condensation of water with following soot restructuring.

[1] Chen, Chao, et al. "Single parameter for predicting the morphology of atmospheric black carbon." Environmental science & technology 52.24 (2018): 14169-14179
[2] Ivanova, Ella et al. "Kinetic model for competitive condensation of vapor between concave and convex surfaces in a soot aggregate." Aerosol Science and Technology 55.3 (2020): 302-315.