Egor Demidov

Heterogeneous nucleation of a non-wetting vapor on NaCl aerosol nanoparticles and its implications on cloud forming and optical properties

Egor V. Demidov, Alexei F. Khalizov

ACS NERM, Oral presentation, 2022


Atmospheric aerosols are significant contributors to climate forcing both directly, through scattering and absorption of sunlight, and indirectly, by modifying the clouds. Aerosols in the atmosphere undergo processing via several pathways, including condensation of various chemicals on particle surfaces. Upon condensation and formation of a coating layer, cloud forming and optical properties of particles change. In most cases, condensation will result in formation of a uniform coating layer, resulting in a core-shell coated particle morphology. However, when a particle is non-wettable by the coating, heterogeneous nucleation will occur on a surface defect, and coating will then grow near the defect without fully encapsulating the seed particle until critical coating mass is reached. Understanding which type of particle-coating morphology is formed for certain aerosol-vapor combinations is critical for predicting climate change accurately, as optical properties and the ability of particles to form cloud droplets would depend on particle-coating morphology. We investigated the differences in optical and cloud forming properties of different particle-coating morphologies experimentally by exposing non-wettable NaCl and wettable polystyrene latex (PSL) aerosols to supersaturated vapors of dioctyl sebacate (DOS). Coating morphology was then verified by measuring mass and mobility diameter of coated particles and calculating the mass-mobility exponent. For core-shell coatings, the mass mobility exponent is expected to be close to 3. A lower exponent indicates deviation from spherical geometry. Another method used to verify if salt particles were fully encapsulated in the coating material was exposing the aerosol to elevated humidity above the deliquescence point. A fully encapsulated particle would not be affected by the increase in humidity, while a particle with an exposed surface would absorb water vapor and drastically increase in size. This procedure was used to determine the critical coating mass needed to prevent deliquescence of NaCl nanoparticles. Optical properties of coated particles were then measured at different coating masses. Scattering was also calculated theoretically for core-shell and non-core-shell morphologies and compared to experimental results. PSL particles were closer to results predicted by the core-shell model, while NaCl particles produced results closer to non-core-shell morphology.


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