Toni L5 Flashcards
(25 cards)
Define nucleation
Nucleation is formation of a new thermodynamic phase or structure This process involves the spontaneous formation of a small cluster of the new phase, which then serves as a seed for further growth of the new phase.
What is homogeneous nucleation
The formation of droplets from vapour in a pure environment (in the absence of surfaces or impurities), e.g. condensation in the atmosphere.
What is heterogeneous nucleation
Heterogeneous nucleation. The formation of droplets on a surface or in the presence of an impurity, e.g. soot inclusion
What does Homomolecular nucleation involve
Homomolecular nucleation involves a single species
What does Heteromolecular nucleation involve
Heteromolecular nucleation involves two or more species
What are the 4 types of nucleation
Types of nucleation are therefore homogeneous-homomolecular, heterogeneous-
homomolecular, homogeneous-heteromolecular and heterogeneous-heteromolecular.
What do we mean by gibbs energy for forming a droplet
Energy needed to form a droplet from vapour
What does gibbs energy for forming a droplet tell us
There’s an energy barrier to forming a droplet. A droplet must reach critical radius to grow spontaneously. If supersaturation is too low, the barrier is too high, and nucleation doesn’t happen
What is the critical radius
Minimum droplet size at which spontaneous growth occurs
What happens for values below the critical radius
Droplet evaporates
What happens for values above the critical radius
Droplet grows rapidly (cloud formation)
What does the critical radius control
Whether an aerosol becomes a cloud droplet
What is the critical supersaturation
The critical supersaturation is the minimum RH above 100% required for a particle to activate and grow into a cloud droplet.
What is a cloud condensation nuclei
A CCN is an aerosol particle that can grow into a cloud droplet when RH is high enough.
What is cloud seeding
Cloud seeding is a method of introducing particles into clouds to encourage droplet formation and precipitation. Works by injecting aerosol particles into clouds.These act as CCN This lowers the critical supersaturation needed → activates more droplets → can increase rainfall (sometimes).
Describe aerosol surfaces
Aerosol surfaces are not flat but they are curved
Describe how the curvature of aerosol surfaces affects vapour pressure
Curved surfaces have fewer molecular neighbors.
Easier for molecules to escape (evaporate). Smaller particles = more curved = higher vapor pressure. Explains why smaller droplets are harder to grow — they evaporate more easily.
What does the kelvin equation describe
The Kelvin equation describes how curvature affects the vapor pressure over a small droplet.
What does the kelvin equation tell you
Small droplets (high curvature) have higher vapor pressure — they evaporate more easily. You need a higher RH for a curved droplet to be stable.
What is the kohler equation
The Köhler equation combines the Kelvin effect with Raoult’s law (solute effect) to describe the equilibrium RH over a solution droplet.
How can we measure critical supersaturation
Uses laminar flow chambers:
Controls temperature and RH along a streamline
Compares growth rates and sizes to determine activation RH
What is the meaning of activated fraction
Fraction of aerosols that become cloud droplets
What does the activated fraction depend on
Particle size, solubility, and supersaturation
How do we calculate the activated fraction
Total number of activated particles/ total number of particles
