Toni atmosphere L3 Flashcards
(21 cards)
Why does size of aerosol/particles matter
Particle size determines everything: lifetime, optical properties, deposition, ability to act as CCN.
How do we characterise particle size
Because aerosol populations vary hugely in size, we use log-normal distributions. A log-normal distribution gives a bell shape when plotted in log diameter space, which better reflects real aerosol data.
Describe aerosol number distributions
The aerosol number distributions is usually dominated by the Nucleation mode. As they account for the largest number of particles but the lowest mass
Describe surface area distribution
The surface area distribution is dominated by the accumulation mode
Describe the volume distribution
Most of the aerosol volume and mass is in the coarse mode, although there are very few particles in this mode
Describe nucleation mode aerosol lifetimes
Minutes to hours
Describe accumulation mode aerosol lifetime
Hours to days
Describe coarse mode aerosol lifetimes
Minutes to hours
Describe what forces are acting on an aerosol particle
Gravitational force - pulls them down
Buoyancy - opposes gravity slightly
Drag force - resists motion through the air
What is Stoke’s law
For particles larger than 1micrometer the drag force is given by
Fd = 6𝜋nrvs
N is the dynamic viscosity
Describe how a particle larger than 1 micrometer falls through the atmosphere
When an aerosol falls through the atmosphere it interacts with the surrounding gas molecules. there is a very consistent flow/ bombardment from all sides. And the gas surrounding the particle looks like a continuous fluid
Describe how a small particle falls through the atmosphere
When a small particle falls through the atmosphere most of the gas molecules will miss the particle and there is no longer a continuous bombardment, so smaller particles tend to slip through the atmosphere much faster then we might expect, as they are not getting equal bombardments.
The drag force would then be a lot smaller than expected
How do we alter the equation for drag force for small particles
We need to correct rhe Stoke’s force with the Cunningham slip correction factor
What is the Knudsen number
The distance between gas phase collisions. It is the ratio of mean free path with respect to the radius of the droplet
What is the equation to calculate mean free path
Mean free path = KbT/√2𝜎p
Describe how an aspheric particle influences the settling velocity and therefore the drag force
Settling velocity equation now incorporates a shape correction factor. As the aerosol becomes more asymmetric the shape factor increases and the settling velocity will decrease. The drag force will decrease proportional to the settling velocity
Define brownian diffusion of particles
Brownian diffusion describe how small particles ( < 1µm) randomly move and spread through a fluid due to bombardment by air molecules
What is coalescence
Particles collide and stick → coalescence
What particles is Brownian motion dominant for
Nucleation particles
What particles is diffusion dominant for
Nucleation
What particle size is gravitational settling dominant for
Coarse mode >1 micrometer
