Aerosol-radiation Interactions Flashcards
Why are aerosols in homogeneous throughout the atmosphere?
Because atmospheric lifetime for tropospheric aerosols is only about 1 week
What is the difference between scattering and absorption?
Scattering is reradiated at the same wavelength as incident beam whereas absorption is re-radiated at longer wavelengths
What happens when light impinges on a particle?
-electron charges are excited into oscillatory motion
-excited electrical charges re-radiate energy in all directions (scattering)
-they may convert a part of the incident radiation into thermal energy (absorption)
What are three parameters that characterise the interaction of aerosols with radiation?
-extinction: how effective is the interaction?
-single scattering albedo: how much absorption takes place per single scattering event?
-the scattering phase function: what direction is light scattered?
What is beers law?
τ=ln(I0/Iν)
What symbol denotes single scattering albedo?
ω
What is the single scattering albedo value for a purely scattering albedo?
ω=1
What is the single scattering albedo value for a purely absorbing albedo?
ω=0
When are the back-scatter fraction and the up-scatter fraction the same?
When the sun is overhead
What does the angstrom exponent describe?
Describes the wavelength dependence of the aerosol optical depth
What do we assume when developing a simple forcing model for a purely scattering aerosol? (6)
1) purely scattering aerosol e.g. sulfates, nitrates
2) consider the effects in the solar spectrum only
3) ignore the wavelength dependency of aerosol optical properties
4) ignore the wavelength dependency of atmospheric transmission
5) use global annual-mean average values
6) make simple corrections assuming there is no forcing in cloudy areas from a purely scattering aerosol
How do aerosols increase the reflectance over dark regions?
They have a very large fractional impact on the planetary albedo
Do aerosols increase the reflectance over bright regions?
Almost no impact on planetary albedo
What do we take into account when calculating the direct radiative effect (DRE)?
- the forcing is neglible over bright clouds so we add (1-Ac) term
- earth is only half lit at once so we add -1/2 S0 term
What is a less elegant way of calculating DRE?
Assume Ra^2 terms are neglible due to no multiple scattering
Are anthropogenic aerosols entirely scattering across all wavelengths?
No-they are partially absorbing
What is transmission through aerosol layer Ta equal to?
Ta=1-Ra-Aa
Ra=reflectance of aerosol
Aa=absorption of aerosol
Assuming aerosols are partially absorbing, will aerosols warm or cool the climate over a high reflectance surface?
Warm
The dirty aerosols decrease albedo
Assuming aerosols are partially absorbing, will aerosols warm or cool the climate over a low reflectance surface?
Cool
The dirty aerosols make the dark surface appear grey
What conditions emit black carbon?
Flaming, high temp combustion
What is the structure of black carbon?
Compels fractal chain structures that tend to collapse down to more compact structures as aerosols age
What is Mie scattering theory?
Assumes spherical particles with a well-mixed complex refractive index and an aerosol size distribution
In Mie scattering, what does a greater imaginary part of the refractive index indicate
Greater absorption
In Mie scattering, what does a greater real part of the refractive index indicate
Greater scattering
How are aerosol refractive indices weighted? And why?
Weighted by volume to represent composite particles
what is the only way to accurately measure aerosol refractive index?
to measure how the black carbon and organic and inorganic compound vary as a function of time from emission
do volcanic eruptions warm or cool the planet? positive or negative radiative forcing?
cool and negative radiative forcing
how does the temperature gradient of the troposphere and stratosphere affect aerosol mixing?
-positive temp gradient in startosphere
-this inhibits vertical mixing
-the temperature inversion at the tropopause means that the troposphere and stratosphere air seldom mix
-hence removal processes are much reduced and lifetime of stratospheric aerosols is much longer
how are aerosols removed from stratosphere?
-transfer of stratospheric air to troposphere, where dry and wet removal occurs
what is emitted from explosive volcanoes?
-volcanic ash
-gases/ vapours rich in sulphur
what phase chemistry occurs in the troposphere
aqueous phase, gas phase, neutralisation of acid into a salt
what phase chemistry occurs in the stratosphere
gas phase only
