Aerosol-radiation Interactions

Question 1

Why are aerosols in homogeneous throughout the atmosphere?

Answer 1

Because atmospheric lifetime for tropospheric aerosols is only about 1 week

Question 2

What is the difference between scattering and absorption?

Answer 2

Scattering is reradiated at the same wavelength as incident beam whereas absorption is re-radiated at longer wavelengths

Question 3

What happens when light impinges on a particle?

Answer 3

-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)

Question 4

What are three parameters that characterise the interaction of aerosols with radiation?

Answer 4

-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?

Question 5

What is beers law?

Answer 5

τ=ln(I0/Iν)

Question 6

What symbol denotes single scattering albedo?

Answer 6

ω

Question 7

What is the single scattering albedo value for a purely scattering albedo?

Answer 7

ω=1

Question 8

What is the single scattering albedo value for a purely absorbing albedo?

Answer 8

ω=0

Question 9

When are the back-scatter fraction and the up-scatter fraction the same?

Answer 9

When the sun is overhead

Question 10

What does the angstrom exponent describe?

Answer 10

Describes the wavelength dependence of the aerosol optical depth

Question 11

What do we assume when developing a simple forcing model for a purely scattering aerosol? (6)

Answer 11

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

Question 12

How do aerosols increase the reflectance over dark regions?

Answer 12

They have a very large fractional impact on the planetary albedo

Question 13

Do aerosols increase the reflectance over bright regions?

Answer 13

Almost no impact on planetary albedo

Question 14

What do we take into account when calculating the direct radiative effect (DRE)?

Answer 14

• 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

Question 15

What is a less elegant way of calculating DRE?

Answer 15

Assume Ra^2 terms are neglible due to no multiple scattering

Question 16

Are anthropogenic aerosols entirely scattering across all wavelengths?

Answer 16

No-they are partially absorbing

Question 17

What is transmission through aerosol layer Ta equal to?

Answer 17

Ta=1-Ra-Aa
Ra=reflectance of aerosol
Aa=absorption of aerosol

Question 18

Assuming aerosols are partially absorbing, will aerosols warm or cool the climate over a high reflectance surface?

Answer 18

Warm
The dirty aerosols decrease albedo

Question 19

Assuming aerosols are partially absorbing, will aerosols warm or cool the climate over a low reflectance surface?

Answer 19

Cool
The dirty aerosols make the dark surface appear grey

Question 20

What conditions emit black carbon?

Answer 20

Flaming, high temp combustion

Question 21

What is the structure of black carbon?

Answer 21

Compels fractal chain structures that tend to collapse down to more compact structures as aerosols age

Question 22

What is Mie scattering theory?

Answer 22

Assumes spherical particles with a well-mixed complex refractive index and an aerosol size distribution

Question 23

In Mie scattering, what does a greater imaginary part of the refractive index indicate

Answer 23

Greater absorption

Question 24

In Mie scattering, what does a greater real part of the refractive index indicate

Answer 24

Greater scattering

Question 25

How are aerosol refractive indices weighted? And why?

Answer 25

Weighted by volume to represent composite particles

Question 26

what is the only way to accurately measure aerosol refractive index?

Answer 26

to measure how the black carbon and organic and inorganic compound vary as a function of time from emission

Question 27

do volcanic eruptions warm or cool the planet? positive or negative radiative forcing?

Answer 27

cool and negative radiative forcing

Question 28

how does the temperature gradient of the troposphere and stratosphere affect aerosol mixing?

Answer 28

-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

Question 29

how are aerosols removed from stratosphere?

Answer 29

-transfer of stratospheric air to troposphere, where dry and wet removal occurs

Question 30

what is emitted from explosive volcanoes?

Answer 30

-volcanic ash
-gases/ vapours rich in sulphur

Question 31

what phase chemistry occurs in the troposphere

Answer 31

aqueous phase, gas phase, neutralisation of acid into a salt

Question 32

what phase chemistry occurs in the stratosphere

Answer 32

gas phase only