Aerosol forcing Flashcards
What is radiative forcing?
The change in net irradiance at the tropopause with surface and tropospheric temperatures held fixed (positive values imply a heating of the troposphere)
How will CO2 change in the future?
Considerable uncertainty, don’t know how it will evolve in the future
What is an aerosol?
Suspension of tiny particles in the air, with a diameter between 0.01-10 micrometers
What makes up aerosols?
Sulfates, nitrates, organic compounds, metals, dust, sea salt …
Where do aerosols come from?
- industry
- volcanoes (natural)
- fertilisers
- diesel fuel
- turpines
How many people does EU legislation on aerosols save per year?
800,000
How much soot is produced from biomass burning per year?
54 Tg
How much industrial dust is emitted every year?
100 Tg
How much anthropogenic sulphate is emitted each year?
200 Tg
How much biogenic volatile organic carbon (VOC) is emitted every year?
16-500 Tg
Where are aerosols generated?
- over industry
- natural: forest fires, dust etc.
- overall, anywhere outside of the poles, aerosol is being generated
Which two ways can be classify aerosols?
Primary and Secondary
What is a primary aerosol?
An aerosol that is directly emitted to the atmosphere e.g. sea salt
What is a secondary aerosol?
Originate from gases which then oxidise or nucleate to form the secondary aerosol (important for cloud formation)
What does volatile mean?
Easily evaporated (less volatile means something is more likely to condense)
What two ways are aerosols removed?
Dry and wet processes
How does lifetime of aerosols change if they reach the stratosphere?
Longer life time e.g. after volcanic eruptions, can remain for a few years
What is aerosol mass important for?
Light scattering
What is aerosol number important for?
Effect on clouds (CCN)
How does aerosol mass differ?
Bulk concentration near emission source
How does aerosol number differ?
Homogenised, reasonable spread (secondary aerosols are able to form further away from source)
What is the annual mean mass of PM2.5?
1-100 µg m-3
What is the annual mean surface particle number?
100-10,000 particles m-3
What model predicts annual mean surface particle number?
GLOMAP global aerosol model
What satellites predicts annual mean mass of PM2.5?
MODIS satellite
Where does anthropogenic sulphate originate from?
Burning fossil fuels, Volcanoes and Phytoplankton
How does burning fossil fuels generate sulphate aerosol?
SO2 and CO2 oxidises to Sulphuric acid (H2SO4) which nucleated and condenses to Sulfate aerosol
What is one direct aerosol effect?
Scattering and absorption of solar radiation
What is one indirect aerosol effect?
Changes in cloud properties
Which band of the EM spectrum does aerosol forcing mostly affect?
Short wave radiation
Scattering efficiency of aerosols depends on …
- size (distribution of particle)
- composition (refractive index)
- shape of particle
Absorption efficiency of aerosols depends on …
- mass of absorbing material
- compositions (mixing of absorber and scatterer)
What effect does light scattering aerosol have?
Atmosphere and surface cools
What effect does light absorbing aerosol have?
Atmosphere warms and surface cools
What is the single-scattering albedo (SSA)?
A measure of the relative amount of light scattered and absorbed
What is the average SSA for northern hemisphere aerosol?
˜0.85-0.95
If SSA > 0.95 …
Net negative top of atmosphere (TOA) forcing
If SSA < 0.85 …
Net positive top of atmosphere (TOA) forcing
What is the SSA for black carbon and therefore does it scatter or absorb?
0.2 therefore it absorbs
What is the SSA for sulphate aerosol and therefore does it scatter or absorb?
1.0 therefore it scatters
How is SSA calculated?
Scattering/(scattering+absorption)
What is the net cooling effect of clouds?
27 Wm^-2
Clouds and the aerosol indirect effect …
More aerosol particles… more smaller cloud droplets… clouds more reflective and cool climate (1st effect) … smaller cloud drops take longer to form rain=increased cloud lifetime (2nd effect)
What evidence do ship tracks provide for the aerosol indirect effect?
Pollution from ship … more aerosol … more smaller cloud droplets … cloud more reflective (increased albedo)
-clouds form over ship tracks
What albedo do clouds have?
Very variable: 0.1-0.8
What causes a change in cloud albedo?
A change in cloud droplet number
How can we approximate cloud albedo (A) due to change in cloud droplet number (N)?
∆A/A = (1-A) (∆N/N)
How much does cloud albedo change by due to a change of 1% in cloud droplet number?
Changes by 0.5%
How has pattern of the amount of radiation reaching the Earths surface changed?
Decreased up to the 1980s then began to rise again
What is one possible reason for the rise in radiation reaching the earths surface since the 1980s?
EU legislation on aerosol pollution
What uncertainties are associated with the aerosol indirect effect?
- highly variable clouds
- response of clouds to droplet number is poorly understood
- hard to link changes to changes in aerosol
Roughly, what is the globally averaged radiative forcing due to all aerosol effects over the industrial period?
-2 Wm^-2
