Lecture 8: Aerosols (part 1)
Download
Report
Transcript Lecture 8: Aerosols (part 1)
Aerosols
Atmospheric Aerosols
Bibliography
Seinfeld & Pandis, Atmospheric Chemistry and Physics, Chapt. 7-13
Finlayson-Pitts & Pitts, Chemistry of the Upper and Lower Atmosphere,
Chapt. 9.
Classic papers:
Prospero et al. Rev. Geophys. Space Phys., 1607, 1983; Charlson et al. Nature
1987; Charlson et al., Science, 1992.
Recent Papers:
Ramanathan et al., Science, 2001; Andreae and Crutzen, Science, 1997;
Dickerson et al., Science 1997; Jickells et al., Global Iron Connections Between
Desert Dust, Ocean Biogeochemistry and Climate, Science, 308 67-71, 2005.
Aerosols: General
Comments
Any solid, liquid (or mixture) in the
atmosphere
Sources
Natural
Anthropogenic (urban, construction,
agriculture)
Primary (introduced directly into the
atmosphere)
Secondary (formed in the attmosphere)
Aerosol Effects
Climate
Weather
Visibility
Health Effects
Clouds?
Saharan Dust
affects the West
African Monsoon
Natural Sources and Estimates of Global
Emissions of Atmospheric Aerosols
Source
Amount-range (Tg yr-1)
Amount -best
estimate (Tg yr-1)
Soil Dust
1000-3000
Sea Salt
1000-10000
Botanical Debris 26-80
1500
1300
50
Volcanoes
4-10000
30
Forest Fires
Gas conversion
Photochem
Total
3-150
100-260
40-200
2200-24000
20
180
60
3100
Anthropogenic Sources of Aerosols
Source
Amount Range
(Tg yr-1)
Best Estimate
Direct Emission
50-160
120
Gas to particle
260-460
330
Photochemistry
5-25
10
Total
320-640
460
Reference: W.C. Hinds, Aerosol Technology, 2nd
Edition, Wiley Interscience
Gas-to-particle conversion:
Certain gas phase reactions result in
formation of low-vapor-pressure
reaction products.
Because of their low vapor pressure,
they exist at high supersaturations
and can form particles.
Natural Background
Aerosol
Stratospheric
Major volcanic activity injects sulfur
dioxide (SO2) into the stratosphere
Gas to particle conversion, SO2 into
sulfuric acid (H2SO4)
Tropospheric
Vegetation, deserts and ocean
Primarily in the lowest few km
Mount Pinatubo, 1991
Urban Aerosol
Dominated by anthropogenic
sources
Three Modes
Nuclei
Accumulation
Coarse
Aitken
Large
Giant
What is meant by the size of an aerosol?
What does a size distribution mean?
ORIGIN OF THE ATMOSPHERIC AEROSOL
Aerosol:Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop)
Soil dust
Sea salt
Environmental importance: health (respiration), visibility, radiative balance,
cloud formation, heterogeneous reactions, delivery of nutrients…
AEROSOL NUCLEATION
# molecules 1
2
3
4
DG
Surface
tension
effect
Thermo
driving
force
Critical
cluster size
cluster size
Atmospheric Aerosols
Question?
Considering the Urban Aerosol,
where are most of the particles?
Where is the most mass?
How many 0.01 mm particles are
necessary to have the same mass as
one 1mm particles?
Urban Aerosol Size Distribution
Nuclei Mode (<0.1mm)
Consist of:
Direct combustion particles emitted
Particles formed by gas-to-particle conversion
Usually found near sources of combustion
(e.g. highways!)
Due to their high number concentration:
Coagulate rapidly.
End up in accumulation mode
Relatively short lifetime
Aitken Particles
Accumulation Mode (0.1 μm <
particle size < 2.5 μm)
Includes combustion particles, smog particles,
and coagulated nuclei-mode particles.
(Smog particles are formed in the atmosphere by
photochemical reactions)
Particles in this mode are small but they
coagulate too slowly to reach the coarseparticle mode.
they have a relatively long lifetime in the
atmosphere
they account for most of the visibility effects of
atmospheric aerosols.
The nuclei and accumulation modes together
constitute “fine” particles.
Large Particles
Coarse-particle mode
(particle size > 2.5 μm)
Consist of
Windblown dust, large salt particles from sea
spray,
Mechanically generated anthropogenic
particles such as those from agriculture and
surface mining.
Due to their large size
Readily settle out or impact on surface,
Lifetime in the atmosphere is only a few hours.
Giant Particles
Dynamic Processes of
Atmospheric Aerosol
Formation
Gas to particle conversion
Photochemical processes
Growth
Coagulation, condensation, evaporation
Removal
Settling
Deposition
Rainout, washout
Global Effects of Aerosols
Global Cooling
Direct effect
Indirect effect
Ozone depletion
Polar stratospheric clouds
(PSC)
Surfaces of PSC act to
catalyze Cl compounds to
atomic Cl