Aerosols: Tiny Particles, Big Impact

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CONTENT

•What is an aerosol?•What are the sources of aerosols?• Aerosol Size distribution• The Aerosol Modes• Atmospheric cycling of aerosols.•What role aerosols play in Earth’s atmosphere?• Three types of aerosols which affects the Earth’s climate.• How do aerosols affect the climate?• Atmospheric lifetime of different size particles at different

levels of the atmosphere.•What is "global dimming" and how are aerosols involved?• Aerosol sinks• Health Effects

What is an aerosol??

•An aerosol is generally defined as a suspension of liquid or solid particles in a gas, with particle diameters in the range of 10-9–10-4 m .

•The most evident examples of aerosols in the atmosphere are clouds, which consist primarily of condensed water with particle diameters on the order of approximately 10 mm.

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What are the sources of aerosols?

•Aerosol particles are either emitted directly to the atmosphere (primary aerosols) or produced in the atmosphere from precursor gases (secondary aerosols).

•Primary aerosols consist of both inorganic and organic components. Inorganic primary aerosols are relatively large and originate from sea spray, mineral dust, and volcanoes.

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Sources and appearance of atmospheric aerosols.Top: local and large scale air pollution. Sources include volcanic eruptions (producing volcanic ash and sulphate), sea spray (sea salt and sulphate aerosols), desert storms (mineral dust), savannah biomass burning (BC and OC), coal power plants (fossil fuel BC and OC, sulphate, nitrate), ships (BC, OC, sulphates, nitrate), cooking* (domestic BC and OC), road transport (sulphate, BC, VOCs yielding OC). (A) sulphates, (B) soot, (C) fly ash, a product of coal combustion .

Aerosol Size distribution

All properties of aerosols depend on particle size, thus it is the most important parameter to study the behavior of aerosols.

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• Nucleation(Aitken) mode – 0.01-0.1 m

• Accumulation mode – 0.1-1 m

• Coarse mode - >1 m

The Aerosol Modes

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Atmospheric cycling of aerosols.

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What roles do aerosols play in Earth's atmosphere?•They act as cloud condensation nuclei.

• They alter albedo (both directly and indirectly via clouds) and hence Earth's radiation budget.

• They serve as catalysts or sites for atmospheric chemistry reactions.

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Three types of aerosols significantly affect the Earth's climate.

•Volcanic Aerosol

•Desert Dust

•Human-Made Aerosol

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How do aerosols affect the climate?

• All atmospheric aerosols scatter incoming solar radiation, and a few aerosol types can also absorb solar radiation.

• Scattering Aerosols have cooling effect.

• Absorbing aerosols have warming effect.

• Absorbing aerosols are particularly efficient when positioned above clouds, which are a main contributor to the total reflection of solar radiation back to space.

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The cloud albedo effect

•Aerosols are vital for cloud formation because a subset of them may serve as cloud condensation nuclei (CCN).

• An increased amount of aerosols may increase the CCN number concentration and lead to more, but smaller, cloud droplets for fixed liquid water content.

•This increases the albedo of the cloud, resulting in enhanced reflection and a cooling effect, termed the cloud albedo effect.

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Visibility and Aerosols

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Chemical composition

•Atmospheric aerosols are generally composed of variable amounts of sulphate, nitrate, ammonium, sea salt, crustal elements and carbonaceous compounds (elemental and organic carbon) and other organic materials.

•The main precursors of sulphate component in the troposphere are sulphur dioxide (SO2) emitted from anthropogenic sources and volcanoes, and dimethyl sulphide (DMS) from biogenic sources (marine planktons).

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Average fine particle composition of aerosol types in urban area.

Average fine particle composition of aerosol types in rural continental region

Average fine particle composition of aerosol types in remote region.

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Atmospheric lifetime of different size particles at different levels of the atmosphere .

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What is "global dimming" and how are aerosols involved?

•Since aerosols serve as cloud condensation nuclei, an increase in aerosols could generate more clouds. Clouds, of course, have a very high albedo, and thus tend to reflect even more incoming sunlight back into space. • In each case, some sunlight would be prevented from

reaching Earth's surface, the result would be an apparent dimming of the sun as viewed from the planet's surface.

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Water solubility

•Atmospheric particles can also be categorized by their water solubility, that is, how well they dissolve in water.

•Most water-soluble aerosol components are hygroscopic and they can absorb water.

•Highly soluble particles are for example ammonium sulphate, ammonium nitrate and sodium chloride. These particles are efficient cloud condensation nuclei (CCN).

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Aerosol sinks

•Aerosols are removed from the atmosphere by sedimentation due to gravity and dry deposition also called dry scavenging.

•Aerosols are also removed by collision with rain drops and snow flakes .The removal of aerosols in-cloud and below-cloud by-

a. Precipitation is called wet deposition or wet scavenging.

b. In an annual global mean, wet scavenging is the dominant sink, which removes 80%-90% of the aerosol mass. The remaining 10-20% are removed by dry deposition.

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HEALTH EFFECTS OF AEROSOLS

Our respiratory system is efficient at removing aerosols, but if they fallwithin particular size ranges, are highly concentrated, or toxic, they maycause adverse health effects. They may also deposit on skin or eyes,generally only causing irritation, though more toxic effects may occur.Very small particles may pass through the skin and enter the body thatway. Soluble particles may dissolve and pass through the skin.

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CONCLUSION:

•The effect of aerosols on climate is an area of active research. •The effect of aerosols on clouds and climate is not

as simple as "more aerosols mean more clouds and greater albedo and hence less light at the surface.•Climate scientists must determine what types of

clouds are produced at what altitudes by various combinations of aerosols and thus cooling".

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REFERENCE:-

• http://en.wikipedia.org/wiki/Cloud_condensation_nuclei• https://www.ipcc.ch/publications_and_data/ar4/wg1/en/f

aq-2-1-figure-2.html• https://www.google.co.in/search?q=atmospheric+cycling+

of+aerosols&tbm=isch&imgil=UbIMLVOwLRkiNM%253A%253Bhttps%253A%252F%252Fencrypted• http://www.aerosols.wustl.edu/education/AerosolBasics/

What%20is%20an%20aerosol.htm• http://www-das.uwyo.edu/~geerts/cwx/notes/chap02/aer

osol&climate.html• http://aerosol.ees.ufl.edu/atmos_aerosol/section02.html• http://www.google.co.in/url?

sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CFsQFjAG&url=http%3A%2F%2Fwww.cdc.gov%2Fniosh%2Ftopics%2Faerosols%2F

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