Tropospheric Aerosols

Tropospheric Aerosols

Devang.S MSc. Previous M.S.University

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Global warming induced by greenhouse gases gets most of the press coverage, but it is not the only climate change issue our planet is dealing with. Emissions of tiny particles, called aerosols, into our atmosphere are disrupting our climate as well.

Let’s take look at 

What are aerosols ?

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What are different type of aerosols ?

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Where do they come from ?

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What roles do aerosols play in Earth’s atmosphere?

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What is “global dimming” and what role do aerosols play in this phenomenon

Aerosols – The basics: Aerosols are 1.Small particles that drift in atmosphere 2. Aerosols are not gases they are usually solids or tiny droplets of liquid 3.They are sufficiently small and light that they do not fall out of the air under influence of gravity 4.Some aerosols remain for few hours while others can stay airborne for years 5. They range in size from about 10 nanometers to 100 microns in diameter

Transport of Aerosols 

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Aerosols are transported by the airflows they encounter during the time they spend in the atmosphere. The transport can be over inter-continental or even global scales. E.g.. Sulpher dioxide (SO2) emitted from power plants in the United Kingdom can be deposited as sulfate far inland in continental Europe.

Average Residence Time, seconds

Average Residence Time of Aerosols in Atmosphere

10

Jaenicke, 1980

8

10 7 10

6

10

5

10

4

Tropopause

Middle Troposphere

1Day Below1.5km

1000 100 10 0.0001

0.001

0.01

0.1

1

Radius, micrometer

10

100

1000

Means Atmospheric Aerosol are 

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The ensemble of all liquid/solid systems suspended in the atmosphere, except water/ ice clouds. Water and Ice clouds are conventionally excluded because of their tight involvement with the hydrological cycle, short lifetimes and involvement in long- range latent energy transport.

Aerosols :

Sources and Types Natural Source - Continental/ Desert Aerosols - Marine Aerosols - Volcanic Aerosols - Organic Forest Hazes  Human Source - Industrial Aerosols - Smoke/Biomass Burning Aerosols 

Sources

Dust

Forest Fires

Hydrocarbons, NOx, SO2, NH3, POA Soot

Resuspension Oxidation

on

H2SO4, HNO3, Organic aerosol

Terpenes

O

ti a d xi

Activation

Sea-salt

Continental/ Desert Aerosols 

Large wind storms over deserts and other arid regions can lift dust particles high into the atmsophere, allowing them to drift downwind hundreds of kilometers from their source

Marine Aerosols

Sea spray accounts for the second most common natural source of aerosols in the form of various types of salts.

Volcanic Aerosols 

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Large volcanic eruptions spew vast clouds of fine ash particles into the air, sometimes reaching the stratosphere. Ash from very large eruptions can stay aloft for months to a few years, and can spread around the globe

Organic Forest Hazes 

Likewise, land-based vegetation emits gases, such as volatile organic compounds (VOSs), that contribute to secondary emissions of aerosols.

Industrial Aerosols 

Anthropogenic (human produced) aerosols make up about 10 percent of the amount of aerosols in our atmosphere. Tiny particles of black carbon, or soot, are a major component of smoke produced by many kinds of burning. Coal-burning power plants generate lots of black carbon and often loft it high into the atmosphere as emissions from tall smokestacks. Internal combustion engines in cars, trucks, and construction vehicles also emit plenty of black carbon. Diesel engines are especially prolific producers of this type of aerosol. Humans activities also increase the amount of mineral dust aerosol generation

Smoke/Biomass Burning Aerosols

Concept of Emission of Aerosols 

Primary emission-

Are aerosols sources that directly project aerosols In the air 

e.g. Dust, volcanic ash and Black CO2 

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Secondary emission –

refers to substances that are not aerosols when they are originally emitted, but later undergo some reaction in the atmosphere that transforms them into aerosols 

sulfur dioxide (SO2) gas is emitted by volcanoes, forest fires, coal-burning power plants, refuse incineration, and other sources. In the atmosphere, chemical reactions and combination with water can transform sulfur dioxide into sulfuric acid droplets, a liquid aerosol that helps form acid rain e.g.

Roles of Aerosols in Earth’s atmosphere? 

1. 2. 3.

The presence of aerosols in Earth's atmosphere influences climate in three key ways. Aerosols alter albedo (reflectivity). Formation of clouds of various types. Various types of chemical reactions in the atmosphere.

To know Roles of Aerosols in Earth’s atmosphere we should know

Role 1: Aerosols alter albedo (reflectivity)

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Changing the amount of solar energy that reaches the planet's surface and the amount that is absorbed at various levels within the atmosphere.

2. Cloud formation Clouds are nothing more than water vapor that condenses and accretes into a visible form.

Formation of clouds & aerosols

Cloud

Aerosol particle that does not activate

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CCN that activates into a cloud drop

The particles around which cloud droplets coalesce are called cloud condensation nuclei (CCN) or sometimes "cloud seeds“ under normal circumstances, these droplets form only where there is some “disturbance”

In general, aerosol particles provide this "disturbance".

Formation of clouds (animation)

3. Chemical reactions in the atmosphere The dispersal of volcanic aerosols has a drastic effect on the Earth's atmosphere

1. sulphur dioxide (SO2), hydrochloric acid (HCL) and ash

2.“heterogeneous chemistry”

3. which can react with nitrogen in the stratosphere,

To stratospheric ozone destruction.

AEROSOL-CLIMATE CONNECTION

“global dimming” & role of aerosols 

Global dimming is the gradual reduction in the amount of global direct irradiance (radiant emittance, and radiant exitance ) at the Earth

's surface

Effect of“global dimming” over India 

The aerosols that contribute to poor air quality over much of the Indian subcontinent are believed to originate from industrial smoke, vehicle emissions, and biomass burning

04 January 2008

26 January 2008

Polluted haze layer seen by Terra satellite over northern parts of India and Bangladesh.

Summary of Problems 

The effects of aerosols are difficult to assess

Aerosols are a very very minor component of the atmosphere and very difficult to measure  Aerosols are very varied and complex in nature at any time and location  Aerosols are highly inhomogeneously distributed and highly episodic in occurrence Aerosols need to be studied simultaneously from space, air and ground and in the lab. 

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International Research Efforts  

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Some Recent Comprehensive Campaigns. ACE1- US Air Pollution, ACE2- European Air Pollution & Desert Aerosol, ACE3- “ACE-ASIA”) East Asian Aerosols, and Desert Aerosols. TARFOX- US Air Pollution, SCAR- A,- Eastern Seaboard Air Pollution, SCAR- BBiomass Burning in Brazil , SCAR-C- CA Air Pollution SAFARI 2000, etc. …

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Satellite Programs: SAGE, EOS, ADEOS, SEAWIFS, Pegasus, … Robotic Surface Photometer Network: AERONET

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AMIP- Climate Model Comparisons

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AVHRR

MISR

AERONET

Very fine aerosols from the World Trade Center.

References: 

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“The Science of climate change” – J.T. Houghton, L.G.Meria Filhio, B.A.Callander, N.Harris National Aeronautics and Space Administration (NASA) National Research Council (U.S.) Space and Atmospheric Sciences Division, Physical Research Laboratory, Ahmedabad, India