aerosol doc

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GROUP

DISCUSSIONAEROSOL

SUBMITTED BY : SANJEEV CHAHARCLASS : PGDBETROLL NO : 115328


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Introduction

Take a deep breath. Even if the air looks clear, its nearly certain thatyoull inhale tens of millions of solid particles and liquid droplets. These

ubiquitous specks of matter are known as aerosols, and they can be found

in the air over oceans, deserts, mountains, forests, ice, and every

ecosystem in between. They drift in Earths atmosphere from the

stratosphere to the surface and range in size from a few nanometersless

than the width of the smallest virusesto several several tens of

micrometersabout the diameter of human hair. Despite their small size,

they have major impacts on our climate and our health.

Aerosols are defined as a suspension of particles and droplets in the size

range between 0,001 m and 100 m in a surrounding gas phase. The

total mass of particles and droplets is indicated asparticulate matter

(PM). Since fine particles smaller than 10 m are only partly precipitated

in the nose, they can be inhaled and transported to the human lungs.

Different specialists describe the particles based on shape, size, and

chemical composition. Toxicologists refer to aerosols as ultrafine, fine, or

coarse matter. Regulatory agencies, as well as meteorologists, typically

call them particulate matterPM2.5 or PM10, depending on their size. In

some fields of engineering, theyre called nanoparticles. The media often

uses everyday terms that hint at aerosol sources, such as smoke, ash, and

soot.


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Aerosol Types and Origin


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The bulk of aerosolsabout 90 percent by masshave natural origins.

Volcanoes, for example, eject huge columns of ash into the air, as well as

sulfur dioxide and other gases, yielding sulfates. Forest fires send partiallyburned organic carbon aloft. Certain plants produce gases that react with

other substances in the air to yield aerosols, such as the smoke in the

Great Smoky Mountains of the United States. Likewise in the ocean,

some types of microalgae produce a sulfurous gas called dimethylsulfide

that can be converted into sulfates in the atmosphere.

Sea salt and dust are two of the most abundant aerosols, as sandstorms

whip small pieces of mineral dust from deserts into the atmosphere and

wind-driven spray from ocean waves flings sea salt aloft. Both tend to be

larger particles than their human-made counterparts.


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As volcanoes erupt, they blast huge clouds into the atmosphere. These

clouds are made up of particles and gases, including sulfur dioxide (SO2).

Millions of tons of sulfur dioxide gas from a major volcanic eruption can

reach the stratosphere. There, with the help of water vapor (H2O), the

sulfur dioxide converts to tiny persistent sulfuric acid (H2SO4) aerosols.

These aerosols reflect energy coming from the sun, thereby preventing the

sun's rays from heating Earth's surface. Volcanic eruptions are thought tobe responsible for the global cooling that has been observed for a few

years after a major eruption. The amount and global extent of the cooling

depend on the force of the eruption and, possibly, on its location relative

to prevailing wind patterns.


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Natural and anthropogenic sources


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Formation of aerosol

Atmospheric aerosols originate from the condensation of gases and

from the action of the wind on the Earth's surface. Fine aerosol

particles (less than 1 mm in radius) originate almost exclusively

from condensation of precursor gases. A key precursor gas

is sulfuric acid (H2SO4), which is produced in the atmosphere by

oxidation of sulfur dioxide (SO2) emitted from fossil fuel

combustion, volcanoes, and other sources. H2SO4 has a low vapor

pressure over H2SO4-H2O solutions and condenses under all

atmospheric conditions to form aqueous sulfate particles.

The composition of these sulfate particles can then be modified by

condensation of other gases with low vapor pressure including NH3,

HNO3, and organic compounds.

Organic carbon represents a major fraction of the fine aerosol and is

contributed mainly by condensation of large hydrocarbons of

biogenic and anthropogenic origin.

Another important component of the fine aerosol is soot produced

by condensation of gases during combustion. Soot as commonly

defined includes both elemental carbon and black organic

aggregates.

Mechanical action of the wind on the Earth's surface emits sea

salt, soil dust, and vegetation debris into the atmosphere. These

aerosols consist mainly of coarse particles 1-10 mm in radius.

Particles finer than 1 mm are difficult to generate mechanically


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because they have large area-to-volume ratios and hence their

surface tension per unit aerosol volume is high. Particles

coarser than 10 mm are not easily lifted by the wind and have short

atmospheric lifetimes because of their large sedimentation

velocities.

Characteristics of atmospheric aerosols

A. Size Number Distribution

If your concern is the mass of some pollutant that is being transported

through the air for biogeochemical cycles, then you want to know the

mean diameter of the particles with the mass orvolume. In other words,

"What size particles carry the most mass?

If your concern loss ofvisibility then you want to know the diameter of

the particles that have the largest cross section orsurface area. In other

words, "What size particles cover the largest surface area?"

If your concern is cloud formation or microphysics then you want to

know the range of diameters with the largest number of particles. In

other words, "What is the size of the most abundant particles?"

If your concern is human health then you need to know about both the

mass and number of the particles, because only a certain size particle

can enter the lungs

B. Chemical Composition


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The bimodal nature of the size-number distribution of atmospheric

particles suggests at least two distinct mechanisms of formation, and the

chemical composition of the particles reflects their origins.

Fine particles have a diameter smaller than about 2.5 mm, and areproduced by the condensation of vapors, accumulation, and coagulation.

They have a chemical composition that reflects the condensable trace

gases in the atmosphere: SO2, NH3, HNO3, VOCs, and H2O. The

chemical composition is water with SO4-2, NO3

-, NH 4+, Pb, Cl-, Br-,

C(soot), and organic matter; where biomass burning is prevalent, K+.

Coarse Particles have a diameter greater than about 2.5 mm, areproduced by mechanical weathering of surface materials. Their lifetimes,

controlled by fallout and washout, are generally short. The composition

of particles in this size range reflects that of the earth's surface - silicate

(SiO2), iron and aluminum oxides, CaCO3 and MgCO3; over the oceans ,

NaCl.

Modes of aerosol

1. Aitken mode Ranges from 0.01-0.1 m diameter

2. Accumulation mode 0.1-1 m

3. Coarse mode >1 m

4. Nucleation mode-


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Sinks of aerosol

Once aerosol is suspended in the atmosphere, it is altered, removed ordestroyed. It cannot stay in the atmosphere indefinitely, and average

lifetimes are of the order of a few days to a week. Clearly the lifetime of

any particular particle depends on its size and location. Larger aerosol

settle out of the atmosphere very quickly under gravity, and some surfaces

are more efficient at capturing aerosol than others. We will first examine

some removal pathways before looking at how aerosol may be expected to

change during the course of its atmospheric residence.

Wet deposition

Wet deposition is the name given to deposition pathways involving water.

They include rainout, washout, sweepout and occult deposition. Brief

qualitative explanations of these will be given, as the primary focus of the

work referred to in this report is dry deposition.

Rainout

Rainout describes the removal of a cloud condensation nucleus. As

described in section 3, aerosol act as nuclei for the condensation of cloud


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droplets. In clouds producing rain, some of these drops grow to such a

large size that they fall (gravitationally settle) to the surface as rain drops.

The aerosol (condensation nuclei) deposited in this way are said to have

been rained out.

Washout

Washout describes the removal of aerosol by cloud droplets. If an aerosol

is incorporated into an already existing cloud drop, and that drop grows

large enough to fall as rain, the particle is said to have been washed out.

Note that the difference between washout and rainout is the required pre-

existence of a collecting drop for washout.

Sweepout

Another fairly closely related wet deposition process is sweepout.

Aerosol remaining below the cloudbase of a raining cloud can impact into

falling raindrops. If the impact leads to incorporation of the aerosol into

the drop, the aerosol is deposited with the raindrop, the condensation

nucleus, and any other washed or swept out particles.

Although the final fate of rained, washed and swept out particles is the

same, the three processes are distinct because the efficiency of each, and

the size and amount of aerosol swept out by each process is calculated

differently. The distinction is therefore mainly useful in modelling work

where the total deposition due to all three processes is of interest.

Occult Deposition

Occult deposition is a slightly more complicated concept than the other

three wet processes examined. Impaction efficiency is the likelihood that a

particle will strike a surface feature encountered in a flow, rather than be

deflected around the object. It is a strong function of size, with larger


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aerosol being more likely to impact on a surface feature than smaller

particles.

Aerosol can be incorporated into droplets in clouds making contact with

the surface of the ground (e.g. fog, orographic clouds). The impaction

efficiency of droplets is higher than that of the aerosol they nucleate on.

This produces an enhanced probability of impaction for such aerosol

incorporated into drops. Sticking efficiency is the probability that an

impacted object will not bounce off and be instantly resuspended.

Providing the sticking efficiency of cloud drops is not significantly lower

than that of the nucleating aerosol, (it is not) clouds contacting the ground

can give rise to an enhanced deposition rate for small aerosol.

Dry Deposition

Dry deposition pathways are the group of deposition mechanisms that

transport pollutants (in this case particles) directly to the surface without

the aid of precipitation. Through the boundary layer there are two drydeposition mechanisms. Each will be described briefly here.

Gravitational Settling

This process is possibly the simplest of all the deposition processes to

describe. It simply means a particle falling under gravity. Very large

particles fall, reaching a terminal velocity, which can be found byequating the force due to gravity by the drag force (from Stokes law) and

solving for velocity. It falls through the boundary layer at this rate until it

strikes the surface.


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Turbulent Deposition

Turbulence is the most effective dry vertical transport mechanism in the

boundary layer

Environmental effects of aerosol


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Form haze that reduces visibility

Climate effects

Absorb and scatter solar radiation and outgoing IR radiation.

Act as CCN, affecting cloud formation and properties.

LARGEST sources of uncertainty in assessments of anthropogenicclimate change.

Uncertain whether aerosol increase cause a net increase or decrease

in average global temperature.

Health and other effects


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Properties Net effect Aerosol type M ain Sourc

Reflect Cool the earth Desert dust, dry lake beds

sunlight sulfate sm og industry

A bsorb H eat the earth & B lack car bon biom ass bu

sunlight atmosphere dirty engines

reduce cloudiness

Cloud brighter clouds sulfate smog industr

Conden sation less precipitation sm oke fires

Nuclei

Ae rosol, their sources and effec ts on clim ate


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