Air Pollution

THE ATMOSPHERE OF EARTH

Atmosphere is made almost entirely of Nitrogen and OxygenOther gases and particles are very small in amount

Constituent Formula Percent by volume

Nitrogen N2 78.08

Oxygen O2 20.95

Argon Ar 0.93

Carbon dioxide CO2 0.035

Neon Ne 0.0018

Helium He 0.0005

Methane CH4 0.00017

Krypton Kr 0.00011

Nitrous oxide N2O 0.00003

Hydrogen H2 0.00005

Ozone O3 0.000004

Composition of clean, dry air in troposphere (excluding very small amount of water vapour and particulate matter)

THE ATMOSPHERE OF EARTHAge of earth: 4.6 billion years (approximately)

Initial atmospheric gas composition: Helium and compound of hydrogen, methane and ammonia

Early atmosphere is thought to have escaped into space

Subsequent volcanic activities: CO2, water vapour, compounds of N2 and S released to atmosphere over time

Formation of molecular oxygen (O2): Photo-dissociation of water vapourPhotosynthesis by plant evolving under water

Photosynthesis increased: resulted in increased formation of O2 and it followed with formation of O3

LAYERS OF THE ATMOSPHERE

Convenient to think the atmosphere made of horizontal layers

Each layer is characterised by the temperature profile

Layers:Troposphere – decreasing temp. with altitudeStratosphere - increasing temp. with altitudeMesosphere - decreasing temp. with altitudeThermosphere- increasing temp. with altitude

Transition altitudes separating these layers:Tropopause – Between Troposphere and StratosphereStratopause – Between Stratosphere and MesosphereMesopause – Between Mesosphere and Thermosphere

Thermosphere

Mesosphere

Stratosphere

Troposphere

Mesopause

Stratopause

Tropopause

120

100

80

60

40

20

-20 0 20-80 -60 -40

0

-100

Temperature (oC)

Alt

itu

de

(km

)

ATMOSPHERE SHOWING FOUR MAJOR LAYERS

TROPOSPHERE

Consists of 80% of mass of the atmosphere

Virtually all water vapour, clouds and precipitation occurs in this layer

Usually very turbulent place, because of strong vertical air movement that lead to rapid and complete mixing

Altitude:Midlatitudes – 10 to 12 kmPoles – 5 to 6 kmEquator – 18 km

Temp profile:Decreases 5 to 7oC per km (wet or saturated adiabatic lapse rate)

STRATOSPHEREStable layer of very dry air

Pollutants finding way to stratosphere may remain there for many years

When they drift back to troposphere they are diluted and removed by precipitation

Short wavelength ultraviolet energy is absorbed by O3 present in this layer

causes heating of airresults in temperature inversionstratosphere becomes stable

Troposphere + Stratosphere: accounts for 99% of mass of atmosphereextend up to 50 km above earth

MESOSPHERE

Air mixes in this layer fairly readily

THERMOSPHERE

Heating of this layer is due to absorption of solar energy by atomic oxygen

IONOSPHERE:A relatively dense band of charged particles within thermosphere

AIR POLLUTION

Definition

Air pollution is defined as the presence of contaminants in air such as dust,

fumes, gases, mist, odour, smoke or vapour in such quantities and

characteristics for a particular duration which may be injurious to human, plant

or animal life or to the property or which unreasonably interfere with

comfortable environment of life and property

Air Pollutant

Primary Pollutant Secondary Pollutant

Particulate matter

Gas

Primary air pollutants i. Substances that are emitted directly into the atmosphere from identifiable

sources as a result of combustion (automobile exhaust and emissions from thermal power plants), evaporation (volatile substances such as gasoline, paints and cleaning fluids), grinding (vehicle wheel-road surface interaction) and abrasion (ploughing)

ii. Particulate matter, CO, NOx, SOx, organic compounds and radioactive compounds

Secondary air pollutantsi. produced in the air by the interaction among two or more primary

pollutants, or by interaction with normal atmospheric constituents, with or without photo-activation

ii. Substances that are created by various physical processes and chemical reactions that take place in the atmosphere

iii. ozone (O3), Formaldehyde (HCHO), Peroxyl Acetyl Nitrate (PAN) and photochemical smog

PRIMARY POLLUTANT GENERATION THROUGH COMBUSTION

Complete combustion of pure hydrocarbon fuel

CH4 + 2 O2 CO2 + 2 H2O

Incomplete combustion of fuel (Reasons: temp or oxygen availability not enough, fuel not given enough time to burn completely)

Result: Some of the carbon will be released as CO, instead of CO2

Incomplete burning of fuel

Result: Emission of partially combusted hydrocarbons

CH4 + O2 mostly (CO2 + 2 H2O) + traces of (CO + HC)

PRIMARY POLLUTANT GENERATION THROUGH COMBUSTION

Combustion takes place in air (N2-78% & O2-21%), not in pure oxygen environment

If temperature of combustion is high, some N2 in air reacts with O2 in air and forms various nitrogen oxides:

Air (N2 + O2) + Heat NOx

Most of the fuels are not pure hydrocarbon; they contain other elements such as N, S,Pb (in petrol) and other unburnable materials called ash.

Incomplete combustion in air (not in pure O2) of fuels that are not pure hydrocarbons:

Fuel (H, C, S, N, Pb, ash) + air (N2 + O2)

Emissions (CO2, H2O, CO, NOx, SOx, Pb, particulates) + Ash

GENERATION OF SECONDARY AIR POLLUTANT

HC and other organic compounds that readily vaporize are called Volatile OrganicCompounds (VOCs). VOCs react with NOx in the presence of sunlight to produce photochemical smog:

VOCs + NOx + Sunlight Photochemical smog (O3 + etc.)

The above reactions are greatly simplified; however they do introduce the 6 principalurban air pollutants:

CO, NOx, SOx, Pb, O3 and PM (ash and unburnt hydrocarbons)

OZONE (O3)

Ground level O3 is harmful to our health

Stratospheric O3 protects our health by shielding us from ultraviolet radiation from the sun.

SOURCE TYPES

Mobile sourceModes of transportation

highway vehicle, rail, aircraft, boat and ship

Stationary sourceIndustry

power plants, metal processing plants, petroleum production and refineries, chemical plants (prominent one is electric power plants)

CARBON MONOXIDE (CO)

CO is a colourless, odourless, tasteless, poisonous gas

SourceProduced out of a combustion that involves one or more of the following:

insufficient oxygenlow combustion temperaturefuel not given enough time to burn

The parameters do not meet more often in mobile source than in stationary sources:Result: CO emission is high in mobile sources

77% of total CO emission comes from transport sector

EffectNo adverse impact on plants and materials at the level that occur in urban airHowever, CO is an asphyxiant (gives suffocation due to lack of oxygen)

Interferes with the blood’s ability to carry O2 from lungs to different organs and tissues

CO + Hb (haemoglobin) COHb (Carboxyhaemoglobin)

CARBON MONOXIDE (CO)

Hb has a much greater affinity for CO than it does for O2

Consequence: Even a small amount of CO can seriously reduce the amount of O2 conveyed throughout the body;

Brain function affected and heart rate increases in an attempt to offset the O2 deficit.

Typical CO Levels

Place CO Concentration (ppm)

Near busy roadways 5 to 50

Congested highways 100

Cigarette smoke contains 400

Inside bars and restaurants where smoking is permitted

20 to 30

OXIDES OF NITROGEN (NOx)

7 oxides of nitrogen are known: NO, NO2, NO3, N2O, N2O3, N2O4 and N2O5

Important air pollutants: NO and NO2 (combined referred to as oxides of nitrogen, NOx)N2O: Green house gas (GHG)

Thermal NOx: Formed, when N2 and O2 in the combustion air react in high temperature (about 1000oK) and N2 gets oxidised.Fuel NOx: Results from oxidation of nitrogen compounds bound chemically in fuel molecules (coal has nitrogen compounds about 3% by weight).

Transport sector contributes almost half of the NOx emissions.

EFFECTS:NO – Has no known adverse health effects at concentrations found in the atmosphereNO oxidises to NO2 and it has many adverse health effects

Adverse effects of NO2

Irritates lungs, causes bronchitis and pneumonia and results in lower resistance to respiratory functionsReacts with hydroxyl radicals (OH) in the atmosphere to form nitric acid (HNO3); HNO3 corrodes metal surface and contributes to the acid rain problem

OXIDES OF SULPHUR (SOx)

SOx emission sources90% fossil fuel combustion from stationary sources (85% of this is emitted from power plants)3% from highway vehiclesSignificant non-combustion sources of sulphur emission:

Petroleum refiningCopper smeltingCement manufacturing

Sulphur in fossil fuelsCoal: 1 – 6 % (About 50 % organic sulphur is chemically bound to coal, other half is physically trapped in the non-carbon portion of coal and gets removed by pulverisation or washing of coal before combustion.)

Petroleum: Trace amount to about 5%All most all sulphur are removed during the process of refiningGasoline has < 1ppm sulphur

SO2 emissionWhen S containing fuels are burned, S is released as SO2 and SO3.With moisture, these form H2SO4.SO2 changes to SO4 particles in a few daysPrincipal removal process is wet deposition (with precipitation)

Adverse effects of SO2

Highly water soluble: absorbed in the moist parts of the upper respiratory tracts – less harmful

When entrained in aerosols, can penetrate to deeper parts of respiratory system and can damage lungs. Synergy of adverse impacts of particle and SO2 will be more damaging. (Every major air pollution episode has resulted from the combination of SO2 and PM)

Acidification damages plants by affecting their ability to extract nutrients from the soil because nutrients get leached from the soil.

Sulphurous pollutants can discolour paint, corrode metals and cause organic fibers to weaken.

Prolonged exposure to sulphates causes damage to building marbles, limestone and mortar as carbonate in these materials are replaced by sulphates.

Calcium sulphate (gypsum, CaSO4) is water soluble and easily washes awayIt leaves a pitted eroded surfaceMany historic building across world are getting rapidly damaged due to this exposure

OXIDES OF SULPHUR (SOx)

𝐶𝑎𝐶𝑂3 + 𝐻2𝑆𝑂4 = 𝐶𝑎𝑆𝑂4 + 𝐶𝑂2 + 𝐻2𝑂

VOLATILE ORGANIC COMPOUNDS (VOC)

Atmospheric VOC consists of:

Unburnt hydrocarbons emitted form tailpipes and smoke stacks when fossil fuels are not completely combustedGaseous hydrocarbons that evaporate from solvents, fuels and other organics

Natural sources: Isopropene emitted from deciduous trees(minor contribution) Pinene and limonene emitted from conifers

Transport sector – responsible for about 1/3 of anthropogenic VOC emissionsIndustrial sector - responsible for about 2/3 of anthropogenic VOC emissions

ENVIRONMENTAL EFFECT

VOCs react with NOx in the presence of sunlight to produce photochemical smog:

VOCs + NOx + Sunlight Photochemical smog (O3 + etc.)

PHOTOCHEMICAL SMOG AND OZONE

NOx, VOC and sunlight, when come together, initiate a complex set of reactions

Secondary pollutants, known as photochemical oxidantsO3 is the most abundant photochemical oxidant

produce

NO – NO2 – O3 photochemical reaction sequence (without added hydrocarbons)

N2 + O2 2NO (Formation of NO during combustion)

2NO + O2 2NO2 (Nitric oxide getting oxidized to NO2)

NO2 + hν NO + O (Photolysis: a photon with right amount energy decomposes NO2 and produces a free atomic oxygen, hν represents a photon)

O + O2 + M O3 + M (Free atomic oxygen combines with diatomic oxygen (O2) to form ozone (O3)

O3 + NO NO2 + O2 (Ozone convert NO back to NO2)

NO2 tends to create O3, whereas, NO tends to destroy O3

sunlight

EFFECT OF PHOTOCHEMICAL OXIDANTS

Respiratory effects: coughing, shortness of breath, headache, chest tightness and eye, nose and throat irritation

Symptoms can be severe for asthmatics

Long term exposure can lead to permanent scarring of lung tissues, loss of lung function

Damage to tree foliage and growth rate

Reduced yield of major agricultural crops such as wheat, soybeans and peanuts

PARTICULATE POLLUTANTS

Definition A particle consists of a single continuous unit of solids or liquid containing many molecules held together by intermolecular forces and primarily larger than molecular dimensions (< 0.001µm).

A particle may also be considered to consist of two or more such unit structure held together by inter-particle adhesive forces such that it behaves as a single unit in suspension or upon deposit.

SourceParticulate matter results from the disintegration of solids.

Transport, construction and industrial activities, volcanic eruption, desert storms, sea salts, secondary aerosols and resuspension of crustal matter are some of the major sources of air pollution.

The finer the particles the more is its retention time in atmosphere.

PARTICULATE MATTERDefinitionAtmospheric particulate matter (PM) consists of any dispersed matter, solid or liquid, in which the individual aggregates range from molecular clusters of 0.005 µm diameter to coarse particles up to about 100 µm.

Several terms for PM

Aerosol: Any tiny particles, liquid or solid, dispersed in the atmosphere

Dust: Solid particle caused by grinding or crushing operations

Fume: Solid particle formed when vapour condenses; size: 0.3 µm to 3 µm

Mist/Fog: Liquid particles suspended in air; particles or droplets formed by

condensation of vapour; mist particles may coalesce; normally < 10 µm in size.

Smoke/Soot: Particles composed primarily of carbon that result from incomplete

combustion; normally < 1 µm in size.

Smog: A combination of smoke and fog.

Fly ash: Noncombustible parts of coal that comes out with flue gas: size: 1 µm to 100

µm

PARTICULATE MATTER SIZE

Equivalent aerodynamic diameter:

For a particle having irregular shape, the equivalent aerodynamic diameter of the particle is the diameter of a spherical particle having the same settling velocity as that of the irregular shaped particle.

Airborne dustSuspended particulate matter (SPM or TSPM): > 10 µm (up to 100 µm)

Respirable particulate matter (RSPM, RPM or PM10): < 10 µm

PM10 = PMcoarse + PMfine

= PM2.5-10 + PM2.5

PARTICULATE MATTER

Settling velocity of the spherical particle

When particle reaches terminal velocity, the gravitational force pulling it down is balanced by the force due to buoyancy and frictional drag force.

For particles < 30 µm and density much greater than air, simplified stokes can be applied for drag force.

Gravitational force = Drag force

m = mass of particle (g)g = gravitational acceleration (9.8 m/s2)d = particle diameter (m)ρ = particle density (g/m3)η = viscosity of air (0.0172 g/m.s)v = settling velocity (m/s)

mg= π6d3ρg= 3πηvd

v = d2ρg18η

Drag force

Gravitational force

d

SETTLING VELOCITIES OF PARTICULATE MATTER

Particle size Approximate settling velocity (cm/s)

0.1 µm 4 x 10-5

1 µm 4 x 10-3

10 µm 0.3

100 µm 30

Numerical: Settling velocity of a spherical particle

Find the settling velocity of a spherical droplet of water with diameter 2µm, and estimate the residence time of such particles if they are uniformly distributed in the lower 1000m of atmosphere.

Solution:With density of water equal to 106 g/m3

Using simple box model to estimate the residence time of particles uniformly distributed in a box of atmosphere with height h (m),

Residence time = h/v = 1000/(1.27 x 10-4 m/s) = 7.9 x 106 s = 91 days

v = d2ρg18η = (2× 10−6 m)2 × ሺ106 𝑔/𝑚3ሻ× (9.8 𝑚/𝑠2)18× 0.0172 g/𝑚𝑠

v = 1.27× 10−4 𝑚/𝑠

AIR POLLUTION AND METEOROLOGY

Principal parameters that influence the dispersion of air pollutant:

Primary parametersWind direction and speedTemperatureAtmospheric stabilityMixing height

Secondary parametersPrecipitationHumiditySolar radiation

These parameters vary as a function of latitude, season and topography


The ease with which pollutants can disperse vertically into the atmosphere is largely determined by the rate of change of air temperature with altitude.

Lapse rate

The rate of change of air temperature with altitude is called lapse rate

A parcel of air moving upward experiences less pressure, expands and cools

A parcel of air moving downward comes under more pressure, gets compressed and temperature of the air parcel increases

Therefore we can imagine that as temperature, pressure and volume of the air parcel are changing, its surroundings are adding or subtracting energy from the air parcel.


Adiabatic lapse rate

However, had this energy transfer in the form of heat not taken place between the air parcel and surrounding, the process would be adiabatic and at that condition the rate of change of temperature of the air parcel with altitude is known as adiabatic lapse rate.

When the air is assumed to be dry, this is called dry adiabatic lapse rate

Dry adiabatic lapse rate, Гd = - dT/dZ = 9.76 oC/km (≈ 10 oC/km)

Saturated (Wet) adiabatic lapse rate

If the air has enough water vapour in it so that condensation takes place as the air parcel is raised and cooled, latent heat will be released.

This added heat will not allow the parcel to cool as rapidly as the dry one.

The lapse rate of such air containing water vapour is called Saturated adiabatic lapse rate.

This lapse rate is variable as amount of moisture that air can hold before beginning of condensation is a function of temperature.

A reasonable average value for troposphere:

Saturated adiabatic lapse rate, Гs = - dT/dZ = 6 oC/km



Ambient lapse rate

A number of factors, such as wind speed, sunlight and the geographical features cause the lapse rate in the real atmosphere to vary from the dry adiabatic lapse rate.

This lapse rate in the real atmosphere is called ambient lapse rate (Г).

This is also called environmental lapse rate.

The difference between the ambient lapse rate and adiabatic lapse rate determines the stability of the atmosphere.

ATMOSPHERIC STABILITY

The tendency of the atmosphere to enhance or resist vertical motion is termed as stability.

Three stability categories

Stable atmosphere:The thermal structure of the atmosphere inhibits mechanical turbulenceDiscourages the dispersion and dilution of pollutants

Unstable atmosphere:Mechanical turbulence is enhanced by thermal structureRapid vertical mixing of air takes placeEncourages pollutant dispersalIncreases air quality

Neutral atmosphere:The thermal structure neither enhances nor resists mechanical

turbulenceLimited pollutant dispersion; dispersion is mainly due to diffusion

Primarily stability depends on Lapse rate.

NEUTRAL ATMOSPHERE

Ambient lapse rate = Dry adiabatic lapse rate

Air parcel experiences no buoyant force

No upward or downward movement of air parcel

Neutral atmosphere condition prevails

Very little pollutant dispersion500

Dry adiabatic lapse rate

Ambient lapse rateHei

ght

(m)

Temperature (o C)

19 20 21 22

400

300

200

100

dT/dZ = -10 oC/km

UNSTABLE ATMOSPHERE

Ambient lapse rate > Dry adiabatic lapse rate (the lapse rate is said to be superadiabatic)

Air parcel experiences buoyant force

Upward movement of air parcel

Air from different altitude mix thoroughly

Very effective condition for pollution dispersion

500

Dry adiabatic lapse rate

Ambient lapse rate

Hei

ght

(m)

Temperature (o C)

19 20 21 22

400

300

200

100

dT/dZ = -12.5 oC/km

STABLE ATMOSPHERE

Ambient lapse rate < Dry adiabatic lapse rate (the lapse rate is said to be subadiabatic)

Very little vertical mixing of pollutants

Pollutants disperse very slowly; results in pollutant build up

500 Dry adiabatic lapse rate

Ambient lapse rate

Hei

ght

(m)

Temperature (o C)

19 20 21 22

400

300

200

100

dT/dZ = -0.5 oC/km

NUMERICALS ON ATMOSPHERIC STABILITY

Q.Given the following temperature and elevation data, determine the stability of the atmosphere.

Elevation (m) Temperature (oC)2.00 14.35324.00 11.13

A.ΔT/ΔZ = (11.13-14.35)/(324.00-2.00) oC/m = - 3.22/322.00

= - 0.01 oC/m = - 10 oC/km

This is same as the dry adiabatic lapse rate.

Therefore, atmospheric stability is neutral.

INVERSION

Two special cases of subadiabatic lapse rate

Isothermal lapse rate Temperature inversionWhen there is no change of temp. When temperature increases with elevation with elevation

Atmosphere is stable Atmosphere is stable

Both cases discourage pollutant dispersion

500Dry adiabatic lapse rate

Ambient lapse rate (Temperature inversion)H

eigh

t (m

)

Temperature (o C)

19 20 21 22

400

300

200

100

Ambient lapse rate (Isothermal)

Superadiabatic

Dry adiabatic

Inve

rsio

n

Su

badiabatic

Iso

ther

mal

Stable

Sta

ble

Sta

ble

Neutral

Unstable

T = 9.8 oC

Temperature, oCT

Ele

vati

on

, km

0.5

1.0

1.5

RELATIONSHIP OF THE AMBIENT LAPSE RATES WITH THE DRY ADIABATIC LAPSE RATE

TEMPERATURE INVERSION

An extreme case of atmospheric stability

Ambient temperature increases with altitude, called negative lapse rate

Results in a virtual lid on the upward movement of the pollutant

TYPES OF INVERSION

Radiation InversionSubsidence InversionFrontal Inversion

RADIATION INVERSION

Surface of the earth cools after sunset by radiation energy towards space

On a clear night, surface more rapidly radiates energy to space and ground cooling occurs much more rapidly

As the ground cools, the temp of the air in contact with the ground also drops

Therefore low level air close to ground is colder than the air above it, a case of temperature inversion

Radiation inversion begin to form at about dusk

As the evening progresses, the inversion extends to higher and higher elevation

Radiation inversion is more prevalent in winter season and in nights with clear sky

On a cloudy night, earth’s radiation gets absorbed by water vapour, which in turn reradiates some of the heat energy back to the ground.

RADIATION INVERSION

Next day morning sunlight warms the ground and destroys the inversion

Often begin at about the early evening traffic build up and therefore traps the pollutant emitted from the traffic and increases the concentration of pollutants to which commuters are exposed

SUBSIDENCE INVERSION

Associated with high pressure weather system, known as Anticyclone

Air in the middle of the high pressure zone descends, gets compressed and temperature of air rises

Air on the edges are rising and getting cooled

FRONTAL INVERSION

When a cold air mass passes under a warm air mass, the inversion is called frontal inversion

Frontal inversion has short life

It tend to be accompanied by precipitation which cleanses the air

MIXING DEPTH/MIXING HEIGHT

That height above earth’s surface to which related pollutants will extend, primarily through the action of the atmospheric turbulence

OR

The height above the earth’s surface up to which pollutants are diluted and dispersed in the available atmospheric condition; i.e. significant mixing of the pollutants takes place

Varies between 100m and 500m above the ground level

Depends on ambient lapse rate at a given place and time

Usually related to wind direction, wind speed and turbulence

It depends on basic meteorological parameters, surface turbulent fluxes and physical parameters, and follows a diurnal cycle.

The mixing height cannot be observed directly by standard measurements

MAXIMUM MIXING DEPTH (MMD)

MMD MMD

MMD

DRY ADIABATIC LAPSE RATE

AMBIENT LAPSE RATE

AL

TIT

UD

E

TEMPERATURE

HIGHER MMDUnstable atmosphereSummer seasonDay time

LOWER MMDStable atmosphereWinter seasonNight time

THE GREENHOUSE EFFECT

Wein’s displacement rule: (gives the wavelength at which a blackbody spectrum peaks as a function of its absolute temperature)

λmax (μm) = 2898/T(K)

SUN: A blackbody with temp. 5800 KSpectrum peaks at 0.5 μm

EARTH: Average temp. about 288 K Spectrum peaks at 10.1 μm

Short wavelength radiation:Nearly all incoming solar energy has wavelength < 3 μm

Long wavelength or Thermal radiation:Nearly all outgoing solar energy has wavelength > 3 μm

Incoming solar radiation just outsideof the earth’s atmosphere

Radiation from the earth’s Surface at 288 K

Wavelength (μm)

2000

0 1

λmax

Incoming extraterrestrial solar radiation, 5800 K(short wavelengths)

Inte

nsi

ty (

W/m

2)

/ μ

m

1000

32 4

Outgoing radiation from Earth’s surface, 288 K(long wavelengths)

0

Wavelength (μm)

λmax

10 20 30 40 50

Inte

nsi

ty (

W/m

2)

/ μ

m

20

30

10


The infrared (IR) portion of the spectrum lies between 0.7 μm and 100 μm.

Some of the incoming solar radiation is IRAll the outgoing thermal radiation is IR

The Process

Radiation from Earth surface

Attempts to pass through atmosphere

Affected by gases and aerosols in the air

Result:Radiant energy pass through atmosphere unaffectedRadiant energy gets scattered by reflectionRadiant energy gets absorbed and do not escape the lower atmosphere

(when frequency of molecular oscillation of gases is close to the frequency of the passing radiant energy)

Key phenomenon for greenhouse effect


The most important areWater vapour (H2O), CO2, CH4, N2O, O3, O2 (molecular oxygen)

Water vapour: strongly absorbs at < 8 μm and >18 μm(the most important greenhouse gas)

CO2: strong absorption centered at 15 μm and at 2.7 μm and 4.3 μm

O3: absorption band between 9.5 μm and 10.6 μm

O2 and O3: absorbs all incoming solar radiation < 0.3 μm (ultraviolet)(cause of stratospheric ozone depletion)

Atmospheric radiative windowWavelength band between 7 μm and 12 μm allows outgoing thermal radiation

Greenhouse gas: Radiatively active gases that absorb wavelengths longer than 4 μm

Absorption of long wavelength energy by atmospheric gases

Surface radiation

H2O, CO2, CH4, N2O, O3

Through atmosphericradiative window

Reradiated IR fromAtmosphere to space

Reradiated IR fromAtmosphere to earth

SPACE

ATMOSPHERE

EARTH

Most of the long wavelengthenergy radiated from the earth’ssurface is absorbed by greenhouse gases in the atmosphere

GREENHOUSE EFFECT

Absorption of radiation by greenhouse gases heats the atmosphere

The greenhouse gases act as a thermal blanket around the globe

Raises the earth’s surface temperature

Greenhouse effect is based on the concept of a conventional greenhouse withGlass acting much like the aforementioned gases

Glass:

Easily transmits short wavelength solar energy into the greenhouse

Absorbs almost all of the longer wavelengths radiated by the greenhouse interior

GREENHOUSE EFFECT and TEMP OF EARTH

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Air Pollution