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
AIR POLLUTION AND METEOROLOGY
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.
AIR POLLUTION AND METEOROLOGY
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
AIR POLLUTION AND METEOROLOGY
AIR POLLUTION AND METEOROLOGY
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 GREENHOUSE EFFECT
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 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