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1. Define air pollution.
Air pollution occurs when the air contains gases, dust, fumes or odour in harmful
amounts. That is, amounts which could be harmful to the health or comfort of humans
and animals or which could cause damage to plants and materials.
The substances that cause air pollution are called pollutants. Pollutants that are pumped into our atmosphere and directly pollute the air are called primary pollutants.
Primary pollutant examples include carbon monoxide from car exhausts and sulfur
dioxide from the combustion of coal.Further pollution can arise if primary pollutants in the atmosphere undergo
chemical reactions. The resulting compounds are called secondary pollutants.
Photochemical smog is an example of this.
2. What are the compositions of Earth’s atmosphere?
A. Nitrogen - 7! - "ilutes oxygen and pre#ents rapid burning at the earth$s surface.%i#ing things need it to ma&e proteins. Nitrogen cannot be used directly from the air. The
Nitrogen 'ycle is nature$s way of supplying the needed nitrogen for li#ing things.
(. )xygen - *+! - sed by all li#ing things. ssential for respiration. t is necessary for
combustion or burning.
'. Argon - /.0! - sed in light bulbs.
". 'arbon "ioxide - /./1! - Plants use it to ma&e oxygen. Acts as a blan&et and pre#ents
the escape of heat into outer space. 2cientists are afraid that the buring of fossil fuels such
as coal and oil are adding more carbon dioxide to the atmosphere.
. 3ater 4apor - /./ to 5./! - ssential for life processes. Also pre#ents heat loss from
the earth.
F. Trace gases - gases found only in #ery small amounts. They include neon, helium,
&rypton, and xenon.
CONSTITUEN
T
CE!IC"#
S$!%O#
!O#E &E'CENT
Nitro(en N* 7./5
O)*(en )* */.057
"r(on Ar /.015
Car+on Dio)i,e ')* /./16
Neon Ne /.//+*
elium e /.///6*
!ethane '5 /.///+7
-r*pton 8r /.///++
*,ro(en * /.////6
Nitrous O)i,e N*) /.////1
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enon 9e /.////+
O/one )1 trace to
/.////
0. Temperature structure an, "tmospheric re(ion
The #ertical distribution of temperature, pressure, density, and composition of the
atmosphere constitutes atmospheric structure. These :uantities also #ary with season and
location in latitude and longitude, as well as from night to day; howe#er under the topic
of atmospheric structure, the focus is on the a#erage #ariations with height abo#e sea
le#el.Atmospheric layers are characteri<ed by #ariations in temperature resulting
primarily from the absorption of solar radiation; #isible light at the surface, near
ultra#iolet radiation in the middle atmosphere, and far ultra#iolet radiation in the upper
atmosphere.
The arth$s atmosphere has layers, which are actually characteri<ed by how the
temperature of the atmosphere changes with altitude.
The "ifferent %e#els of the Atmosphere are=
Troposphere= This is the lowest atmospheric layer and is about se#en miles >++ &m? thic&.
@ost clouds and weather are found in the troposphere. The troposphere is thinner at the
poles >a#eraging about &m thic&? and thic&er at the e:uator >a#eraging about +&m
thic&?. The temperature decreases with altitude.
2tratosphere= The stratosphere is found from about 7 to 1/ miles >++-5 &ilometers?
abo#e the arthBs surface. n this region of the atmosphere is the o<one layer, which
absorbs most of the harmful ultra#iolet radiation from the 2un. The temperature increases
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slightly with altitude in the stratosphere. The highest temperature in this region is about
1* degrees Fahrenheit or / degrees 'elsius.
@esosphere= The mesosphere is abo#e the stratosphere. ere the atmosphere is #ery
rarefied, that is, thin, and the temperature is decreasing with altitude, about C+1/
Fahrenheit >-0/ 'elsius? at the top.
Thermosphere= The thermosphere starts at about 66 &ilometers. The temperature is :uite
hot; here temperature is not measured using a thermometer, but by loo&ing at the motion
and speed of the rarefied gases in this region, which are #ery energetic but would not
affect a thermometer. Temperatures in this region may be as high as thousands of degrees.
xosphere= 4ery high up, the arth$s atmosphere becomes #ery thin. The region where
atoms and molecules escape into space is referred to as the exosphere. The exosphere is
on top of the thermosphere.
onosphere= The ionosphere o#erlaps the other atmospheric layers, from abo#e the arth.
The air is ioni<ed by the 2unBs ultra#iolet light. These ioni<ed layers affect the
transmittance and reflectance of radio wa#es. The ionosphere is bro&en down into the ",
and F regions. The brea&down is based on what wa#elength of solar radiation is
absorbed in that region most fre:uently. The " region is the lowest in altitude, though it
absorbs the most energetic radiation, hard x-rays. The " region doesn$t ha#e a definite
starting and stopping point, but includes the ioni<ation that occurs below about 0/&m.
region pea&s at about +/6&m. t absorbs soft x-rays. The F region starts around +/6&m
and has a maximum around //&m. t is the highest of all of the regions. xtreme ultra-
#iolet radiation >4? is absorbed there.)n a more practical note, the " and regions reflect A@ radio wa#es bac& to
arth. Dadio wa#es with shorter lengths are reflected by the F region. 4isible light,
tele#ision and F@ wa#elengths are all too short to be reflected by the ionosphere. 2o your
t.#. stations are made possible by satellite transmissions.
. Eolution of Earth’s (ases
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The e#olution of the atmosphere could be di#ided into four separate stages=
+. )rigin*. 'hemicalE pre-biological era
1. @icrobial era, and
5. (iological era.
The first three steps were discussed in detail. The composition of the present atmosphere
howe#er re:uired the formation of oxygen to sufficient le#els to sustain life, and re:uired
life to create the sufficient le#els of oxygen. This era of e#olution of the atmosphere is
called the (iological ra.
The (iological ra - The Formation of Atmospheric )xygen
The biological era was mar&ed by the simultaneous decrease in atmospheric carbon
dioxide >')*? and the increase in oxygen >)*? due to life processes. 3e need to
understand how photosynthesis could ha#e led to maintenance of the G*/! present-dayle#el of )*. The build up of oxygen had three maHor conse:uences that we should note
here.
Firstly, u&aryotic metabolism could only ha#e begun once the le#el of oxygen had built
up to about /.*!, or G+! of its present abundance. This must ha#e occurred by G* billion
years ago, according to the fossil record. Thus, the eu&aryotes came about as a
conse:uence of the long, steady, but less efficient earlier photosynthesis carried out by
Pro&aryotes.
)xygen increased in stages, first through photolysis >Figure +? of water #apor and carbon
dioxide by ultra#iolet energy and, possibly, lightning= *) -I J )
produces a hydroxyl radiacal >)? and ')* -I ')J ) produces an atomic oxygen >)?.
The ) is #ery reacti#e and combines with the )=
) J ) -I )* J
The hydrogen atoms formed in these reactions are light and some small fraction excape to space
allowing the )* to build to a #ery low concentration, probably yielded only about +! of the
oxygen a#ailable today.
2econdly, once sufficient oxygen had accumulated in the stratosphere, it was acted on by
sunlight to form o<one, which allowed coloni<ation of the land. The first e#idence for #ascular
plant coloni<ation of the land dates bac& to G5// million years ago.
Thirdly, the a#ailability of oxygen enabled a di#ersification of metabolic pathways, leading to a
great increase in efficiency. The bul& of the oxygen formed once life began on the planet,
principally through the process of photosynthesis= ')* J *) K--I '+*) J )* where
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carbon dioxide and water #apor, in the presence of light, produce organics and oxygen. The
reaction can go either way as in the case of respiration or decay the organic matter ta&es up
oxygen to form carbon dioxide and water #apor.
%ife started to ha#e a maHor impact on the en#ironment once photosynthetic organisms e#ol#ed.
These organisms fed off atmospheric carbon dioxide and con#erted much of it into marinesediments consisting of the innumerable shells and decomposed remnants of sea creatures.
3hile photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to
produce oxygen. The oxygen did not build up in the atmosphere for a long time, since it was
absorbed by roc&s that could be easily oxidi<ed >rusted?. To this day, most of the oxygen
produced o#er time is loc&ed up in the ancient banded roc& and red bed roc& formations
found in ancient sedimentary roc&. t was not until G+ billion years ago that the reser#oirs of
oxidi<able roc& became saturated and the free oxygen stayed in the air. The figure illustrates a
possible scenario.
3e ha#e briefly mentioned the difference between reducing >electron-rich? and oxidi<ing
>electron hungry? substances. )xygen is the most important example of the latter type of
substance that led to the term oxidation for the process of transferring electrons from reducing to
oxidi<ing materials. This consideration is important for our discussion of atmospheric e#olution,
since the oxygen produced by early photosynthesis must ha#e readily combined with any
a#ailable reducing substance.
3e ha#e been able to outline the steps in the long drawn out process of producing present-day
le#els of oxygen in the atmosphere. 3e refer here to the geological e#idence.
%an,e, Iron 3ormations
3hen the oceans first formed, the waters must ha#e dissol#ed enormous :uantities of reducing
iron ions, such as Fe*J. These ferrous ions were the conse:uences of millions of years of roc&
weathering in an anaerobic >oxygen-free? en#ironment. The first oxygen produced in the oceans
by the early pro&aryotic cells would ha#e :uic&ly been ta&en up in oxidi<ing reactions with
dissol#ed iron. This oceanic oxidi<ation reaction produces Ferric oxide Fe*)1 that would ha#e
deposited in ocean floor sediments. The earliest e#idence of this process dates bac& to the
(anded ron Formations, which reach a pea& occurrence in metamorphosed sedimentary roc& at
least 1.6 billion years old. @ost of the maHor economic deposits of iron ore are from (anded ron
formations. These formations, were created as sediments in ancient oceans and are found in
roc&s in the range * - 1.6 billion years old. 4ery few banded iron formations ha#e been found
with more recent dates, suggesting that the continued production of oxygen had finally exhausted
the capability of the dissol#ed iron ions reser#oir. At this point another process started to ta&e up
the a#ailable oxygen.
'e, %e,s
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)nce the ocean reser#oir had been exhausted, the newly created oxygen found another large
reser#oir - reduced minerals a#ailable on the barren land. )xidi<ation of reduced minerals, such
as pyrite Fe2* , exposed on land would transfer oxidi<ed substances to ri#ers and out to the
oceans #ia ri#er flow. "eposits of Fe*)1 that are found in alternating layers with other sediments
of land origin are &nown as Ded (eds, and are found to date from *./ billion years ago. The
earliest occurrence of red beds is roughly simultaneous with the disappearance of the banded iron
formation, further e#idence that the oceans were cleared of reduced metals before )* began to
diffuse into the atmosphere.
Finally after another +.6 billion years or so, the red bed reser#oir became exhausted too
>although it is continually being regenerated through weathering? and oxygen finally started to
accumulate in the atmosphere itself. This signal e#ent initiated eu&aryotic cell de#elopment, land
coloni<ation, and species di#ersification. Perhaps this period ri#als differentiation as the most
important e#ent in arth history.
3hile photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to produce oxygen. The oxygen did not build up in the atmosphere for a long time, since it was
absorbed by roc&s that could be easily oxidi<ed >rusted?. To this day, most of the oxygen
produced o#er time is loc&ed up in the ancient banded roc& and red bed roc& formations
found in ancient sedimentary roc&. t was not until G+ billion years ago that the reser#oirs of
oxidi<able roc& became saturated and the free oxygen stayed in the air. The figure illustrates a
possible scenario.
3e ha#e briefly mentioned the difference between reducing >electron-rich? and oxidi<ing
>electron hungry? substances. )xygen is the most important example of the latter type of
substance that led to the term oxidation for the process of transferring electrons from reducing tooxidi<ing materials. This consideration is important for our discussion of atmospheric e#olution,
since the oxygen produced by early photosynthesis must ha#e readily combined with any
a#ailable reducing substance.
3e ha#e been able to outline the steps in the long drawn out process of producing present-day
le#els of oxygen in the atmosphere. 3e refer here to the geological e#idence.
The O)*(en Concentration &ro+lem
3hy does present-day oxygen sit at */!L This is not a tri#ial :uestion since significantly loweror higher le#els would be damaging to life. f we had K +6! oxygen, fires would not burn, yet at
I *6! oxygen, e#en wet organic matter would burn freely.
The Earl* Ultraiolet &ro+lem
The genetic materials of cells >"NA? are highly susceptible to damage by ultra#iolet light atwa#elengths near /.*6 Mm. t is estimated that typical contemporary microorganisms would be
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&illed in a matter of seconds if exposed to the full intensity of solar radiation at these wa#elength.
Today, of course, such organisms are protected by the atmospheric o<one layer that effecti#ely
absorbs light at these short wa#elengths, but what happened in the early arth prior to thesignificant production of atmospheric oxygenL There is no problem for the original non-
photosynthetic microorganisms that could :uite happily ha#e li#ed in the deep ocean and in
muds, well hidden from sunlight. (ut for the early photosynthetic pro&aryotes, it must ha#e beena matter of life and death.
t is a classical chic&en and egg problem. n order to become photosynthetic, early
microorganisms must ha#e had access to sunlight, yet they must ha#e also had protection against
the 4 radiation. The oceans only pro#ide limited protection. 2ince water does not absorb #erystrongly in the ultra#iolet a depth of se#eral tens of meters is needed for full 4 protection.
Perhaps the organisms used a protecti#e layer of the dead bodies of their brethren. Perhaps this is
the origin of the stromatolites - algal mats that would ha#e pro#ided ade:uate protection forthose organisms buried a few millimeters in. Perhaps the early organisms had a protecti#e 4-
absorbing case made up of disposable "NA - there is some intriguing e#idence of unused
modern elaborate repair mechanisms that allow certain cells to repair moderate 4 damage totheir "NA. owe#er it was accomplished, we &now that natural selection wor&ed in fa#or of the
photosynthetic microorganisms, leading to further di#ersification.
3luctuations in O)*(en
The history of macroscopic life on arth is di#ided into three great eras= the Paleo<oic, @eso<oic
and 'eno<oic. ach era is then di#ided into periods. The latter half of the Paleo<oic era, includesthe "e#onian period, which ended about 1/ million years ago, the 'arboniferous period, which
ended about */ million years ago, and the Permian period, which ended about *6/ million years
ago.
According to recently de#eloped geochemical models, oxygen le#els are belie#ed to ha#eclimbed to a maximum of 16 percent and then dropped to a low of +6 percent during a +*/-
million-year period that ended in a mass extinction at the end of the Permian. 2uch a Hump in
oxygen would ha#e had dramatic biological conse:uences by enhancing diffusion-dependent processes such as respiration, allowing insects such as dragonflies, centipedes, scorpions and
spiders to grow to #ery large si<es. Fossil records indicate, for example, that one species of
dragonfly had a wing span of * +E* feet.
eochemical models indicate that near the close of the Paleo<oic era, during the Permian period,global atmospheric oxygen le#els dropped to about +6 percent, lower that the current
atmospheric le#el of *+ percent. The Permian period is mar&ed by one of the greatest extinctions
of both land and a:uatic animals, including the giant dragonflies. (ut it is not belie#ed that thedrop in oxygen played a significant role in causing the extinction. 2ome creatures that became
specially adapted to li#ing in an oxygen-rich en#ironment, such as the large flying insects and
other giant arthropods, howe#er, may ha#e been unable to sur#i#e when the oxygen atmosphereunderwent dramatic change.
4. &ressure5 Densit* an, !i)in( 'atio
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!i)in( 'atio
The mixing ratio '9 of a gas 9 >e:ui#alently called the mole fraction? is defined as thenumber of moles of 9 per mole of air. t is gi#en in units of molEmol >abbre#iation for
moles per mole?, or e:ui#alently in units of #E# >#olume of gas per #olume of air? since
the #olume occupied by an ideal gas is proportional to the number of molecules.Pressures in the atmosphere are sufficiently low that the ideal gas law is always obeyed to
within +!.
The mixing ratio of a gas has the #irtue of remaining constant when the air density
changes >as happens when the temperature or the pressure changes?. 'onsider a balloon
filled with room air and allowed to rise in the atmosphere. As the balloon rises it expands,
so that the number of molecules per unit #olume inside the balloon decreases; howe#er,the mixing ratios of the different gases in the balloon remain constant. The mixing ratio is
therefore a robust measure of atmospheric composition.
ases other than N*, )*, Ar, and *) are present in the atmosphere at extremely lowconcentrations and are called trace gases. "espite their low concentrations, these trace
gases can be of critical importance for the greenhouse effect, the o<one layer, smog, andother en#ironmental issues. @ixing ratios of trace gases are commonly gi#en in units of
parts per million #olume > ppm# or simply ppm?, parts per billion #olume > ppb# or ppb?,
or parts per trillion #olume > ppt# or ppt?; + ppm# O +x+/- molEmol, + ppb# O +x+/-0molEmol, and + ppt# O +x+/-+* molEmol. For example, the present-day ')*
concentration is 16 ppm# >16x+/- molEmol?.
Ta+le 161 !i)in( ratios of (ases in ,r* air
7as!i)in( ratio
8mol9mol:
Nitrogen >N*? /.7
)xygen >)*? /.*+
Argon >Ar? /.//01
'arbon dioxide >')*? 16x+/-
Neon >Ne? +x+/-
)<one >)1? /./+-+/x+/-
elium >e? 6.*x+/-
@ethane >'5? +.7x+/-
8rypton >8r? +.+x+/-ydrogen >*? 6//x+/-0
Nitrous oxide >N*)? 1*/x+/-0
Num+er Densit*
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The number density n9 of a gas 9 is defined as the number of molecules of 9 per unit
#olume of air. t is expressed commonly in units of molecules cm-1 >number of
molecules of 9 per cm1 of air?. Number densities are critical for calculating gas-phasereaction rates. 'onsider the bimolecular gas-phase reaction
>D+?
The loss rate of 9 by this reaction is e:ual to the fre:uency of collisions betweenmolecules of 9 and multiplied by the probability that a collision will result in chemical
reaction. The collision fre:uency is proportional to the product of number densities
n9n. 3hen we write the standard reaction rate expression
where & is a rate constant, the concentrations in brac&ets must be expressed as number
densities. 'oncentrations of short-li#ed radicals and other gases which are of interest primarily because of their reacti#ity are usually expressed as number densities.
Another important application of number densities is to measure the absorption or scattering of a light beam by an optically acti#e gas. The degree of absorption or
scattering depends on the number of molecules of gas along the path of the beam and
therefore on the number density of the gas. 'onsider in this atmosphere an opticallyacti#e gas 9. A slab of unit hori<ontal surface area and #ertical thic&ness d< contains
n9d< molecules of 9. The integral o#er the depth of the atmosphere defines
the atmospheric column of 9 as
This atmospheric column determines the total efficiency with which the gas absorbs or
scatters light passing through the atmosphere. For example, the efficiency with which the
o<one layer pre#ents harmful solar 4 radiation from reaching the arth$s surface isdetermined by the atmospheric column of o<one.
The number density and the mixing ratio of a gas are related by the number density of air
na >molecules of air per cm1 of air?=
The number density of air is in turn related to the atmospheric pressure P by the ideal gas
law. 'onsider a #olume 4 of atmosphere at pressure P and temperature T containing N
moles of air. The ideal gas law gi#es
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where D O .1+ Q mol-+ 8-+ is the gas constant. The number density of air is related to N
and 4 by
where A# O ./*1x+/*1 molecules mol-+ is A#ogadro$s number. 2ubstituting
e:uation (1.5) into (1.4) we obtain=
and hence
&artial &ressure
The partial pressure P9 of a gas 9 in a mixture of gases of total pressure P is defined as
the pressure that would be exerted by the molecules of 9 if all the other gases wereremo#ed from the mixture."alton$s law states that P9 is related to P by the mixing ratio
'9 =
For our applications, P is the total atmospheric pressure. 2imilarly to (1.6) , we use the
ideal gas law to relate P9 to n9=
The partial pressure of a gas measures the fre:uency of collisions of gas molecules with
surfaces and therefore determines the exchange rate of molecules between the gas phaseand a coexistent condensed phase. 'oncentrations of water #apor and other gases that areof most interest because of their phase changes are often gi#en as partial pressures.
'loud formation in the atmosphere ta&es place when P*) P*),2AT, and it is
therefore important to understand how P*),2AT depends on en#ironmental #ariables.
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From the phase rule, the number n of independent #ariables determining the e:uilibrium
of c chemical components between a number p of different phases is gi#en by
n the case of the e:uilibrium of li:uid water with its #apor there is only one componentand two phases. Thus the e:uilibrium is determined by one single independent #ariable;
at a gi#en temperature T, there is only one saturation #apor pressure P*),2AT>T? for
which li:uid and gas are in e:uilibrium.
n weather reports, atmospheric water #apor concentrations are fre:uently reported as
the relati#e humidity >D? or the dew point >Td?. The relati#e humidity is defined as=
so that cloud formation ta&es place when D +//!. The dew point is defined as the
temperature at which the air parcel would be saturated with respect to li:uid water=
. "raw carbon, nitrogen and oxygen cycles
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Car+on C*cle
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Nitro(en C*cle
O)*(en C*cle