Electives 1

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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



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



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



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



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



)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



&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



!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*



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



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.

http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap1.html#55274









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



Car+on C*cle



Nitro(en C*cle

O)*(en C*cle

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