ee 2001 atmosphere lecture 1

7/28/2019 EE 2001 Atmosphere Lecture 1

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PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES

Radiation Safety and Radioactive Waste ManagementCourse

(Lecture 1)

BASICS CONCEPTS OF ATMOSPHERIC DISPERSION

The increasing civilization of man unfortunately brings with it an increased disturbance of its surrounding

environment. Continuous and progressive industrialization, advancing transportation technology coupled with rapid

growth of urban areas has led to ever-increasing amounts of pollutants around. In order to assess the impact it is

necessary to understand the physical nature and composition of the surroundings and the parameters, which affect or

alter the composition. The whole scene all around is mainly composed of three predominant features [figure 1.1] that

is the dry part of the land i.e. continents called lithosphere, the wet part i.e. oceans and seas called hydrosphere,

and the upper envelop of air called atmosphere. With these attached some geo-chemical cycles such as air cycles,

water cycle or rock or cycle. These cycles are not restricted to any one of the part but can be assigned to that part that

has the maximum residence time of that cycle in it.

Figure: 1.1 The general parts of the environment around us on this globe

THE ATMOSPHERE

The earths atmosphere is an envelope of gases extending to a height of about 2000 km. The density of these gases

decreases with increasing altitude to such an extent that one half of the total mass of the atmosphere is found in the

lower 5 km. Strictly speaking, the atmosphere of a geographical region cannot be isolated from the remaining part of

the globe. Thus whatever pollutant is emitted at one part of the globe may affect the inhabitants of the other part due

to mixing and dispersion in the atmosphere. Radioactivity released in Chernobyl accident may be one of the example

in which due to the activity fall out on grass lands in Scandinavian countries, grazed by cows, entered in the food

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chain and was detected in the dried milk at various ports. Thus before explaining pollutant dispersion in the

environment it is better to understand atmosphere itself, its nature, composition and phenomenon occurring within it.

Exploration of atmosphere began in 17th century. As instruments developed to measure different elements they helped

to formulate the laws to explain the phenomenon occurring in the atmosphere. In 1593 Galileo made the early version

of thermometer, in 1643 Torricelli invented the first barometer for air pressure and in 1661 Robert Boyles found the

pressure-volume relationship of gases. During 18th century instrument, instruments further developed, improved and

standardized and extensive collection of data began. The collected data at ground level was insufficient and provided

a limited understanding. In late 17th century the collected data at higher altitude was only based on observation taken

on mountain. In 1752 Benjamin Franklin used kite and discovered that lightening is an electrical discharge. Kites

were than used to measure higher altitude temperature. In 18th century manned balloon were used to study the

composition and properties of upper atmosphere. Manned balloon were dangerous and seldom exceeded height of 5 to

8 km. Unmanned balloon could go higher but there was no assurance that instruments carried aloft would be

recovered. Airplanes were used in World War II and subsequently after the war the airplanes were also used to collect

the data from upper atmosphere. Newly developed weather radar and satellites are now being used, TIROS-I being

the first meteorological satellite send to space in 1960.

Atmosphere begins from earth surface but the question arises that where it ends and space begins. To examine

vertical extent the data related to pressure changes with altitude may help. At sea levels pressure would be about

1000 mb (or 1.01 x 105) N/m2. Obviously the pressure at higher altitude is lower. Actually the pressure of atmosphere

is an indicative of the density or concentration of molecules in air at the height in question. Based on the ratio of

pressure at a certain altitude to pressure at the surface of the earth it is obvious that half of the atmosphere is

concentrated within the first 5.6 km. 90% of the total air mass is located in the first 16 km only. And after about 100

km the pressure left would be 0.00003 mb.

In earlier 20th

century Balloon and kites revealed that atmospheric temperature drops with altitude. Although therewas no data available above 10 km still it was believed that this trend would be continued until absolute zero at the

end of the atmosphere and beginning of the space. In 1902, French scientist Leon Phillip Teisserenc de Bort refuted

the notion that temperature decreases continually with altitude. Based on the observation of 200 balloon launching, he

found that temperature stopped decreasing and leveled off at a height of about 10 to 12 km. This surprising discovery

was at first doubted, but subsequent data gathering confirmed his findings. Later, through the use of rocket sounding

techniques, the temperature structure of the atmosphere up to great heights became clear.

The Vertical Structure of the Atmosphere:

The variation of temperature with altitude is given in figure 1.2. Based on this temperature profile the atmosphere is

divided into various strata or sub-layers such as Troposphere, Stratosphere, Mesosphere and Thermosphere. The

properties of each of these layers have an inherent direct or indirect relevance to the ecosystem and the environment.

The Troposphere:

It is the lower most layer and is characterized by the steady state average decrease in temperature at 6.5 oC per

kilometer. This is due to the fact that radiation from the sun penetrates the atmosphere to warm the oceans and

ground, and the surface of the globe in turn transfers energy to the low levels of the troposphere. Convection and

radiation transmit this energy upward, so there is an average decrease in atmospheric temperature with increase in

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altitude. Troposphere has an average altitude of 15 kilometer but this altitude may vary with different latitudes (e.g.

about 10 km at poles, about 12 kilometer at mid latitudes, and about 18 km at tropical regions). The upper boundary

of the troposphere is termed as tropopause. This is a 10-20 km wide barrier at - 60oC, which prevents the escape of

water vapor. Water vapor condense at this low temperature and thus tropopause preserve the water on earth.

The Stratosphere:

Above tropopause is the stratosphere. This often shows two regions of distinct temperature variation. The lowest mayhave a temperature essentially independent of altitude, a continuation of the condition of tropopause. The other region

of the stratosphere has a temperature gradient and temperature increases with height. The rise in temperature within

stratosphere is thought to be associated with the absorption of ultra-violet radiation from the sun by ozone, which is

formed in the upper atmosphere at an altitude of about 30 km, principally by the photo-dissociation of molecular

oxygen. The stratosphere is thus an important part of atmosphere providing natural shield around the earth due to

presence of ozone. It extends vertically upward to cover a height of about 15km. At about 50 km from earth surface

the temperature is increased to about 20oC, which marks the upper boundary of stratosphere called Stratopause.

The Mesosphere:

Just above 50 km is a region known as the mesosphere. Here the density of air and hence the ozone concentration

decreases rapidly with height and become insufficient to support effective photochemical reactions. Hence the

temperature steadily decreases with altitude due to decreased absorption of solar radiation by ozone, reaching about -

100oC at the upper boundary of mesosphere which is called mesopause.

The Thermosphere:

The region overlying the mesopause, extending upward from about 85 km is again a portion of the atmosphere in

which solar energy is converted into sensible heat. This is thermosphere where there are fewer than 1019 molecules

Lecture delivered by Dr. Naseem Irfan(iii) of iv

Figure 1.2 Atmospheric strata(based on tempeature proflie)


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per cubic meter as compared with about 2.5 x 1025 at sea level. The intense ultraviolet radiation penetrating to the

lower thermosphere causes photo-dissociation of O2 and photo-ionization of N2 and atomic oxygen. Thus

thermosphere is characterized by a steady rise in temperature with altitude. The temperature at 200 km exceeds 500 oC

and at the upper boundary (1000km) it exceeds 1225oC. Although temperatures rise to extremely high values of more

than 1000oC, such temperatures are not strictly comparable with that experienced near the surface of the earth.

Temperature is defined in terms of the average speed at which molecules are moving. Because the gases of the

thermosphere are moving at very high speeds, the temperature is obviously very high. The gases are so sparse,

however, that very few of these fast-moving air molecules would collide with a foreign body; therefore, only an

insignificant quantity of energy would be transferred. Thus, the temperature of a satellite orbiting the earth in the

thermosphere is determined chiefly by amount of solar radiation it absorbs and not by temperature of the surrounding

air; if an astronaut inside were to expose his or her hand, it would not feel hot outside the space capsule. The

thermosphere is also known as ionosphere and is the highest layer recognized.

Lecture delivered by Dr. Naseem Irfan(iv) of iv

ee 2001 atmosphere lecture 1

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