Zooming in Our Atmosphere
Dr. Md. Mominul Islam
Professor
Department of Chemistry, University of Dhaka, Dhaka 1000
E-mail: mominul@du.ac.bd
Our sky is blue in color. Oh, how blue our clear sky is! Blue and blue, lots of blue, but
these blue are only to see! Perhaps for this, Rabindranath Tagore quoted“Who can strain the blue from the sky?”
Indeed, blue color is the beauty of our atmosphere and by our eyes we can only see this
beauty. But one can discover many things hiding inside the blue sea of atmosphere in a
microscopic view.
All planets have their own atmosphere, like our Earth does. 4.6 billion year ago, our
atmosphere (was) started to form. Solar nebula, rock-forming elements present as gaseous state
at high temperature, condensed into small solid grains. The grains grew by collision to
centimeter-sized particles. Planetesimals, small-body precursors to the planet formation, are
then formed by the accumulation of grains. The asteroids that form the planet were grown by
the accretion of grains on the planetesimals. Asteroids collided to form the planets. Meteorites,
solid rocking forming minerals with gases, collided further with asteroids and finally they
condensed to solid phase elements and non-condensable elements, creating a solid-body of a
planet surrounded by a gaseous atmosphere. Like other planets, the Earth and its atmosphere
also formed.
Our first atmosphere contained only hydrogen (H) and helium (He). These elements are
known as the most abundant elements of the solar nebula. The second atmosphere evolved due
to the outgassing from the Earth mantle by volcanoes, fumaroles, stream walls, and geysers.
Before the formation of molecular oxygen (O2), it is named as prebiotic atmosphere.
Figure 1. Our solar system.
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What is the age of molecular oxygen (O2)? O2 is 2.3 billion year old. During prebiotic
period, hydroxyl radical (OH) outgassed from the earth mantle reacted with H2, CH4, NH3, N2
and H2S (reduced gases) to form oxidized products such as H2O, CO, CO2, NO2 and SO2. Our
atmosphere was reducing, but it is now oxidizing in nature (Figure 1). In fact, the atmosphere
of the planets situated near to the sun is chemically oxidizing, but those situated after the Earth
are still reducing in character, i.e., they still contain reduced gases. Thus, one may say that the
sun is burning up (oxidizing) the atmosphere of his planets and so far it could finish his job up
to the Earth’s atmosphere! Now we may just guess, after the atmospheres of Mercury, Venus
and Earth, the next target of the sun would be to oxidize the atmosphere of Mars and so on! In
fact, atmosphere of Mars contains about 96% of CO2 as contained our atmosphere at its earlier
stage (see Figure 2) and trace amount of methane with other gases.
The estimated change in composition of Earth’s
atmosphere against time is represented in Figure 2. Up
to 2.3 billion year ago, the components of our
atmosphere were mainly of CO2, N2 and H2. After the
formation of O2, further changes in the composition
occurred. However, we all know well about the
composition of our present atmosphere as shown in
the pie chart of Figure 3. In this figure, the structure
Figure 2. Change in composition of gases
of atmosphere and temperature profile against
of the Earth’s atmosphere with time.
altitude, and human activities occurring inside the
atmosphere are also exhibited. If we zoom in it, we may see many more matters which are of
in molecular dimension are floated in the air other than the essential components of the
atmosphere.
Figure 3. Structure and compositions and temperature profile of the atmosphere.
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It is commonly known to all that none can survive by any means without essential gases
present in our atmosphere. But it may be out of our guess that these gases are playing a great
role in keeping warm the vicinity of the Earth surface. The equilibrium temperature of the Earth
is estimated to be 18 C that is naturally 18 C below the freezing point of water and would
not support most life on the Earth. Fortunately, the actual globally averaged near-surface air
temperature is 15 C. The difference in temperature between the predicted temperature and the
actual one by 33 C is resulted from the fact that our atmosphere is transparent to most
incoming solar radiation, but selectively absorb outgoing thermal-infrared (IR) radiation. Some
of absorbed thermal-IR radiations are reemitted back, warming up the surface of Earth. The
resulting 33 C increase in temperature over the equilibrium temperature of the Earth is called
the natural greenhouse effect.
Figure 4. Types and sources of air pollutants
Can we guess how many types of chemical species are present in our atmosphere? And
how many varieties of chemical reactions take place in the atmosphere? For instance, some of
these may be seen in Figure 4. This figure is self-explaining. Is it not? Zooming in the
atmosphere, we may actually find many chemical entities present inside as stated above. Some
of them are recognized as pollutants and these pollutants are categorized as primary and
secondary on the basis of their sources. Pollutants emit from the Earth’s surface due to the
anthropogenic activities is called primary. These primary pollutants undergo further chemical
reaction inside the atmosphere to produce some other chemical entity. If the newly formed
species are harmful, then these are called as secondary pollutants.
In this regard, the following matters about the chemistry of atmosphere are worthy to note.
More than 400 chemical species have been identified in the exhaust of a heat engine such as
engine of an automobile. One can now imagine how many similar activities are being done by
us and how many varieties of chemical species are being left to the atmosphere. However, these
chemical species mix with other species present in our atmosphere. Some of them are highly
reactive and some of them are stable. For instance, the life time of a radical is less than a second,
but the average life time of N2O in the atmosphere is 120 years! That is, these species may
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react with some other species present in some places of the atmosphere to form some new
chemical species (see in Figure 4). This process is called sink process. At this stage, we will
not be surprised to know that there around 6000 elementary chemical reactions are revealed to
take place in our atmosphere. Thus, we may call our atmosphere as a big chemical reactor. In
this big reactor, sunlight induces photochemical reactions. The chemistry of a photochemically
active species in nighttime is, therefore, far different from that occurs during daytime.
Now let us go back to the story of role of atmosphere in warming up the vicinity of the
Earth surface. Natural greenhouse effect arisen due to the presence of indispensable
components of our present atmosphere as mentioned above is certainly a gift for us. The
temperature of our atmosphere is the highest at the surface of Earth (see in Figure 3) among
the other zones of troposphere, stratosphere and mesosphere. In this profile, we can see that
temperature decreases with increasing the altitude and then increases in the stratospheremesosphere. The highest temperature of stratosphere is comparable with that at the vicinity of
the Earth’s surface.
Figure 5. Pollutants and heat trapped in the atmosphere.
The gases produce due to anthropogenic activity on/around the Earth’s surface go to the
atmosphere by vertical and horizontal (geostrophic) flow driven by both temperature (negative)
gradient, i.e., gases naturally move from warmer place to colder place, and the circular motion
of the Earth. Obviously these gases would not go far away from the Earth’s surface due to
gravitational force and the positive temperature gradient near to the stratosphere, i.e., the gases
do not move from colder place to warmer place. What happens then? These gases are being
trapped between regions of troposphere and stratosphere. Thus, we may consider these zones
of our atmosphere as a sandwich of which the middle part is colder and contains trapped gases.
The trapped gases adsorb thermal IR and warming up our atmosphere by reemitting process of
IR in addition to the natural greenhouse effect, regulating in so-called global warming. In this
regard, it may be noted that CH4 molecule contribute to the global warming by 21-times higher
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than CO2. N2O and CFC-12, the life times of them are more than 100 years, contribute to the
global warming by more than 300- and 6000-times, respectively, higher than CO2. For stopping
global warming further, it is now a challenge to clean up our atmosphere.
By replying one famous question about the wind we may conclude here. Alan Alexander
Milne, British author (1882-1956), challenged through his famous poem “Wind on the Hill”:
No one can tell me,
Nobody knows,
Where the wind comes from,
Where the wind goes
Modern science and sophisticated theoretical and instrumental resources have assisted us
to answer the question of Milne. We can now real-time monitor and analyze the flow of air and
particles and gases present in the air by various measurement platforms. These platforms
include stationary station, vehicle-based mobile station, and ship-borne, balloon-borne,
aircraft-borne and other space-borne platforms, i.e., rocket platforms and satellites. Modern
mathematical theories and computer simulation facilities have been in fact helping us to model
and reveal the origins of gases and route of flow of air over the atmosphere. Nowadays,
excellent effort on atmospheric chemistry research has been devoted by many research groups
worldwide. The Nobel Prize in Chemistry 1995 was consequently awarded to Paul J. Crutzen,
Mario J. Molina and F. Sherwood Rowland jointly for their work in atmospheric chemistry,
particularly for revealing the formation-decomposition mechanism of ozone layer.
References
1. Mark Z. Jacobson, “Atmospheric Pollution: History, Science and Regulation”, Cambridge University
Press, 2002.
2. Daniel J. Jacob, “Introduction to Atmospheric Chemistry”, Princeton University Press, 1999.
3. John Wright, “Environmental Chemistry”, Routledge, Taylor & Francis Group, 2003.
4. Dwayne E. Heard, “Analytical Techniques for Atmospheric Measurement”, Blackwell Publishing,
2006.
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