8 Atmosphere and Its Properties

Prof. K.S. Gupta

epgp books

Contents

  1. Introduction
  2. Mass of the Atmosphere
  3. Chemical Composition
  4. Vertical Profile of Atmospheric Pressure/Variation in Atmospheric Pressure
  5. Air Mixing Ways: Homosphere and Heterosphere
  6. Macroscopic Mixing/Bulk Fluid Motion/Turbulent Mixing
  7. The Vertical Temperature Profile
  8. Troposphere
  9. Stratosphere
  10. Mesosphere
  11. Thermosphere/Ionosphere
  12. Adiabatic Lapse Rate
  13. Saturated Adiabatic Lapse Rate
  14. Temperature/Thermal Inversion
  15. Earth’s Atmosphere in Relation to Venus and Mars
  16. References

Introduction

The air-envelop surrounding the Earth in which we live is called atmosphere. Actually the envelop is thin layer of air, which is called as atmosphere. It is believed to extend to a height of about 60000 km from Earth’s surface. Where, most of the mass of the atmosphere is found near the planetary surface. However, altitude increases the density of the air decreases and beyond 500 km there is very little air. The structure of the atmosphere and the properties of its different atmospheric layers greatly determine the atmospheric chemistry. Our atmosphere is unique and its properties are such that among all the known planets and other heavenly bodies, life exists and sustains only on Earth. The major constituents are nitrogen, oxygen and CO2 followed by other gases in smaller amounts and traces. About 99.9% of mass air is concentrated in troposphere and stratosphere( up to ~50 km) and half of the mass of air is in first 6 km from the Earth’s surface.

 

The importance of oxygen needs no elaboration. All living organism, including humans, require oxygen for respiration, and the conversion of chemical energy stored in food for use by living beings for life sustaining functions. Carbon dioxide is in photosynthesis to produce food and replenish the oxygen supply. Nitrogen is important in several ways, such synthesis of proteins, nucleotides, fertilizers, etc.

 

Mass of the Atmosphere

Atmospheric pressure is defined as the force per unit area exerted on a unit area of the surface by the weight of the air above the surface of the Earth. The mean atmospheric pressure at sea level is 1 atmosphere or 760 torr or 1.013 × 105 Pa or Pascal or 1.013 bar. At the surface of the Earth, mean global atmospheric pressure Pearth is 984 hPa, slightly less than at mean sea level pressure due to higher elevation of Earth.

Chemical Composition

The relative average concentrations of major gaseous constituents of air at sea level are in Table 1.

Table 1. Composition of dry unpolluted air (Brimblecombe, 1996).

Gases Percent/ppm
Nitrogen 78.084%
Oxygen 20.946%
Argon 0.9345%
Carbon dioxide 0.039%
Others traces

 

As we shall learn later that the trace atmospheric constituents are responsible for air pollution and each of the gaseous constituents, such as, CO, SO2, NOx, methane , VOCs etc need separate studies.

 

Vertical Profile of Atmospheric Pressure/Variation in Atmospheric Pressure

  With increase in altitude, the pressure continuously decrease due to decrease in gravitational attraction. Te variation of pressure with altitude is shown in Fig. 1.

 

  At a height of 5 km from Earth surface, pressure is nearly half the value at surface.This thinning of air with altitude induces breathlessness. At Mt. Everest air pressure drops to about 265 mbar. So mountaineers need to use bottled oxygen. That is why the air pressure in aircrafts is maintained artificially at 1atmosphere. Some features of atmosphere are as follows:

  1. The gaseous components are compressible.
  2. The gas molecules do not settle down under the influence of gravity because translational kinetic energy competes with sedimentation forces and keeps the gas molecules afloat. Had it not been tall the gases would have been virtually within few centimeters at the surface with vacuum overhead.
  3. The pressure and density decrease exponentially with increase in altitude as defined by Barometric Law equation(3).

Fig 1. The variation of atmospheric pressure with altitude

Since the pressure at Earth surface is equal to one atmosphere,Eq.(6) becomes Eq.(7).

It can be seen from Eq. (6) that a scale height is a distance over which the pressure decreases by a factor of e , i. e., ~ 2.71828. Assuming the temperature to be invariant with height(which is not true), the plot of Z and ln Pz is expected to be linear having a Slope = – H. The value of H is found to be ~7.4

 

. This value corresponds to the scale height at mean atmospheric temperature of 250oC. We have assumed the temperature to be constant, but in reality it is not. However, the effect of change in T is small because it is used in Kelvin scale. The real ln P versus altitude plot shows come curvature due to variation in temp. So H varies from 6 km at 210 K to 8.5 km at 290 K

The scale height has been calculated from Eq. (3) using s 28.97 ( ~ 29) as the mean molar mass of dry air, m.

 

Air Mixing Ways: Homosphere and Heterosphere

Interestingly up to an altitude of ̴100 km, air behaves as a single homogeneous component of molecular mass, M = 28.8 (~29) and the composition of gases remains unchanged. This is contrary to Barometric law, which requires the concentration of heavier species such as N2, O2, to decrease more rapidly than lighter elements like He and H2 with increase in height. Above 100 km, the concentration of heavier elements starts decreasing more rapidly than the lighter elements and air composition starts changing. And the percentage of lighter gases starts increasing.

 

This observed behavior in air composition above and below 100 km is dependent on which of the two mixing ways, viz., mixing by molecular diffusion and macroscopic mixing dominate.

 

Mixing by Molecular Diffusion: The main features are:

  1. It discriminates the molecules on the basis of their molecular masses in accordance with Barometric law..
  2. Gravitational attraction tries to separate the molecules on molecular scale during diffusion.
  3. Mean free path, λm, increases on increasing altitude due to decrease in pressure.

                        λm = (1/21/2лd2)(kT/P)

where d = diameter, k =Boltzman constant and P = pressure. Mean free path is the average distance traveled by a moving particle between successive collisions.

 

Macroscopic Mixing/Bulk Fluid Motion/Turbulent Mixing

This kind of mixing occurs on macro scale due to convection, turbulence, small eddies or fluctuations. It does not discriminate among gas molecules on the basis of their molecular massesThe fluid mixing length is a distance that a fluid parcel will maintain its original character before mixing in the surrounding fluid.. Fluid Mixing Length, λF, tends to decrease with increase in altitude. Thus it is the distance travelled during a Transport Event. In atmosphere, whereas the molecular diffusion attempts to separate the molecules on the basis of their molecular masses, on the other hand the fluid motion tries to redistribute the gas molecules that are being separated by gravitational attraction.

 

Up to 100 km, the fluid motions dominate since λF > λm. And the composition of air remains unchanged. So air behaves as a single component. This part is called Homosphere. Between 100 -120 km, λF and λm are almost of the same size( 0.1 – 1.0 m). This region is known as turbopause.

 

Above 120 km, mixing by molecular diffusion dominates as λm is much larger than λF. Here, composition is based on molecular mass. So above 120 km, the proportion of lighter gases starts

increasing. This region is called Heterosphere.                                                                                         .

At the top of the atmosphere only lighter components such as H, H2 and He are present. Some

of these escape in outer space depending up on escape velocity.

 

The Vertical Temperature Profile

As shown in Fig. with increase in height from the Earth surface, the change in temperature is not form. This temperature change pattern in different regions is basis of dividing atmosphere in four layers. The lowest layer is troposphere, followed by stratosphere, mesosphere and the last layer thermosphere. The thickness and boundary of each layer is not same throughout the globe and change with season and also during day.

Troposphere

Its height or thickness is about 8-9 km at poles and 16-18 km at equator. The average thickness is about 12 km. Some of its features are as follows:

  1. contain most of the atmospheric gases. About one –half of the total mass of atmosphere is found in lower 5 km.
  2. By far the troposphere is the most important layer as all weather phenomena occur in this layer.
  3. Its height is about 9-18 km which varies with latitude, being lower at poles and higher at equator.
  4. It contains most of water. The total mass of atmospheric vapors is equivalent to a precipitation of 2.5 cm rain over the whole of earth’s surface. The average rainfall is about 90 cm per year. Hence there are over 36 evaporation-precipitation cycles per annum. Average residence time for a water molecule is about 10 days.
  5. In general, there is a steady decrease in temperature in troposphere with increase in altitude. The rate of decrease of temperature (T) with altitude(z), dT/dz, is called lapse rate. It has a value of about 6.5 oC per km.
  6. Tropopause – the portion of the atmosphere at which the negative temperature gradient of the troposphere changes to constant temperature is known as tropopause.
  7. The exchange of material through tropopause in either direction is slow.
  8. The cooler air of the atmosphere, which lies over troposphere acts as a lid. The life time of the materials is of the order of months.
  9. Heating is through convection. Solar radiation is not directly absorbed by atmospheric gases in troposphere.
  10. The air in the troposphere is more unstable due to strong convection. Almost all the water vapor in the atmosphere exists within this layer; therefore, common weather phenomena such as clouds, fog, rain, and snow, occur only in this layer and mostly in its lower part.
  11. Mixing time of a given hemisphere is of the order of weeks. Complete exchange between two hemispheres requires months.
  12. In troposphere, solar radiation is not absorbed by air to any significant extent so there is no heating. The solar radiation first reaches the Earth surface, heat it, then the air which comes in contact with hot Earth get heated. Thereafter, convection is set in motion and heating of air starts. So lower air is hotter than the air overlain. That is one of the simplest reason to explain decrease in temperature with increase in altitude in troposphere. The issue would be discussed further in details in a next Section in this Module.

 

Stratosphere

The stratosphere extends from top of the troposphere to about 50 km altitude above the earth surface. It is characterized by increase in temperature with height due to absorption of solar radiation by ozone layer present in stratosphere. At low height, there are insufficient high energy solar radiation (<242nm) to dissociate O2 to form O3, and at high altitude(80 km), there are insufficient O2 to form O and react with O2 to generate O3. At 50 km height, [O2] and Solar radiation both are optimum and maximum temperature is at this point. Below and above this either O2 or high energy radiation is insufficient, so there is maximum in temperature at about 50 km and it decreases above or below it. Heating of stratosphere is due two reasons. First, the formation of O3 by a series of a Chapman mechanism rereleases chemical energy in the form of heat. Secondly, solar radiation (UV-B) absorbed by ozone are also liberated as heat.

Mesosphere

The region of the mesosphere is about 50 to 80 kilometers in altitude. The temperature decreases continuously with increase in height up to the top of the mesosphere about 80 km. Here the temperature is found to be as low as – 95 °C or even lower. The lowest temperature whole of atmosphere of Earth is found in mesosphere. Lapse rate is about 3.75oC per km. The boundary between stratosphere and mesosphere is called mesopause. Cooling in          mesosphere is due to lack of such reactions which would absorb high energy solar radiation to very low concentration of O2. O3 is also not formed here.

 

Thermosphere/Ionosphere

The region above the top of mesosphere, i. e., 80 km is thermosphere or ionosphere, where the temperature begins to rise again. At low sun activity this layer may extend up to 400 km, and during high sun activity period the layer can extend up to 500 km in altitude. There is steady rise in temperature and at 200 km the temperature is > 500 0C and at 700-800 km the temperature is more than 1000 0C. The gas molecules encounter strong UV radiation here first. These are absorbed by gaseous molecules which are ionized resulting in ions and free electrons in the air. Therefore, this layer is also called the ionosphere. This layer is very effective in reflecting radio waves and so very vital for communication. Some reactions are listed below (Atkins, 1998).

 

Adiabatic Lapse Rate

The decrease in temperature with increase in height can be calculated as follows. Imagine a rising parcel of dry air. The parcel is in equilibrium with its surroundings, but thermally isolated from it and behaves adiabatically. Since on increasing altitude, the pressure decreases, the parcel of air expands and does work against surroundings consuming its internal energy, E.

We know that

From Eqs. (21 – 23),

where ρ = density, g = acceleration due to gravity, A = area, dz = change in height, force =mass ×g, mass = density× volume and volume = dz × A.

Γd or -dT/dz is known as dry adiabatic lapse rate for a dry parcel . From the values of g and cp( for air), lapse rate is found to be 9.8 oC/km. Notably Γd does not depend on atmospheric conditions.

 

Saturated Adiabatic Lapse Rate, Ts

  When condensable vapors are present, latent heat is released and this adds complications. For a saturated vapor, decrease in temperature is accompanied by release of latent heat. And so the decrease in temperature with attitude is lower than in the case of dry air parcel. Thus, saturated adiabatic lapse rate, Ts, is lower than Td . Its value lies in Earth’s atmosphere between 4 -7 oC/km.

 

  By comparing the atmospheric lapse rate, Γatm,, with Γd , the vertical stability of the atmosphere with respect to buoyancy can be predicted.

  The resistance of the atmosphere to vertical motion is the stability. The degree of stability or instability of the atmosphere at any particular instant determines the upward or downward vertical movement. This depends up on the actual lapse rate is greater or less than the dry adiabatic lapse rate. The departure of this from the dry adiabatic lapse rate determines whether the atmosphere is unstable or stable. the Unstable atmosphere encourages the vertical movement of air. When the air is stable, vertical movement of air is hindered.

 

  In sum, if Γatm,, is less than Γd and temperature decreases with altitude, the atmosphere is stable, and if if Γatm,, is less than Γd but there is temperature inversion that is temperature increases with altitude then the atmosphere is very stable. On the contrary, if if Γatm,, is more than Γd the atmosphere is then unstable, and if Γatm,, is much more than Γd then the atmosphere is very unstable. Finally, when Γatm,, = Γd atmosphere is neutral. In this case the parcel of air will have same the same density and temperature as the surroundings.

 

Temperature/Thermal Inversion

   Temperature inversion occurs when the instead of decreasing with altitude the temperature starts increasing. It is called temperature inversion. In such a situation, air above the ground is warmer than the air below. Earth has the quality of heating faster as well as cooing faster.than the surrounding air. On a clear night ,when the air near the ground cools rapidly it loses heat and the Earth surface becomes cooler quickly than the air above because its ability to lose heat slowly it retains and remains warmer than the Earth surface. There are other ways of inversion also.

 

   Temperature inversion greatly influences the meteorology as the atmosphere becomes stable It blocks the dispersion of pollutants by restricting vertical mixing. By not allowing the dispersion of pollutants, inversion helps in the development of the significant amounts of smog. Particularly in winter, many Indian cities see the accumulation of pollutants released by automobiles and other human activities. London’s Great Smog and Mexico’s similar problems are extreme examples of smog being impacted by the presence of an inversion layer. This is a problem all over the world though and cities like Los Angeles, California; Mumbai, India; Santiago, Chile; and Tehran, Iran, frequently experience intense smog when an inversion layer develops over them. Inversion is involved in other weather phenomena such as freezing rain, thunderstorms and tornadoes also.

 

Earth’s Atmosphere in Relation to Venus and Mars

   Wayne(2000) has presented an interesting comparison of atmospheres of planets and the reason why Earth sustains life. Table presents the atmospheric concentrations of N2, O2 and CO2 and temperature in Venus, Earth and Mars.

 

Table Amounts of nitrogen, oxygen, and carbon dioxide and average surface temperature at Earth , Venus and Mars

Planet CO2, ppm N2, ppm O2, ppm Surface temperature
Venus 965000 35000 < 0.3 735 K
Earth 400 780840 209460 288 K
Mars 953200 27000 1300 223 K

 

   In Earth’s atmosphere, concentrations of N2 and O2 are very high and that of CO2 very low. Temperature of Venus is very high and that of Mars low as compared to Earth. The concentration of O2 maintained at high concentration in Earth due to the its replenishment by biological processes for example, photosynthesis in plants., despite its removal by reactions with nitrogen,, H2 and with a large number of other elements and compounds, The CO2 at lower level is due to its consumption by plants and many other processes. In fact the presence of biosphere is responsible for current status of Earth. In its absence it would be like Venus and Mars.

you can view video on Atmosphere and Its Properties

References

  1. P. Brimblecombe (1996) Air Composition and Chemistry, Cambridge University Press, Cambridge.
  2. J. E. Andrews, P. Brimblecombe, T. D. Jickells , P. S. Liss and B. Reid(2004), An Introduction to Environmental Chemistry, Blackwell, Oxford
  3. H. A. Bridgeman (1990), Global Air Pollution: Problems for 1990s. CBS Publishing Co., New Delhi.
  4. D.J. Jacob (1999), Introduction to Atmospheric Chemistry, Princeton University Press, Princeton
  5. J. H. Seinfeld and S.N. Pandis, (1998), Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, Wiley, New York.
  6. J. Finlayson-Pitts and J.N. PITTS,  Jr. (1986),  Atmospheric Chemistry:  Fundamentals and Experimental Techniques, John Wiley, New York.
  7. R.P. Wayne (2000), Chemistry of Atmospheres , Oxford University Press, Oxford.
  8. P.V. Hobbs, (2000), Introduction to Atmospheric Chemistry, Cambridge University Press, Cambridge
  9. P. Brimblecombe (1996) Air Composition and Chemistry, Cambridge University Press, Cambridge.
  10. Colin Baird (1998), Environmental Chemistry, W.H. Freeman, New York.
  11. R. F. Phelan and R. N. Phelan(2013), Introduction to Air Pollution Chemistry- A Public Health Perspective, Jones and Bartlett, Burlington, MA.
  12. J. Gerard(2012), Introduction to Environmental Chemistry, Jones and Bartlett, New Delhi
  13. Michael Jenkins. Cite/attribute Resource. admin. (2005, November 08). Unit 7: Atmospheric Stability and Instability.