Atmospheric Chemistry

Discussion Questions

The Atmosphere

The atmospheric chemistry studies the chemical composition of the natural atmosphere, the way gases, liquids, and solids in the atmosphere interact with each other and with the earth's surface and associated biota, and how human activities may be changing the chemical and physical characteristics of the atmosphere. It is interesting to note that the 1995 Nobel Prize in Chemistry 1995 was awarded to the atmospheric scientists P. Crutzen, M. Molina and F. S. Rowland.

How do atmospheric scientists view the atmosphere?

For convenience of study, atmospheric scientists divide the atmosphere as if it consists of 4 layers. The division is mainly due to temperature variations as the altitude increases. The 4 layers according to the variation of temperature are. Ionosphere (Aurora) or Thermosphere
Mesosphere
Stratosphere
Troposphere
Above 100 km is the thermosphere and ionosphere where the temperature increases from 200 K at 100 km to 500 K at 300 km. The temperature goes even higher as the altitude increases. activity as the altitude decrease. In the outer space, most particles consist of single atoms, H, He, and O etc. At lower altitude (200 - 100 km), diatomic molecules N2, O2, NO etc are present. The ionosphere is full of electrically charged ions. The UV rays ionizes these gases. The major reactions are

In the ionosphere:
O + h v ® O+ + e
N + h v ® N+ + e
In the neutral thermosphere:
N + O2 ® NO + O
N + NO ® N2 + O
O + O ® O2
Beyond the neutral thermosphere is the ionosphere and exosphere. These layers are of course interesting for space explorations and environmental concerns and space sciences. The atmosphere in the outer space is more like a plasma than a gas.

Below the thermosphere is the mesosphere (100 - 50 km) in which the temperature decreases as the altitude increase. In this region, OH, H, NO, HO2, O2, and O3 are common, and the most prominent chemical reactions are:

H2O + h v ® OH + H
H2O2 + O ® OH + OH.

Below the mesosphere is the stratosphere, in which the temperature increases as the altitude increase from 10 km to 50 km. In this region, the following reactions are common:

NO2 ® NO + O
N2O ® N2 + O
H2 + O ® OH + H
CH4 + O ® OH + CH3

Air flow is horizontal in the stratoshpere. A thin ozone layer in the upper stratosphere has a high concentration of ozone. This layer is primarily responsible for absorbing the ultraviolet radiation from the sun. The ozone is generated by these reactions:

O2 + h v ® O + O
O2 + O ® O3

The troposphere is where all weather takes place; it is the region of rising and falling packets of air. The air pressure at the top of the troposphere is only 10% of that at sea level (0.1 atmospheres). There is a thin buffer zone between the troposphere and the next layer called the tropopause.

The major components in the region close to the surface of the Earth are N2 (78%), O2 (21%), Ar (1%) with variable amounts of H2O, CO2, CH4, NO2, NO2, CO, N2O, and O3. The ozone concentration in this layer is low, about 8% of the total ozone in the atmosphere is in the troposphere.

All the weather phenomena happen in the troposphere. If you want to know the current weather condition, check the AccuWeather network for a good view of the troposphere.

What gases are pollutants in the atmosphere?

From the atmospheric science viewpoint, interactions of all gasses amoung themselves and their interaction with the environmental elements are of interest. However, for identification purposes, we need to identify the gases produced by man-made process (industry).

Some of the gases due to human activities are:

Water vapour is also considered a greenhouse gas, but it is also generated by nature contineously due to radiation from the Sun. Of course, when water vapour condense into a liquid, much energy is released in the exthermal process. Condensation of watervapour causes storms and many of the weather phenomena.

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