4.2 Reactions of atmospheric oxygen

Oxygen in the troposphere plays an important role in the processes taking place on earth’s surface. Oxygen takes part in energy producing reaction like burning of fossil fuels, e.g., oxidation of methane:

.

(R4.1)

Atmospheric oxygen is also used by aerobic organisms in the degradation of organic compounds or in oxidative weathering processes:

,

(R4.2)

.

(R4.3)

The carbon dioxide and water can be utilized by the green plants in the process of photosynthesis and oxygen is again returned back to the atmosphere (Figure 4.2):

.

(R4.4)

Chemical structure of glucose.

Figure 4.2: Chemical structure of glucose.

Because of the effect of ionizing radiation, oxygen in the higher atmosphere can exist in some other forms, which are different from those stable forms in lower levels. Therefore, in addition to O2 (molecular oxygen), the upper atmosphere has also O (oxygen atom), O2*(excited oxygen molecule) and O3 (ozone). The atomic oxygen is very reactive and stable primarily in the thermosphere. It is obtained by a photochemical reaction

.

(R4.5)

Due to this photodissociation of oxygen according to the above reaction and high energy solar radiation molecular oxygen is practically do not exist at very high altitudes. Less than 10% of the oxygen in the atmosphere is present as molecular oxygen (O2) at altitude exceeding approximately 400 km. Excited oxygen atom can be formed according to reaction

.

(R4.6)

The excited oxygen atom emits visible light at 636 nm, 630 nm and 558 nm wavelengths. Ultraviolet radiation (UV) reacts with oxygen atoms and forms oxygen ions (O+)

.

(R4.7)

This positively charged oxygen atom can react further at the higher atmosphere

,

(R4.8)

.

(R4.9)

The species O2* is also formed in the ionosphere by the absorption of UV radiation

.

(R4.10)

Ozone (O3) is one the most important chemical species in the atmosphere (Figure 4.3), because it absorbs harmful ultraviolet radiation and thus protecting living beings from the lethal effects of this ionizing UV radiation. Ozone absorbs UV light strongly in the region of 220 nm and 330 nm. The absorption causes decomposition of ozone. Life existed only in the oceans early in the planet’s history where the harmful UV radiation was absorbed and scattered by the water. Photosynthesis in ocean plants then converted CO2 and H2O into O2 (R4.4). When the ocean became saturated with oxygen, the amount of oxygen in the atmosphere gradually increased. Gas-phase oxygen was consumed by two mechanisms: (i) new life forms used O2 and sugars for metabolism, and (ii) O3 was formed by the photochemical destruction of molecular oxygen. As concentrations of ozone in the atmosphere began to increase, less UV radiation reached the Earth’s surface. Because the ozone absorbed the DNA damaging (ionizing) radiation, therefore, life could slowly begin to exist on the Earth’s surface (Figure 4.4).

Chemical structure of ozone.

Figure 4.3: Chemical structure of ozone.

Origin of life on the Earth.

Figure 4.4: Life evolution on the Earth.