How does the ozone layer work?

The ozone layer is an area of ​​the upper part of the stratosphere containing some levels of ozone (O3) higher than the rest of the atmosphere. While the average concentration of ozone in the atmosphere is 0.3 ppm, in the ozone layer can reach 10 ppmwhich makes it contain 90% of all atmospheric ozone.

In addition to ozone, the stratosphere also contains high amounts of gaseous oxygen (Otwo), water vapor and other inorganic gases, mainly nitrogen oxides, halogen oxides, colorofluorocarbons (CFCs), methane and sulfur gases, all of them long-lived gases from the troposphere.

The radiation that reaches us from the Sun covers a wide range of wavelengths. In addition to visible light, it also includes infrared radiation and ultraviolet radiation (UV). Although ultraviolet radiation is necessary for many biological processes, for example humans use it to synthesize vitamin D, it is also very harmful.

If the Earth's surface received the total UV radiation that reaches the planet, most living things would probably die. Fortunately, the ozone layer acts as a screen and filters most of the UV radiation.

UV radiation is divided into several types based on its wavelength. The most harmful the extreme ultraviolet and UV-Care almost completely absorbed by the atmosphere:

  1. GRAPE: 315 – 400nm. It is not absorbed into the ozone layer
  2. UV-B: 280 – 315nm. It is absorbed by ozone in large quantity.
  3. UV-C: 100 – 280nm. It is absorbed almost entirely by molecular oxygen and ozone.
  4. extreme ultraviolet: 10 – 100nm. It is absorbed almost entirely by atmospheric nitrogen.

Ozone-oxygen cycle and UV protection

The conditions that exist in the upper part of the stratosphere make it possible for a cyclic process of ozone-oxygen interconversion that absorbs much of the UV radiation. In this process we can distinguish three stages:

  1. ozone creation
  2. Ozone-oxygen cycle
  3. ozone removal

ozone creation

In the upper part of the stratosphere, ultraviolet radiation from the Sun breaks down molecular oxygen molecules (photolysis) and two separate oxygen atoms are obtained. Atomic oxygen is highly reactive and readily binds to molecular oxygen molecules (Otwo) to form ozone.

EITHERtwo + ℎνuv → 2 O

O + Otwo → or3

Ozone-oxygen cycle

The O molecules3 they are quite unstable and are quickly broken down by ultraviolet radiation back into atomic oxygen and Otwo:

EITHER3 + ℎνuv → ortwo + O

The atomic oxygen produced reacts rapidly with a molecule of Otwo to produce again or3:

O + Otwo → OR3 + ANDK

where EK is the excess of energy released as heat. Due to the instability of O3 and the reactivity of atomic oxygen, the cycle is much faster that the stages of creation and withdrawal of O3. This maintains the ozone layer in stable conditions and is capable of protecting the earth's surface of most UV radiation and warm the stratosphere.

ozone removal

Ozone removal occurs through the formation of molecular oxygen, which can occur in several ways:

EITHER3 + O → 2 Otwo

Y

2 O → Otwo

The concentration of ozone in the stratosphere depends on the rate of creation by ultraviolet radiation and the rate of removal. Since the concentration of atomic oxygen is very low, the withdrawal is a very slow process, which allows the high concentrations of ozone found in the stratosphere to be reached.

Some free radicals, such as hydroxyl (OH), nitric oxide (NO), and halogens such as chlorine (Cl) or bromine (Br), catalyze the reaction and can increase withdrawal rate.

Hydroxyl and nitric oxide are naturally present in the stratosphere but some human activities can increase their concentration. One of the worst pollutants for the ozone layer are halogenated hydrocarbons, such as chlorofluorocarbons (CFCs)which also catalyze the withdrawal reaction and are one of the main causes of the depletion of the ozone layer.

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