The upper layers of the Earth’s atmosphere, including the thermosphere and exosphere, extend beyond the mesosphere. They contain sparse air molecules and play a crucial role in phenomena like auroras.
Gas A, found in the upper layers of the atmosphere, is a deadly poison but is essential for all living beings. The amount of this gas started declining sharply in the 1980s. a. Identify Gas A. How is it formed at higher levels of the atmosphere? b. Why is it essential for all living beings? State the cause for the depletion of this gas.
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Gas A: The gas being referred to is likely ozone (O3).
Formation at higher levels of the atmosphere:
Ozone is primarily formed in the stratosphere, the second major atmospheric layer, through the photochemical reaction involving oxygen molecules:
O2 + UVradiation → 2O
O + O2 → O3
In simpler terms, high-energy ultraviolet (UV) radiation from the Sun causes the dissociation of oxygen molecules (O2) into individual oxygen atoms (O). These oxygen atoms then combine with oxygen molecules to form ozone (O3).
b. Importance for all living beings:
Ozone plays a crucial role in protecting life on Earth by absorbing the majority of the Sun’s harmful ultraviolet (UV) radiation. High-energy UV radiation can cause damage to living tissues, including DNA mutations that may lead to skin cancer. Ozone acts as a shield, preventing a significant portion of these harmful UV rays from reaching the Earth’s surface.
Cause for depletion:
The depletion of ozone in the upper atmosphere, particularly in the ozone layer, is mainly attributed to the release of human-made chemicals called ozone-depleting substances (ODS). The most notable of these substances are chlorofluorocarbons (CFCs), halons, carbon tetrachloride, and methyl chloroform.
Once released into the atmosphere, these ODS can reach the stratosphere, where they undergo photochemical reactions that release chlorine and bromine atoms. These atoms catalytically destroy ozone molecules:
Cl + O3 → ClO + O2
ClO + O → Cl + O2
The net result is the depletion of ozone, leading to the formation of the so-called ozone hole, particularly over Antarctica. The decrease in ozone concentration in the upper atmosphere allows more harmful UV radiation to reach the Earth’s surface, posing significant risks to human health and the environment. The 1987 Montreal Protocol is an international agreement aimed at phasing out the production and consumption of ozone-depleting substances to address this issue.