The hydroxyl radical, OH
, is formed at low altitudes via thereaction of excited oxygen atoms with water:
The Correct Answer and Explanation is:
Correct Answer:
The hydroxyl radical (OH) is formed at low altitudes through the following chemical reaction:
O(¹D) + H₂O → 2OH
Explanation :
The hydroxyl radical, written as OH, plays a critical role in atmospheric chemistry, especially in the lower atmosphere or troposphere. It is often called the “detergent” of the atmosphere because it helps break down pollutants and greenhouse gases like methane and carbon monoxide.
This radical is formed when an excited oxygen atom, symbolized as O(¹D), reacts with water vapor (H₂O). The oxygen atom in the excited singlet state (O(¹D)) is usually produced from the photodissociation of ozone (O₃). When high-energy ultraviolet (UV) sunlight strikes an ozone molecule, it can split into an oxygen molecule (O₂) and an excited oxygen atom:
O₃ + UV light → O₂ + O(¹D)
This excited oxygen atom is highly reactive. When it encounters a water vapor molecule in the atmosphere, it reacts quickly to produce two hydroxyl radicals:
O(¹D) + H₂O → 2OH
This reaction is a major natural source of OH radicals at low altitudes, particularly in moist environments where water vapor is abundant. The hydroxyl radical is extremely reactive and short-lived. It initiates many chemical reactions in the air, especially the degradation of volatile organic compounds (VOCs) and some greenhouse gases. These reactions often produce more radicals and influence the formation of ozone in the lower atmosphere.
Even though the hydroxyl radical exists in very low concentrations, its impact on atmospheric composition and air quality is significant. By helping to remove harmful substances from the air, OH radicals contribute to maintaining the chemical balance in the atmosphere and controlling the lifespan of many pollutants.
