Which of the following in the stratosphere screens out a large fraction of ultraviolet rays from the Sun? A) CO₂, B) O₃, C) O₂, D) Cl₂, or E) NO₂.
The stratosphere is a region of the atmosphere. It lies between the troposphere and the mesosphere. The stratosphere begins at higher altitudes closer to the equator and lower altitudes close to the poles, between seven and 20 kilometres up. The stratosphere is warmer than the air below because certain gases in the stratosphere absorb extra radiation from the Sun.
Our job is to identify which of the five candidates is responsible for screening out ultraviolet rays in the stratosphere. Our candidates are CO₂, carbon dioxide; O₃, ozone; O₂, often simply called oxygen but otherwise called dioxygen; Cl₂, which is chlorine; and NO₂, nitrogen dioxide. What should jump out at this point is ozone, which gives its name to the ozone layer.
The ozone layer is a part of the stratosphere, with a relatively high concentration of ozone. The ozone layer absorbs nearly all of the ultraviolet radiation from the Sun, helping to protect plant and animal life from dangerous radiation. And our protector is the magical molecule ozone. Carbon dioxide and oxygen are present in meaningful quantities in the atmosphere, but neither absorbs ultraviolet light. And while chlorine gas does absorb ultraviolet light, as we know, in the synthesis of haloalkanes, it’s not present in the atmosphere. And lastly, nitrogen dioxide, while it does absorb UV light, actually can make ozone when mixed with oxygen. And it’s more commonly associated with acid rain because it reacts with water to produce mixtures of nitric and nitrous acid.
Therefore, of the five substances we’ve been given, the one that in the stratosphere screens out a large fraction of ultraviolet rays from the Sun is O₃.