Ozone and the Stratosphere

Last updated on September 16, 2023

Ozone and the Stratosphere: A Deeper Dive

The stratosphere, extending from approximately 15 km (9 miles) to the stratopause at 50 km (31 miles), plays a crucial role in our atmosphere. As we ascend beyond the tropopause, the boundary between the troposphere and stratosphere, temperatures begin to rise once again. Within this region, a fascinating process involving ozone takes place, impacting both the Earth's climate and the beauty of our sky.

In the upper stratosphere, oxygen molecules (O2) absorb short wavelength ultraviolet radiation, causing them to dissociate into highly reactive oxygen atoms. These atoms then diffuse through the stratosphere and at heights of mainly 30-50 km, they combine with additional oxygen molecules to form ozone (O3). Ozone acts as a potent absorber of longer wavelength UV radiation (200-340 nm), effectively shielding the Earth from harmful ultraviolet rays.

The presence of ozone in the stratosphere is responsible for the increasing temperature observed with height. Without this crucial molecule, the mixing between the troposphere and stratosphere would occur at a much faster rate, leading to a significantly different atmospheric structure. The ozone layer not only helps regulate the temperature of the stratosphere but also prevents harmful UV radiation from reaching the Earth's surface.

The process of ozone formation in the stratosphere is relatively slow due to the rarity of two-body collisions in this region. For ozone to form, a collision involving a third body is required to remove excess energy and momentum. This third body, denoted as 'M,' can be either a nitrogen or another oxygen molecule. Given the rarified conditions of the stratosphere, two-body collisions are infrequent, making three-body collisions even more so. As a result, the formation of ozone takes time.

Interestingly, the maximum density of ozone is found at an altitude of approximately 20-25 km. This is because the destruction processes of ozone occur at a slower rate in this region. Ozone is destroyed through various catalytic chain reactions involving oxygen molecules (O2), nitrogen compounds (NO), and hydroperoxyl radicals (HO2). Additionally, man-made chlorine, bromine, and nitrogen compounds act as potent catalysts for ozone destruction.

The presence of ozone in the stratosphere not only protects us from harmful UV radiation but also contributes to the mesmerizing colors observed during twilight. When light from the Sun passes through the Earth's atmosphere, it undergoes scattering, resulting in the scattering of shorter wavelengths of light (blue and violet) more than longer wavelengths (red and orange). This selective scattering, combined with the absorption of red light by ozone, gives rise to the deep blue-violet hues that grace our twilight sky.

In conclusion, the intricate relationship between ozone and the stratosphere is a vital component of our atmosphere. The formation and destruction of ozone molecules play a critical role in maintaining the structure of our atmosphere and protecting us from harmful UV radiation. Understanding the processes occurring within the stratosphere not only deepens our knowledge of atmospheric optics but also highlights the interconnectedness of Earth's systems.

Total lunar eclipse March 3, '07 imaged in Germany by Eva Seidenfaden (atmospheric optics site) . The blue at the edge of the Earth's umbral shadow is produced by ozone in Earth's stratosphere. In addition to being a strong ultraviolet absorber, ozone absorbs red light. The upper part of the moon is illuminated by refracted rays that have made a long slanting passage through our stratosphere. The reds towards the umbra centre are from light refracted through the denser troposphere. Image ©Eva seidenfaden, shown with permission.

As we climb beyond the tropopause, temperatures start to increase again. We are now in the stratosphere, a region extending from a nominal 15 km (9 mile) up to the stratopause at 50 km (31 mile).Oxygen molecules, O2, iIn the upper stratosphere absorb short wavelength ultraviolet radiation (<200 nm) and dissociate into highly reactive oxygen atoms*. The atoms diffuse through the stratosphere and at heights of mostly 30-50 km many eventually combine** with more oxygen molecules to produce the reactive oxygen allotrope, ozone.Ozone, O3 ,is a strong absorber of longer wavelength (200-340 nm) UV radiation and the absorbed energy heats the atmosphere. The ozone layer is responsible for the stratosphere's increasing temperature with height***. Without ozone, mixing between the troposphere and stratosphere would be much faster and the structure of our atmosphere quite different.The ozone layer prevents harmful UV from reaching the earth’s surface and is partly responsible for the deep blue-violet beauty of the twilight sky.* Oxygen atoms have quite different properties to oxygen molecules.** The reaction of O atoms with an oxygen molecule requires a collision involving a third body to remove excess energy and momentum otherwise the newly formed ozone molecule would almost immediately decompose. The third body, 'M' in the diagram, can be a nitrogen or another oxygen molecule. In the rarified conditions of the stratosphere two body collisions are infrequent and three body ones even more so. Ozone formation is therefore slow.*** The temperature maximum is at ~50km where the low density air requires very little energy to raise its temperature. The greatest ozone densityis at 20-25km because the ozone destruction processes are slower there. Ozone is destroyed by several catalytic chain reactions involving O2, NO and, at lower altitudes, HO2 radicals. Man-made chlorine, bromine and nitrogen compounds are also potent ozone destruction catalysts.

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