Ozone Depletion

When someone talks about ozone depletion, the person is usually referring to ozone reduction in the stratosphere (about 20km to 50km above the ground).

What is Ozone

Ozone (O3) is a gas molecule that is made up of 3 oxgyen atoms.

Unknown to many, ozone has many effects, including “good” ones and “bad” ones, under different circumstances.

In the stratosphere, the naturally occurring ozone gas is “good”, as it plays an important role in shielding the earth and the organisms living on it from the harmful effects of ultraviolet B rays emitted from the sun. The harmful effects of these radiations are described in greater details below.

Stratospheric ozone is formed when ultraviolet radiation from the sun breaks down an oxygen molecule (O2) to form two oxygen atoms. One of these highly reactive oxygen atom (O) then combines with another oxygen molecule (O2) to form an ozone molecule (O3). In this way, stratospheric ozone is continuously being formed in the presence of sunlight. At the same time, ozone is continuously being “destroyed” as it interacts chemically with other chemicals in the stratosphere in the presence of sunlight. Subsequently, an equilibrium in the amount of ozone is achieved in the stratosphere.

On the contrary, at earth’s lower atmosphere (where you and I live), ozone that is formed (also known as ground-level ozone) is “bad”, as it is the main component in smog, and is corrosive. Ground-level ozone is formed when automobile-industry-produced carbon monoxide (CO) gas, nitrogen oxide (NOx) gases and gaseous volatile organic compounds (VOCs; e.g. xylene) interact chemically in the presence of sunlight, or high-voltage electric arc (e.g. spark plugs).

In this article, we will be focusing on the “good” ozone in the stratosphere and its depletion.

Cause of ozone depletion

Ozone depletion takes place when the natural balance between the production and destruction of stratospheric ozone tilts towards greater destruction than production.

With ozone depletion, the density of ozone for an entire column above a point on the earth is significantly reduced, especially in the lower stratosphere. This thinning in the ozone layer at the lower stratosphere level (the upper stratosphere level is less affected) probably gives rise to the notion of a “hole” in the ozone layer.

The creation of the first Chloroflourocarbons (CFCs) in 1928 was one of the major reasons for the creation of the ozone hole in the recent history of mankind. First used as a non-toxic, non-flamable refrigerant, CFC was soon also used as a propellant for aerosol sprays and in the cleaning of delicate electronic items. In fact, according to the NASA Advanced Supercomputing Division (NAS), the worldwide consumption of CFCs reached over a billion kilograms in 1988.

These CFC compounds are highly stable. When released into the air, they take about 5 to 7 years to reach the stratosphere from the ground, and are not washed back to Earth by rain. Because they are highly non-reactive, they could stay in the upper atmosphere for as long as a century.

Under intense ultraviolet radiation from the sun in the stratosphere, chlorine (Cl) atoms are dissociated from the CFC compounds. These chlorine atoms act as a catalyst for the break down of ozone (O3) molecules into oxygen (O2) molecules. The process can be represented via the following chemical equation:

Cl + O3 ---> ClO + O2 ClO + O3 ---> 2 O2 + Cl.

As a catalyst, the chlorine atoms are not being consumed in the process (as you can see in the equation above). Instead, they remain in the higher atmosphere for a long time and have the capability to break down hundreds of thousands of ozone molecules each, before they finally combine with other substances to form less reactive compounds (eg. HCl) and get removed from the stratosphere by rain.

The reactions of Cl and ozone is enhanced in the presence of polar stratospheric clouds (PSCs) that form in extreme cold conditions, such as over Antarctic during its winters and springs. The PSCs enhance the formation of the reactive Cl radicals from the halocarbons in the stratosphere. Nonetheless, it is during the springs, in the presence of sunlight, that the greatest rates of ozone depletion take place.

According to NAS, the ability of CFC to break down ozone was documented in a laboratory study report in 1974 by Molina M.J. and Rowland F.S. Further studies followed. The demonstration that 7% of the ozone layer would be destroyed within sixty years motivated the US to ban CFCs in aerosol sprays in 1978. This ban caused CFC used to drop in the late 1970s. However, CFC was still being used for other purposes in US and in other parts of the world. And in fact, according to the report “The Evolution of Policy Responses to Stratospheric Ozone Depletion” by Peter M. Morrisette, on-aerosol use of CFCs in the mid-1980s rose again to as high as the mid-1970 levels.

It wasn’t until the late 1980s before the situation took a turn for the better. In 1985, twenty countries (including most of the major CFC producers) signed the Vienna Convention for the Protection of the Ozone Layer. Subsequently, a framework was put in place for the international community to come together to draw up regulations on ozone-depleting substances (ODS). The Antarctic ozone “hole” was discovered in the same year, reviving public interest and pressure on the governments on the issue of ozone depletion.

In 1987, 43 countries signed the Montreal Protocol (that precedes international treaties like the Kyoto Protocol) to limit CFC production. This protocol was revised several times, with the latest revision being made in Copenhagan in November 1992. In this most recent revision, CFCs and related halocarbons were to be phased out by 2030. The sharing of technologies and expertise was mandated so that countries could speed up the replacement and disposal of CFCs from their land. Signed by more than 100 countries (which consume about 95% of the world’s CFCs), the protocol also imposed trade sanctions on countries not signing the protocol for CFCs, Halons and products containing these substances.

During this period, a major study showed that global ozone had dropped by 5.5% between 1969 to 1993 (based on NAS references, Science: 260, 1993).

Today, the amount of chlorine in the atmosphere is falling, according to National Geographic. However, scientists estimate that it won’t be another fifty years before the level of chlorine in the atmosphere returns to its natural pre-1928 levels (assuming no more chlorine is added to the atmosphere for the next 50 years).

Other factors affecting stratospheric ozone levels

Besides the presence of chlorine from man-made halocarbons in the stratosphere, there are other natural factors that can influence ozone levels in the stratosphere.

Natural events like volcanic eruptions can introduce substantial amounts of chlorine into the atmosphere, causing localized ozone depletion in the ozone layer directly above the eruption. Nonetheless, such natural cause of ozone depletion is not anywhere near the impact man has made on the stratospheric ozone. The effect of even a large volcanic eruption on global ozone levels is usually modest -- at less than three percent, and last for at most 3 years.

Because ozone is created in the stratosphere in the presence of ultraviolet radiation from the sun, changes in the sun’s activities (e.g. sunspots, solar storms, etc) can also alter the balance between the production and destruction of ozone molecules in the stratosphere.

Tropical winds in the lower stratosphere can alter ozone levels in a particular area as it moves the ozone through the atmosphere. Nonetheless, ozone is neither destroyed nor created in the process. The global ozone level remains the same.

The link between ozone depletion and global warming

Ozone depletion and global warming are two separate phenomenon, but they are also related.

According to NASA Goddard Institute for Space Studies (GISS), while global warming has been associated with the warming of the Earth’s surface, it has been associated with the cooling of the upper stratosphere. In turn, a cooler stratosphere promotes the formation of polar stratospheric clouds, which enhances the ozone destructive effect of free Cl radicals in the stratosphere. In other words, the rate of ozone depletion would be increased in the presence of greenhouse gases in the troposphere.

In turn, with ozone depletion, there is cooling in the stratosphere, because there is less ozone to absorb the sun’s ultraviolet (UV) rays and the infrared rays coming from the troposphere. This cooling in turn aggravates the ozone reduction problem, as lower temperatures promote the formation of polar stratospheric clouds.

As for the troposphere (lowest layer of the Earth’s atmosphere where we live in), ozone depletion has two opposing effects. The reduced levels of ozone increases the warming of the troposphere as a result of greater exposure to the sun’s UV rays now that the protective stratospheric ozone shield is reduced. Increased UV in the troposphere also enhances the formation of tropospheric ozone, which in turns acts like a greenhouse gas that contributes to warming of the troposphere. Yet at the same time, the cooler stratosphere from the reduced ozone levels emits less long-wave radiation onto the troposphere, reducing the heating of the latter. According to the IPCC, the observed outcome of the two opposing effects of ozone depletion is the overall slight cooling of the troposphere.

Effects of ozone depletion

The ozone layer in the stratosphere absorbs more than 97% of the sun’s ultraviolet rays and prevents the harmful rays from reaching the troposphere, where most of the planet’s living things inhibit. With ozone depletion, the protective shield we have in our stratosphere is reduced, and the earth’s surface is exposed to more ultraviolet radiation.

In turn, increased exposure to ultraviolet rays has been associated with increased incidences of skin cancer and cataracts, as well as incidences of mutation in both humans and animals. According to GreenPeace, the radiation has also been found to be capable of disrupting the mechanisms of photosynthesis in plants.

According to National Geographic, excessive UV rays may also negatively affect the reproduction of phytoplankton, as such affecting the population of other organisms that depend on the phytoplankton for food.

As mentioned above, the presence of UV rays in the troposphere also provides the energy for the creation of ozone at the lower atmosphere, and this tropospheric ozone in turn contributes to smog – a health hazard in cities.

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