Ozone: When is is good and when is it bad?
When most people think of ozone they think of the large layer of ozone around the Earth that has a hole in it. This is correct and that is ozone. This is the good ozone. There is also bad ozone, found at the Earth's surface. However, both have the same chemical composition and structure because they are actually the same thing. The key differences that make them good and bad is the location in which they are found and how they are produced.
The Good Ozone
Good ozone exists in the stratosphere. This is the atmospheric layer directly above the one we live in here at the surface. It ranges from about 10 km above the surface to 50 km above the surface and it benefits all life on Earth in 2 significant ways.
- It absorbs much of the incoming ultraviolet radiation and prevents it from reaching the surface
- During this absorption it produces and holds heat in our atmosphere
Without both of these vital functions, life could not exist on Earth. Excessive ultraviolet radiation (especially UV-B) is very damaging to plants and animals. In humans it is strongly associated with incidences of skin cancer. Without the warming provided by the ozone layer, the Earth would be too cold to sustain plant and animal life. It would be uninhabitable and barren.
This good ozone is very good ozone indeed. However, we have not always taken great care of it.
The Hole in the Ozone Layer
The hole in the ozone layer is a widespread, acute thinning of the ozone layer over Antarctica. It is the worst in the springtime with slight recoveries, or less depletion, other times of year. The main cause of the thinning has been attributed to chlorofluorocarbons (CFC). CFCs were used around the world for a several purposes including refrigerants, blowing agents, and propellants. They were not very toxic or reactive, so they made good candidates for all these applications.
What the world didn't know at the time was how these compounds would react in the upper atmosphere, if released. And they would frequently be released to the atmosphere.
If we look at one CFC in particular, it can tell us a lot about how the destruction of ozone occurs. CFC-11, also known as trichlorofluoromethane has the chemical formula CCl3F. What is important here is the carbon-chlorine bond and the fact that there are three of them. When this CFC reaches the upper atmosphere, the strength of the radiation hitting it increases and it can much more easily break off one of the chlorine atoms. At this point there are two compounds: CCl2F and Cl. This free chlorine atom is called a radical and that will be important in a minute.
In the stratosphere, ozone is destroyed all the time. Ultraviolet radiation strikes ozone atoms, photolyzing them, and one of the oxygen atoms breaks off. This leaves O2 and O. These molecules can reform in a subsequent reactions as well. Given the relative quantities of both and relative rates of reactions, the ozone layer is usually in a steady-state equilibrium.
However, if we introduce CFCs into the upper atmosphere, we run into problems. By nature, there are more O2 molecules than O molecules. O molecules are more reactive and quickly react with other substances around them. So, if CFCs are now present in abunance and photolyze, producing free Cl molecules, they can readily react with the free O molecules. This produces ClO. This isn't a particularly bad product, not at all. But it isn't very reactive. It won't just be photolyzed, freeing both molecules. This means much of the free oxygen that would form ozone is trapped as ClO.
As ozone is naturally destroyed, and the free oxygen atoms are trapped, it can't be replenished as quickly. Thus, we get a thinning of the ozone layer, all thanks to a few chemicals we thought were pretty useful.
The reason the hole is centered over the south pole, where there isn't really any pollution, is due to global circulation patterns. Much of the emissions circulate in the troposphere to the equator where they move up to the stratosphere and towards the poles. The consistently low temperatures over Antarctica also helped the process occur and become so out of hand.
The Bad Ozone
The bad ozone exists in the troposphere, the layer of the atmosphere at the surface that we all live in. Tropospheric ozone is harmful to plants, animals (including humans), rubbers, textiles, and other materials of a similar nature. Ozone may damage living tissue causing irreparable harm. It may cause plants to close their stomata and interfere with photosynthesis. In humans it may cause severe problems to the respiratory system as well as other suspected long term effects. It may also weaken rubbers and elastic materials, damaging their cellular structure.
While ozone in the stratosphere is naturally occurring and helpful, ozone in the troposphere is anthropogenic and harmful. It is created when ultraviolet light reacts with hydrocarbons and oxides of nitrogen. These are emitted by cars, factories, and other sources of fossil fuel combustion. Urban areas are particularly vulnerable because of increased traffic and congestion.
The reactions have multiple, complex steps that work in a loop. However, it all boils down to a few reactions.
With the presence of an oxide of nitrogen, NO2 for example, the following reaction may occur.
NO2 + hv --> NO + O
O + O2 + M --> O3
The hv just means light is present to photolyze the molecule and the M is a placeholder because it can be a variety of compounds that assist in the reaction.
There are also several reactions that involve hydrocarbons reacting with nitrogen and producing the compounds that will ultimately produce the above reaction. This ozone can be destroyed by photolyzation as well, but given an abundance of nitrogen oxides and hydrocarbons the concentrations tends to remain high.
This is the bad ozone, and this is how it happens. It is at the surface, directly harming us and directly caused by us.
Smog, Not the Best Term
Smog is an old term that was created mixing the words 'smoke' and 'fog'. While smog may appear to be either of these things, or a combination of them, it is not. Smog, or more specifically photochemical smog, is actually just suspended particulate matter and tropospheric ozone. The particulate matter may be from automobiles, factories, dust in the wind, or the reactions that formed the ozone. It looks hazy because it is masking and refracting the light, like smoke or fog would.
So Now What?
Now I have hopefully explained the difference between the two types of ozone and how they both interact in our environment. Possibly you have also noticed that we humans directly influence them both in significant ways. We produce chemicals that destroy the good ozone and burn fossil fuels, carelessly fueling production of the bad ozone. This combination makes for a pretty serious problem.
The good news is that since the Montreal Protocol CFCs have pretty much been banned and done away with globally. The ozone layer is starting to replenish slowly. There are still chemicals that harm it that we still produce as well as those we produced years ago that are still persisting, but the outlook is good if the right measures are enforced to aid recovery.
The bad news is that tropospheric ozone levels are increasing as well which can spell disaster for urban populations. As population increases automobiles, industry, and power generation do as well. This all contributes to more of the ingredients needed in the troposphere to make ozone. Stricter regulations are needed to minimize emissions and improve air quality in cities all around the world. The costs to human health, and the more quantifiable healthcare costs to society, could be quite significant with little to no action by governments, industries, and the people as stakeholders and consumers.
Increase stratopsheric ozone, decrease tropospheric ozone. Use public transit and install solar panels everywhere!