# 4.4: Stratospheric Ozone Formation

Ozone is ozone no matter where it is in the atmosphere. Good ozone is good only because it is in the stratosphere where we cannot breathe it (Figure $$\PageIndex{1}$$). Bad ozone also absorbs solar ultraviolet light, but it is down near Earth's surface where we can breathe it. For UV protection, we are interested in the total number of ozone molecules between us and the Sun. 90% of ozone molecules are in the stratosphere and 10% are in the troposphere - some down near Earth's surface where we can breathe them. There are important issues affecting human and ecological health for both good ozone and bad ozone. For good ozone, the most important issues are the reduction of ozone globally, the Antarctic Ozone Hole, and Arctic ozone loss that is caused by chlorofluorocarbons. Reduced ozone means more solar UV gets to the ground causing more skin cancer. For bad ozone, the most important issues include the production of too much ozone in cities and nearby regions that is caused by too many pollutants from traffic, industrial processes, power generation, and other human activities. Increased ozone means more people have respiratory and heart problems. Let's look at both the good and the bad, starting with the stratospheric ozone.

Figure $$\PageIndex{1}$$: A cross-section of the typical vertical ozone profile for the tropics. Credit: World Meteorological Organization

Click for a text description of the Cross Section image.

Explanation diagram of the typical vertical ozone profile for the tropics

Stratospheric Ozone (the ozone layer, ~15-35 km) contains 90% of atmospheric ozone beneficial role: acts as primary UV radiation shield Current issues

• long-term global downward trends
• springtime Antarctic ozone hole each year
• springtime arctic ozone losses in several recent years
• episodes of high surface ozone in urban and rural areas

Tropospheric Ozone (0-15 km) contains 10% of atmospheric ozone harmful impact: toxic effects on humans and vegetation current issues

To get the total amount of ozone between us and the Sun, we simply add up the ozone amount starting at the surface and going up to the top of the ozone layer. Note how much more ozone there is in the stratosphere. At higher latitudes, the bottom of the stratospheric ozone layer is at approximately 10-12 km. Recall the following image from Lesson 2:

Potential temperature (solid lines, K) as a function of latitude and altitude. Note that the decrease in potential temperature with height is small in the troposphere and large in the stratosphere. Credit: W. Brune, after Andews, Holton, and Leovy

The process of stratospheric ozone formation starts with ozone (O3), which is made by ultraviolet sunlight in the stratosphere (but not the troposphere, as we shall see). The two reactions are:

$O_{2}+\operatorname{hard} U V \rightarrow O+O$

$O+O_{2}+N_{2} \rightarrow O_{3}+N_{2}$

Note that N2 doesn't really react in this last chemical equation, but instead, simply bumps into the O3 molecule as it is being formed and stabilizes it by removing some of the energy from O3. We call O3 an oxidant because it can react with some compounds and oxidize them.

This O3 can be broken apart by ultraviolet light to make O2 and O. Usually O combines with O2 to form Oin this way: O+O2+N2→O3+N2, so nothing really happens, except that the solar energy that breaks apart the O2 ends up as extra energy for the O3 and for the colliding N2 and, as a result, ends up warming the air. Sometimes O collides with O3 and reacts: O+O3→O2+O2. Putting all of the reactions together, we can see the chemical lifecycle of ozone in the stratosphere. This set of reactions was proposed in the 1930s by Chapman:

Stratospheric Ozone and Atomic Oxygen Production, Cycling, and Loss
O2 + hard UV → O + O production
2(O + O2 + N→ O+ N2) cycling
O3 + UV → O2 + O cycling
O + O→ O+ O2 loss
Net: UV → higher T

These four reactions could produce the basic characteristics of the ozone layer as it was in the 1940s through the 1970s. However, this theory produced peak ozone levels that were 50 milli-Pascals, not the 25-30 milli-Pascals seen in the first figure above. Thus, the measured levels of stratospheric ozone were about half of those predicted by Chapman's theory - it was a real puzzle. However, in the 1970s, scientists proposed new sets of reactions by other gases that accomplished the same results as the loss reaction shown above. A famous example involved chlorine, which comes mostly from human-made chlorofluorocarbons (CFCs):

The Stratospheric Chlorine Catalytic Cycles That Destroy Ozone
CFCs + UV → product + Cl production
Cl + O→ ClO + O2 cycling
ClO + O → Cl + O2 cycling
Cl + CH→ HCl + CH3 loss
Net O3+O: O3 + O → O2 + O2

During the cycle, chlorine (Cl) and chlorine monoxide (ClO) aren’t destroyed but instead are just recycled into each other. With each cycle, two ozone molecules are lost (one directly and a second because O almost always reacts with O2 to form O3). This cycle can run for hundreds of thousands of times before Cl gets tied up in HCl. So ClO and Cl levels of tens of parts per trillion of air (10-12) are able to destroy several percent of the few parts per million of O3. Sherry Rowland and Mario Molina figured this cycle out and wrote a paper about it in 1974. They received a Nobel Prize in Chemistry in 1995 for this work. When catalytic cycles involving chlorine, nitrogen oxides, and OH are included with the theory, the agreement between the theory and the measurements gets much better.

The chlorine catalytic cycle that destroys ozone. Credit: UCAR

Note that the total ozone amount at midlatitudes is greater than the amount in the tropics. This should seem strange to you because the solar UV that is part of the Chapman mechanism is strongest in the tropics. Why do you think that total ozone is distributed this way?