Ozone:

Environmentalists are concerned about the hole in the ozone layer. At the same time, ozone is suspected of being hazardous to human health, and many cities rate the ozone level each day in order to keep citizens informed about whether, for example, they should run on a treadmill in the gym instead of running outside in the afternoon, especially for those with asthma. All in all, this little O₃ molecule has sparked quite a bit of controversy.

So is ozone good, bad, or neither? Well the answer is....

It depends on where you're talking about.

More specifically, "where" means the distance away from the earth.

(This node is written from a scientific/chemistry background. I'll put in the reactions, but just skip over them if you're not interested - it will be understandable without them.)

Regions of the earth's lower atmosphere

The earth's lower atmosphere can be divided into two sections. The 10-16 km or so closer to the earth is called the troposphere, and the region farther away from the earth is called the stratosphere. This diagram shows a side view of the layers of the atmosphere and the earth's surface:

upper atmosphere

------------------------------------------------------------------------ even farther away from earth's surface

stratosphere, more than 10-16 km from earth's surface

------------------------------------------------------------------------ 10-16 km altitude

troposphere, closer to earth's surface

__________________________________________ surface of earth
earth, people, E2, etc

Quick answer: Ozone in the troposphere is bad because it is detrimental to human health. Ozone in the stratosphere is good because the layer of ozone located there blocks harmful uv rays from the sun. It is very, very important to remember that the stratosphere and the troposphere do not mix, so these regions can be treated as completely separate systems.

1. Ozone in the stratosphere

Oxygen in the stratosphere undergoes reaction via the Chapman Mechanism:

1) O₂ + sunlight --> 2O

2) O + O₂ + M --> O₃ + M (M is a third body; it can be anything, even other O₃.)

3) O₃ + sunlight --> O + O₂

4) O₃ + O --> 2O₂

Reaction 4, which breaks down ozone, is quite likely to occur through a catalytic free radical mechanism:

4a) X + O₃ --> XO + O₂

4b) XO + O --> X + O₂

X can be any free radical catalyst, e.g., OH•, NO, Cl•, Br•, ClO•, BrO•, etc. (If you don't know what a free radical is, there's a great write-up about it, but for this explanation, free radicals are typically very reactive chemical species.)

Nitrogen oxides help to prevent the destruction of ozone by scavenging, or reacting with, these harmful free radicals. With the free radicals used up in other reactions, they're no longer available for destroying the ozone layer.

So what's the big deal about aerosol cans?

Aerosol cans can contain CFCs that can eventually produce halide (Cl•, Br•, etc.) radicals.

These reactions take place all over the world, but ultimately the hole in the ozone layer occurs only at the poles. Why?

Let's say you're at a mid-latitude type of location, for example, in St. Louis. In St. Louis, even though winter is cold, sunlight (or a lame excuse for a little sunlight) makes the atmosphere relatively warm during the day. So the reactions listed above can continue to occur, re-producing ozone at about the same rate that it is depleted. No problem.

The weather behaves a little differently in Antarctica. Darkness falls over the continent for half of the year. In the meantime, winds carry pollutants from the rest of the world to Antarctica. NO_y compounds (nitrogen and oxygen-containing compounds) need to react with these pollutants to prevent the destruction of the ozone layer. However, the temperature drops much lower in Antarctica than in St. Louis! In Antarctica, clouds called polar stratospheric clouds (PSCs) form. This doesn't happen in St. Louis because it's too warm. Basically, PSCs are clouds that have frozen.

But before these giant ice configurations finish freezing, almost all of the NO_y is transferred into the water vapor that's in the air. Without NO_y present, (reaction) can no longer take place to lead to the replenishing of ozone.

One example of a possible reaction cycle is:

5) ClO• + NO₂ + M --> ClONO₂ + M

6) ClONO₂ + HCl --> Cl₂ (gas that remains in the atmosphere) + HNO₃ (freezes in clouds)

When the sun comes out in the spring, the sunlight can react with chlorine gas in the atmosphere to produce free radicals.

7) Cl₂ + sunlight --> 2Cl• - The product is a free radical that can destroy ozone!

When the spring comes, it's a race: UV rays from sunlight will start producing chlorine radicals, but can the warmth from the sunlight melt clouds quickly enough to release nitrogen oxides to scavenge the free radicals?

Ready.

Set.

Go!

--------------------------------------------------------------------------------------| Finish

--------------------------------------------------------------------------------------| UV rays

----------| PSC melting

(not to scale)

Ok, so the UV rays won.

Antarctica is a giant cold land mass that keeps the air above it very cold. The clouds don't melt fast enough to release the NO_y back into the atmosphere.

Now, chemicals in the air can not react with nitrogen oxides, and as you can see from Reaction 7, this limitation can cause a problem.

Starting around October during the Antarctic spring, the ozone layer at the South Pole develops a hole.

The same phenomenon occurs at the North Pole, but to a much lesser extent. The North Pole has no giant land mass to keep the atmosphere as cold during the spring, so it heats up faster, releasing nitrogen oxides back into the air. So the North Pole has a much smaller ozone layer hole.

2. Ozone in the troposphere

Ozone in the troposphere has a bunch of adverse effects on human health that I'll let someone else write about. Two main byproducts of industry create more ozone in the troposphere: volatile organic compounds (VOCs) and NO_x.

Volatile organic compounds are defined as organic liquids and solids which have relatively high vapor pressures at room temperature (typically > 0.0007 atm).

NO_x compounds are compounds that contain nitrogen and oxygen only. (e.g. NO, NO₂, etc)

VOCs and NO_x in different combinations can affect the ozone in the troposphere differently. Reducing VOC emmisions never hurts in improving ozone standards, but reducing NO_x without reducing VOCs can actually raise the ozone level in a few cases.

Thus a good control strategy for reducing tropospheric ozone will involve the proper combination of reductions in NO_x and VOCs.

From information from more specific research on combinations of NO_x and VOC levels, scientists have compiled charts with lines called isopleths that indicate levels of ozone. These isopleth charts, which are a type of contour map, can verify the potential for an ozone increase if only NO_x reductions take place, depending on the initial state of the system.

Source:

Turner, Jay, Ph.D. Environmental Chemistry (Chemical Engineering 443) lecture, Washington University, dates November 3, 10, 12, 19, 24, December 1, 2003