1. The Atmosphere and Ozone

2. Our Atmosphere Like the Skin of an Apple
As large as it seems when you look up, our atmosphere is actually very thin, analogous to the skin of an apple. This means that pollutants may have a very significant effect on global atmospheric chemistry.
Atmospheric pressure falls off approximately exponentially with altitude.  The pressure falls off by a factor of two for every ~5 km (3 miles or ~16,000 feet).  Thus, the pressure is approximately:
1 atm at 0 km
0.5 atm at 5 km
0.25 atm at 10 km
0.125 atm at 25 km
The lowest layer of the atmosphere is called the "troposphere".  The height of the troposphere depends on latitude, but is about 12 km at mid latitudes.  About 90% of all air molecules reside within the troposphere.  The diameter of the Earth is 12,756 km.  Thus, the atmosphere is about 1/1000 of the diameter of the Earth -- or about the same as the skin on an apple.
When one realizes how thin the atmosphere actually is, it is easier to understand why the old saying, "dilution is the solution to pollution", doesn't really apply.  As we shall see, air pollution can be transported long distances and certainly cross international borders.  Acid rain in Nova Scotia, Canada, for example, where there is very little industry, was found to be mostly due to emissions in the northeastern U.S.  And, it has been found that a few ppb of the ozone measured along the western U.S. coast can be attributed to ozone formation in China, nearly half way around the world.

3. Layers of the Atmosphere
Notice the change in temperature in relation to altitude.
The production of ozone causes the temperature
to rise with increasing altitude in the stratosphere.
This diagram shows the layers of the atmosphere and how temperature varies with altitude.  The layers of the atmosphere are defined by inflections in the temperature profile.  Within the troposphere, temperature decreases with increasing altitude.  This decreasing temperature with increasing altitude is the natural trend -- as air rises it expands and thus cools.  But, this cooling is reversed due to the presence of ozone in the stratosphere.  The ozone absorbs UV light from the sun, thereby heating the air molecules and reversing the cooling trend.  At the top of the troposphere, the temperature starts to increase with increasing altitude. This inflection point is the "tropopause".
Throughout the stratosphere, temperature increases with increasing altitude.  Thus, a “temperature inversion” exists with warmer air always lying above colder air.  Because the dense, cold air lies beneath warmer and less dense air, there is no tendency for vertical mixing of air, and we say that the air is "stratified", hence the name "stratosphere".  Because of the temperature inversion, pollutants that make it to high levels in the stratosphere remain there for a very long time.
The ozone concentration begins to drop off near the top of the stratosphere (near 50 km).  Once the ozone concentration is low, the natural trend of decreasing temperature with increasing altitude begins again.  This inflection point (change from increasing temperature with increasing altitude to decreasing temperature with increasing altitude) occurs at the top of the stratosphere, or "stratopause".
At the top of the stratosphere, the atmosphere begins to be heated again due to absorption of x-rays from the sun.  This results in another temperature inversion that defines the thermosphere.  The temperature within the themosphere can be thousands of degrees during solar flares.  The high temperature results because there are very few molecules in this region to absorb the x-rays (i.e., the heat capacity is very low).
The exosphere is everywhere above the Earth's atmosphere.
Air pollutants can affect the chemistry of the troposphere and the stratosphere.  No harmful effects of air pollutants at elevations above the stratosphere have been identified. 

4. The Difference Between Stratospheric and Tropospheric Ozone
Too much ozone here… Cars, trucks, power plants and industry all emit air pollution that forms ground-level ozone. Ozone is a primary component of smog.
The GO3 Project is primarily concerned with ground-level ozone, i.e., ozone in the troposphere.  Ozone at ground level is considered "bad ozone" because it is in contact with the biosphere where it is damaging to human health, crops and natural ecosystems.
Note that on this and the next two slides the ozone layer is shown as a thin layer on top of the stratosphere.  Actually, ozone is distributed throughout the stratosphere and peaks near the middle of the stratosphere.

5. The Difference Between Stratospheric and Tropospheric Ozone
Too little there… In the past, many popular consumer products like air conditioners, refrigerators and aerosol propellants made use of CFCs. Over time, these chemicals have damaged the Earth’s protective ozone layer.
Ozone in the stratosphere is considered to be "good ozone" because it protects the Earth from most of the sun's harmful UV radiation.  Actually, even the natural ozone shield is a little "leaky" in that it allows some UV radiation to reach the Earth's surface where it causes sunburn and skin cancer.  Some air pollutants released to the atmosphere through human activity, especially the chlorofluorocarbons (CFCs) used as aerosol propellants and as refrigerants, have resulted in partial depletion of the ozone layer. 

6. The Difference Between Stratospheric and Tropospheric Ozone
Remember…
Ozone is Good Up High and Bad Nearby 3
Ozone is the stratosphere is good because it protects us from biologically damaging UV light, but ozone at ground level is bad because it has serious health effects and damages materials such as rubber, crops and natural ecosystems such as forests. 3
Lesson 2: Ozone Formation in the Troposphere

7. Formation of Ground Level Ozone
Ground level or “bad” ozone is not emitted directly into the atmosphere, but is created by chemical reactions of oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight.
In this course we will learn how ozone, a secondary pollutant, is formed at ground level from other air pollutants.

8. Ground Level Ozone Ingredients
Carbon Source
CO, CH4 & VOCs
Oxides of Nitrogen (NOx)
NO, NO2
Sunlight O3
There are three ingredients for making ozone in the troposphere: 
1. A carbon source:  CO, Methane (CH4) or a Volatile Organic Compound (VOC)
2. Oxides of Nitrogen:  NO & NO2
3. Sunlight
The oxides of nitrogen act as catalyst to oxidize the carbon source and produce ozone.  The energy for the reaction is derived from sunlight.  Thus, no ozone is made at night.
ozone precursors

9. A Closer Look at CO (Chemists only) 2014 ESS no need
The series of reactions with CO that leads to the formation of ground level ozone: (we will explore this in more depth in later sections)
CO + OH → CO2 + H
H + O2 → HO2
HO2 + NO → OH + NO2
NO2 + hv → NO + O
O + O2 → O3
Net: CO + 2 O2 → CO2 + O3
Sunlight
Only cover if chem students are in class. Carbon monoxide (CO) is the simplest example of how a carbon source (CO) is catalyzed by NO to be oxidized to CO2 and form ozone (O3) in the process.
In the first reaction, CO reacts with the hydroxyl radical (OH) to form CO2 and a hydrogen atom (H).  The hydroxyl radical is a water molecule with one hydrogen atom removed.  The hydroxyl radical exists at very low concentrations in the atmosphere (much less than a part per trillion), and it is formed from ozone ... so it takes some ozone to form more ozone.  The OH radical is a catalyst.  A catalyst is something that speeds up a reaction but is not itself consumed in the reaction.  Note that although OH is destroyed in the first reaction, it is produced in the third reaction, so it is not consumed.
Hydrogen atoms are extremely unstable and rapidly react (within nanoseconds) with surrounding oxygen molecules to form hydroperoxy radicals, HO2, as seen in the second reaction.  This is always the fate of a hydrogen atom in the troposphere where the oxygen content is high.
In the third reaction, the HO2 radical reacts with NO to form NO2 and OH.  (As mentioned earlier, this regenerates the OH radical, so no OH is consumed in the reaction).
NO2 is reddish-colored gas, meaning that it absorbs blue light.  When NO2 absorbs a photon of light (h) in the fourth reaction, it produces NO and an oxygen atom (O).  Since the NO is regenerated in this step, NO also is a catalyst.  Like H atoms, O atoms rapidly react with surrounding oxygen molecules, in this case to form ozone (O3).
If you add up all of the reactions, canceling species that occur on both the left and right sides of the reactions, the net result is:  CO + 2 O2    CO2 + O3.  This overall reaction does not contain either OH or NO in it, meaning that their concentrations remain unchanged.  The net effect is that CO is oxidized by two molecules of oxygen to form carbon dioxide and ozone.
Although this may seem a bit complicated, the overall effect is that a carbon source CO reacts with sunlight to form ozone.  And, the reaction is catalyzed by NO.  In fact, CO and O2 can be mixed and kept in a vessel for years with no direct reaction.
Other carbon sources, such as methane and VOCs, react in a similar way, but the reaction mechanism is much more complicated, as we shall see.  In all cases, NO is oxidized to NO2 by a peroxy radical derived from the carbon source, and NO2 photolyzes to regenerate NO and form ozone.

10. The Role of Sunlight in the Formation of Ozone(Chemists only)
In step four you see the need for sunlight to break the nitrogen dioxide apart.
CO + OH → CO2 + H
H + O2 → HO2
HO2 + NO → OH + NO2
NO2 + hv → NO + O
O + O2 → O3
This is one of the reasons that sunlight is needed to produce ozone. Sunlight also is needed to make the OH radical.
Step 4
Sunlight N O O
This slide focuses in on the last two reactions of this mechanism and shows why sunlight is needed to produce ozone.  The ozone is formed from the O atom that is split from nitrogen dioxide by absorption of sunlight.  When a photon of light is absorbed, one of the N-O chemical bonds is broken.  The O atom produced then rapidly (within nanoseconds) reacts with nearby oxygen atoms to form ozone.
It should be noted that the presence of OH radicals in the atmosphere also depends on sunlight, so there is no OH in the atmosphere at night.  Hydroxyl radicals are sometimes called the "scavengers of the atmosphere" because they react with almost everything.  Without hydroxyl radicals, reduced compounds produced by bacteria and plants -- like methane, VOCs and hydrogen sulfide -- would build to extremely high concentrations in the atmosphere, perhaps to toxic levels.
Hydroxyl radicals are produced by absorption of UV light by ozone:
O3 + h    O2 + O*
O* + H2O    2 OH
Here, O* is an electronically excited oxygen atom, called "O singlet D", or more technically O(1D2).  These highly excited oxygen atoms have all of their electrons paired (compared to ground-state oxygen atoms which have two unpaired electrons).  This provides O* enough internal energy to rip a hydrogen atom off a water molecule, thereby producing two hydroxyl radicals.
Ironically, a leaky stratosphere combined with some ozone in the troposphere provides the possibility of OH radicals to "clean" the troposphere of toxic gases.  Without OH radicals, the atmosphere would soon become a toxic soup.  This is point not often discussed ... but neither a perfect ozone shield that lets no UV light reach the surface nor a troposphere free of ozone is desirable for the life forms that currently exist at the Earth's surface.
You might want to discuss with your students how life might have evolved differently if the atmosphere had a stratosphere with much more ozone and/or a troposphere with much less ozone. N O + O2 = O3 O
Step 5

11. + = O2 O O3 Formation of Ozone (O3) The Single Oxygen Atom is Lonely
A single Oxygen atom (O) in the atmosphere will quickly find an O2 and bind with it to form Ozone (O3). O2 is obviously quite abundant in the troposphere, but a single O is extremely rare. + = O2 O O3
This slide emphasizes how ozone is formed.  This is the only reaction that forms ozone in the atmosphere.  Ozone is always formed by an oxygen atom combining with O2.  In the troposphere, the O atoms are always formed by the photolysis of NO2:
NO2 + h    NO + O
We shall find that in the stratosphere, the O atoms are formed by the photolysis of O2:
O2 + h    2 O

12. Temperature Inversions Can Trap Air Pollution at Ground Level
During a temperature inversion, the cold air stays near the ground, because cold air sinks and stays there.
In the normal situation, hot air rises as shown below.
The natural tendency of the atmosphere is for temperature to decrease with increasing altitude.  This is because air warmed at the Earth's surface is less dense than the cooler air above and thus rises.  But as the air rises it also expands because of the lower pressure at higher altitude, and as air expands it cools.
Under some meteorological conditions, warm air occurs above cold air, a situation called a "temperature inversion".  Pollutants are then trapped near the surface and continue to build up in concentration.  This results in the most serious air pollution events.  Temperature inversions occur when warm air masses (warm fronts) move over colder air masses and also when the surface is cooled by radiative emission faster than it is warmed by solar radiation.  The latter occurs more often in the winter.  Temperature inversions often occur at night, and it takes some time for the solar warming to break up the inversion layer the next morning.

13. Ozone Transport Downwind
Rural areas can suffer from high ozone that is transported by the wind from large cities. It can also take time for ozone to form, so a city might not see much ozone, but the town downwind of it will see the highest concentrations. This also depends on geographical situation.
It takes some time, up to a few hours, for the photochemical reactions to produce the maximum level of ozone.  And, very high levels of NOx can actually slow down ozone formation by combining with some of the free radical intermediates.  For these reasons, often the highest ozone concentrations occur downwind of the urban/industrial area where the primary pollutants are emitted. 

14. Ozone and Rain
Ozone concentration in the air is not significantly decreased by rain itself, but can be decreased due to its partner – clouds. The clouds cover the sun and reduce the amount of sunlight needed to form ozone.
Unlike Ozone, SO2 and NO2 are soluble in water and react with rain drops to form Sulfuric and Nitric Acid, which is better known as acid rain.
Ozone is only slightly soluble in water, so it is not effectively removed by rain.  Rain does remove many of the ozone precursors and, more importantly, reduces temperature and sunlight so ozone formation is greatly reduced.
Rain =
Clouds =
Less Sunlight & UV Rays =
Less Ozone

15. Temperature Inversions Can Trap Air Pollution at Ground Level
What a temperature inversion can look like: all the pollutants are trapped near the ground
The accumulation of particulate matter, which scatters light and reduces visibility, makes inversion layers visible over many urban areas.  Inversion layers can be quite noticeable during airline flights during takeoff and landing.