Now we're ready to spend a couple of lectures on air pollutants. 

We listed the 5 most abundant gases in the atmosphere below.  Several important trace gases have been added to the list in class.  Trace gases are gases found in low concentrations (and often the concentrations are variable).  Low concentrations doesn't mean they aren't important, however.


Water vapor, carbon dioxide, methane, nitrous oxide (N2O = laughing gas), chlorofluorocarbons, and ozone are all greenhouse gases.  Increasing atmospheric concentrations of these gases are responsible for the current concern over climate change and global warming.  We'll discuss this topic and learn more about how the greenhouse effect actually works later in the course.

Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants. 

Give ozone some special attention, it has sort of a Dr. Jeckyl and Mr. Hyde personality
(i)  Ozone in the stratosphere (a layer of the atmosphere between about 10 and 50 km altitude) is beneficial because it absorbs dangerous high energy ultraviolet (UV) light coming from the sun.  Without the protection of the ozone layer, life as we know it would not exist on the surface of the earth.  Chlorofluorocarbons are of concern in the atmosphere because they destroy stratospheric ozone.

(ii)  In the troposphere (the bottom 10 kilometers or so of the atmosphere) ozone is a pollutant and is one of the main ingredients in photochemical smog (Los Angeles-type smog).

(iii)  Ozone is also a greenhouse gas.

In the next two lectures we'll learn about a few of the main air pollutants.

Air Pollution is a serious health hazard in the US and around the world (we'll mainly discuss outdoor pollution, but indoor air pollution is also a problem).


Keep in mind that many of these numbers are difficult to measure and some may contain a great deal of uncertainty.  The row that is highlighted, toxic agents, contains estimates of deaths caused by indoor and outdoor air pollution, water pollution, and exposure to materials such as asbestos and lead both in the home and at the work place.  It is estimated that 60% of the deaths are due to exposure to particulate matter, something that we will examine briefly in the next lecture.


Air pollution is a serious hazard worldwide.  Indoor air pollution is, in many places, a more serious threat than outdoor air pollution.

The Blacksmith Institute listed the Top 10 polluted places in the world in a 2007 report.  The report has received a lot of worldwide attention.  If you go to this address, you can view the report online or download and print a copy of the report. 

We'll start with carbon monoxide.  Some basic information is shown below.  You'll find additional information at the Pima County Department of Environmental Quality website and also at the US Environmental Protection Agency website.


We will mostly be talking about carbon monoxide found outdoors, where it would rarely reach fatal concentrations.  Indoors is a serious hazard indoors also where it can (and does) build up to deadly concentrations.  ( several people were almost killed in Tucson in December 2010)

Carbon monoxide is insidious, you can't smell it or see it and it can kill you (Point 1).  Once inhaled, carbon monoxide molecules bond strongly to the hemoglobin molecules in blood and interfere with the transport of oxygen throughout your body.  The article above mentions that the CO poisoning victims were put inside a hyperbaric (high pressure) chamber filled with pure oxygen.  This must force oxygen into the blood and displace the carbon monoxide.

CO is a primary pollutant (Point 2 above).  That means it goes directly from a source into the air,  CO is emitted directly from an automobile tailpipe into the atmosphere for example. The difference between primary and secondary pollutants is probably explained best in a series of pictures.







Nitric oxide, NO, and sulfur dioxide, SO2, are also primary pollutants.  Ozone is a secondary pollutant (and here we are referring to tropospheric ozone, not stratospheric ozone).  It doesn't come directly from an automobile tailpipe.  It shows up in the atmosphere only after a primary pollutant has undergone a series of reactions.

Point 3 explains that CO is produced by incomplete combustion of fossil fuel (insufficient oxygen).  Complete combustion would produce carbon dioxide, CO2.   Cars and trucks produce much of the CO in the atmosphere in Tucson.

Vehicles must now be fitted with a catalytic converter that will change CO into CO2 (and also NO into N2 and O2 and hydrocarbons into H2O and CO2).  In large metropolitain areas such as Pima County, vehicles must also pass an emissions test every year and special formulations of gasoline (oxygenated fuels) are used during the winter months to try to reduce CO emissions. 

In the atmosphere CO concentrations peak on winter mornings (Point 4).  Surface temperature inversion layers form on long winter nights when the sky is clear and winds are calm.  The ground cools quickly and becomes colder than the air above.  Air in contact with the cold ground ends up colder than air above. 

In a temperature inversion layer, air temperature increases with increasing altitude (Point 5).  This produces a very stable  layer of air at ground level (stable means the air is stagnant, there are no up or down air motions).  A very reasonable wintertime morning temperature profile in Tucson is shown below:


Temperature increases from 47o F at the ground (Point A) to about 60o F at 1000 feet altitude (Point B), that's the stable inversion layer.  Temperature begins to decrease with increasing altitude above Point B.
There is very little vertical mixing in a stable air layer.



When CO is emitted into the thin stable layer (left figure above), the CO remains in the layer and doesn't mix with cleaner air above.  CO concentrations build.

In the afternoon, the ground warms, the inversion layer disappears, and the atmosphere becomes more unstable.  CO emitted into air at the surface mixes with cleaner air above.  The CO concentrations are effectively diluted.




Thunderstorms contain strong up (updraft) and down (downdraft) air motions.  Thunderstorms are a sure indication of unstable atmospheric conditions.  When the downdraft winds hit the ground they spread out horizontally.  These surface winds can sometimes reach 100 MPH, stronger than many tornadoes. 

The concentrations of several of the main pollutants are monitored in large cities in the US and around the world.  In Tucson, carbon monoxide, ozone, and particulate matter are of primary concern and daily measurements are reported in the city newspaper.

Let's imagine that the average carbon monoxide concentration in Tucson air yesterday was 3 ppm.  Is this a high and unhealthy value or was the air quality OK?  We need some more information.  We need to know what an acceptable concentration level for carbon monoxide is.  The EPA has done just that


The six main pollutants are listed above (there are many more).  The concentration of lead in air has decreased significantly since lead was removed from gasoline (the following quote is from a Wikipedia article on gasoline: "In the US,standards to phase out leaded gasoline were first implemented in 1973 ..... In 1995, leaded fuel accounted for only 0.6% of total gasoline sales ...... From 1 January 1996, the Clean Air Act banned the sale of leaded fuel for use in on-road vehicles. Possession and use of leaded gasoline in a regular on-road vehicle now carries a maximum $10,000 fine in the US.")

Rather than report the actual measured values, an Air Quality Index value is reported instead.    The AQI is the ratio of the measured to accepted concentrations multiplied by 100%. 

If we plug in the 3 ppm value mentioned above for carbon monoxide, the AQI value would be

The air quality in this case would be good.  Air becomes unhealthy when the AQI value exceeds 100%.  The units "ppm", by the way, stands for "parts per million."  A CO concentration of 3 ppm would mean that in 1 million air molecules 3 of them would be carbon monoxide.

Current Air Quality Index values for Tucson are available online.

So are we have been talking about carbon monoxide found in the atmosphere.  Carbon monoxide is also a serious hazard indoors where is can build to much higher levels than would ever be found outdoors.  You may remember having heard about an incident at the beginning of the school year in 2007 where carbon monoxide from a malfunctioning hot water heater sickened 23 Virginia Tech students in an apartment complex.  The CO concentration is thought to have reached 500 ppm.  You can get an idea of what kinds of health effects concentrations this high could cause from the figure below.


To get an idea of what effects 500 pm CO concentrations could cause, we will follow the 400 ppm line (shaded orange) from left to right.  At exposure times less than 1 hour you should experience no symptoms.  Beginning at 1 hour you might experience headache, fatique, and dizziness.  Exposures of a few hours will produce throbbing headache, nausea, convulsions, and collapse.  The 400 ppm trace level approaches the level where CO would cause coma and death.  At Virginia Tech several students were found unconscious and one or two had stopped breathing but they were revived.

Carbon monoxide alarms are relatively inexpensive and readily available at any hardware store.  They will monitor CO concentrations indoors and warn you when concentrations reach hazardous levels. Indoors CO is produced by gas furnaces and water heaters that are either operating improperly or aren't being adequately vented to the outdoors.  A few hundred people are killed indoors by carbon monoxide every year in the United States.  You can learn more about carbon monoxide hazards and risk prevention at the Consumer Product Safety Commission web page.

Before discussing tropospheric ozone, which is a pollutant, a quick reminder that there are both "good" and "bad" types of ozone. 

Ozone has a Dr. Jekyll and Mr. Hyde personality.  Ozone in the stratosphere (the ozone layer) is beneficial, it absorbs dangerous high energy ultraviolet light (which would otherwise reach the ground and cause skin cancer, cataracts, and many other problems).  It is also easy to make ozone in the stratosphere; molecular oxygen and ultraviolet light are all that are required.

Ozone in the troposphere is bad, it is a pollutant.  That is the stuff we will first be concerned with here.  Tropospheric ozone is a key component of photochemical smog (also known as Los Angeles-type smog).  Making ozone in the troposphere is also a more complex process as we will see below.  We'll make some photochemical smog as a classroom emonstration.  This will require ozone (and a hydrocarbon of some kind).  We'll use the simple stratospheric recipe for making ozone in the demonstration rather than the more complex tropospheric process..


At the top of this confusing figure you see that a more complex series of reactions is responsible for the production of tropospheric ozone.  The production of tropospheric ozone begins with nitric oxide (NO).  NO is produced when nitrogen and oxygen in air are heated (in an automobile engine for example) and react.  The NO can then react with oxygen to make nitrogen dioxide, a poisonous brown-colored gas..  Sunlight can dissociate (split) the nitrogen dioxide molecule producing atomic oxygen (O) and NO.  O and O2 react in a 4th step to make ozone (O3).  Because ozone does not come directly from an automobile tailpipe or factory chimney, but only shows up after a series of reactions, it is a secondary pollutant.   Nitric oxide would be the primary pollutant in this example.

NO is produced early in the day (during the morning rush hour).  The concentration of NO2 peaks somewhat later.  Peak ozone concentrations are usually found in the afternoon.  Ozone concentrations are also usually higher in the summer than in the winter.  This is because sunlight plays a role in ozone production and summer sunlight is more intense than winter sunlight.

As shown in the figure below, invisible ozone can react with a hydrocarbon of some kind which is also invisible to make a product gas.  This product gas sometimes condenses to make a visible smog cloud or haze.  The cloud is composed of very small droplets or solid particles.  They're too small to be seen but they are able to scatter light - that's why you can see the cloud.

The class demonstration of photochemical smog is summarized below.  We begin by using the UV lamp to create and fill the flask with ozone.  Then a few pieces of fresh lemon peel were added to the flask.  A whitish cloud quickly became visible (the cloud is colored brown in the figure below).