In this lecture we will conclude our discussion of air pollutants.  Here's some basic information about sulfur dioxide.

Sulfur dioxide is produced by the combustion of sulfur-containing fuels such as coal.  Combustion of fuel also produces carbon dioxide and carbon monoxide.  People probably first became aware of sulfur dioxide because it has an unpleasant smell.  Carbon dioxide and carbon monoxide are odorless.  That is why sulfur dioxide was the first pollutant people became aware of.

Volcanoes are a natural source of sulfur dioxide.



The Great London smog is still one of the two or three deadliest air pollution events in history.  Because the atmosphere was stable, SO2 emitted into air at ground level couldn't mix with cleaner air above, and the SO2 concentration was able to build to dangerous levels.  4000 people died during this 4 or 5 day period and as many as 8000 additional people died in the following weeks and months. 

Some of the photographs below come from articles published in 2002 on the 50th anniversary of the event. 
 

from:
http://news.bbc.co.uk/1/hi/uk/2542315.stm

from:
http://news.bbc.co.uk/1/hi/health/2545747.stm

from:
http://news.bbc.co.uk/1/hi/england/2543875.stm

from:
http://www.npr.org/templates/story/story.php?storyId=873954

The sulfur dioxide didn't kill people directly.  Rather the SO2 aggravated an existing condition of some kind and hastened their death.  The SO2 probably also made people susceptible to bacterial infections such as pneumonia.   This link discusses the event and its health effects in more detail.
London type smog which contains sulfur dioxide and is most common during the winter is very different from photochemical or Los Angeles type smog.  Los Angeles type smog contains ozone and is most common in the summer.

Some other air pollution disasters also involved high SO2 concentrations.  One of the deadliest events in the US occurred in 1948 in Donora, Pennsylvania.



"This eerie photograph was taken at noon on Oct. 29, 1948 in Donora, PA as deadly smog enveloped the town. 20 people were asphyxiated and more than 7,000 became seriously ill during this horrible event." (from: http://oceanservice.noaa.gov/education/kits/pollution/02history.html)


(from: http://www.eoearth.org/article/Donora,_Pennsylvania)

"When Smoke Ran Like Water," a book about air pollution is among the books that students can check out, read, and report on to fulfill part of the writing requirements for the classroom version of this course (instead of doing an experiment report).  The author, Devra Davis, lived in Donora Pennsylvania at the time of the 1948 air pollution episode.

Sulfur dioxide is one of the pollutants that can react with water in clouds to form acid rain (some of the oxides of nitrogen can react with water to form nitric acid).  The formation and effects of acid rain are discussed below.


Note that clean unpolluted rain has a pH less than 7 and is slightly acidic.  This is because the rain contains dissolved carbon dioxide gas.  We'll see how this happens in a classroom demonstration.  Acid rain is often a problem in regions that are 100s even 1000s of miles from the source of that sulfur dioxide that forms the acid rain.  Acid rain in Scandinavia came from industrialized areas in other parts of Europe.


Some of the problems associated with acid rain.

The last pollutant that we will cover is Particulate Matter (PM) - small solid particles or drops of liquid (but not gas) that remain suspended in the air (sometimes referred to as aerosols).  The designations PM10 and PM25 refer to particles with diameters less than 10 micrometers and 2.5 micrometers, respectively.  A micrometer is one millionth of a meter.  The drawing below might give you some idea of what a 1 micrometer particle would look like (actually it would probably be too small to be seen without magnification).


Particulate matter can be produced naturally (wind blown dust, clouds above volcanic eruptions, smoke from lightning-caused forest and brush fires).  Human activities also produce particulates.

Particles with dimensions of 10 micrometers and less can be inhaled into the lungs (larger particles get caught in the nasal passages).  Inhaled particulates are a health threat.  The particles can cause cancer, damage lung tissue, and aggravate existing repiratory diseases.  The smallest particles can pass through the lungs and get into the blood stream (just as a molecule of oxygen does) and damage other organs in the body.

The figure below identifies some of the parts of the human lung mentioned in the figure above.

 
Crossectional view of the human lungs
from: http://en.wikipedia.org/wiki/Lung

1 - trachea
2 - mainstem bronchus
3 - lobar bronchus
4 - segmental bronchi
5 - bronchiole
6 - alveolar duct
7 - alveolus
from http://en.wikipedia.org/wiki/Image:Illu_quiz_lung05.jpg

Note the PM10 annual National Ambient Air Quality Standard (NAAQS) value of 50 micrograms/meter3 at the bottom of the figure before last.  The following list shows that there are several cities around the world where PM concentrations are 2 or 3 times higher than the NAAQS value.




There was some concern during the most recent summer Olympic Games that the polluted air in Beijing would keep athletes from performing at their peak.  Chinese authorities restricted transportation and industrial activities both before and during the games in an attempt to reduce pollutant concentrations.  Rainy weather during the games may have had the greatest effect, however. 


Clouds and precipitation are the best way of cleaning pollutants from the air.
Before beginning this section, you should familiarize yourself with the concept of light scattering by looking at the Light Scattering Activity.
Particulates can affect visibility and can make the sky appear hazy.  To understand this we need to look at how air molecules and particles scatter sunlight.


Rays of sunlight are passing through clean air above.  You wouldn't ordinarily be able to see the sunlight unless you looked back along one of the rays (a) in the figure, i.e. back toward the sun.  You'd see the sun in that case (at least up until you caused some serious damage to your eyes).  If you look away from the sun toward the sky (b) you see blue light.  This is light that is being scattered by air molecules.

Sunlight is white light, which tells you it is a mixture of all the colors.  Because air molecules are small (relative to the wavelength of visible light) they scatter shorter wavelengths more readily than longer wavelengths.  When you look away from the sun and toward the sky you see this scattered light, it has a deep blue color.  This is basically why the sky is blue.  If the earth didn't have an atmosphere (or if air molecules didn't scatter light) the sky would be black.

Scattering of sunlight by air molecules turns distant mountains blue and eventually makes them fade from view.




A nearby mountain might appear dark green or brown.  You are mostly seeing light reflected off the mountain.  As the mountain gets further away you start seeing increasing amounts of blue light (sunlight scattered by air molecules in between you and the mountain) being added to and mixed in with the brown and green reflected light.  As the mountain gets even further the amount of this blue light from the sky increases.  Eventually the mountain gets so far away that you only see blue sky light and the light reflected by the mountain itself becomes so weak it can't be seen. 




We've added some particles to the air in the picture above.  Particles also scatter light.  But because the particle size is about equal to or somewhat greater than the wavelength of visible light the particles scatter all the colors equally.  The light scattered by particles is white.  This is basically why clouds are white.

As the amount of particulate matter in the air increases the color of the sky changes from deep blue to whitish blue.  The higher the particle concentration, the whiter the sky becomes.

Have a look at the color of the sky before and after a rainstorm.  Before the storm, the air will be full of particulate pollution and will appear whitish blue.  After the storm, after the rain has removed a lot of the pollutants, the sky often has a much deeper blue color.
The next set of figures tries to explain how particles in the air can affect visibility.



The air is free of particles in this first picture.  You're looking at a relatively nearby mountain.  The "side view" at left explains that you are able to see the mountain because it reflects sunlight back toward you.  The picture at right is what you see when you look at the mountain.  We are ignoring any light scattered by air molecules.



Now some particulates have been added to the air.  They scatter sunlight, the scattered light is white (it's highlighted in yellow in the picture at left for emphasis).  So now you still see the brown and green reflected light but also some white scattered light.  Some, unrealistically big, spots of white light have been added to the picture at right. 




More particles have been added to the air.  That means there will be more scattered light.

We'll add even more particles to the air in the next picture.  Because there is more scattered light (more spots of white light in the picture of the mountain) you have a harder time seeing the mountain.



Eventually all you see is the scattered light.  The mountain fades from view.