Wed., January 20, 2016

Skeye - "Ruthless" (4:45), "Love in Vain" (5:33), "Wicked Game" (3:58)
"Old Love" (4:02), "Going Home" (5:54), "Creep" (4:02), "Mad World" (4:14), "Hallelujah" (6:51)

Trace gases in air - pollutants and greenhouse gases

We are going to add a bunch of trace gases to the list of the 5 main gases in our present day atmosphere.  These are gases that are found in very low concentrations in air.  The concentrations often vary with location and time.  The fact that the concentrations are low doesn't mean the trace gases are not important.  This week and next Monday, we'll be concentrating on one sub-group of trace gases, air pollutants.  Air pollution (both indoors and outdoors) probably kill a few million people every year across the globe.





Water vapor, carbon dioxide, methane, nitrous oxide (N2O = laughing gas), chlorofluorocarbons, and ozone are all greenhouse gases.  The greenhouse effect warms the earth and 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 dioxide will probably be the subject of a future 1S1P Assignment).

Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants.  We'll cover 3 of these in more detail.

Ozone 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 (deadly) high energy ultraviolet (UV) light coming from the sun.  Without the protection of this ozone layer, life as we know it would not exist on the surface of the earth.  It was only after ozone started to buildup in the atmosphere that life could move from the oceans onto land.  Chlorofluorocarbons are of concern in the atmosphere because they destroy stratospheric ozone.

(ii)  Ozone in the troposphere (the bottom 10 kilometers or so of the atmosphere and where we live) ozone is a pollutant and is one of the main ingredients in photochemical smog.

          (iii)  Ozone is also a greenhouse gas.


Air Pollutants

Today through next Monday we will be looking at four air pollutants.  They are carbon monoxide, tropospheric ozone, sulfur dioxide (all three are gases).  We'll also cover particulate matter.  They're listed below together with an idea of the number of main points you should remember and understand about each. 







Today's class will also feature a light scattering demonstration.  It's a fairly simple concept and explains how/why we are able to see things like smog, clouds, and particulate matter in the air.  We will also produce some photochemical smog in a demonstration Friday (safely confined in a glass bottle).  You'll be able to see the smog because of scattering of light.

Air Pollution is a serious health hazard in the US and around the globe  (click here to download a copy of the information below).  The lists below give some idea of how serious a threat it is.
  


The top list shows the external or environmental agent that causes death.  The second list is the physiological or internal bodily function that ultimately leads to your demise.  Keep in mind that many of these numbers are difficult to measure and some may contain a great deal of uncertainty (they are also somewhat out of date).  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 in a little more detail next week.


Air pollution is a serious hazard worldwide.  Interestingly indoor air pollution is, in many places, a more serious threat than outdoor air pollution.  In that regard, here's a link to an article titled "Open-Fire Stoves Kill Millions.  How Do We Fix It?" (it appeared in the Dec. 2012 issue of Smithsonian Magazine). 

I'm not sure how the researchers determine that 150,000 people are killed by climate change every year.

The Blacksmith Institute  (now Pure Earth 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 (click on 2007 at the top edge of the page) you can view the report online or download and print a copy of the report.  Click here to see a 2013 update of the list.  These lists are included just in case you are interested.  And note all types of pollution are considered, not just air pollution.

This past summer I heard mention of a recent study (ref) that estimates air pollution kills about 4000 people per day in China.  Here's an August 18 article from the Washington Post that discusses the study.  Much of this is particulate pollution which is something we'll cover next week.  In addition to being a health hazard, particulates can have a dramatic effect on visibility.

Carbon Monoxide (CO)


We'll start our section on air pollutants with carbon monoxide.  You'll find additional information on carbon monoxide and other air pollutants at the Pima County Department of Environmental Quality website and also at the US Environmental Protection Agency website.


The material above is from page 7 in the photocopied ClassNotes.  We will mostly be talking about carbon monoxide found outdoors, where it would only rarely reach fatal concentrations.  CO 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 for example).  Between 1999 and 2010  an average of 430 people were killed per year from unintentional, non-fire-related carbon monoxide poisoning according to the Centers for Disease Control and Prevention (ref).

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 interferes with the transport of oxygen throughout your body.  The article above about carbon monoxide poisoning mentions that the 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.








In addition to carbon monoxide, nitric oxide (NO) and sulfur dioxide (SO2), are also primary pollutants.  They all travel directly from a source (automobile tailpipe or factory chimney) into the atmosphere.  Ozone is a secondary pollutant (and here we mean tropospheric ozone, not stratospheric ozone).  It wouldn't be present in the exhaust coming out of a car's tailpipe.  It shows up in the atmosphere only after a primary pollutant has undergone a series of reactions without other chemical compounds in the air.



Point 3 explains that CO is produced by incomplete combustion of fossil fuel.  Basically there isn't enough oxygen.  More oxygen and complete combustion would produce carbon dioxide, CO2.  

Because cars and trucks produce much of the CO in the atmosphere in Tucson, special formulations of gasoline (oxygenated fuels) are used during the winter months in Tucson to try to reduce CO emissions.  The added ethanol has the effect of adding more oxygen to the combustion process.
 
Vehicles are also 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 Pima County, vehicles must also pass an emissions test every year to insure that the car is burning fuel as cleanly as possible.

The photograph below at left shows the blue flame that results from complete combustion of natural gas (methane) in a bunsen burner.  The air holes on the side of the bunsen burner are open and plenty of air (oxygen) is available during combustion.  The flames on a gas stove or the pilot light in a hot water heat or a furnace should have this blue color.  If the air holes are closed, the flame turns yellow.  There isn't enough oxygen in this case.  This incomplete combustion will produce more carbon monoxide and also a lot of black soot (carbon).  (source of the photo)



Clean (complete) and dirty (incomplete) combustion of natural gas

In the atmosphere CO concentrations peak on winter mornings (Point 4).  The reason for this is surface radiation inversion layers.  They are most likely to form on cold winter mornings.

When we say inversion layer (Point 5), we mean a temperature inversion, a situation where air temperature increases with increasing altitude.  That's just the opposite of what we are used to (you would expect it to be colder at the summit of Mt. Lemmon than here in the Tucson valley).  This produces stable atmospheric conditions which means there is little up or down air motion.

 The lack of vertical air motions means there is very little vertical mixing in a stable air layer.



In the left figure above, notice how temperature increases from 40 F to 50 F in the thin air layer next to the ground.  That's the inversion layer.  Temperature then begins to decrease as you move further up.  When CO is emitted into the thin stable layer during the morning rush hour, the CO remains in the layer and doesn't mix with cleaner air above.  CO concentrations build.  Later in the day the ground and air in contact with the ground warms.  The inversion disappears and air at the ground mixes with cleaner air above.  The evening rush hour adds CO to the air but it is mixed in a larger volume of air and the concentration doesn't get as high.


Thunderstorms like you have been seeing this time of year contain strong up and down air motions.  Thunderstorms are an indication of unstable atmospheric conditions.

Scattering (splattering) of light

We spent the next portion of today's class learning about the scattering of light.  You are able to see a lot of things in the atmosphere (clouds, fog, haze, even the blue sky) because of scattering of light.  We'll try to make a cloud of smog in class on Friday.  The individual droplets making up the smog cloud are too small to be seen by the naked eye.  But you will be able to see that they're there because the droplets scatter light.  That's true also of the little water droplets that make up a cloud.  So we need to take some time for a demonstration to see exactly what light scattering is.

In the first part of the demonstration a narrow beam of intense red laser light was directed from one side of the classroom to the other.


 
We're looking down from above  in the the figure above.  Neither the students or the instructor could see the beam of light.  To see the laser light some of it would need to be traveling toward you rather than from one side of the room to the other.  This figure and the ones that follow are on pps 107a & 107b on the packet of ClassNotes.




The instructor would have been able to see the beam if he had stood at the end of the beam of laser light where it hit the wall and looked back along the beam of light toward the laser.  That wouldn't have been a smart thing to do, though, because the beam was strong enough to possibly damage his eyes (there's a warning on the side of the laser). 

Everybody was able to see a bright red spot where the laser beam struck the wall. 




This is because when the intense beam of laser light hits the wall it is scattered (I think splattered is a more descriptive term).  The original beam is broken up into a multitude of weaker rays of light that are sent out in all directions.  There is a ray of light sent in the direction of every student in the class.  They see the light because they are looking back in the direction the ray came from.  It is safe to  look at this light because the original intense beam is split up into many much weaker beams.

Next we clapped two erasers together so that some small particles of chalk dust fell into the laser beam.  I also sprinkled baby powder into the beam.




Now instead of a single spot on the wall, students saws lots of points of light coming from different positions along a straight segment of the laser beam.  Each of these points of light was a particle of chalk, and each piece of chalk dust was intercepting laser light and sending light out in all directions.  Each student saw a ray of light coming from each of the chalk particles.  With a cloud of chalk dust you are able to see segments of the laser beam.

We use chalk because it is white, it will scatter rather than absorb visible light.  What would you have seen if black particles of soot had been dropped into the laser beam?

In the last part of the demonstration we made a cloud by pouring some liquid nitrogen into a cup of water.  The cloud droplets are much smaller than the chalk particles but are much more numerous.  They make very good scatterers.





The beam of laser light was very bright as it passed through the small patches of cloud.  The cloud droplets did a very good job of scattering laser light.  So much light was scattered that the spot on the wall fluctuated in intensity (the spot dimmed when lots of light was being scattered, and brightened when not as much light was scattered).  Here's a photo I took back in my office.




The laser beam is visible in the left 2/3 rds of the picture because it is passing through cloud and light is being scattered toward the camera.  There wasn't any cloud on the right 1/3rd of the picture so you can't see the laser beam over near Point 1.

The air molecules in the room are actually scattering laser light but it's much too weak for us to be able to see it.  When a stronger light source (sunlight) shines through much more air (the entire atmosphere) we are able to see the scattered light.  The blue light that you see when you look at sky is sunlight being scattered by air molecules.  This will be the topic of another, soon to appear, 1S1P Assignment.

There's a good chance there won't be time to discuss the photographs that follow, but they don't really require much explanation.

Gases like water vapor, oxygen, and nitrogen are invisible.  Some gases are colored and can be seen; some examples are shown below.  I would like to bring some actual samples to class, but all of these gases are toxic and require very careful handling. 





Bromine in both liquid and gaseous phases. Bromine and mercury are the only two elements that exist as liquids at room temperature.  The bromine is in a sealed glass ampoule inside an acrylic cube.  Bromine could be safely brought to class in a container like this.

Webelements.com states: " It is a serious health hazard, and maximum safety precautions should be taken when handling it."  I'm not sure what maximum safety precautions are, that's why I don't bring it to class.

This photo was taken by Alchemist-hp and was Picture of the Day on the English Wikipedia on Oct. 29, 2010.
 Chlorine (Cl2)
I found this image here
 Iodine
Also an element that is normally found in solid form.  The solid sublimes, i.e. it changes directly from solid to gas (you would probably need to heat the solid iodine to produce gas as dense as seen in the picture above).  source of this image

I think we can probably handle iodine safely and might well bring some to class.
 Nitrogen dioxide  (NO2)
An important pollutant.  I used to make this in class but I've read that you can inhale a fatal dose of  NO2 before showing any symptoms.  NO2 also has an anesthetic effect - it can deadens your sense of smell.

source of this image