Friday, Jan. 19, 2018

Dave McGraw and Mandy Fer "Grow" (4:17), "Seritony" (4:52), "Cat Creek" (5:52)

A pretty dramatic change in the weather is due this weekend.





Here's the forecast from the National Weather Service Office in Tucson (captured on Friday morning).



The "bull's eye" center of low pressure in the upper left hand corner of this photograph
is the center of the storm.  Winds blowing on the western side of the storm move cold air southward.  A cold front indicates the front edge of this advancing cold air.  The front will pass through Arizona sometime late Friday or during the day on Saturday
 Top view of a cold front. 
You would expect to see warm winds blowing from the south or southwest before the front passes through and colder winds from the west or northwest once the front moves through.  Gusty winds often accompany passage of the front.  The coldest temperature may occur a day or two after the front has passed through.

The center of the storm system is still off shore and north of the US-Canada border.  The southern end of a cold front radiating out from the low pressure center of the storm will pass through Arizona late Friday or during the day on Saturday.  This will bring a slight chance of some rain (snow in the mountains), gusty winds, and colder temperatures.  Low temperatures Sunday and Morning will be near freezing. 

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).  Carbon monoxide from a malfunctioning heating system is also suspected to have caused the deaths of four people spending the recent holidays in a cabin near Flagstaff (more information).   Between 1999 and 2010  an average of 430 people were killed per year in the US 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 interfere with the transport of oxygen throughout your body.  The article about carbon monoxide poisoning in Tucson 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 molecules.

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.   The distinction between primary and secondary pollutants is a relatively minor point. 








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 with 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 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.  That's what we normally see.  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 Thursday.  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.  The figures below can be found on pps 107a - 107b in the ClassNotes.

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. 



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.  The insert at upper right shows what the instructor would see, a bright spot of light originating at the end of the laser tube itself.   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 red spot of 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. 




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).  Again the insert shows what you would see if you stood at the wall and looked back toward the laser.  Some of the light passes through the cloud so you would still be a spot of red light, but it would be weaker and more diffuse.  Then you would see red scattered light coming from the cloud surrounding the beam of laser light.

Here's a side view photo that 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 is the topic of another 1S1P assignment.

Sulfur dioxide (SO2 )
 We'll turn now to another of the air pollutants, sulfur dioxide (SO2 ).


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 most likely why sulfur dioxide was the first pollutant people became aware of.  Apparently sulfur dioxide is one of the smells in a freshly struck match.

Volcanoes are a natural source of sulfur dioxide.

US NAAQS in the figure above stands for United States National Ambient Air Quality Standards.  Air with a pollutant concentration that exceeds the NAAQS is considered unhealthy.  This is discussed further in an online Supplementary Reading section.

London-type smog




Sulfur dioxide has been involved in some of the world's worst air pollution disasters.  Still the deadliest, as best I can tell, is the Great London Smog of 1952.  At that time people burned coal in their homes and coal was burned in factories.  At the time of the 1952 event, the atmosphere was stable, SO2 and smoke from all the coal fires was being emitted into air at ground level and couldn't mix with cleaner air above.  The SO2 concentration was able to build to dangerous levels.  4000 people died during this 4 or 5 day period.  As many as 8000 additional people died in the following weeks and months.  Perhaps 100,000 people became ill.

The inversion layer in this case lasted for several days and was produced in a different way than the surface radiation inversions we heard about when covering carbon monoxide.  Surface radiation inversions usually only last for a few hours.

The term smog, a contraction of smoke + fog, was invented to describe a mixture of smoke and fog, something that was fairly common in the winter in London.  The 1952 event was an extreme case.  Now we distinguish between "London-type smog" which contains sulfur dioxide and photochemical or "Los Angeles-type smog" which contains ozone.
Most of the photographs below come from articles published in 2002 and 2012, the 50th and 60th anniversaries of the event.   The dramatic drops in visibility are mostly being caused by fog.  Later in the semester we will learn that fog clouds that form in "dirty" contained smoke particles can be thicker than fog that forms in cleaner air.


The caption to this photo from The Guardian reads
"Arsenal goalkeeper Jack Kelsey peers into the fog. 
The 'smog' was so thick the game was eventually stopped."
The smog in this photo is the thickest I was able to find.  Visibility here is perhaps 10 or 20 feet. (source of this image)


Buses had to creep along to avoid hitting someone or something.
from: http://news.bbc.co.uk/1/hi/health/2545747.stm

Someone would often walk out ahead of a bus to be sure the way was clear.
from: http://news.bbc.co.uk/1/hi/england/2543875.stm


You can get a feel for the cause of the smog
in this photograph by Paul Lowry in an article in SAGEMagazine.


Smog masks from this reference
The masks would filter out the smoke but not the sulfur dioxide gas

Even though it is a little off topic, here are some interesting photographs of early and mid 20th century London.

The sulfur dioxide didn't kill people directly.  Rather it would aggravate an existing condition of some kind.  The SO2 probably also made people susceptible to bacterial infections such as pneumonia.  Here's a link that discusses the event and its health effects in more detail.

The Clean Air Act of 1956 in England reduced smoke pollution and emissions of sulfur dioxide. 

Air pollution disasters involving sulfur dioxide have also occurred in the US.  One of the deadliest events was in 1948 in Donora, Pennsylvania.




The reference material that contained this photographed stated "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."

The photograph below shows some of the mills that were operating in Donora at the time.  Not only where the factories adding pollutants to the air they were undoubtedly adding hazardous chemicals to the water in the nearby river.




source of this photo

The US passed its own Clean Air Act in 1963.  There have been several major revisions since then.  The EPA began in late 1970 (following an executive order signed by President Nixon)

"When Smoke Ran Like Water," a book about air pollution is among the books that you can check out, read, and report on to fulfill part of the writing requirements in this class (though I would encourage you to do an experiment instead).  The author, Devra Davis, lived in Donora Pennsylvania at the time of the 1948 air pollution episode.