Mon., Jan. 14 notes

Monday Jan. 14, 2013
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Mumford and Son and "I Will Wait" before class this afternoon.

The cold weather persists. It meant the end of my last year's tomatoes, though they usually don't last nearly this long and I need the room for next year's garden. The tomato seeds that I planted a week or so ago have germinated. I keep them in my office because it's quite a bit warmer than my rental house.


Last summer's tomatoes (killed by the cold weather)	Next summer's tomatoes (growing in my office)

I handed out 10 more sets of Expt. #1 materials. I'll bring another one or two on Wednesday but that will be the last call for Expt. #1.

Have a look at the Report Signup lists and see if your name is there. One of the signup sheets went missing last Friday and I wasn't able to copy those names onto the online signup lists. So if you're name isn't listed please let me know.

At the top of the list below are the 5 most abundant gases in the atmosphere. Several more important trace gases, new information, were added to the bottom of the figure. Trace gases are gases found in low concentrations (and often the concentrations vary with time and location). Low concentrations doesn't mean they aren't important, however.

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 this week.

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.

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 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. 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) 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 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 internal body 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. 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 late 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 the article "Open-Fire Stoves Kill Millions. How Do We Fix It?" that I mentioned in class (it appeared in the Dec. 2012 issue of Smithsonian Magazine).
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I'm not sure how the researchers determine that 150,000 people are killed by climate change every year.

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 (click on 2007 at the top left edge of the page) you can view the report online or download and print a copy of the report. This is just in case you are interested (click on some of the other years also if you do go to the site). And note they are concerned with all types of pollution, not just air pollution.

You may have heard of the record setting levels of air pollution currently affecting Beijing, China. Here's a link to a pretty good collection of photographs. Much of this is particulate pollution which is something we'll cover later this week or next week. In addition to being a health hazard, particulates can have a dramatic effect on visibility.

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.

We will mostly be talking about carbon monoxide found outdoors, where it would 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 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.

In addition to carbon monoxide, nitric oxide (NO) and sulfur dioxide (SO₂), are also primary pollutants. They all go 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 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 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). The reason for this is surface radiation inversion layers. They are most likely to form on cold winter mornings.

In an inversion layer (Point 5) air temperature actually increases with increasing altitude which is just the opposite of what we are used to. This produces stable atmospheric conditions which means there is little up or down air motion.

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 (it then decreases with altitude above that). This is the stable inversion layer. When CO is emitted into the thin stable layer, the CO remains in the layer and doesn't mix with cleaner air above. CO concentrations build.

In the afternoon, the ground warms, and the atmosphere becomes more unstable. Temperatures decrease with increasing altitude in the right figure above. 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.

We'll come back to carbon monoxide on Wednesday. We'll spend the remainder of the 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. I'm going to try to make a cloud of smog in class next week. 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. So we took some time for a demonstration that tried to show you 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.

Looking down on the situation in the figure above. Neither the students or the instructor could see the beam of light. Nobody could see the beam because there weren't any rays of light pointing from the laser beam toward the students or toward the instructor.

The instructor would have been able to see the beam if he had stood at the end of the beam of laser light 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 (splattered is a more descriptive term). The original beam is broken up into a myriad 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 some 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.

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 really lit up 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.

A day or two ago I mentioned that air was invisible.

Well that's not entirely true. Even something as small as air molecules can scatter light. If you shine a strong enough light source through enough air you can see the scattered light.

You can find all kinds of things in the sky: air, particulates, clouds, etc. But first let's imagine there isn't an atmosphere. No air, clouds, particulates, nothing.

If you went outside and looked at the sun (you shouldn't do that of course) you'd see a bright sun against a black background. You'd see the sun because you're looking back in the direction of one of the rays of light coming from the sun.

If you look away from the sun and toward the sky you wouldn't see anything. The sky would appear black. That's because there's nothing to scatter the sunlight. This is just like when you couldn't see the laser beam as it traveled across the front of the classroom. You couldn't see the beam unless something was put into the beam to scatter some of the laser light.

In the next picture we'll add an atmosphere. Just air molecules, no particles, or clouds.

Air molecules scatter light. We didn't see this light in the laser demonstration because the laser light scattered by the air was too weak. But when you're dealing with intense sunlight traveling through a lot more air in the atmosphere you can see the scattered light.

The incoming sunlight is white. White light is a mixture of all the colors. Air molecules scatter the shorter wavelengths (violet blue green) more than the longer wavelengths (yellow orange red). This is depicted above. Air molecules scatter light in this way because they are very small (much smaller than the wavelength of visible light).

Violet has the shortest wavelength and is scattered the most. However there isn't as much violet in sunlight as there is blue and green. There's a lot of green light in sunlight (more than any other color as a matter of fact) but it isn't scattered as readily as blue. So the end result is that we see blue light coming from the sky. This is why the sky is blue. When the air is clean (from of particulates), the sky has a deep blue color. Here's a little more explanation of why the sky appears blue.

Next we'll add a cloud to the picture. As we saw in the laser demonstration, cloud droplets and ice crystals are good scatters of light. Cloud droplets and ice crystals though are much larger than air molecules. Because of this they scatter all the colors in equal amounts.

When white light strikes a cloud, white light is scattered and reflected. This is why clouds are white (with some shades of grey mixed in if the cloud is thick). When you look up at a cloud you see a white cloud (sunlight being scattered by cloud droplets) surrounded by blue sky (sunlight being scattered by air molecules.

What about particles? Particulates are much bigger than air molecules and a little bit smaller than cloud droplets. They scatter light is the same way that cloud droplets and ice crystals do. The scattered light from particles is white.

What do you see now when you look at the sky? It depends on how much particulate matter is in the air. When the air is clean and doesn't contain much particulate matter the sky is a deep blue. As the concentration of particulates increases you mix in more and more white light. The color of the sky can change to a whitish blue when the particulate concentration is high.