Fri., Aug. 24 notes

Friday Aug. 24, 2007

The Experiment #1 materials were handed out in class today.

Here's a review of what we learned last Wednesday
Atmospheric CO₂ concentration was fairly constant between 1000 AD and the mid 1700s. CO₂ concentration has been increasing since the mid 1700s (other greenhouse gas concentrations have also been increasing). The concern is that this might enhance or strengthen the greenhouse effect and cause global warming.

What has the temperature of the earth been doing during this period? There is a two part answer to that question.

First part:
Accurate direct measurements of temperature are available only from the past 150 years or so. The figure below (top of p. 3 in the photocopied Class Notes and also Fig. 14.7 in the text) shows how global average surface temperature has changed during that time period.

This is based on actual measurements of temperature made (using thermometers) at many locations on land and sea around the globe.

The graph doesn't actually show temperature. It shows how much different temperatures at various times beween 1860 and 2000 were compared to the 1961-1990 average. Temperature appears to have increased 0.7^o to 0.8^o C during this period. The increase hasn't been steady as you might have expected given the steady rise in CO₂ concentration; temperature even decreased slightly between 1940 and 1975.

It is very difficult to detect a temperature change this small over this period of time. The instruments used to measure temperature have changed. The locations at which temperature measurements have been made have also changed (imagine what Tucson was like 130 years ago). Average surface temperatures naturally change a lot from year to year. The year to year variation has been left out of the figure above so that the overall trend could be seen more clearly (click here to see a different version of this figure that does show the year to year variation and the uncertainties in the yearly measurements).

2nd part
Now it would be interesting to know how temperature was changing prior to the mid-1800s. This is similar to what happened when the scientists wanted to know what carbon dioxide concentrations looked like prior to 1958. In that case they were able to go back and analyze air samples from the past (trapped in bubbles in ice sheets).

That doesn't work with temperature.

Imagine putting some air in a bottle, sealing the bottle, putting the bottle on a shelf, and letting it sit for 100 years. In 2107 you could take the bottle down from the shelf, carefully remove the air, and measure what the CO2 concentration in the air had been in 2007 when the air was sealed in the bottle. You couldn't, in 2107, use the air in the bottle to determine what the temperature of the air was when it was originally put into the bottle in 2007.

You need to use proxy data. You need to look for something else whose presence, concentration, or composition depended on the temperature at some time in the past.

Here's an example.

Let's say you want to determine how many students are living in a house near the university.

You could walk by the house late in the afternoon when the students might be outside and count them. That would be a direct measurement (this would be like measuring temperature with a thermometer). There could still be some errors in your measurement (some students might be hidden inside the house, some of the people outside might not live at the house).

If you were to walk by early in the morning it is likely that the students would be inside sleeping (or in one of the 8 am NATS 101 classes). In that case you might look for other clues (such as the number of empty bottles in the yard) that might give you an idea of how many students lived in that house. You would use these proxy data to come up with an estimate of the number of students inside the house.

In the case of temperature scientists look at a variety of things. They could look at tree rings. The width of each yearly ring depends on the depends on the temperature and precipitation at the time the ring formed. They analyze coral. Coral is made up of calcium carbonate, a molecule that contains oxygen. The relative amounts of the different oxygen isotopes (atoms of oxygen with different numbers of neutrons in the nucleus) depends on the temperature that existed at the time the coral grew. Scientists can analyze lake bed and ocean sediments. The types of plant and animal fossils that they find depends on the water temperature at the time. They can even use the ice cores. The ice, H₂O, contains oxygen and the relative amounts of various oxygen isotopes depends on the temperature at the time the ice fell from the sky as snow.

Using these proxy data scientists have been able to estimate average surface temperatures for 100,000s of years into the past. The next figure shows what temperature has been doing since 1000 AD. This is for the northern hemisphere only, not the globe.

The blue portion of the figure shows the estimates of temperature derived from proxy data. The red portion are the instrumental measurements made between about 1860 and the present day. There is also a lot of year to year variation and uncertainty that is not shown on the figure above (click here or see Figure 14.6 in the text for a more accurate representation of this curve).

It appears that there has been a significant amount of warming that has occurred in just the last 150 years or so. Many scientists believe that this warming is a result of the increase in atmospheric greenhouse gas concentrations. Others suggest that this change in temperature might be just a natural change in climate (Mother Nature has produced much larger changes than we see here though usually on a much longer time scale), or might be do to other human activities that affect climate (changing land use).

We've only considered a small part of a large debate that involves science, economics, and politics.

Summary
There is general agreement that
    Atmospheric CO₂ and other greenhouse gas concentrations are increasing and that
    The earth is warming

Not everyone agrees
    on the Causes (natural or manmade) of the warming or
    on the Effects that warming will have on weather and climate in the years to come

At this point we made a not so smooth transition to carbon monoxide, an important air pollutant. We'll finish up CO in class on Monday and also cover ozone.

Some basic information about carbon monoxide is shown below (p. 7 in the photocopied Class Notes). You'll find additional information at the Pima County Department of Environmental Quality website and also at the US Environmental Protection Agency website.

Carbon monoxide molecules bond strongly to the hemoglobin molecules in blood and interfere with the transport of oxygen through your body. CO is a primary pollutant. That means it goes directly from a source into the air (nitric oxide, NO, and sulfur dioxide, SO₂, are also primary pollutants). CO is emitted directly from an automobile tailpipe into the atmosphere for example

CO is produced by incomplete combustion of fossil fuel. Complete combustion would produce carbon dioxide, CO₂. Cars and trucks produce much of the CO in the atmosphere. Vehicles must now be fitted with a catalytic converter which will change CO into CO₂ (and also NO into N₂ and O₂). In Pima County vehicles must 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. See if you can figure out why carbon monoxide is often a problem in cities at high altitude (the answer is found at the bottom of today's online notes)

Carbon monoxide is also a serious hazard indoors. Because it is odorless, concentrations can build to dangerous levels without you being aware of it. You can purchase a carbon monoxide alarm that 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 outdoors. Many people are killed indoors by carbon monoxide every year. You can learn more about carbon monoxide hazards and risk prevention at the Consumer Product Safety Commission web page.

In the atmosphere CO concentrations peak on winter mornings. Surface temperature inversion layers form on long winter night when the ground becomes colder than the air above. Air in contact with the cold ground cools and ends up colder than air above. Air temperature increases with increasing altitude in a temperature inversion and this produces a very stable layer of air at ground level.

When CO is emitted into a 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 atmosphere becomes more unstable. CO emitted into air at the surface mixes with cleaner air above. The CO concentrations are effectively diluted and don't get as high as they do in the morning.

A portion of a time lapse cloud move was shown at the end of class. Thunderstorms were developing over the Catalina mountains. Thunderstorms are a visible indication of unstable atmospheric conditions.

You could see the clouds growing vertically in the movie, evidence of rising air motions. Falling precipitation also produces a downdraft, sinking air motions. This downdraft is the source of the strong, often damaging, surface winds that accompany thunderstorms.

The object of this experiment is to measure the percentage concentration of the oxygen in air. Basically a wet piece of steel wool is stuck into a 100 mL graduated cylinder. The cylinder is turned upside down and the open end is immersed in a cup of water. The air in the graduated cylinder is sealed off from the rest of the atmosphere. The oxygen reacts with the steel wool to form rust and is removed from the air sample (it turns from a gas and becomes part of the rust, a solid).

If you simply try to immerse the open end of the cylinder in a cup of water you would find that the water doesn't enter the cylinder. Air pressure keeps the water out. You want the water to enter partway into the cylinder so that the water level can be read on the cylinder scale.

Note that it isn't that the cylinder is full of air that keeps the water out (as shown above at left), there's actually a lot of empty space in the cylinder. Rather it is the fact that the air molecules are moving around inside the cylinder at 100s of miles per hour and they strike the water molecules with enough force that the water can't move into the cylinder.

The solution to this problem is to insert a small piece of flexible tubing into the cylinder as shown above. If you lower the cylinder into the water while keeping the two ends of the tubing out of the water, water will enter the cylinder. When the water level can be read on the scale (ideally between the 90 and 100 ml marks), the tubing is removed. This seals off the air sample and the experiment is underway.

You can carefully rest the cylinder against bottom and side of the cup. Be sure to tell any friends or roommates to leave your experiment materials alone.

Periodically lift the cylinder just enough to be able to read the water level. Don't lift the open end of the cylinder out of the water as this would break the seal and you would need to restart the experiment (extra pieces of steel wool will be available in class should this happen). Also make a note of the time.

After some time you will notice that the water level doesn't change between readings. All of the oxygen in the sample has been removed and the experiment is over. The figure below shows you one way of removing the steel wool (which should then be discarded). Return the materials to class and pick up the supplementary information handout.

Straighten the paper clip supplied with the experiment and then bend about 2/3 rds of it around the end of a pencil to form a corkscrew. Attach the corkscrew to the end of the pencil and then insert it into the cylinder. With a list twisting the corkscrew will snag the steel wool and you will be able to pull it out of the cylinder and dispose of it.

Answer to the question found earlier in the notes:
The air in high altitude cities is thinner (less dense) than at lower altitude. There isn't as much oxygen in a volume of air. With oxygen in short supply, combustion of fuels will more likely be incomplete and will produce CO rather than carbon dioxide.