Background Information

Volcanic eruptions can increase the risk of skin cancer, even when the erupting volcano is a hemisphere away. Besides this personal risk, there is a larger, global risk. Volcanic eruptions may cool the Earth. This is a dramatic twist that would complicate efforts to conclusively determine whether or not greenhouse gases contribute to global warming. A close look at the June 1991 eruption of Mt.Pinatubo, in the Philippines, may help to explain this seemingly remote connection between health, the environment, and volcanos.

When Mt. Pinatubo erupted in the early summer of 1991, it sent clouds of smoke, ash, sulfur dioxide, and water vapor into the atmosphere. Most of the heavier ash settled to the Earth within the first several weeks. By mid-August, however, satellite measurements showed that a band of sulfuric acid droplets in the stratosphere had spread around the Earth in a path on both sides of the equator.

Sulfuric acid is formed when sulfur dioxide, a gas, combines with water. In this case, the tiny sulfuric acid droplets are called aerosols. Aerosol particles that travel around the Earth in the stratosphere are less likely to fall to Earth and therefore, remain aloft for a longer period of time than particles in the troposphere.


The larger aerosol particles will settle out of the atmosphere within about three years. The smallest particles could remain suspended for decades. Some computer models of atmospheric chemistry suggest that a huge increase in sulfuric acid aerosols could thin the protective ozone layer, allowing harmful ultraviolet (UV) radiation to reach the Earth's surface. This increase in surface UV could increase health risks, including skin cancer. In addition to thinning the protective ozone layer, atmospheric aerosols may affect the Earth's temperature. Since more light from the sun is reflected back into space by the increased amount of aerosol particles in the stratosphere, the Earth's lower atmosphere is likely to cool. This cooling effect will complicate efforts to determine whether or not there is a net global warming due to the greenhouse effect.

Atmospheric scientists are studying the effects of Mt. Pinatubo's eruption using lidar, a type of radar that uses pulses of laser light instead of pulses of radio waves. The short pulse of light bounces off particles, molecules, and even insects in the atmosphere. Some of the scattered light returns to its source. Measuring the amount of time it takes for the scattered laser light to return allows us to calculate the distance to the object (in this case, aerosols). The light that returns to the source is called "backscatter." The amount of backscatter indicates the amount of sulfuric acid aerosols in the atmosphere. The larger the backscatter number, the more scattered light that is returning to the lidar after bouncing off aerosols, the more aerosols in the atmosphere.




Procedure

  1. In Figure 6.1, number the horizontal axis (backscatter) of the graph from zero to 3600 by 100s.

  2. Also in Figure 6.1, number the vertical axis (altitude) of the graph from zero to 30 by ones.

    Figure 6.1. Atmospheric Backscatter Data over Boulder, Colorado
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  3. Using the data provided in Table 6.1, plot the data points corresponding to the units of backscatter for each time period. Connect the points with a smooth line. Use a different colored pencil for each time period.

    Table 6.1. Atmospheric Backscatter Data over Boulder, Colorado
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  4. Draw a horizontal dashed line across the graph at ten kilometers. Label the area beneath the line "Troposphere."

  5. Draw a horizontal dashed line across the graph at 11 kilometers. Label the area beneath the line "Tropopause."

  6. Label the upper part of the graph "Stratosphere."

  7. Print a title at the top of the graph.

  8. Place a color coded legend on your graph in the space provided.




Questions

  1. What gas combines with water from a volcanic eruption to form sulfuric acid aerosols?

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  2. At what altitude above the troposphere is the most backscatter from aerosols located?

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  3. What layer of Earth's atmosphere above 10 kilometers has the most aerosol backscatter?

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  4. From January 1991 to August 1991 what happened to the amount of backscatter above an altitude of 10 kilometers?

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    Why?

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  5. What is the maximum (highest) amount of backscatter in the stratosphere for August 1991?

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    For January 1992?

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  6. In what layer of the Earth's atmosphere, above the troposphere, is the largest amount of backscatter located for January 1992 and March 1992?

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  7. Between August 1991 and January 1992, what change in altitude, above the troposphere, occurred for the maximum amount of backscatter?

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  8. Why is the change in altitude significant for the maximum amount of backscatter between August 1991 and January 1992?

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  9. Why does the maximum amount of backscatter occur in January 1992, when the eruption of Mt. Pinatubo occurred in June 1991, six months earlier? (Hint: Think of our location on the globe compared to Mt. Pinatubo.)

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Figure 6.2. Questions Sheet
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Conclusions

Review the problem stated in the workstation screen graphic
at the top of this web page and write your conclusions here.



Figure 6.3. Conclusions Sheet
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For Your Information

The original backscatter data have been multiplied by 1012 to make the numbers easier to manipulate. The actual unit of backscatter is called the "backscatter cross section" (m-1sr-1x10-9), where "sr" is a solid angle called a steradian. To simplify this term, we call it a backscatter unit.




For Your Information

Figure 6.4 gives you information on the various levels and layers in the atmosphere.

Figure 6.4. Names and Heights of the Different Atmospheric Layers
(heights are representative values)
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