Academic journal article The Science Teacher

The Green Room

Academic journal article The Science Teacher

The Green Room

Article excerpt

Good Ozone, Bad Ozone

One Free-Response Question (FRQ) on the Advanced Placement (AP) Environmental Science exam this spring addressed ozone (see "On the web"). Those knowledgeable about atmospheric chemistry might ask: "Stratospheric or tropospheric?" Well, the FRQ asked about both! Why is that tough? Most high school students, even those in advanced science classes, struggle to correctly distinguish between stratospheric ozone and tropospheric ozone. So, let's review.

Stratospheric ozone (aka "good" ozone)

Part of our atmosphere's stratospheric layer contains a naturally high concentration of ozone ([O.sub.3]) molecules. These molecules are in a dynamic equilibrium with oxygen ([O.sub.2]) molecules--constantly decomposing and forming upon exposure to ultraviolet (UV) radiation. Ozone molecules absorb some UV radiation, reducing our exposure at the Earth's surface, where it can cause sunburns, skin cancer, and cataracts. Stratospheric ozone also mitigates UV radiation's inhibition of photosynthesis and plant growth (see "On the web").

The depletion of this "good" ozone layer by emissions of ozone-depleting substances (ODSs), such as chlorofluoro-carbons, is of global concern. In 1987, the Montreal Protocol was passed to reduce ozone depletion. According to the 2010 assessment of the United Nations Environment Programme (UNEP), the protocol "successfully controlled the global production and consumption of ODSs over the last two decades. Nevertheless, ozone depletion will continue for many more decades because several key ODSs last a long time in the atmosphere after emissions end" (see "On the web").

Tropospheric ozone (aka "bad" ozone) In the troposphere--the lowest part of Earth's atmosphere--ozone forms as a secondary air pollutant. Fossil fuel combustion, among other activities, leads to ozone formation through a complex series of photochemical reactions. The United States Environmental Protection Agency (EPA) provides extensive information on ground-level ozone (see "On the web"). "Bad" tropospheric ozone causes throat irritation and worsens respiratory problems in humans and leaf damage and reduced growth in plants. Urban areas experience this ozone as a major component of photochemical smog.

Classroom activities

To further understand stratospheric ozone, send your students on the "Ozone Hole Tour" created by the Centre for Atmospheric Science at Cambridge University (see "On the web"). …

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