Environmental Science and International Politics by David E. Henderson & Susan K. Henderson

Author:David E. Henderson & Susan K. Henderson [Henderson, David E.]
Language: eng
Format: epub
ISBN: 9781469640297
Publisher: Reacting Consortium Press
Published: 2018-03-01T08:00:00+00:00

FIGURE 18 Diagram of the processes involved in the formation of ozone and smog.

Early efforts in the United States to control photochemical smog were focused on removing the VOC part of the chemical equation. Because both VOCs and NOx are required, it was believed that removing the VOCs would stop the cycle and prevent ozone formation. This would not do anything to deal with the acid rain produced, but most acidity in rain was due to SO2, so this was considered an acceptable approach.

The United States instituted a wide range of actions to reduce VOCs. Evaporative loss equipment was required in cars to capture gasoline that evaporated from the gas tank in hot weather. Similar equipment was installed at gas stations and gasoline storage facilities. Most oil-based paints were eliminated and replaced with low VOC latex paint. Industries that had once exhausted solvents into the air were required to capture and recycle them. The effort to control VOCs was successful, but the efforts failed to reduce smog as expected.

The reason that VOC control failed to reduce ozone is related to a central concept of chemistry, the limiting reagent. The best way to introduce this is to recognize that every chemical reaction is a recipe. So let us consider the recipe for s’mores.

1 piece chocolate bar + 1 toasted marshmallow + 2 pieces graham cracker → 1 s’more

As you have probably made s’mores at some point, you experienced the limiting reagent problem firsthand when you ran out of one of the ingredients. So let us look at how a chemist would analyze the problem.

Each chocolate bar provides 3 pieces of chocolate. Each whole graham cracker has 2 pieces. We go to the store and buy 4 chocolate bars, a bag of marshmallows, and a box of graham crackers. We count the marshmallows and find we have 30 of them. The box of graham crackers has 40 whole crackers. The question is how many s’mores can we make?

You intuitively know that with 4 chocolate bars and 3 pieces per bar, we can make at most 12 s’mores (1 piece/s’mores × 3 pieces/bar × 4 bars × = 12 s’mores), which assumes you can keep people from snacking on the chocolate. (Keep your hands off that chocolate!) We have enough marshmallows for 30 s’mores and enough crackers for 40 s’mores. It always seems to work out this way. You make all the s’mores you can, toast the rest of the marshmallows, and end up with a surplus of crackers. Chocolate bars always seem to be the limiting reagent.

In the recipe for ozone, we saw we needed three things: NOx, VOC, and sunlight. We cannot control the sunlight, but the United States has tried to stop the reaction by controlling the VOCs. The reason this strategy did not work was that the VOCs were the graham crackers in this process. There are large natural sources of VOCs. Even if people stopped all their emissions of VOCs, there would still be enough VOCs left to use up all the NOx.

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