Ground-level ozone produces an interesting example of the consequence of isopollution curves that do not always slope downward. Ozone is an air pollutant that results from a chemical reaction involving hydrocarbons (HC) and nitrogen oxides (NOx), both pollutants in their own rights, in the presence of sunlight. The isopollution curves for ozone production mostly slope downward but then bend up at one end. Consider textbook Figure 8.11. It shows two isopollution curves, one with a higher level of ozone than the other. Thus, point A represents higher ozone levels than point B. In practice, air pollution regulators typically reduce ozone by requiring reductions in both NOx and HC. Suppose, however, that ozone levels are at point B, and that a regulator proposes to reduce only NOx levels, because NOx itself causes damages. This phenomenon is known as NOx disbenefit. What happens to ozone levels?

Isopollution lines for ozone production.

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08 0.7 0.6 0.5 Higher ozone 0.4 0.3 Lower ozone 0.2 0.1 0 1 2 34 5 6 789 10