Many pollutants fall back to earth on their own. Carbon dioxide, for example, is absorbed by trees and by the oceans to be used in photosynthesis. Dust particles fall because of their weight.

But another class of pollutants, including methane and other "greenhouse gases," is not scavenged back to the surface. Instead, these need to be broken down chemically into forms that fall like dust, or are washed out of the atmosphere by rain or are similarly cleansed from the atmosphere.

The reactions which these molecules must undergo, called oxidation, need ozone for to work.

"Most of the oxidation takes place in the troposphere," said Jacob. "There are only a few strong radical oxidants that can perform the reactions. They ordinates from ozone."

One negative effect of a loss of tropospheric ozone can be illustrated by the present trend to phase out chloroflorocarbons (CFCs), present in aerosols and Styrofoam, and replace them with hydrochloroflorocarbons, (HCFCs).

"CFCs, which destroy stratospheric ozone, are going out, and HCFCs are coming in," Jacob says. "The HCFCs won't break down the ozone in the stratosphere, because they can be oxidized in the troposphere."

If levels of tropospheric ozone remain high enough, HCFCs will be broken down and scavenged without reaching the stratosphere. But if tropospheric ozone levels drop, HCFCs will reach the stratosphere and cause just as much damage as their predecessors.

"We want to boost the oxidizing power of the atmosphere. If tropospheric ozone levels had not risen over the past 100 years, we'd in trouble," Jacob says. "But at the same time we dump pollutants that need to be destroyed, we dump more ozone to destroy them."

And in a statement which seems to contradict atmospheric chemists, Jacob jokes. "Maybe pollution isn't so nasty after all."