How Ozone Is Formed

Published May 1, 2006

NOx, shorthand for the sum of NO2 + NO, is necessary for the formation of ozone (O3). Even when volatile organic compounds (VOC) are not present, ozone is formed through a series of sunlight-driven reactions among NO2, NO, and oxygen:

  • NO2 + sunlight —> NO + O(1)
  • O + O2 —> O3(2)
  • O3 + NO —> O2 + NO2(3)

This cycle results in relatively low ozone levels. Ozone can’t build up because, although it is formed in reaction (2), it is destroyed in reaction (3).

But add VOC, and ozone builds up. VOC allow NO2 to be regenerated without destroying ozone. That is, VOC allow reaction (3) to be bypassed. OH radicals (also generated by various reactions among pollutants in the atmosphere) convert some VOC to peroxy radicals, which then regenerate NO2 as follows:

  • VOC–OO + NO —> NO2 + VOC–O(4)

… where the two oxygen atoms (“OO”) are the peroxide group attached to a VOC.

Ozone formation depends on the ratio of VOC to NOx (VOC/NOx). At high VOC/NOx ratios, ozone formation is controlled by the amount of NOx available, and reaction (4) is the main route to regenerate NO2 from NO. Under this “NOx-limited” situation, decreasing NOx reduces ozone, while decreasing VOC has little or no effect on ozone.

But at low VOC/NOx, ozone formation is limited by the amount of VOC available for reaction (4), and reaction (3) becomes the main route to regenerate NO2 from NO. In addition, at low VOC/NOx, NO2 competes with VOC to react with OH radicals, slowing the rate at which VOC is converted to peroxy radicals, and thereby slowing the rate of reaction (4).

Under this “VOC-limited” or “VOC-sensitive” condition, reducing VOC reduces ozone, but reducing NOx increases ozone. The NOx reductions increase ozone through two means: First, by slowing down the rate of ozone destruction through reaction (3), and second, by speeding up the rate of NO2 regeneration through reaction (4), allowing each molecule of NOx to make ozone more rapidly.

— Joel Schwartz