Source: Laboratories of Margaret Workman and Kimberly Frye - Depaul University

Ozone is a form of elemental oxygen (O₃), a molecule of three oxygen atoms bonded in a structure that is highly reactive as an oxidizing agent. Ozone occurs in both the stratosphere and the troposphere levels of the atmosphere. When in the stratosphere (located approximately 10-50 km from the earth’s surface), ozone molecules form to the ozone layer and help prevent harmful UV rays from reaching Earth’s surface. In lower altitudes of the troposphere (surface - approximately 17 km), ozone is harmful to human health and is considered an air pollutant contributing to photochemical smog (Figure 1). Ozone molecules can cause damage directly by harming respiratory tissue when inhaled or indirectly by harming plant tissues (Figure 2) and softer materials including tires on automobiles.

Outdoor tropospheric ozone is formed at ground level when nitrogen oxides (NO_x) and volatile organic compounds (VOCs) from automobile emissions are exposed to sunlight. Consequently, health concerns over ozone concentrations escalate in sunny conditions or when and where automobile use is increased.

Reaction: NO₂ + VOC + sunlight → O₃ (+ other products)

Indoor tropospheric ozone is formed when electrical discharges from equipment using high voltages (e.g. ionic air purifiers, laser printers, photocopiers) break down the chemical bonds of the atmospheric oxygen (O₂) in the air surrounding the equipment:

O₂ → 2 O

The free radicals of oxygen in and around electrical discharge recombine to create ozone (O₃).

2 O + 2 O₂→ 2 O₃

Figure 1: Golden Gate Bridge panorama

Characteristic coloration for smog in California in the beige cloud bank behind the Golden Gate Bridge. The brown coloration is due to the NOx in the photochemical smog.

Figure 2: Plants damaged by ozone. Top row is normal, bottom row has been exposed to ozone.