Ozone and the Environment

Ozone (O₃) is an unstable molecule that contains three oxygen atoms. It is a very reactive gas, and even at low concentrations is irritating  and toxic. Although it represents a tiny  fraction of the atmosphere (approximately 0.000002 - 0.000007% at sea level), ozone is crucial for life on earth. At room temperature, ozone is a pale blue gas with a characteristic odor similar to that of the air after a thunderstorm. It condenses to a dark blue liquid at -112 and freezes at -193.

Ozone is highly reactive form of oxygen. It is a very powerful oxidizing agent. It can oxidize many organic compounds and is used commercially as a deodorizing agent, as a germicide and as a bleach for textiles, oils and waxes.

 The presence of ozone in the atmosphere can be positive or negative depending on where it is found. In the troposphere – the level of the atmosphere that contains the air we breathe – ozone is a damaging pollutant. In the stratosphere – 15 to 50 kilometers above the surface of the earth – ozone forms a protective layer which absorbs harmful U.V. rays of the sun.

The atmospheric chemistry involved in ozone formation is complex. In general, ozone can be formed when a mixture of NO₂ and O₂ is exposed to bright light. The reactions involved are as follows:

 Nitrogen dioxide dissociates when is irradiated with bright light:

                                               NO_2(g)    =    NO_(g)       +   O_(g)    

The oxy gen atom formed in this process is very reactive and readily attaches to a molecule of O₂ to form ozone:

                                                  O_(g)  +   O_2(g)    =    O_{3 (g)}

NO₂ is formed from NO which is a gas produced during the operation of internal combustion engines:

                                             N_2(g)  +  O_2(g)  + heat   =   2 NO_(g)

                                           2 NO_(g)  +  O_2(g)   =   2 NO_2(g)

On sunny days where  NO₂  pollution from cars is high, the concentration of ozone in the air can reach levels that are dangerous for plants and animals.

Ozone injury on a pumpkin leaf (taken from USDA, Agricultural Research Center)

In most countries an "ozone alert" is issued when the average concentration of ozone over an eight hour period is over 100 ppb. For example, a pollution scorecard including ozone is shown for the Niagara Falls region. Trends for ground-level ozone concentrations for the last three decades are given by EPA.

Los Angeles, along with Long Beach and Riverside, Calif. remain a metropolitan area with one of the highest levels of ozone pollution in the U.S. Nick Ut/AP (taken from iwatchnews, see relevant article)

  Ozone has several toxic effects. Air containing 1000 ppb by volume ozone has a distinct odor and can cause severe irritation and headache. It irritates the eyes, the upper respiratory system and the lungs. Chromosomal damage has also been observed in subjects exposed to ozone. Ozone generates free radicals in tissue. These reactive species can cause lipid peroxidation, oxidation of sulhydryl (-SH) groups and other destructive oxidation processes.

In contrast to the harmful effects of ozone in the air we breathe, the effects of ozone in the upper atmosphere are essential to the survival of life on earth. In the upper atmosphere (specifically the stratosphere) ozone absorbs harmful ultraviolet radiation and serves as a radiation shield, protecting living beings on the earth from the effects of excessive amounts of such radiation.
The ozone in the stratosphere is produced by photochemical reactions involving oxygen O₂ . When diatomic oxygen in the stratosphere absorbs ultraviolet radiation with wavelengths less than 242 nm, it breaks apart into two oxygen atoms.

  O_2(g)  +   h.ν    =    O _{(g)  +}  O (g)

The resulting oxygen atoms combine with O2 molecules and M molecules (where M can be N2, O2 or another species) to form ozone. The chemical reaction is shown below:

O(g) +  O_2(g)  +  M    =    O_{3 (g)  +}  M (increased energy)

The above reaction is exothermic. The heat given off  is absorbed by M molecules and enable the ozone formed to stay together. The net effect of the previous two reactions is the conversion of three molecules of  O_2(g) to two molecules of ozone with the simultaneous conversion of  light (ultraviolet light) to heat.

2 O3 (g)  =  3 O2(g)

 Thus, ozone in the stratosphere prevents high energetic radiation from reaching the earth's surface and converts the energy of this radiation to heat. If this light were not absorbed by ozone, severe damage would result to expose forms of life on earth.