Chemistry of Ozone Formation
Ozone forms readily in the stratosphere as incoming ultraviolet radiation breaks molecular oxygen (two atoms) into atomic oxygen (a single atom). In that process, oxygen absorbs much of the ultraviolet radiation and prevents it from reaching the Earth’s surface where we live.

In the language of a simplified chemical formula,

When an electrically excited free oxygen atom encounters an oxygen molecule, they may bond to form ozone.

Destruction of ozone in the stratosphere takes place as quickly as formation of ozone, because the chemical is so reactive. Sunlight can readily split ozone into an oxygen molecule and an individual oxygen atom.

When an electronically excited oxygen atom encounters an ozone molecule, they may combine to form two molecules of oxygen.

The ozone formation-destruction process in the stratosphere occurs rapidly and constantly, maintaining an ozone layer.

In the troposphere near the Earth’s surface, ozone forms through the splitting of molecules by sunlight as it does in the stratosphere. However in the troposphere, nitrogen dioxide, not molecular oxygen, provides the primary source of the oxygen atoms required for ozone formation. Sunlight splits nitrogen dioxide into nitric oxide and an oxygen atom.

A single oxygen atom then combines with an oxygen molecule to produce ozone.

Ozone then reacts readily with nitric oxide to yield nitrogen dioxide and oxygen.

The process described above results in no net gain in ozone. Concentrations occur in higher amounts in the troposphere than these reactions alone account for. In the 1950s, chemists discovered that two additional chemical constitutents of the troposphere contribute to ozone formation. These constituents are nitrogen oxides and volatile organic compounds, and they have both natural and industrial sources.

Nitrogen oxides (NO_x) Nitric oxide and nitrogen dioxide are together known as NO_x and often pronounced “nox.” Sources of NO_x include lightning, chemical processes in soils, forest fires, and the intentional burning of vegetation to make way for new crops (biomass burning). NO_x also come from smokestack and tailpipe emissions as by-products of the combustion of fossil fuels (coal, oil, and natural gas) at high temperatures. Coal-fired power plants are the primary sources of NO_x in the United States. Automobiles, diesel trucks and buses, and non-road engines (farming and construction equipment, boats, and trains) also produce NO_x.

Refineries generate large amounts of nitrogen oxides in the process of distilling gasoline and other petroleum products. Another major source is the burning of oil and gasoline in both power plants and automobiles. These nitrogen oxides form a link in a chain of chemical reactions that form ozone in the lower atmosphere. (Photograph copyright Philip Greenspun)

Volatile organic compounds (VOCs) such as hydrocarbons. “Volatile” refers to an extreme readiness to vaporize. Some plants and bacterial processes in soils emit VOCs. (The smell of a pine forest comes from a hydrocarbon called alpha-pinene.) VOCs also come from gasoline combustion and from the evaporation of liquid fuels, solvents and organic chemicals, such as those in some paints, cleaners, barbecue starter, and nail polish remover.

Evaporation of solvents and organic chemicals from some kinds of paint contribute volatile organic compounds (VOCs) to the air. VOCs participate in ozone formation. (Photograph copyright Philip Greenspun)

Ozone formation in the troposphere requires both NO_x and VOCs. In a highly simplified version of tropospheric ozone-forming reactions,

The formula above represents several series of reactions that do not lend themselves to simple depiction. They involve the oxidation of VOCs in reactions that also involve NO_x. Hydroxyl catalyzes some of the key reactions, and dozens of other chemical species take part. The result is ozone, nitrogen dioxide (available for more ozone formation), the regeneration of hydroxyl (available to catalyze more ozone formation), and some other chemical species. The specific ratio of NO_x to VOC determines the efficiency of the ozone formation process.

The efficiency of ozone formation rises and then falls as the ratio of nitrogen oxides (NO_x) to volatile organic compounds (VOCs) increases in an idealized plot. Higher NO_x emissions result in less efficient ozone production. The modeler who made this plot intentionally left off all units of measurement, because the ratio itself is more important than the specific concentrations of the compounds. Authorities who want to control ozone production must take the ratio into account. (Graph courtesy Jim Meagher, NOAA Aeronomy Laboratory)

Livestock such as cattle and hogs emit significant amounts of methane, one of the chemicals involved in ozone production. One cow produces an average of 0.23 kg (0.5 lb) of methane per day. Earth's total population of 1.4 billion cows produce 322 million kg (708 million pounds) of methane per day. Termite mounds and rice paddies are also significant producers of methane. (Photograph courtesy USDA On-line Photography Center)

Ozone formation with the hydrocarbon methane provides a useful example of the general pattern that most such reactions follow. The methane example is somewhat simpler and easier to follow than the others, described in steps.

Most ozone formations in the troposphere involve non-methane hydrocarbons. The chemistry of ozone formation from non-methane hydrocarbons follows the general pattern described above but is much more complex. NO_x and VOCs together include about 120 different chemical compounds, and hundreds of chemical reactions can take place. Some of the participating chemicals may be intercepted part of the way through the process by reactions with other chemicals in the atmosphere, and may form intermediate compounds that act as temporary reservoirs for varying amounts of time.

Swirling cloud masses over the Tasman Sea between Australia and New Zealand illustrate the fluidity of our dynamic atmosphere. Winds and weather conditions such as air temperature and humidity influence ozone chemistry. (Photograph courtesy NASA JSC Gateway to Astronaut Photography of Earth)

An additional challenge arising for anyone tracking tropospheric ozone-forming reactions is that they entail interactions between different phases of matter (gas, liquid, and particles known as aerosols), and can occur on various kinds of aerosol surfaces in the atmosphere. Changing environmental conditions such as air temperature and humidity also affect ozone chemistry. Furthermore, many of the chemicals involved have very short lifetimes before they react with other chemicals to form new compounds. Scientists face myriad challenges in their pursuit of understanding tropospheric ozone chemistry.

References
Finlayson-Pitts, Barbara J. and Pitts, James N., Jr. 1999. Chemistry of the Upper and Lower Atmosphere. (Academic Press) P. 583

Fishman, Jack. 1990. Global Alert: The Ozone Pollution Crisis. (New York and London: Plenum Press)

Fishman, Jack, et al., NASA Langley Research Center, Hampton, VA. 1999. Surface Ozone Measurements: Exciting Science for All Seasons All the Time.

Madronich, Sacha. 1993. Tropospheric photochemistry and its response to UV changes. In The Role of the Stratosphere in Global Change. Vol. 18. NATO-ASI Series, Editor M-L. Chanin. (Amsterdam: Springer-Verlag) Pp. 437-61

Turco, Richard P. 1996. Earth Under Siege. (New York: Oxford University Press)

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Chemistry in the Sunlight
• Introduction
• Ozone, Space, and Time
• Chemistry of Ozone Formation

Related Articles:
• Stratospheric Ozone
• Highways of a Global Traveler: Tracking Tropospheric Ozone
• The Ozone We Breathe