Chemistry of Ozone
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
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
an electronically excited oxygen atom encounters an ozone molecule, they
may combine to form two molecules of oxygen.
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 (NOx)
Nitric oxide and nitrogen dioxide are together known as NOx
and often pronounced “nox.” Sources of NOx include
lightning, chemical processes in soils, forest fires, and the
intentional burning of vegetation to make way for new crops (biomass
burning). NOx 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 NOx in the United States. Automobiles,
diesel trucks and buses, and non-road engines (farming and construction
equipment, boats, and trains) also produce NOx.
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 NOx 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 NOx.
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 NOx 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 (NOx) to volatile
organic compounds (VOCs) increases in an idealized plot. Higher
NOx 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 700 million kg (317
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. NOx 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
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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