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Basics on Ozone
08.24.05
 
WHAT IS OZONE?
Ozone (O3) is a gas composed of three oxygen atoms that occurs both in the Earth’s upper atmosphere and at ground level. Ozone can be good or bad, depending on where it is found.


Animation above: This animated image shows formation and destruction of ozone.


GOOD OZONE
Ozone occurs naturally in the Earth’s upper atmosphere – 10 to 30 miles above the Earth’s surface, where it shields us from the Sun’s harmful ultraviolet rays.

BAD OZONE
At ground level ozone is created by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of heat and sunlight. Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC that help to form ozone. Sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. As a result, it is known as a summertime air pollutant.

WHICH AREAS ARE SUBJECT TO BAD OZONE?
Many urban areas tend to have high levels of "bad" ozone, but even rural areas are also subject to increased ozone levels because wind transports ozone and pollutants that form it hundreds of miles away from their original sources.

WHAT IS THE OZONE LAYER AND WHY IS IT IMPORTANT?
The stratospheric ozone layer shields life on Earth from harmful solar ultraviolet (UV) radiation. About 90% of the planet's ozone is in the ozone layer. Stratospheric ozone is a naturally-occurring gas that filters the Sun's ultraviolet (UV) radiation. Excess exposure to UV radiation is harmful to agriculture, the marine food chain and causes skin cancer and eye problems.

Ozone Hole Watch-Latest View of Antarctic Ozone + View this video

WHAT CHEMICAL PROCESS OCCURS IN THE STRATOSPHERE WITH OZONE?
Intense UV radiation in the upper atmosphere produces ozone (O3).The radiation breaks typical oxygen molecules (O2) into free oxygen atoms (O). A free oxygen atom (O) can then join with an oxygen molecule (O2) to form a molecule of ozone (O3). Ozone absorbs UV, shielding the Earth from harmful rays.

WHAT DESTROYS OZONE?
Chemical reactions involving gases such as chlorine, bromine, nitrogen, and hydrogen destroy ozone. The ozone depletion over Antarctica results from the combined actions of very cold conditions, the return of sunlight in the Antarctic spring, and ozone depleting chemicals, which mostly come from human-produced compounds.

WHAT IS A DOBSON UNIT?
A Dobson unit is a measure of the amount of ozone in the atmosphere. The unit is named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone (~1920 - 1960). He designed the 'Dobson Spectrometer' - the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity of solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which are not.