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Ozone (O3) is a trace gas of the troposphere, with an average concentration of 20-30 parts per billion by volume (ppbv), with close to 100 ppbv in polluted areas. . Ozone is also an important constituent of the stratosphere, where the ozone layer exists. The troposphere is the lowest layer of the Earth's atmosphere. It extends from the ground up to a variable height of approximately 14 kilometers above sea level. Ozone is least concentrated in the ground layer (or planetary boundary layer) of the troposphere. It's concentration increases as height above sea level increases, with a maximum concentration at the tropopause. About 90% of total ozone in the atmosphere is in the stratosphere, and 10% is in the troposphere. Although tropospheric ozone is less concentrated than stratospheric ozone, it is of concern because of its health effects. Ozone in the troposphere is considered a greenhouse gas, and may contribute to global warming.

Photo-chemical and chemical reactions involving ozone drive many of the chemical processes that occur in the troposphere by day and by night. At abnormally high concentrations brought about by human activities (the largest source being emissions from combustion of fossil fuels), it is a pollutant, and a constituent of smog.

Photolysis of ozone occurs at wavelengths below approximately 310-320 nano-meters. This reaction initiates the chain of chemical reactions that remove carbon monoxide, methane, and other hydrocarbons from the atmosphere via oxidation. Therefore, the concentration of tropospheric ozone affects how long these compounds remain in the air. If the oxidation of carbon monoxide or methane occur in the presence of nitrogen monoxide (NO), this chain of reactions has a net product of ozone added to the system.

Contents

 * 1Measurement
 * 2Formation
 * 3Health effects
 * 4Problem areas
 * 5Climate change
 * 6See also
 * 7References
 * 8Further reading
 * 9External links

Measurement
Ozone in the atmosphere can be measured by remote sensing technology, or by in-situ monitoring technology. Because ozone absorbs light in the UV spectrum, the most common way to measure ozone is to measure how much of this light spectrum is absorbed in the atmosphere. Because the stratosphere has higher ozone concentration than the troposphere, it is important for remote sensing instruments to be able to determine altitude along with the concentration measurements. The TOMS-EP instrument aboard a satellite from NASA is an example of an ozone layer measuring satellite, and TES is an example of an ozone measuring satellite that is specifically for the troposphere. Lidar is a common ground based remote sensing technique to measure ozone. TOLnet is the network of ozone observing lidars across the United States.

Ozonesondes are a form of in situ, or local measurements. An ozonesonde is an ozone measuring instrument attached to a meteorological balloon, so that the instrument can directly measure ozone concentration at the varying altitudes along the balloon's upward path. The information collected from the instrument attached to the balloon is transmitted back using radiosonde technology. NOAA has worked to create a global network of tropospheric ozone measurements using ozonesondes.

Ozone is also measured in air quality environmental monitoring networks. In these networks, in-situ ozone monitors based on ozone's UV-absorption properties are used to measure ppb-levels in ambient air.

Formation
The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs), react in the atmosphere in the presence of sunlight, specifically the UV spectrum. NOx, CO, and VOCs are considered ozone precursors. Motor vehicle exhaust, industrial emissions, and chemical solvents are the major anthropogenic sources of these ozone precursors. Although the ozone precursors often originate in urban areas, winds can carry NOx hundreds of kilometers, causing ozone formation to occur in less populated regions as well.

The chemical reactions that produce tropospheric ozone are a series of interrelated cycles (known as the HOx and NOx cycles); They start with the oxidation of carbon monoxide (CO) or VOCs (such as butane). To begin the process, CO and VOCs are oxidized by the hydroxyl radical (•OH) to form carbon dioxide (CO2), and water (H2O) in the CO oxidation case. These oxidizing reactions then produce the peroxy radical (HO2•) that will react with NO to produce NO2. NO2 is subsequently photolyzed during by daytime, thus resulting in NO and a single oxygen atom. This single oxygen atom reacts with molecular oxygen O2 to produce ozone.

An outline of the chain reaction that occurs in oxidation of CO, producing O3:

The reaction begins with the oxidation of CO by the hydroxyl radical (•OH). The radical adduct (•HOCO) is unstable and reacts rapidly with oxygen to give a peroxy radical, HO2•:


 * •OH + CO → •HOCO
 * •HOCO + O2 → HO2• + CO2

Peroxy-radicals then go on to react with NO to produce NO2, which is photolysed by UV-A radiation to give a ground-state atomic oxygen, which then reacts with molecular oxygen to form ozone.


 * HO2• + NO → •OH + NO2
 * NO2 + hν → NO + O(3P), λ<400 nm
 * O(3P) + O2 → O3
 * note that these three reactions are what forms the ozone molecule, and will occur the same way in the oxidation of CO or VOCs case.

The amount of ozone produced through these reactions in ambient air can be estimated using a modified Leighton relationship. The limit on these interrelated cycles producing ozone is the reaction of •OH with NO2 to form nitric acid at high NOx levels. If nitrogen monoxide (NO) is instead present at very low levels in the atmosphere (less than 10 approximately ppt), the of peroxy radicals (HO2• ) formed from the oxidation will instead react with themselves to form peroxides, and not produce ozone.