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photodissociation region

A region at the boundary of a molecular cloud where ultraviolet light dominates, dissociating (breaking up) the molecules into fragments such as radicals and ions. These fragments are very ...

photodissociation region

photodissociation region   Quick reference

A Dictionary of Astronomy (3 ed.)

Reference type:
Subject Reference
Current Version:
2018

... region A region at the boundary of a molecular cloud where ultraviolet light dominates, dissociating (breaking up) the molecules into fragments such as radicals and ions. These fragments are very reactive and generate a rich and complex chemistry. The ultraviolet light may come from an external source such as a star or nebula, or from an embedded source such as a young star within the...

photodissociation region

photodissociation region  

A region at the boundary of a molecular cloud where ultraviolet light dominates, dissociating (breaking up) the molecules into fragments such as radicals and ions. These fragments are very reactive ...
Ozone

Ozone   Reference library

Encyclopedia of Climate and Weather (2 ed.)

Reference type:
Subject Reference
Current Version:
2011

...addition, the energetically excited monatomic oxygen produced by the photodissociation of ozone is necessary for the formation of radicals such as nitric oxide (NO) and hydroxyl (OH), which in turn are involved in important chemical reactions. Moreover, ultraviolet absorption by ozone is the main agent of solar heating in the middle atmosphere, which is responsible for the increase in air temperature with height above the lower atmosphere. This temperature structure results in a stable region called the stratosphere. At the height of production, ozone abundance...

Planets

Planets   Reference library

Encyclopedia of Global Change

Reference type:
Subject Reference
Current Version:
2005

...its surface at one time. Water vapor was very abundant in Venus's lower atmosphere, however. The latent heat released when this water vapor condensed to form clouds caused the tropospheric cold trap—the cold region at the top of the convective layer—to move to a very high altitude. The stratosphere became moist, and this in turn allowed photodissociation and hydrogen escape to proceed very rapidly. Once Venus lost its water, carbon dioxide could no longer combine with surface minerals to form carbonates, so that volcanic carbon dioxide accumulated to form the...

Ozone

Ozone   Reference library

Encyclopedia of Global Change

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Environmental Science, Social sciences, Environment
Length:
6,530 words
Illustration(s):
7

...other minor constituents of the atmosphere that also affect the ozone balance. Ozone in the “Natural” Stratosphere In the strato sphere, the net production of oxygen atom results almost exclusively from the photodissociation of molecular oxygen at UV wavelengths shorter than 242 nanometers: O 2 + hν(λ < 242nm) ⟶ O + O . (2) This photodissociation process, which is the source of odd oxygen (i.e., O and O 3 ) in the stratosphere, has a rather long time constant (defined as the time delay required to reduce the molecular oxygen concentration by a factor of...

Atmosphere

Atmosphere   Reference library

Encyclopedia of Climate and Weather (2 ed.)

Reference type:
Subject Reference
Current Version:
2011
Subject:
Science and technology, Earth Sciences and Geography, Social sciences, Environment
Length:
4,482 words
Illustration(s):
2

...which can be thought of as the reverse of the reaction above. However, a small fraction (around 0.5 percent) of the organic matter produced by photosynthesis is buried in sediments, leaving an equivalent amount of O 2 free in the atmosphere. Oxygen can also be produced by photodissociation (splitting by sunlight) of water vapor, followed by the escape of the hydrogen atoms to space, but this mechanism is capable of generating only trace amounts of O 2 . The question of when significant concentrations of O 2 first appeared in the atmosphere has occupied...

Atmosphere Structure and Evolution

Atmosphere Structure and Evolution   Reference library

Julianne I. Moses

Encyclopedia of Global Change

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Environmental Science, Social sciences, Environment
Length:
3,328 words
Illustration(s):
5

...+ * → H + + H* N — O + H + * → O + + H* N — 2. Dissociative O 2 1 + e → O* + O* N Energy divided equally  recombination OH + + e → O + H* N H takes nearly all the energy 3. Impact dissociation N 2 + e* → N* + N* N e* may be a photoelectron or an accelerated electron  Photodissociation O 2 + hv → O* + O* N — 4. Ion-neutral reaction O + H 2 → OH + + H* N — 5. Sputtering or O + O + * → O* + O + * N Sputtering requires kilovolt or greater energies  Knock-on O* + H → O* + H* N Knock-on requires much less 6. Solar-wind pickup O + hv → O + + e I Also...

Ozone

Ozone   Reference library

The Oxford Companion to Global Change

Reference type:
Subject Reference
Current Version:
2009

...per cubic centimeter, or relative values of 4–8 parts per million, corresponding to 4–8 molecules of ozone for one million air molecules. The remaining 10% of the ozone is in the lower region of the atmosphere, the troposphere, which extends from the Earth's surface up to 10–12 kilometers. Ozone is continuously formed at altitudes above 30 kilometers by the photodissociation of molecular oxygen (O 2 ) by solar radiation. The ozone balance in the stratosphere is maintained by destruction of ozone through catalytic cycles that involve nitrogen, hydrogen,...

Planetary Atmospheres

Planetary Atmospheres   Reference library

Encyclopedia of Climate and Weather (2 ed.)

Reference type:
Subject Reference
Current Version:
2011

...pressure of 90 bars, is enshrouded by clouds and is composed mainly of CO 2 , along with a few percent of molecular nitrogen. Smaller amounts of SO 2 (150 parts per million) and H 2 O exist in the troposphere, and trace amounts of CO exist above the clouds, derived from photodissociation of CO 2 . The thick cloud deck, composed of sulfuric acid droplets, covers nearly 100 percent of the planet; it has a high albedo (reflectance) over visible wavelengths. Near the planet surface below this dense atmosphere, meridional (equator-to-pole) temperature differences...

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