Thursday, June 21, 2007

Ozone and the light from above: The upper atmosphere

Through a number of previous postings, the Earth's upper atmosphere has come up as an important topic. The upper atmosphere is connected with the lower atmosphere (troposphere), but it's also interesting in its own right and deserves a posting all its own. The comparison of Earth's atmosphere with other planets will underline its importance.*

This is the atmosphere's vertical structure:

Vertical structure of the atmosphere
In short, the upper atmosphere is everything above the troposphere, from the tropopause and on into outer space. The highest layer (not shown) is called the exosphere.

Essential characteristics of the upper atmosphere. With two key points in mind, you'll understand most of what makes the upper atmosphere special.

The first is that the upper atmosphere is strongly heated by direct radiation from the Sun. That's different from the troposphere, which is heated largely from the surface. Of course, this heat flow does ultimately come from the Sun, and clouds do directly absorb some of the incoming solar radiation. But that's a secondary effect compared to the re-radiation from the surface as infrared (IR).

This direct absorption of incoming solar radiation is so strong that the temperature stops "lapsing" with altitude; that is, in parts of the upper atmosphere (stratosphere and exosphere), the temperature rises with altitude - there's that much solar radiation being directly absorbed. One of these temperature inversions starts at the tropopause and extends through the lower stratosphere; that's the inversion that puts a "lid" on the "boiling" convection of the tropopause. No upward vertical heat transport is possible by matter under such conditions, and indeed, upward heat transport is all radiative in the upper atmosphere. There are virtually no IR-active gases (water vapor, CO2) up there anyway, so the IR radiation proceeds almost without obstacle into space.**

Without the strong direct absorption of incoming solar radiation, the temperature would just keep declining with altitude, as pressure and density actually do. (In hydrostatic equilibrium, a negative pressure gradient is needed at every altitude to counter the downward pull of gravity.) The upper atmosphere would get colder and colder as it gets more and more rarefied farther from the surface. Instead, because of the rising temperature, the upper atmosphere at very high altitudes (exosphere) merges into the rarefied, hot plasma of the solar wind, the rapidly expanding outermost layer of the Sun's atmosphere. That is, the Earth's upper atmosphere and the Sun's mingle high above our heads, an important fact whose significance will become apparent.

The second point concerns the nature and effects of this directly absorbed radiation: it's energetic ultraviolet (UV) radiation - photons with energies higher and wavelengths shorter than those of optical radiation - that dissociates and ionizes the rarefied molecules and atoms of high altitudes. "Dissociation" means that molecules are broken down into their constituent atoms; "ionization" means that molecules and atoms are stripped each of one or more negatively charged electrons and themselves become positively charged. (These separated charges are ions. Without ionization, atoms and molecules are electrically neutral, as their internal charges balance each other.) A gas of electrically charged atoms and molecules is a plasma.

Although most of the Sun's radiation output takes place in the optical (visible) range, a significant fraction of it occupies shorter wavelength ranges. The "softest" (least energetic) part of the solar UV gets through to the ground; that's how you get a suntan. The "harder" (more energetic) part of the solar UV is absorbed by the upper atmosphere, thankfully - otherwise, we and other life forms on the surface would be toast. The absorbers are molecular oxygen (O2) and a famous by-product of O2 and UV called ozone (O3).

The modern convention for light wavelengths is nanometers (nm), billionths of a meter (millionths of a millimeter and thousandths of a micron). (Ten angstroms is one nm.) Visible light runs from 400 nm (deep violet) to 700 nm (deep red). The "soft" UV range that gets to the surface is higher than 320 nm. The "mid" UV range that ozone absorbs lies between 240 and 320 nm, at altitudes between 10 and 50 km. Diatomic oxygen absorbs "hard" UV smaller than 240 nm, at altitudes above 90 km.

Photochemistry of oxygen and ozone. The whole complex of radiation and forms of oxygen is a big tangle, but can be boiled down to a couple essentials.

In the high upper atmosphere (mainly above 90 km - the thermosphere), diatomic oxygen (O2) absorbs the hardest part of the solar ultraviolet: O2 + UV photon -> O + O. (Below 180 nm wavelength, diatomic nitrogen N2 also does some of the absorbing.) Some of the monatomic oxygen (O) floats down and combines with O2 to form ozone (O3). The third O in ozone is only loosely bound.

The ozone then absorbs two distinct types of radiation. Like Schroedinger's cat, O3 exists simultaneously in two quantum states: all three O's equally orbiting the others; and two O's deeply bound (quasi-O2), with a loose O in a "far orbit." †

Like the hardest UV, the intermediate UV breaks the "strong" O2 bond embedded within the O3; but the intermediate UV photon doesn't have to be quite so energetic, because some break-up energy can be taken from the third O. It's crucial for life, because it means that most of the solar UV is absorbed in the mid- to lower stratosphere, leaving only the "softest" component to reach us on the ground. While small doses of the soft UV (360-400 nm) benefit us by producing vitamin D in the skin, UV in high doses, especially below 340 nm, is very harmful.

Finally, the so-called mid-IR radiation (around 10,000 nm) from the Earth's surface breaks the "weak" bond that binds the third O to the O2. Below the bottom of the stratosphere (about 10 km), this absorption destroys virtually all O3 (by O3 + IR photon -> O2 + O, with the loose O's recombining to form O2). Only a tiny equilibrium O3 residual remains in the lower atmosphere.††

Ozone destruction and the polar ozone "hole." The absorption of all but the "soft" solar UV radiation by naturally existing O2 and O3 is critical for life on the surface, shielding living tissue and DNA from most solar UV radiation. Trace atmospheric chemicals from volcanic and man-made sources can enhance the destruction of ozone in the lower stratosphere, although it should be clear that these only add to the main ozone destruction channel of IR absorption. The critical point is that the intermediate UV penetrates the atmosphere more deeply when there's less ozone in the lower stratosphere. More of it gets through to the ground. The "hard" UV is absorbed by O2 in any case.

But the full situation is trickier than it first appears. The same Sun whose intermediate UV is blocked by O3 at lower altitudes also produces that O3 in the first place from O2 by its hard UV at a higher altitude. We should expect then that, in polar night, O3 should vanish from the stratosphere, and it does. Only when the Sun returns in the polar spring does its UV start producing O3 again. So the existence of an ozone "hole" in polar winter is not any surprise or cause for panic; that's the way it should be.

The right questions are, do these trace chemicals delay the formation of the equilibrium ozone layer in the polar spring? And, are the accelerated breakdown and formation retardation of O3 by trace chemicals allowing in significantly more intermediate solar UV a cause for concern? Maybe. However, measured UV levels at the ground (outside the polar regions) have been mostly falling for several decades. The panic over ozone depletion in the 1970s and resurgent in the early 1990s was overblown - a forerunner-in-miniature of the much larger "global warming" hysteria now playing on your TV.
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* The two classic books of Craig, the more technical The Upper Atmosphere: Meteorology and Physics and the popular The Edge of Space: Exploring the Upper Atmosphere, are out of print and somewhat dated but still excellent introductions to the topic.

** This does not preclude horizontal heat transport by matter (convective currents in air = winds). Indeed, the lower stratosphere features some very strong horizontal winds (like the jet stream).

† Chemistry students should think of a close cousin, bond hybridization.

†† A significant part of the trace tropospheric ozone is produced by lightning and ground activities. It has a distinctive tangy smell familiar to students of photochemistry and users of ionizing air filters.

Ozone itself is powerfully oxidizing and poisonous to humans in significant amounts. Sustained breathing of trace amounts produces headaches and nausea.

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