The Ozone Hole-Ozone

Ozone

named from the Greek "ozein" for smell. Pronunciation: 'O-"zOn

Function: noun

Etymology: German Ozon, from Greek ozOn, present participle of ozein to smell

Ozone is a molecule that contains three atoms of oxygen and thus has the formula O3.

Ozone was first discovered in 1839 by German scientist Christian Friedrich Schonbein.

Ozone is a molecule made up of three oxygen atoms which is naturally formed by the photolysis of normal oxygen by ultraviolet solar radiation at wavelengths below 242.5 nm in the stratosphere. A certain amount of ozone is also produced in the troposphere in a chain of chemical reactions involving hydrocarbons and nitrogen-containing gases. Though ozone is a minor atmospheric constituent, with an average concentration of about 3 parts per million volume (ppmv), the radiation properties of this greenhouse gas make it a significant contributor to the radiative energy balance of the atmosphere, and an important regulator of the ultraviolet solar radiation received at the Earth's surface. Most of the atmospheric ozone (90 per cent) is located in the stratosphere with a maximum concentration of between 17 and 25 km , depending on latitude and season, where its presence causes stratospheric temperature inversion and results in maximum temperature at the stratopause. In addition to its radiation properties, ozone reacts with many other trace species, some of which are anthropogenic in origin. The geographical and vertical distributions of ozone in the atmosphere are determined by a complex interaction of atmospheric dynamics and photochemistry.

Ozone near the ground is monitored because it is a product of industrial and urban pollution. Measurements of tropospheric and stratospheric ozone are used for the verification of models that simulate the photochemistry or general circulation of the real atmosphere. Ozone is also measured to determine attenuation of the ozone layer by man.made gases, to validate model estimations of changes in ozone and to confirm the efficiency of the Montreal Protocol on Substances that deplete the Ozone Layer and its Amendments. This monitoring of the ozone layer requires high.quality, long-term records of ozone at stations with well-maintained instruments, which are crucial for reliable trend analyses.

A member of the Zhong Shan station crew with the Brewer spectrophotometer.

There are basically three characteristics of atmospheric ozone that are routinely measured and reported by ground and satellite monitoring systems, namely:

(a) Surface ozone;

(b) Total ozone;

Surface ozone expresses the concentration of local ozone in the layer a few metres above the ground at a particular site on the Earth’s surface. Surface ozone measurements are commonly given in units of partial pressure or mixing ratio (by either mass or volume).

Total ozone refers to the total amount of ozone contained in a vertical column in the atmosphere above the ground extending from the earth’s surface to the upper edge of the atmosphere. Commonly used units of total ozone are (a) column thickness of a layer of pure ozone at standard temperature and pressure (STP) and (b) vertical column density (number of molecules per area).

The vertical profile of ozone expresses ozone concentration as a function of height or ambient pressure. The amount of ozone at each height or pressure level in the atmosphere is commonly expressed as partial pressure, mixing ratio or local concentration (number density). The integral of the ozone profile from the ground to the top of the atmosphere is the total column amount of ozone.

The following are definitions of other terms used frequently in this context:

Aerosols: A suspension, in a gaseous medium, of solid particles, liquid particles, or solid and liquid particles

Relative optical air mass: The ratio of the slant path of solar radiation through the atmosphere (or through a layer) to the vertical path.

Dobson unit (DU): A measure of total ozone equalling a thickness of 10.5 m of pure ozone at STP (commonly used but not a unit in the International System of Units).

Milliatmosphere centimetre (m-atm-cm): A measure of total ozone equal to 10.3 cm of pure ozone at STP (1 m-atm-cm is is equivalent to 1 DU).

Ozone: An unstable blue gaseous allotrope of oxygen and a strong oxidant. It absorbs selectively radiative energy in the 100.340- and 550.650-nm bands of the solar spectrum and at 4 700, 9 600, and 14 100 nm in the infrared.

Spectrophotometer: An instrument for creating a spectrum and measuring the spectral radiance at selected wavelengths.

Ultraviolet (UV): Electromagnetic radiation in the 100.400-nm range which is often divided into UVA (315.400 nm), UV.B (280.315 nm) and UV.C (100.280 nm).

Umkehr: An optical effect appearing close to sunrise or sunset when relative zenith sky radiances increase with increasing solar zenith angle. By taking a series of zenith measurements with spectrophotometers at selected UV wavelengths, it is possible to infer the vertical distribution of ozone. These ground.based measurements are performed only for clear skies.

Ozone - Good Up High Bad Nearby

NASA Graphic

It is a pale blue, relatively unstable molecule made up of three oxygen atoms. The ozone molecule is angular, polar, and diamagnetic. Both oxygen bond lengths (1.28 angstroms) are identical. It is formed from molecular oxygen (O2) by ultraviolet and extreme ultraviolet photolysis followed by recombination of atomic oxygen (O) with O2.The name ozone is derived from a Greek word meaning "to smell". It may also be formed by passing an electrical discharge through gaseous oxygen. It is characterized by a unique odor that is often noticed during electrical storms and in the vicinity of electrical equipment. In fact, the term ozone is derived from the Greek word ozein which means "to smell." The density of ozone is about 2.5 times that of O2. At -112 degrees C it condenses to a deep blue liquid. It is a powerful oxidizing agent and, as a concentrated gas or a liquid, is highly explosive. Excess oxygen atoms, also known as free radicals, oxidize materials that they contact and are associated with the aging process.

Credit:University of Alaska

Ozone is a relatively simple molecule, consisting of three oxygen atoms bound together. Yet it has dramatically different effects depending on where ozone resides, it can protect or harm life on Earth. High in the atmosphere about 15 miles (24 km) up ozone acts as a shield to protect Earth's surface from the sun's harmful ultraviolet radiation. Without this shield, we would be more susceptible to skin cancer, cataracts, and impaired immune systems.

Closer to Earth, in the air we breathe, ozone is a harmful pollutant that causes damage to lung tissue and plants. Near Earth's surface, where ozone comes into direct contact with life forms, it primarily displays a destructive side. At ground level, ozone is a health hazard it is a harmful pollutant that causes damage to lung tissue and plants- it is a major constituent of smog.

Earth’s atmosphere

NASA GSFC Graphic

Because it reacts strongly with other molecules, large concentrations of ozone near the ground prove toxic to living things. Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC, also known as ozone precursors.

Environmental Protection Agency graphic

Strong sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. Many urban areas tend to have high levels of "bad" ozone, but other areas are also subject to high ozone levels as winds carry NOx emissions hundreds of miles away from their original sources.

At higher altitudes, where 90 percent of our planet's ozone resides, it does a remarkable job of absorbing ultraviolet radiation. In the absence of this gaseous shield in the stratosphere, the harmful radiation has a perfect portal through which to strike Earth. The amounts of "good" and "bad" ozone in the atmosphere depend on a balance between processes that create ozone and those that destroy it. An upset in the ozone balance can have serious consequences for life on Earth. Scientists are finding evidence that changes are occurring in ozone levels—the "bad" ozone is increasing in the air we breathe, and the "good" ozone is decreasing in our protective ozone shield.

Launching an ozonesonde from the Amundsen-Scott Base at the South Pole. The launch is done by Nick Morgan while a NOAA colleague is taking the photo. At this station polyethylene balloons are used since they consistently reach sufficient altitude even under cold conditions. Rubber balloons tend to become brittle at low temperatures and often burst at relatively low altitude.

Factors influencing Ozone concentrations

Stratospheric sulfate aerosols: large explosive volcanoes are able to place a significant amount of aerosols into the lower stratosphere, as well as some chlorine. Because more than 90% of a volcanic plume is water vapor most of the other compounds, including volcanic chlorine, get ''rained-out'' of the stratosphere. The effects of a large volcano on global weather are significant, which in turn can affect localized weather patterns such as the antarctic ozone hole. Many observations have linked the 1991 Mt. Pinatubo eruption to a 20% increase in the ozone hole that following spring[Solomon et al. 1993]) . The effects of a large volcanic eruption on total global ozone are more modest (less than 3%) and last no more than 2-3 years.
Stratospheric winds: every 26 months the tropical winds in the lower stratosphere change from easterly to westerly and then back again, an event called the Quasi-biennial Oscillation (QBO). The QBO causes ozone values at a particular latitude to expand and contract roughly 3%. Since stratospheric winds move ozone, not destroy it, the loss of one latitude is the gain of another and globally the effects cancel out.
Greenhouse gases: to the degree that greenhouse gases might heat the planet and alter weather patterns, the magnitude of the stratospheric winds will certainly be affected. Some of the more popular scenarios of global warming predict cooler stratospheric temperatures, leading to more polar stratospheric clouds and more active chlorine in the area of the antarctic ozone hole.
Sunspot cycle: ozone is created by solar UV radiation. The amount of UV radiation produced by the sun is not constant but varies by several percent in a roughly 11year cycle. This 11year cycle is related to magnetic changes within the sun which increase the solar UV output, and is heralded by an increase sunspots which appear on the surface of the sun. Comparisons of yearly ozone concentrations show a small 11 year variation in global ozone of about 2%. Episodes of unusual solar activity, solar storms and large solar flares, could certainly alter this value.
Stratospheric chlorine, coming mostly from man-made halocarbons. Careful subtracting of other natural factors yields a net decrease of 3% per decade in global ozone,1978-1991; due most likely to catalytic degradation by stratospheric chlorine

Ozone is measured throughout the atmosphere with instruments on the ground and on board aircraft, high-altitude balloons, and satellites. Some instruments measure ozone locally in sampled air and others measure ozone remotely some distance away from the instrument. Instruments use optical techniques, with the Sun and lasers as light sources, or use chemical reactions that are unique to ozone. Measurements at many locations over the globe are made regularly to monitor total ozone amounts.

Credit: EPA,NASA,NOAA