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; 

(c) The vertical profile of 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.