named from the Greek “ozein” for smell. Pronunciation: ‘O-“zOn
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
Ozone was first discovered in 1839 by German scientist Christian Friedrich
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,
(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
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
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.