Concentration of Ozone in the Atmosphere

Ozone is a tri-atomic molecule of oxygen instead of normal two. It is formed from oxygen naturally in the upper levels of the Earth's atmosphere by high-energy ultraviolet (UV) radiation from the Sun. The radiation breaks down oxygen molecules, releasing free atoms, some of which bond with other oxygen molecules to form ozone.

About 90 per cent of all ozone formed in this way lies between 10 km and 50 km above the Earth's surface - this part of the atmosphere is called the stratosphere. Hence, this is known as the 'Ozone Layer'. Even in the ozone layer, ozone is present in very small quantities; its maximum concentration, at a height of about 17-25 km, is only ten parts per million. The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson.

Since solar radiation is the strongest over the tropics, the global ozone is formed in tropics. However, strong solar radiation also causes rise of air to high altitudes and ozone is transported away from the equator towards the poles where it accumulates in the cold sub polar regions. At the equatorial region formation and photochemical depletion of ozone take place simultaneously and ozone cannot accumulate in this region. In the polar region there is accumulation of ozone because photochemical depletion is low and due to transport of ozone from equator. Therefore, in winter the highest ozone values are observed over the Polar regions as long as there is no other disturbing influence.

Total ozone at any point from Earth's surface to Stratosphere is quantified in Dobson Units (DU). One hundred DU equals the quantity of ozone that would form a layer of 1mm thick at sea level if compressed at Standard Temperature and Pressure (STP).

Typical distribution of ozone is about 240 DU near the equator with a maximum of 440 DU during early spring at high latitudes in the Arctic and 360 DU in the Antarctic. When the concentration of ozone, over any area, falls below 220 DU, we call it Ozone Hole.

Ozone is an unstable molecule. High-energy radiation from the Sun not only creates ozone, but also breaks it to oxygen, recreating molecular oxygen and free oxygen atoms. The concentration of ozone in the atmosphere depends on a dynamic balance between creation and destruction of ozone.

Measurement of Ozone in the Atmosphere

Ozone is spread from the surface of Earth upto the top of stratosphere, 50 km as a very thin layer. The question often asked is how is the concentration of ozone in this thin layer is measured and quantified with a reasonable accuracy.

Atmospheric ozone is measured both by remote sensing and by in-situ techniques.

Generally, the following three characteristics of atmospheric ozone are routinely measured by various monitoring systems:

• Surface Ozone
• Total Ozone over an area and
• The vertical profile of Ozone.

Surface ozone is generally measured by in-situ techniques using optical, chemical or electro chemical methods. The most convenient method is the optical method which depends upon the strong absorption of UV light at 254 nanometer (nm). The absorption is measured in a UV cell at 254 nm against another cell containing air free from ozone. By comparing the two irradiation signals, it is possible to determine the concentration of ozone from 1 to 1000 parts per trillion (ppt) by volume.

Total ozone is measured by remote sensing methods using ground based instruments, measuring the intensity of absorption spectrum of ozone between 300 and 340 nm using direct sun or direct full moon light and satellite based instruments, measuring the solar UV radiation scattered back to space by the Earth's atmosphere. The most commonly ground based instruments used by World Meteorological Organization (WMO) global ozone network are the Dobson and Brewer Spectrophotometers.

The most accurate and the best defined method for determining total ozone is to measure direct solar radiation from ground at UV wave bands between 305 and 340 nm.

Dobson instrument measures spectral intensities at three wave length pairs and the Brewer spectrophotometer at five operational wavelengths. Moon light as a source of UV radiation can also be used, but the accuracy is reduced due to lower intensity of light. For accuracy and comparison, all spectrometers are calibrated regularly at National Oceanic and Atmospheric Administration (NOAA) subtropical high altitude observatory at Mauna Loa, Hawaii where other interfering air pollutants like Sulfur dioxide (SO₂), Nitrous Oxide (NO_x), aerosols etc. are absent.

Vertical profile of ozone is measured with (a) Ozonesondes (b) Ground-based Dobson and Brewer Spectrophotometers using light from zenith sky during twilight using the Umkher inversion method and (c) laser radars (Light Detection and Ranging (LIDAR).

Ozonesondes measure the concentration of ozone as a function of height by a wet chemical method (ozone liberating iodine when bubbled through potassium iodide solution and measured electro chemically) during its balloon borne ascent to an altitude of about 35 km (mid stratosphere). They operate regularly in all climatic regions and have been the backbone of ozone profiling since 1960.

The latest method of vertical profiling of ozone is the LIDAR system in which a short laser pulse at a wave length in the UV ozone absorption spectrum is sent towards the zenith. Back scattered radiation is measured as a function of time which gives the height and its intensity gives the concentration of ozone. Two wavelengths are used, one of which is absorbed by ozone, and the other is not which serves as a reference. The concentration of ozone measured at different heights thus gives the vertical profile.

Ozone Measurement over India

Indian Meteorological Department (IMD) is the nodal agency to measure the ozone levels in India. IMD has established a National Ozone Centre. The centre maintains and control a network of Ozone Monitoring Stations located at New Delhi, Ranichauri, Guwahati, Nagpur, Pune, Kodaikanal, Thiruvananthapuram, Vanarasi, Port Blair and Maitri (Antarctica). The ozone data measured is regularly sent to World Ozone Data Centre, Canada.

Ozone Monitoring Network

National Ozone Centre of India Meteorological Department (IMD) is designated as secondary regional ozone centre for Regional Association II (Asia) of World Meteorological Organization. The centre maintains a network of ozone monitoring stations including Maitri and Bharati in Antarctica.

• Total Columnar ozone measurement using Brewer and Dobson spectrometer at five locations.
• Surface Ozone monitoring through a network of ten stations.
• Measurement of Vertical Distribution of Ozone using IMD Ozonesonde

Source : The Montreal Protocol India's Success Story