Total Column Ozone in the Atmosphere

By: V K Soni and R R Kumar
The Montreal Protocol and its amendments have played an important role in restricting the depletion of stratospheric ozone caused by anthropogenic ozone-depleting substances. The India Meteorological Department is monitoring the total column ozone, which shows that photochemistry is the dominant factor controlling its concentration. Research has also revealed that stratospheric ozone concentrations over different regions along the same latitude have varying recovery rates.
Pollution

Total column ozone is the total amount of ozone in a column extending vertically from the earth’s surface to the top of the atmosphere. It is measured using ground-based stations and satellites and is reported in Dobson units (DU). The ozone hole is defined in terms of reduced total column ozone—less than 220 DU.

Stratospheric ozone is beneficial for humans and other life forms as it absorbs harmful ultraviolet (UV) radiation emitted by the sun and stops it from reaching the surface of the earth. As solar radiation passes through the atmosphere, all UV-C and approximately 90 per cent of ozone, water vapour, oxygen and carbon dioxide absorb UV-B radiation. The atmosphere doesn’t affect UV-A much. Thus, the UV radiation reaching earth is mostly UV-A with a small amount of UV-B. The increased exposure to UV-B radiation enhances the risk of skin cancer, cataracts and suppressed immune system in humans. Excessive UV-B exposure can also harm fauna, single-cell organisms and aquatic ecosystems.

Ozone is found primarily in two regions of the atmosphere—about 10 per cent of atmospheric ozone is in the troposphere while the remaining 90 per cent resides in the stratosphere, primarily between the top of the troposphere till about 50 km altitude. The presence of a large amount of ozone in the stratosphere, therefore results in it being referred to as the ‘ozone layer’. Some stratospheric ozone is regularly transported down into the troposphere and can occasionally influence ozone amounts at the earth’s surface, particularly in remote, unpolluted regions ofthe globe.

Stratospheric ozone is formed naturally as a result of chemical reactions involving solar UV radiation (below 242 nm) and oxygen molecules. The solar UV radiation breaks apart one oxygen molecule (O2) to produce two oxygen atoms (2O) (Fig. 1). The oxygen atom is highly reactive and almost immediately combines with an oxygen molecule to produce an ozone molecule (O3). The reactions occur only in the presence of UV sunlight in the stratosphere—since the UV solar radiation amounts are highest in the tropical regions, major ozone production occurs in the tropical stratosphere.

Interestingly, UV radiation sunlight also destroys ozone. Therefore, the total stratospheric ozone is balanced by its simultaneous destruction. Ozone reacts continuously with UV radiation and a wide variety of natural and anthropogenic chemicals in the stratosphere. In 1970, Paul Crutzen proposed the following catalytic reaction that results in the destruction of ozone in the stratosphere  (Crutzen, 1974):

X + O3 ð XO + O2

O3 + UV ð 2O2

O + XO ð X + O2

Net Reaction: 2O3 + UV  3O2

Here, X is an atom or molecule that acts as a catalyst to convert one ozone molecule to oxygen molecule. The catalyst X is not lost in the net reaction, and can therefore continue to destroy a large number of ozone molecules. The most important catalyst X includes chlorine (Cl), hydroxyl (OH), nitric oxide (NO), and bromine (Br). These catalysts reach the stratosphere through ozone-depleting substances (ODSs). Chlorofluorocarbons (CFCs) which contain chlorine, fluorine, and carbon—CFC-11 (CFCl3) and CFC-12 (CF2Cl2) and are the most common compounds which significantly contribute to ozone depletion. Thus, the delicate balance between the production and destruction of ozone is disrupted by human activities.

The Montreal Protocol for the protection of the ozone layer is the most successful environmental international agreement to date and it has been ratified by the 197 countries of the United Nations. All CFCs have been phased out since January 2010. The Antarctic ozone hole in the stratosphere is in fact recovering. The Montreal Protocol, therefore, helped in averting a severe ozone depletion, especially in the polar regions.

Changes in the Total Column Ozone

Total column ozone at any location on the globe is found by measuring all the ozone in the atmosphere directly above that location. Total column ozone values are reported as DU and defined as the thickness of vertical column of unit area, conceptualising all the ozone from the earth’s surface till the atmospheric limits condensed to standard temperature and pressure (STP). An ozone measurement of 100 DU would be 1 mm thick at STP. The values of total column ozone typically vary from 200 to 300 DU over
the globe.

Most ozone is formed in the tropics, but it is rapidly transported to higher latitudes by the Brewer–Dobson large-scale circulation. Changes in total column ozone play a significant role in the extent of UV radiation reaching the earth’s surface, along with solar elevation, aerosols, clouds, ground albedo and altitude. The radiative property of ozone maintains the thermal structure of the atmosphere. Additionally, changes in ozone have a significant impact on climate when they occur in the upper troposphere/lower stratosphere (UT/LS) region.

Total column ozone significantly varies over latitudes with the highest values occurring at the middle and high latitudes. This is a result of stratospheric winds circulating in the air, moving the tropical ozone-rich air towards the poles. Except for the Antarctic in spring, smallest values of total ozone are found in the tropics in
all seasons.

The natural variations of total column ozone on daily to weekly time scales can be attributed to two reasons. First, natural air motions mix and combine air between regions of high ozone values and those that have low ozone values in the stratosphere. Air motions increase the vertical thickness of the ozone layer near the poles, which increases the value of total ozone in those regions. However, tropospheric weather systems can temporarily reduce thickness of the stratospheric ozone layer in a region, lowering the total column ozone. Second, variations are seen because of changes in the balance of atmospheric chemicals as air moves to different locations over the globe. Additionally, reductions in UV radiation from the sun in its 11-year cycle also reduces the production
of ozone.

In 1985, it was observed that since the 1970s total column ozone over the Halley Bay Station of British Antarctic Survey decreased abruptly in spring (September to November), while no depletion was observed in other seasons (Farman, 1985). Global satellite data also confirmed that the depletion of stratospheric ozone extended over the Antarctic vortex which is a large circumpolar region that includes most of the southern polar latitudes. The depletion of stratospheric ozone was seen to have aggravated since 1985. Observations from vertical profiles also suggest that the depletion of ozone is essentially between 10 and 25 km, which normally contains most of the total column ozone (Fig. 2).

Measuring the Total Column Ozone

The Dobson spectrophotometer is the standard instrument to measure the total column ozone. The instrument measures UV light from the sun at 2 to 6 different wavelengths from 305 to 345 nm. By measuring UV light at different wavelengths, the amount of ozone can be calculated. For instance, the 305 nm wavelength is strongly absorbed by ozone, while the 325 nm one is not absorbed at all. Therefore, the ratio between the two light intensities is a measure of the amount of ozone in the column.

The India Meteorological Department (IMD) has been monitoring total column ozone since 1950, using the Dobson spectrophotometer. Figure 3 shows the mean monthly variation of total column ozone over New Delhi and figure 4 shows the long term variation of annual mean total column ozone from 1958 to 2015. The peak total column ozone values are observed in summer while the lowest occur in winter. In New Delhi an increasing trend in total column ozone is seen. The geostationary meteorological satellite INSAT-3D is used to monitor total column ozone over the tropical Indian region using clear sky infrared radiances (Fig. 5).

As per the Scientific Assessment of Ozone Depletion: 2018, by the World Meteorological Organisation (2018), ground and space based observations indicate that there is no statistically significant trend in near global (60°S–60°N) total column ozone between 1997 and 2016—just a slight increase between 0.3 and 1.2 per cent per decade since 1997, with uncertainties of about 1 per cent per decade in the tropics, where the ozone loss caused by halogen-driven reasons is of lesser magnitude in the lower stratosphere, total column ozone has not varied significantly with ODS concentrations. Outside the tropics, between 2014 and 2017, total ozone columns from ground and space based observations remain lower than the total column ozone between 1964 and 1980 by approximately 2.2 per cent for the near-global average (60°S–60°N), about 3 per cent in the northern hemisphere mid-latitudes (35°N–60°N) and nearly 5.5 per cent in the southern hemisphere mid-latitudes (35°S–60°S) (World Meteorological Organisation, 1985). Updated Chemistry Climate Model (CCM) projections, are fully compliant with the Montreal Protocol and assuming the baseline estimate of the future evolution greenhouse gases (RCP-Representative Concentration Pathway-6.0), have confirmed that the Antarctic ozone hole is expected to close gradually with total column ozone in the spring returning to its value observed in 1980 shortly after the mid-century (about 2060). Increase in greenhouse gas concentrations will not significantly affect the time of recovery of the ozone hole. It is expected that by 2100 the stratospheric ozone column will recover and also exceed the average values between 1960 and 1980 in the Arctic. Springtime Arctic ozone is expected to be higher by about 35 DU for RCP-4.5 and about 50 DU for RCP-8.5.

Way Forward

Extensive research has highlighted the role of stratospheric ozone as a core component of the global climate system. The complex coupling of ozone, atmospheric chemistry and transport and climate change is not fully understood yet. Therefore,a combination of ground based, airborne and satellite measurements together with a strong modelling component provide a suitable platform to predict the nature and impact of future changes to our environment. Such systematic observations are critical to monitor and understand long-term changes in the ozone layer as well as changes in atmospheric composition and climate. Aligned to this global goal, the satellite and ground based measurements are being continuously strengthened in India. In one such initiative, the IMD has recently started measuring the vertical distribution of ozone at the Indian station Bharati in  the Antarctica.

 

References

Crutzen P. J., 1974. Photochemical Reactions initiated by and Influencing Ozone in Unpolluted Tropospheric Air. Tellus, pp. 47–57.

Farman J.C., Gardiner B.G., Shanklin JD., 1985. Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction, Nature, 315, pp. 207-210.

World Meteorological Organization, Scientific Assessment of Ozone Depletion: 2018, Global Ozone Research and Monitoring Project – Report No. 58, 588 pp., Geneva, Switzerland, 2018.

World Meteorological Organization,1985. Atmospheric ozone: assessment of our understanding of the processes controlling its present distribution and change. Its Global Ozone Research and Monitoring Project, Report 1985 16, 1095.

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