Earth Hour Special. Earth s Atmosphere

Transcription

1 Earth Hour Special Stratospheric ozone acts as a shield, which protects the Earth's surface from the sun's ultraviolet radiation. Hence a decrease in stratospheric ozone will result in an increase in UV-B radiation, which will have negative impacts on human health such as suppressed immune system, serious sunburn, cataracts, reduced vitamin D synthesis, skin cancer and will also inhibit photosynthesis in plants. However, ozone being toxic to the living system, elevated tropospheric ozone is associated with persistent decrease in lung function, pneumonia, influenza, asthma and decrease in crop yield apart from increase in air temperature, as it is a green house gas. Earth s Atmosphere Atmosphere is the medium for life on the surface of the planet and is the1 transition zone between Earth and Space. It comprises of a mixture of gases exposed to the electromagnetic spectrum of the sun. The atmosphere does not have a well defined outer boundary, but its density decreases progressively with altitude and ultimately merges with the space environment. The atmosphere can be divided into different regions on the basis of temperature profile. In the lowermost region from the ground to about 15 km (~17 km at the tropics), it is found that the temperature decreases with height. This region is called the Troposphere. The height at which this temperature reaches a minimum is called Tropopause, signifying the end of Troposphere. The region above the Tropopause from 15 to 55 km is called the Stratosphere. Here the temperature increases with height. Heating of this region is caused by the absorption of ultraviolet radiation from the Sun by ozone. Stratopause signifies the end of Stratosphere. Atmospheric Ozone Ozone is a relatively unstable molecule found in the Earth's atmosphere, which is made up of three atoms of oxygen and constitutes a tiny fraction of the atmosphere (approx. 3 molecules of ozone for every 10 million air molecules). One of the most important constituents in the stratosphere is ozone, because it is the only atmospheric species that effectively absorbs ultraviolet solar radiation from 200 to 300 nm, thus protecting plant and animal life from exposure to harmful radiation. About 90% of ozone is found in the stratosphere and the remaining 10% in the troposphere. The only process known to produce a significant amount of ozone in the stratosphere is the photolysis of molecular oxygen by ultraviolet radiation followed by recombination of atomic oxygen with molecular oxygen. The band dissociation energy of oxygen is 5.1 ev, which corresponds to a wavelength of nm. Atmospheric ozone is continuously destroyed by the action of ultraviolet, visible and near infrared radiation. The threshold for ozone dissociation is 1.1 ev corresponding to a wavelength of 1180 nm. Hence ozone dissociation can take place at all levels in the atmosphere right down to the surface. In the troposphere, ozone is produced photo chemically through the standard OH - CO NO reaction. The amount of ozone in the atmosphere depends on a balance between processes that

2 create ozone and those that destroy it. An upset in the ozone balance can have serious consequences for life on the Earth. Absorption of sunlight by ozone in the ultraviolet wavelength range is responsible for stratospheric heating, and determines the temperature structure of the middle atmosphere. Changes in middle atmospheric ozone concentrations result in an altered radiative input to the troposphere and to the Earth s surface, with implications on the energy balance and the chemical composition of the lower atmosphere. Thus there is a great need to monitor the atmospheric ozone concentration. Figure 1: Vertical ozone profile. Figure 2: Depth of penetration of the solar UV irradiance (incident on the top of the Earth's atmosphere), as allowed by the ozone layer. Two factors, which are necessary for the production of stratospheric ozone, are presence of molecular oxygen and intensity of solar UV irradiation with wavelength λ < 240 nm. At high altitudes the solar intensity is high but the concentration of oxygen molecules is low. At low altitudes, there are plenty of oxygen molecules but less radiation to dissociate it. Somewhere in between a compromise is reached between the two factors, which maximizes the production of ozone at around km altitude. The formation of ozone layer in this manner was first explained by Chapman. Figure 3: Production and destruction of ozone

3 The total columnar ozone concentration in the atmosphere is expressed in Dobson Units (DU). 1 DU is about 2.7 x molecules/cm 2. The total columnar ozone concentration is least (250 DU) at the tropics where it is produced and highest ( DU) at the poles to where it is transported by the stratospheric circulation known as the Brewer-Dobson circulation. Surface ozone levels from 120 to 200 parts per billion by volume (ppbv) or more are harmful to human beings. The ambient air quality standards formulated and adopted by the United States Environment Protection Agency for ozone is 120 ppbv for 1-hour average and 80 ppbv for 8-hour average. Environmental issues associated with ozone The dual role of ozone leads to two important environmental issues. Importance of monitoring of stratospheric ozone levels on a global basis has increased, in view of the findings of Molina and Rowland in 1974, that substances called chlorofluorocarbons (CFC s), which are used extensively in refrigerators, as propellants in aerosol spray cans, in foam manufacture and in solvents could play a major role in the destruction of stratospheric ozone, and it gave rise to the possibility of a long term global ozone depletion. Its possible impact on the biosphere put ozone on the center stage of atmospheric research over the next few decades. Figure 4: Destruction of stratospheric ozone by CFC s

4 The discovery of the Antarctic ozone hole in the mid eighties by Farman, Gardiner and Shanklin, highlighted the gravity of the problem. Every year about 60% of the total amount of overhead ozone gets depleted over some parts of Antarctica during spring time (August - November). This is because the winters in Antarctic regions are extremely cold, which leads to the formation of polar stratospheric clouds (PSC s). Chemical reactions which normally do not occur in the gas phase occur on the surface of PSC s. These reactions convert inactive forms of chlorine that do not affect ozone into reactive chlorine species, which can easily get dissociated into chlorine radicals and catalytically destroy ozone in the presence of weak sunlight during spring time. These reactions are so fast that a major part of the ozone content over Antarctica between 12 and 20 km is destroyed within few weeks during spring. The main factors affecting the Antarctic ozone loss are high chlorine and bromine levels, extremely low temperature during winter, and relative isolation of the polar region from the mid latitudes by a strong circumpolar wind belt called polar vortex, which restricts the ozone rich air from the mid latitudes from mixing with the air inside the polar region where ozone loss has taken place. The vortex in the Antarctic region is stronger, develops lower temperature and persists longer than the Artic vortex. Therefore the ozone depletion is higher in Antarctic compared to Artic region. Figure 5: Ozone abundances in the Antarctic (South Pole) and Artic (North Pole) regions. The low ozone amount observed over the Antarctic continent from the ground-based balloon and spectrometer measurements were further validated by NASA's Total Ozone Mapping Spectrometer (TOMS) on board the Nimbus-7 satellite. These studies led to the United Nations Environment Programme (UNEP) initiative to protect the ozone layer and a protocol outlining proposed protective actions followed. The Montreal Protocol, signed in September 1987, was a benchmark treaty calling for a 50% reduction in CFC production by 2000, and an eventual complete phase-out of ozonedepleting chemicals. The United States ratified the Montreal Protocol in The implementation of

5 Montreal Protocol entered into force in January A total of 193 nations including India, have since become parties to the treaty as of June 25, The Montreal Protocol is widely regarded as one of the most successful and effective environmental treaties in existence because of its strict targets and enforcement mechanisms. The concern about elevated tropospheric ozone, which is a key component of photochemical "smog," and is observed not only in industrial areas and cities but also in rural areas, is equally alarming. Monitoring of atmospheric ozone There have been significant changes and improvements in the techniques for ozone measurements since the last few years. Measurement of ozone from satellite platforms, balloon, lidar, aircraft and ground based platforms involve remote sensing techniques. Vertical profiles of stratospheric ozone can be obtained with electrochemical sonde by sending balloon borne sensors into the stratosphere and making direct measurements. In situ surface ozone is measured by electrochemical method. Dobson Spectrophotometer, Brewer spectrometer and MICROTOPS Sun photometer are ground-based instruments, which are widely used for measuring the total ozone content in the atmosphere. A number of remote sensing instruments such as Upper Atmosphere Research Satellite (UARS), Stratospheric Aerosol and Gas Experiment (SAGE), Solar Back Scatter Ultraviolet technique (SBUV), Total Ozone Mapping Spectrometer (TOMS), Ozone Monitoring Instrument (OMI), Microwave Limb Sounder (MLS) aboard NASA's satellites and Global Ozone Monitoring Experiment (GOME) aboard the ERS-2 (European Remote Sensing) satellite help researchers to monitor the chemical make-up of the atmosphere. Ever since the discovery of the ozone hole in 1985, NASA's Total Ozone Mapping Spectrometers on board the Nimbus-7 satellite (from 1978 to 1993), Meteor - 3 satellite (from ), Advanced Earth-Observing Satellite (from ) and Earth Probe satellite (from ) have been key instruments for monitoring ozone levels throughout the globe. However the limitations in using satellite data is that, it is difficult to precisely calibrate such instruments. Comparison with the in situ ground-based measurements provides a check on the accuracy of the satellite measurements and gives a better understanding of the atmospheric chemistry through computer models. Variations in atmospheric ozone Total ozone content (TOC) in the atmosphere exhibits high natural variability in both space and time. TOC variability at a particular location is strongly influenced by the movement of air from one region to another. Thus, ozone exhibits temporal variability over hourly, diurnal, synoptic (3 5 days), weekly, seasonal, and long-term (5 20 years) timescales. Variation in atmospheric ozone content may be due to : 1.Diurnal variation: The variation in ozone content in the upper stratosphere (above 40 km) occurs with the daily rising and setting of the sun. The diurnal variation indicates a rapid decay at sunrise, followed by a daytime minimum and a rapid rise at sunset due to recombination of atomic oxygen forming ozone. The photo dissociation processes within the odd oxygen family governs this daily ozone cycle. Unlike the diurnal pattern in the upper stratosphere, the diurnal pattern of surface and total ozone in a typical urban location is low at sunrise; increases to a maximum in the afternoon and

6 thereafter decreases in the evening. This observed pattern is due to increase in solar radiation from morning to afternoon hours causing corresponding increase in ozone by photochemical formation. The NO X family mainly governs these small diurnal variations in ozone. 2. Seasonal variability: The annual or seasonal cycle exhibits a general pattern that repeats every year. 3. Latitudinal variation: The columnar ozone concentration increases with increasing latitude. 4. Transport of air from one region to another due to the effect of weather disturbances, winds and jet streams. 5. El Niño southern oscillation (ENSO): The El Niño current arises as the ocean and atmosphere interact to balance earth s thermal energy. Normally, the waters of the eastern Pacific Ocean near South America are quiet cool as a result of upwelling ocean currents. However in an El Niño period, the Walker Cell (trade winds) weakens, allowing the warmer waters of the western pacific to migrate eastwards. This change in temperature of the sea surface water causes large-scale shifts in the global circulation patterns in the troposphere and the lower stratosphere, which in turn affects the transport of ozone in these regions. 6. Quasi-Biennial Oscillation (QBO): The QBO is an oscillation in the average zonal winds in the tropical stratosphere. Roughly every 27 to 30 months, the tropical stratospheric winds are observed to shift from westerly to easterly and then back again. This circulation enters the lower stratosphere in the tropics and leaves the middle stratosphere in the extra tropics. Air masses having different ozone concentration are transported between the two latitude regions. This induced (latitude / altitude) meridional circulation moves ozone poor air into the tropics and ozone rich air out of the tropics during one phase of QBO and then reverses this process during the other. Ozone in the tropics thus decreases and then increases as we go through the QBO cycle. 7. Solar ultraviolet variations: The output of solar UV radiation is influenced by magnetically active regions (sun spots) on the sun. The period when sunspot activity is at its greatest during the 11 year cycle, is a solar maximum, while the period when sunspot activity is at its least is a solar minimum. The modulations in the solar UV output have a direct effect on ozone photochemistry and even column amount of ozone. The production rate of ozone is high when solar activity is at its maximum and it decreases during solar minimum. Moreover, during solar minimum condition, there is an increase in low energy cosmic rays in the earth s atmosphere, resulting in an increase in NO x and decrease in ozone. 8. Solar storms: Solar storms lead to the ejection of large amount of high-energy protons which penetrate the earth s magnetic field near the poles. These protons penetrate into the atmosphere causing ionization of air molecules. As the ionized particles recombine, they produce nitrogen and hydrogen oxides, which can affect ozone through the NO X and HO X catalytic cycles. These effects are short lived because the hydrogen oxides, which cause the primary ozone loss, recombine within hours. The effect of NO X can persist for several months. 9. Lightning: Lightning is an important elevated source of NO X. The total amount of NO X produced by lightning is proportional to the lightning flash frequency. This increases ozone production potential at high altitudes. 10. Volcanic eruption: Volcanic eruptions are potential sources of SO 2, HCl and H 2 O in the lower stratosphere. SO 2 in the stratosphere gets converted into H 2 SO 4 and condenses into small aerosol particles, which provide an increased scope for heterophase chemistry and enhanced ozone depletion.

7 11. Possible effect of earthquakes: Observed increase in ozone concentration following strong earthquakes having large magnitude and shallow depth of focus during winter is believed to be mainly due to the effect of dynamical disturbances and transport processes. The ozone concentration is normally observed to be low on the day of earthquake, increases gradually after the earthquake, reaching a maximum within 3 to 11 days and thereafter decreases to its normal value. 12. Catalysts: A catalyst is a substance that facilitates a chemical reaction, but which itself remains unchanged by the end of the reaction, so that it can take part in a similar reaction again. In the catalytic process, the ozone molecule is lost while the catalyst (CFC s, chlorine, nitrogen, bromine or hydrogen) is reformed to potentially destroy another ozone molecule. 13. Forest emissions: Plants emit isoprene (C 5 H 8 ) and monoterpene (C 10 H 16 ), through their leaves and needles, in response to stress such as drought, mechanical disturbances such as rain and also during normal growth. These hydrocarbons emitted from forests react with NO x produced from automobile exhausts and factory byproducts transported from the cities, to produce tropospheric ozone, which is a pollutant and green house gas. 14. Aircraft emissions: The subsonic aircraft fleet adds NO X to the lower most stratosphere, where large-scale dynamics tend to prevent advection to higher altitudes. The net ozone production rate increases rapidly with NO X until a maximum is reached. At NO X concentration > 500 parts per trillion by volume (pptv), the net rate of ozone production is expected to decrease with increasing NO X. Depending on the background concentration of NO X, addition of NO X from aviation can increase or decrease the net ozone production rate. 15. Nuclear Testing: During atmospheric testing of nuclear bombs, the temperature within the fireball of a nuclear explosion is sufficient to convert atmospheric nitrogen and oxygen into nitrogen oxides.the fireball can then reach the middle stratosphere and deposit the nitrogen oxides, which could participate in catalytic ozone destruction. 16. Automobile exhaust: Nitrogen oxides and hydrocarbons present in automobile exhaust combine in the presence of sunlight to produce tropospheric ozone. 17. Emission from fire works: In addition to emitting light in the visible region, flammable materials present in fireworks emit radiation in the ultraviolet region at high temperature. Consequences of this are, that the high energy UV radiation are absorbed by molecular oxygen present in the air, resulting in the splitting of molecular oxygen into atomic oxygen, which in turn reacts with molecular oxygen to produce ozone. 18. Possible effect of emission from coal fires: When a coal seam is exposed to favorable conditions (temperature, moisture, oxygen etc.) it ignites spontaneously at low temperature and continues to burn for tens to hundreds of years, depending primarily on the availability of coal and oxygen. The smoke plume from these coal fires contain gases such as methane, oxides and dioxides of carbon, nitrogen and sulphur, some of which act as precursors of tropospheric ozone. Moreover, when these gases are released into the air, they gradually infiltrate all parts of the atmosphere, including the stratosphere, where they are broken down by the high levels of solar UV radiation, freeing extremely reactive NO and OH, which may take part in a complex series of reactions, leading to stratospheric ozone depletion. Sulphur dioxide is rapidly converted into sulphuric acid aerosol, increasing the rate of ozone depletion.

8 Ozone scenario in India In India, ozone is being monitored since past several years at India Meteorological Department, a number of research institutes and universities across the country, which include measurements of total ozone with Dobson and Brewer Spectrometer as well as profiles of ozone from ozonesonde and surface ozone using electrochemical method. There have also been a number of major campaigns like INDOEX (Indian Ocean Experiment) and BOBMEX (Bay of Bengal Monsoon Experiment), which provide insights of seasonal changes in ozone. These measurements show that although the total ozone content has not changed significantly over the past three decades, there has been a small reduction in stratospheric ozone and substantial increase in tropospheric (surface) ozone. Unlike the places located at higher and middle latitudes, where severe ozone depletion has been reported, the total ozone and stratospheric ozone content in India from satellite measurements show a negligible decreasing trend from Thus it is noteworthy to observe that the problem of ozone depletion is not so alarming in India. Acknowledgment: The images presented in this article were created by NASA and NOAA and have been taken from their websites. Suggested Readings [1] Attri, A. K., Kumar, U., and Jain, V. K., Microclimate: Formation of ozone by fireworks. Nature 411, 1015, 2001 [2] Chapman, S., A theory of upper atmospheric ozone. Mem. Royal Meteorol Soc., 3, 130,1930. [3] Crutzen, P. J., Photochemical reactions initiated by and influencing ozone in unpolluted tropospheric air. Tellus, 26, 4757, [4] Farman, J. C., Gardiner, B. G., and Shanklin, J. D., Large losses of total ozone in Antarctica reveals seasonal CLO X / NO X interactions. Nature, 315, , [5] Ganguly, N. D., Ranjan, R., Joshi, H. P., and Iyer, K. N., Diurnal and seasonal variation of ozone at Rajkot. Indian Journal of Radio and Space Physics. 35 (3), , [6] Ganguly, N. D., and Iyer, K. N., Long term trend in ozone and erythemal UV at Indian latitudes. Journal of Atmospheric chemistry, 55 (3), , [7] Ganguly, N. D., Trend of tropospheric ozone in Indian forests from Current Science, 93 (12), , 2007 [8] Ganguly, N. D., Low level of stratospheric ozone near the Jharia coal field in India, Journal of Earth System Sciences, 117 (1), 79 82, 2008 [9] Ganguly, N. D., Variation in atmospheric ozone concentration following strong earthquakes. International Journal of Remote Sensing, 30 (2), , 2009 [10] Mandal, T. K., Beig, G., and Mitra, A. P., Ozone and UV scenario over India. Scientific Report no.22, Center on Global change, NPL, New Delhi, [11] Molina, M. J., and Rowland, F. S., Stratospheric sinks for chlorofluromethanes. Nature, 249, 810, 1974.

9 [12] Niemeier, U., Granier, C., Kornblueh, L., Walters, S., and Brasseur, G. P., Global impact of road traffic on atmospheric chemical position and on ozone climate forcing.j. Geophys.Res., 111, D09301, doi: /2005JD006407, [13] Prakash, A., Remote sensing - GIS based Geoenvironmental studies in Jharia Coalfield, India, with special reference of coalmine fires. Ph.D. Thesis, Department of Earth Sciences, UOR, Roorkee, India, 194, 1996 [14] Singh, H. R., Environmental Biology (S.Chand and company Ltd., India), 121, [15] Street, R.A., Hevitt, C. N., and Mennicken, S., Isoprene and monoterpene emissions from a Eucalyptus plantation in Portugal, J. Geophys. Res., 102, , 1997 [16] The Earth s Atmosphere and ozone. [17] The Stratospheric Ozone Electronic Textbook [18] Ruderman, M. A. and Chamberlain, J. W., Origin of the Sunspot modulation of Ozone, its implications for Stratospheric NO injection. Planet.Space Science, 23, 247, [19] Wilson, W. J., Schwartz, P. R. and Hulbert, E. O., Diurnal variations of mesospheric ozone using millimeter wave measurements. Proceedings of quadrennial ozone symposium, Boulder, Colorado, , Dr. Nandita Ganguly is currently a lecturer of Physics at St.Xavier s College, Ahmedabad. She completed her M.Sc. and M. Phil in Physics from Gujarat University, Ahmedabad and Ph.D. in Atmospheric Sciences from Saurashtra University, Rajkot under the guidance of Prof. K. N. Iyer. Her research interests include the study of atmospheric ozone, aerosols and related fields. She has several papers in National and International Journals to her credit and is a Fellow of the Society of Earth Scientists. ganguly.nandita@gmail.com