ozone layer

Origin of ozone

Ozone-oxygen cycle in the ozone layer.

The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sidney Chapman in 1930. Ozone in the Earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O₂), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O₂ to create ozone, O₃. The ozone molecule is also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O₂ and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere, the region from about 10 to 50 kilometres (33,000 to 160,000 ft) above Earth's surface. About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 kilometres (12 and 25 mi), where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only 3 millimeters thick.^[3]

[edit] Ultraviolet light and ozone

Levels of ozone at various altitudes and blocking of ultraviolet radiation.

UV-B energy levels at several altitudes. Blue line shows DNA sensitivity. Red line shows surface energy level with 10% decrease in ozone

Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation coming from the sun. UV radiation is divided into three categories, based on its wavelength; these are referred to as UV-A (400–315 nm), UV-B (315–280 nm), and UV-C (280–100 nm). UV-C, which would be very harmful to all living things, is entirely screened out by ozone at around 35 kilometres (115,000 ft) altitude. UV-B radiation can be harmful to the skin and is the main cause of sunburn; excessive exposure can also cause genetic damage, resulting in problems such as skin cancer. The ozone layer is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface. Nevertheless, some UV-B reaches the surface. Most UV-A reaches the surface; this radiation is significantly less harmful, although it can potentially cause genetic damage.

[edit] Distribution of ozone in the stratosphere

The thickness of the ozone layer—that is, the total amount of ozone in a column overhead—varies by a large factor worldwide, being in general smaller near the equator and larger towards the poles. It also varies with season, being in general thicker during the spring and thinner during the autumn in the northern hemisphere. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity.
Since stratospheric ozone is produced by solar UV radiation, one might expect to find the highest ozone levels over the tropics and the lowest over polar regions. The same argument would lead one to expect the highest ozone levels in the summer and the lowest in the winter. The observed behavior is very different: most of the ozone is found in the mid-to-high latitudes of the northern and southern hemispheres, and the highest levels are found in the spring, not summer, and the lowest in the autumn, not winter in the northern hemisphere. During winter, the ozone layer actually increases in depth. This puzzle is explained by the prevailing stratospheric wind patterns, known as the Brewer-Dobson circulation. While most of the ozone is indeed created over the tropics, the stratospheric circulation then transports it poleward and downward to the lower stratosphere of the high latitudes. However in the southern hemisphere, owing to the ozone hole phenomenon, the lowest amounts of column ozone found anywhere in the world are over the Antarctic in the southern spring period of September and October.

Brewer-Dobson circulation in the ozone layer.

The ozone layer is higher in altitude in the tropics, and lower in altitude in the extratropics, especially in the polar regions. This altitude variation of ozone results from the slow circulation that lifts the ozone-poor air out of the troposphere into the stratosphere. As this air slowly rises in the tropics, ozone is produced by the overhead sun which photolyzes oxygen molecules. As this slow circulation bends towards the mid-latitudes, it carries the ozone-rich air from the tropical middle stratosphere to the mid-and-high latitudes lower stratosphere. The high ozone concentrations at high latitudes are due to the accumulation of ozone at lower altitudes.
The Brewer-Dobson circulation moves very slowly. The time needed to lift an air parcel from the tropical tropopause near 16 to 20 kilometres (9.9 to 12 mi) is about 4–5 months (about 30 feet (9.1 m) per day). Even though ozone in the lower tropical stratosphere is produced at a very slow rate, the lifting circulation is so slow that ozone can build up to relatively high levels by the time it reaches 26 kilometres (16 mi).
Ozone amounts over the continental United States (25°N to 49°N) are highest in the northern spring (April and May). These ozone amounts fall over the course of the summer to their lowest amounts in October, and then rise again over the course of the winter. Again, wind transport of ozone is principally responsible for the seasonal evolution of these higher latitude ozone patterns.
The total column amount of ozone generally increases as we move from the tropics to higher latitudes in both hemispheres. However, the overall column amounts are greater in the northern hemisphere high latitudes than in the southern hemisphere high latitudes. In addition, while the highest amounts of column ozone over the Arctic occur in the northern spring (March–April), the opposite is true over the Antarctic, where the lowest amounts of column ozone occur in the southern spring (September–October).

causes of water pollution

• Agriculture runoff - carrying fertilizers, pesticides/insecticides/herbicides and other pollutants into water bodies such as lakes, rivers, ponds). The usual effect of this type of pollution consists in algae growing in affected water bodies. This is a sign of increased nitrates and phosphates in water that could be harmful for human health.
• Storm water runoff – carrying various oils, petroleum products and other contaminants from urban and rural areas (ditches). These usually forms sheens on the water surface.
• Leaking sewer lines – may add trihalomethanes (such as chloroform) as well as other contaminants into groundwater ending up contaminating surface water, too. Discharges of chlorinated solvents from Dry Cleaners to sewer lines are also a recognized source of water pollution with these persistent and harmful solvents.
• Mining activities – mining activities involve crushing the rock that usually contains many trace metals and sulfides. The left material may easily generate sulfuric acid in the presence of precipitation water. Please, read more at Mining Sites.
• Foundries – have direct emissions of metals (including Hg, Pb, Mn, Fe, Cr and other metals) and other particulate matter into the air. Please, read more at Foundry.
• Industrial discharges – may add significant pollution to water bodies, but are usually regulated today. Please, read more at Industrial Sites.
• Accidental leaks and spills – associated with handling and storage of chemicals may happen anytime and, although they are usually contained soon after they occur, the risk of polluting surface and groundwater exist. An example are ship accidents such as Exxon Valdez disaster which spilled large amounts of petroleum products into the ocean;
• Intended/illegal discharges of waste – while such occurrences are less common today, they may still happen due to the high cost of proper waste disposal; illegal waste discharges into water bodies were recorded all over the world;
• Burning of fossil fuels – the emitted ash particles usually contain toxic metals (such as As or Pb). Burning will also add a series of oxides including carbon dioxide to air and respectively water bodies.

what are the different water resources?

Water Resources in India Rivers, estuaries, lakes and dams and other water bodies comprise the water resources in India. These water resources of India are helpful in enriching the land and irrigational purposes. en.wikipedia.org/wiki/Water_resources

	•	Water Projects in India		•	Dams in India		•	Lakes of India
	•	Estuaries of India		•	Indian Ports		•	Indian Rivers
	•	Indian Maritime Trade		•	Buckingham Canal		•	Hot Springs in Orissa

India is pregnant with an affluent and huge variety of natural resources, including water are regarded as one of the most essential properties. Its improvement and control largely affects in agricultural output. For a stable environmental and economic development, management of water resources should be integrated as per the National Water Policy, 2002. An annual rainfall of around 4000 km3 occurs in India. The rainfall in India shows very high sequential and spatial inconsistency, though Mousinram in Cherrapunjee gets the world`s highest rainfall, yet it also experiences water shortages during the other seasons.

The Indian rivers receive a total average annual flow of 1953 km3 per year. It is estimated that entire annual usable ground and surface water resources is around 396 km3 and 690 km3. With improvements in standard of living and growing number of population, demand for water resources has also increased along with reducing availability of water through out the nation. Moreover, due to rising pollution levels, the quality of water resources is also declining. Thus the change in climatic conditions, might affect the annual rainfall and availability of water.

In India, the monsoon period is generally lasts for around three to four months. A vast portion of the nation faces shortage of surface water resources for most part of the year. Even areas like Konkan and Meghalaya, which receive adequate rainfall, face deficiency during winter and summer months. Although in the coastal and northern plains, water resources are abundant, other regions supply is remarkably insufficient. In general, in particular areas, ground water can be drained from a depth of almost 15 metres. Even water resources that are safe for ingestion can not be provided to most of the villages in the rural areas. Further more, villagers have to cover vast distances to gather water. Thus supply of water for irrigation and agriculture is also inadequate. Total water resources are around 167 million hectare-metres in India, which has been derived after considering the total area and the average annual rainfall around 50 cm. out of this, only 66 million hectare-metres are available for irrigation.

In 1954, surface water exploration programme was instigated and drilling actions were under taken in river basins and in lower Himalayan areas. This programme gained further momentum during the 1990s, with the introduction of open-hole drilling equipments. Drilling was carried on in areas of Arunachal Pradesh and Jammu and Kashmir. Around 27,500 wells have been established by drilling around the nation. Moreover, numerous wells were drilled to counter water deficiency, particularly in drought prone regions. These well are managed by the State Governments for providing water to the public.

Moreover, canals, rivers, ponds, reservoirs, tanks and other small water bodies form Inland Water resources in India. These are mostly present in the regions of Uttar Pradesh, Madhya Pradesh, Jammu and Kashmir, Tamil Nadu, Andhra Pradesh, Karnataka, Rajasthan, West Bengal, Gujarat, Maharashtra and Orissa, amongst others. The entire length of canals and rivers are 31.2 thousand km in Uttar Pradesh and Orissa has the highest total area for salty water bodies.

Around 9.7 million hectare-metres of water was available for agricultural reasons before 1951. But by 1973, almost 18.4 million hectare-meter of water resources was being supplied for agriculture and irrigation. In India, the utilisable ground water resources are considered to be approximately around 40 million hectare-metres. Yet only 10 million hectare-metres are being utilised currently. The residual 30 million hectare-metres are stocked in pipelines for employment.

The Indian water resources are key natural resources, a fundamental human requirement and a valuable asset for the nation. Thus great importance should be given to the proper improvement and competent utilisation of such water resources. Few programmes and policies, such as, policy guidelines, technical examination, sectoral planning, techno-economic appraisal and coordination of projects, have been undertaken by the Government for regulation and growth of water resources. These support particular projects, facilitate exterior assistance and help in solving regional disputes related to water, provide assistance in formation of policies and management of irrigation and expansion and improvement of water resources in India.