Global warming is often attributed to more generation of green house gases like CO2, Nitrous Oxide, Methane and depletion of ozone in the stratosphere. Green house gases, each containing a minimum of 3 component atoms, absorb infrared radiation from earth's surface, vibrate and re-emit the radiation absorbed by them. This absorption-emission-absorption cycle in the lower atmosphere is supposed to keep heat near the surface of the earth and insulate against the cold of the space. Ozone is formed high in the atmosphere when intense light from the sun, especially ultra violet-B light with wave length of 280-315 nm, causes oxygen to break into constituent atoms and reform as ozone molecules which shield people, trees, crops, property and microbes from the harmful effects of UV light from the sun. Green house effect ensures earth's temperature is maintained at comfortable levels but too much of these emissions can heat up the surface of this planet to unbearable levels which is not desirable. A 2C rise in earth's temperature is considered to be lethal to 30% of the living creatures on this planet
Green house effect of gases and vapors vary considerably with water vapor being the most efficient one with 36-72% while CO2 contributes to 9-26%, Methane 4-9% and ozone 3-7%. During the last one and a half century the concentration of these gases has increased dramatically mostly due to human activity. CO2 level recorded an increase from 280 ppm to 387 ppm, Methane 700 ppb to 1745 ppb, NO 270 ppb to 314 ppb and Chlorofluoro Carbon from zero to 533 ppt. CO2 emission increase is mostly due to burning of fossil fuels for a variety of tasks in the modern world including energy generation, automobile running, industrial operations etc. Massive deforestation taking place around the world reduces the 'sink' effect trees have, by converting CO2 into carbohydrates through photosynthesis, allowing the gas to stay for long time in the atmosphere causing increased heat radiation. Though the temperature rise was only 1 C during the last one century, its effect on life has been phenomenal. Further rise of 2 C during this century is predicted which can have disastrous consequences if not checked in time,
Ozone, besides its role as a protectant against UV radiation reaching earth's surface, is also a green house gas. Ozone layer in the stratosphere, 10-50 km above earth's surface, acts as a shield and is considered good and beneficial. This layer absorbs 93-99 % of sun's high frequency UV light and the ozone content here is around 2-8 ppm. Any drastic reduction of ozone can cause increased radiation hazard to the population and hence to be avoided. Free radical catalysts like NO, hydroxyls, atomic bromine and chlorine generated by human activity can destroy ozone significantly. Chlorine and Bromine atoms are liberated by the action of UV light on man made chemicals like Chlorofluorocarbons (CFC) or Bromofluorocarbons and each radical has the potential to destroy 100,000 molecules of ozone. The present depletion rate is about 4% a decade and international community is taking action to phase out the use of these halogenated gases over a period of time. Other industrial gases like Halon, Carbon Tetrachloride, Methyl Chloroform etc extensively used in consumer products and industrial applications are also under the scanner to phase them out eventually so that the 'good ozone" is protected.
Presence of ozone in earth's atmosphere is considered bad as they cause shortness of breath, coughing, wheezing, headache, nausea, throat and lung irritation. Here again the background ozone concentration formed by a series of atmospheric chemical reactions involving nitrous oxide and volatile organic compounds (VOC), if kept at low levels, may not be dangerous. When ground level ozone levels go up, it can affect plant life as well as the human beings as it is a photochemical oxidant. VOCs are invariably generated by gasoline stations, motor vehicles, airplanes, trains, boats, petroleum tankers, oil refineries and higher their levels in the air more will be the formation of ozone. Ozone is a key component in the smog that reduces visibility during summer months. Strong sun light and hot weather conditions cause ground level ozone to form in harmful concentrations in the air.
There is natural generation of ozone due to some plants which produce biogenic volatile organic compounds (BVOC)like mono terpenes and isoprenes in significant amounts that can react with ozone precursors to form ozone at ground level. This has posed a dilemma to policy makers regarding the negative aspects of urban forestry when high biogenic hydocarbon emitting trees are planted in urban areas as CO2 sink as well as for their cooling effect. High BVOC emitting trees include Casuarina, Eucalyptus, Poplar, Oak, Willow etc. However BVOC emission accounts for only about 10% of the total VOCs generated and urban forestry is still an attractive proposition from CO2, ozone and temperature angles and if low BVOC emitting plants are chosen ozone problem may not be that significant. Ozone formation due to reaction between NO and VOCs is temperature dependent and higher the temperature more will be the ozone generation. A mature tree on an average can absorb about 22-23 kg of CO2 in an year and a 10% increase in urban canopy can reduce ozone levels by 4 ppm besides reducing the temperature by 4-5C. Ground level ozone beyond 0.075 ppm is considered undesirable in many countries.