ATMOSPHERIC OZONE DEPLETION

   INTRODUCTION

 Ozone is a naturally-occurring gas that can be good or bad for our health and the environment depending on its location in the atmosphere.  In the layer near the Earth?s surface the troposphere ground level or ?bad? ozone is an air pollutant that is a key ingredient of urban smog.  But higher up, in the stratosphere, ?good? ozone protects life on Earth by absorbing some of the sun?s UV rays.  An easy way to remember this is the phrase ?good up high, bad nearby.?  

The stratospheric ozone layer forms a thin shield in the upper atmosphere, protecting life on Earth from the sun?s ultraviolet (UV) rays.  It has been called the Earth?s sunscreen.  In the 1980s, scientists found evidence that the ozone layer was being depleted.  Depletion of the ozone layer results in increased UV radiation reaching the Earth?s surface, which in turn leads to a greater chance of overexposure to UV radiation and the related health effects of skin cancer, cataracts, and immune suppression.  This fact sheet explains the importance of protecting the stratospheric ozone layer. 

   Ozone Layer Depletion

Compounds that contain chlorine and bromine molecules, such as methyl chloroform, halons, and chlorofluorocarbons (CFCs), are stable and have atmospheric lifetimes long enough to be transported by winds into the stratosphere. When these ozone-depleting su bstances (ODS) break down in the atmosphere, they release chlorine or bromine, which attack ozone.  Each chlorine or bromine atom reacts with ozone, repeatedly combining with and breaking apart as many as 100,000 ozone molecules during its stratospheric life. 

CFCs, which have a long history of use as refrigerants, solvents, foam-blowing agents and in other applications, have been almost completely phased out worldwide.  In addition, restrictions are now in place to phase out hydro chlorofluorocarbons (HCFCs), compounds used as substitutes for the more damaging CFCs.  The U.S. will phase out HCFCs completely in 2030.  

   Good Ozone and bad Ozone

High concentrations of ozone are often found in the troposphere in polluted areas such as Los Angeles, California. Although ozone near the ground will also absorb DNA-damaging ultraviolet radiation, it is toxic to plants and humans. Therefore, ozone can be thought of as ?good? in the stratosphere and ?bad? in the troposphere (where we would breathe it).

   Chemistry of Ozone layer

Ozone, a molecule consisting of three oxygen atoms, was first discovered in the 1830s by the German scientist Christian Schonbein. He identified a new compound in laboratory experiments using oxygen, and named the molecule ?ozein,? meaning ?to smell? in Greek. In 1881, John Hartley experimented with ozone and found that it strongly absorbed ultraviolet light. He compared the absorption spectrum of ozone to the spectrum of sunlight as seen from the Earth?s surface and found that they matched exactly.

O3 + light ? O2 + O

O + O3 ? 2 O2

 Net: 2 O3 + light ? 3 O2

  The Ozone Hole

When gas phase catalytic ozone destruction cycles involving NOx, ClOx, BrOx, and HOx were first included in photochemical atmospheric models, global ozone levels were predict- ed to drop 5–10% over the next 100 years. This decrease was predicted because CFCs were increasing rapidly in concentration, causing the amount of chlorine in the atmosphere to increase. Due to the concentration of this gases the ozone layer becomes depleted which is called ozone hole.

Cause of Ozone Layer Depletion

?        Chlorofluorocarbons (CFCs).

?        Methyl bromide (CH3Br)

?         Halons

?        Carbon tetrachloride (CCI4).

?        Hydrochloroluorocarbons (HCFCs)  

Effect of Ozone Depletion

a. Effects on Human and Animal Health

Increased penetration of solar UV-B radiation is likely to have profound impact on human health with potential risks of eye diseases, skin cancer and infectious diseases. UV radiation is known to damage the cornea and lens of the eye.

b. Effects on Terrestrial Plants                                            

 It is a known fact that the physiological and developmental processes of plants are affected by UV-B radiation.

c.  Effects on Aquatic Ecosystems                                  

While more than 30 percent of the world?s animal protein for human consumption comes from the sea alone, it is feared that increased levels of UV exposure can have adverse impacts on the productivity of aquatic systems.

d. Effects on Bio-geo-chemical Cycles                          

Increased solar UV radiation could affect terrestrial and aquatic bio-geo-chemical cycles thus altering both sources and sinks of greenhouse and important trace gases, e.g. carbon dioxide (CO2), carbon monoxide (CO), carbonyl sulphide (COS), etc.

e.  Effects on Air Quality                                         

Reduction of stratospheric ozone and increased penetration of UV-radiation result in higher photo dissociation rates of key trace gases that control the chemical reactivity of the troposphere. This can increase both production and destruction of ozone and related oxidants such as hydrogen peroxide which are known to have adverse effects on human health, terrestrial plants and outdoor materials.

f.   Effects on Materials                                                        

An increased level of solar UV radiation is known to have adverse effects on synthetic polymers, naturally occurring biopolymers and some other materials of commercial interest. UV-B radiation accelerates the photo degradation rates of these materials thus limiting their lifetimes.

g.      Effects on Climate Change

Atmospheric ozone has two effects on the temperature balance of the Earth. It absorbs solar ultraviolet radiation, which heats the stratosphere. It also absorbs infrared radiation emitted by the Earth's surface, effectively trapping heat in the troposphere.