Causes

CAUSES - Chlorine in the atmosphere

Chlorine in the atmosphere can be contributed in two ways: manmade and natural. Manmade chlorine comes from Chlorofluorocarbons (CFCs) and natural chlorine is released during volcanic eruptions or through the production of sea salt. Majority of CFCs released are manmade (75%<)

CFCs

CFCs are mainly used in our daily lives, ranging from air conditioners to aerosal sprays and styrofoam. CFCs are used for household appliances as they are low in toxicity, nonflammable, non-corrosive and non-reactive with other chemical substances. As such, they serve their purposes as user-safe materials. However, CFCs when released in the stratosphere do not have such desirable chemical properties unlike they have on land. CFC’s take about 15 years to reach the stratosphere, and stay there for about 100 years. In this time they destroy up to 100,000 molecules of ozone.

CFC REACTIONS
In the stratosphere, CFCs come into contact with short wavelength ultraviolet radiation which is able to split off chlorine atoms from the CFC molecules
u.v radiation
CCl3F(g) --------------->CCl2F(g) + Cl(g)
These chlorine atoms destroy the ozone layer

Cl(g) + O3(g) -------------> ClO(g) + O2(g)

ClO(g) + O(g) -------------> O2(g) + Cl(g)
http://www.ausetute.com.au/cfcozone.html

Free-radical catalysts:
These are atoms, molecules and or ions with unpaired electrons; elements which have not achieved the stable octet configuration. As such they are highly reactive. While most of the Nitric Oxide Radical (NO-) and Hydroxyl Radical (OH-) are produced naturally, human activities have caused the amount of Chlorine (Cl-) and Bromine (Br-) to increase. These elements are found in certain stable organic compounds especially in chlorofluorocarbons.

CFCl3 + ultraviolet rays → CFCl2 + Cl-
The liberation of the chloride atoms can destroy the ozone molecules in a number of ways. For example:

Cl + O3 → ClO + O2
When the Cl atoms combine with the ozone molecules, they form chlorine monoxide (CIO) and oxygen. Thereby breaking down oxygen.
Then the chlorine monoxide in turn can react with ozone molecules to form another chloride atom and two oxygen molecules.
ClO + O3 → Cl + 2 O2

This diagram illustrates the prediction by the NASA of the ozone layer in the present and in the future if CFCs are not banned.
Author: NASA
Source: http://earthobservatory.nasa.gov/images/imagerecords/38000/38685/ozone_wa_midlat.jpg
Website taken from: http://en.wikipedia.org/wiki/File:Future_ozone_layer_concentrations.jpg

On a per atom basis, the bromide atom is more efficient in breaking down the ozone. Fortunately, at present, the amount of chloride atom is more than the amount of bromide atoms.
This image shows the global monthly average total ozone amount for the time period 1979 through the end of 2001. The green line shows the results from Nimbus-7 TOMS instrument. The red line shows the results from the Meteor-3 TOMS instrument. The blue line shows the results from the Earth Probe TOMS instrument, which developed a fault in 2002.

Author: NASA
Source: http://toms.gsfc.nasa.gov/multi/ozone_time_series.jpg
Website taken from: http://www.newworldencyclopedia.org/entry/Image:TOMS_Global_Ozone_65N-65S.png

Ultraviolet rays:
The primary source of Ultraviolet rays is the sun.

This diagram shows the different “types” of ultraviolet rays and its affect on the different layers of the atmosphere.

Created by: Center for Global Environmental Research, National Institute for Environmental Studies Japan
Website taken from: http://www.theozonehole.com/uvrays.htm

Based on the length of their wavelength, ultraviolet rays are being classified into 3 different categories

1. UVA wavelengths(320-400 nm). Most UVA radiation are able to reach the earth's surface and can contribute to tanning, skin aging, eye damage, and and many other health issues. It is only affected by the Ozone Layer minimally
2. UVB wavelengths(280-320 nm).The amount of UVB radiation is mostly reflected away by the ozone layer. However, a decrease in stratospheric ozone mean that an increase of UVB radiation can reach the earth's surface, causing sunburns, snow blindness, and a variety of skin problems including skin cancer and premature aging.
3. UVC wavelengths (100-280 nm). Only a little UVC radiation can reach the surface of the earth. This is because the Ozone Layer blocks most of it away.
Reference: http://www.theozonehole.com/uvrays.htm

The ozone layer absorbs most of the ultraviolet rays from the sun.