Fig. 4.3 shows the structure of the atmosphere and the ozone layer. The ozone layer in the stratosphere protects life on Earth by absorbing harmful ultravio...

(a) Layer of the atmosphere containing the ozone layer [1]

The stratosphere. [1]

The ozone layer is located approximately 15 to 35 km above the Earth's surface, within the stratosphere (which extends from about 12 km to 50 km altitude).

(b) Role of the ozone layer in protecting life on Earth [3]

1. The ozone layer absorbs most of the harmful ultraviolet (UV) radiation from the Sun before it reaches the Earth's surface. Ozone molecules (O\(_3\)) are particularly effective at absorbing UV-B and UV-C wavelengths. [1]
2. Without this protective layer, increased UV radiation reaching the surface would cause significantly higher rates of skin cancer, cataracts and immune system suppression in humans. [1]
3. UV radiation also damages the DNA in plant cells, reducing crop yields and forest productivity, and harms phytoplankton in the oceans. Phytoplankton are the base of marine food chains and are responsible for a large proportion of global oxygen production, so their decline would have cascading effects throughout marine ecosystems. [1]

(c) How CFCs cause ozone depletion [3]

1. Chlorofluorocarbons (CFCs), released from products such as aerosol sprays, old refrigerators, air conditioning units and foam packaging, are chemically stable in the lower atmosphere and slowly rise into the stratosphere. [1]
2. In the stratosphere, intense UV radiation breaks apart CFC molecules, releasing free chlorine (Cl) atoms. [1]
3. Each chlorine atom acts as a catalyst: it reacts with an ozone molecule (O\(_3\)), converting it to ordinary oxygen (O\(_2\)) and chlorine monoxide (ClO). The ClO then reacts with another oxygen atom to regenerate the free chlorine, which goes on to destroy more ozone. A single chlorine atom can destroy thousands of ozone molecules before it is eventually removed, making CFCs extremely destructive even in small quantities. [1]

(d) Why the Montreal Protocol is considered a successful agreement [3]

1. The Montreal Protocol (1987) established a binding international framework to phase out the production and use of CFCs and other ozone-depleting substances. Nearly every country in the world ratified it, making it one of the most widely supported environmental treaties in history. [1]
2. As a direct result of the phase-out, atmospheric concentrations of CFCs have decreased measurably, and the Antarctic ozone hole has begun to show signs of recovery. Scientists project that the ozone layer will return to pre-1980 levels by the middle of this century. [1]
3. It demonstrated that international cooperation can successfully address a global environmental problem when there is clear scientific evidence of the threat, political will to act, and the availability of viable alternative substances and technologies to replace the harmful ones. [1]