6.4 - stratospheric ozone

Sun’s radiation types

• UVA

  • 320-400 nm

• UVB

  • 280-320 nm

• UVC

  • 100-280 nm

UVA

• 95% of radiation

• penetrate deep into skin causing premature aging and DNA damage

UVB

• 5%

• partially absorbed by ozone layer

• can cause skin cancer, sunburn and eye damage

UV effects on photosynthetic organisms

• restrains photosynthesis in phytoplankton

• damage to plant tissue

• damage to symbiotic algae living in corals

UV impact on society

• reduced food security

• declines in fish populations

• concerns of health risks

• stimulation of vitamin D → healthy bones

• bacterial sterilisation

• phototherapy

stratospheric ozone

formed by chemical reactions involving UV sunlight and oxygen molecules

• acts as critical shield for Earth’s life

• blocks harmful UV radiation at 10-50 km above surface

ozone formation

• UV photons hit ozone molecules O3

• breakdown into oxygen molecule O2 and oxygen atom O

• converts UV energy into heat

• O → O2 recombine to form new ozone

ozone depleting substances

• CFC chlorofluorocarbons → refrigerants

• HCFC hydrochlorofluorocarbons

• halon → fire extinguishers

• methyl bromide → pesticides

• nitrogen oxides → bacterial breakdown

• carbon tetrachloride → cleaning substances

chlorine effect on ozone layer

• UV breaks off chlorine atom

• chlorine atom attacks ozone molecule

• breakdown and ozone destruction

• result in O2 molecule and chlorine monoxide molecule

• chlorine monoxide is attacked by free oxygen

• release of chlorine atom and formation of oxygen molecule

• chlorine atom is now free to attack and destroy more

Antarctic ozone hole

• largest September-october

process of the antarctic ozone hole

• in winter (June) extreme low temperatures allow polar stratospheric clouds to form

• they convert chlorine into reactive form

• sunlight returns in spring and reactive chlorine rapidly destroys ozone

• by summer (January) temperature rises → PSC disappear → reduced chlorine activity

Dobson units

measure of ozone molecules concentration in a column of air from Earth’s surface to the top of the atmosphere

• x < 220 severe ozone destruction

volcanic eruptions effect

enhance ozone depletion by injecting sulfur compounds into stratosphere

• contribute to formation of additional surfaces for chemical reactions on PSCs

strategy for reducing pollution

• altering human activity

  • replace old refrigerators

  • use propane and butane instead CFC in aerosol

  • environmentally safer pesticides

• controlling release of pollutants

  • recover and recycle CFC to prevent release

  • ensuring compliance

• clean up and restoration