FSN 1500 Week 12 Stratospheric Ozone Depletion, Ground-Level Ozone and the Electromagnetic Spectrum
Introduction • Although the lay press doesn’t report on stratospheric ozone depletion like it did 5-10 years ago, this issue is still a significant environmental topic with global implications (see slide) • It’s also important that you realize the difference between stratospheric ozone (beneficial) and ground-level ozone (detrimental)
Background • Atmospheric scientists partition the Earth’s atmosphere into five layers based on differences in their physical and chemical properties; the two layers closest to the Earth are the troposphere and the stratosphere (see figure)
Troposphere • Troposphere - extends from the Earth’s surface to an altitude of about 15 kilometers (km); contains about 90% of all the air in the entire atmosphere; with increasing altitude air temperature decreases
Stratosphere Stratosphere - extends from the top of the troposphere to an altitude of about 50 km; contains about 90% of the atmosphere’s naturally occurring ozone; with increasing altitude the air temperature increases Why does the air temperature increase with increasing altitude in the stratosphere? How does stratospheric ozone benefit life on Earth?
Stratospheric Ozone • Stratospheric ozone benefits humans by absorbing the bulk (~ 95%) of the ultraviolet (UV) radiation released from the Sun • Ozone - a pale blue, gaseous molecule composed of three atoms (triatomic) of oxygen (O3) • The average concentration of ozone in the stratosphere is about 10 parts per million (ppm)
Stratospheric Ozone • Another measure of ozone concentration is the Dobson Unit - defined as a slab of ozone of 0.01 mm thickness that would encircle the Earth; the 10 ppm average ozone concentration is equivalent to 300 Dobson units (DU) • If the entire stratosphere’s ozone was compressed, a 3 millimeter (mm) thick slab of ozone would encircle the Earth (see slide)
Stratospheric Ozone • In reality, there is no physical ozone layer - the ozone is distributed throughout the stratosphere with a higher concentration found at 12 - 30 km (commonly termed the ozone layer) • The phrase “ozone hole” is also misleading; this describes a volume of the stratosphere that is markedly depleted in ozone • Most studies now recognize an ozone hole as a section of the stratosphere where ozone concentrations lie below 220 Dobson units
Stratospheric Ozone • Why discuss this topic? A significant body of evidence suggests stratospheric ozone levels are declining. • Studies suggest a worldwide average 6 - 10% decline in stratospheric ozone levels during the past three decades • Although first recognized over Antarctica, some degree of stratospheric ozone loss has affected all latitudes (see figures)
Antarctic Ozone 2010: ~21 2010: 118 Source: NASA
Stratospheric Ozone • Be aware; this is another topic that is frequently discussed by the news media - not always correctly!
Stratospheric Ozone • Possible consequences of continued stratospheric ozone depletion? • 1) Increase in skin cancer incidence as increased proportions of UV light strike Earth’s surface; the more energetic UV light can penetrate more deeply than visible light and could mutate skin cell DNA
Stratospheric Ozone • Some dose-response models suggest a 1- 2 percent increase in skin cancer incidence for every 1 percent decline in stratospheric ozone levels • Worldwide, reported skin cancer cases have been rising at a faster rate than predicted during the last 30 years. What could be some other contributing factors?
Stratospheric Ozone • 2) Declines in stratospheric ozone may lead to increases in cataracts and other eye damage; the more energetic UV light could also damage the eyes • As a group optometrists are reporting a higher occurrence frequency of the predicted eye damage
Stratospheric Ozone • Optometrists and ophthalmologists recommend wearing sunglasses with 100% UV filtration when you venture into daylight • How could you put yourself at further risk for eye damage if all you wore was a pair of dark sunglasses with no UV protection?
Stratospheric Ozone • Eyeglasses, contact lenses, vehicle windshields, and commercial and residential windows now all available with UV filter options • 3) Declines in the autoimmune response of humans would be expected if stratospheric ozone levels continue to decline
Stratospheric Ozone • Sunburn definitely lowers the concentration and function of disease-fighting white blood cells in the body for up to 24 hours after sun exposure • 4) Plant productivity projected to decline if stratospheric ozone levels continue to decline
Stratospheric Ozone • The more energetic UV light can damage plant tissue just as easily as human tissue • 5) Indirectly, declines in stratospheric ozone could lead to an enhanced greenhouse effect and global warming. How? The lower the plant productivity the less CO2 removed from the air; the more air CO2 the more radiated heat can be absorbed.
Stratospheric Ozone • Studies suggest that ozone is created and destroyed naturally in the stratosphere according to four primary, simultaneously occurring reactions: • O3 (g) + UV ----> O2 (g) + O + heat • O2 (g) + O -----> O3 (g) • O3 (g) + O ------> 2 O2 (g) • O2 (g) + UV -----> O + O
Stratospheric Ozone • If the preceding reactions occur normally, stratospheric ozone levels should have a concentration of 300 DU • Since the mid-1970s, evidence has accumulated linking industrial gas emissions to accelerated rates of stratospheric ozone depletion
Stratospheric Ozone • Major human impact? Apparently, the release of chlorofluorocarbon (CFC) gases to the atmosphere • CFCs - a family of gases colloquially known as freons, all of them are composed of various numbers of chlorine, fluorine, and carbon atoms bonded together (see figure)
Stratospheric Ozone • The CFCs were first produced in large volumes by Dupont chemists in the mid 1930s as a substitute refrigeration gas for the toxic chloromethane and ammonia gases that were used in very small scale at that time • The CFCs are a perfect refrigeration gas because they’re nontoxic, noncorrosive and highly chemically unreactive
Stratospheric Ozone • The primary use of CFCs have always been as a refrigerant gas; their wide-scale implementation allowed the US to dominate the world’s economy for close to 50 years • When CFCs escape from a refrigeration unit, they rise (less dense than air or carried upwards by air currents) toward the stratosphere with virtually no reactivity
Stratospheric Ozone • Laboratory studies suggest it commonly takes one to two (perhaps as long as ten) years for CFCs to rise to the stratosphere and that some of them may have residence times in the troposphere from 25 – 400 years! • When the CFCs reach the stratosphere they react with UV light according to the reaction: CFCs + UV ---> Cl + F + C
Stratospheric Ozone • Apparently it’s the freed Cl and F atoms that disrupt the natural ozone cycle • How? Cl + O3 ---> ClO + O2 (destroys ozone) and ClO + O ---> Cl + O2 (releases more free Cl) • These two reactions occur more quickly than the four natural reactions we examined earlier (see slide)
CFCs in Ozone Destruction Source: Images courtesy NASA.
Stratospheric Ozone • Lab studies suggest that one Cl atom may destroy as many as 100,000 ozone molecules before it’s naturally purged • These same studies suggest a batch of CFCs could facilitate ozone depletion for 75 - 100 years!
Stratospheric Ozone • Predicted outcome? Stratospheric ozone levels would continue to deplete • The evidence accumulated in the late 1970s and 1980s sparked action via the signing of the Montreal Protocol in 1987 by 43, almost exclusively Western Hemisphere, countries
Stratospheric Ozone • The signatories agreed to reduce CFC production to one-half their current levels by the end of 1999 • Many less technologically advanced countries (China, India, the USSR) resisted signing, arguing that their minor emissions of CFCs didn’t cause the problem and that they had the right to acquire the West’s living standards
Stratospheric Ozone • By 1990, more data led to revision of the Montreal Protocol - over 100 nations (exceptions being China, India, Russia) agreed to cease CFC production by the end of 1996 with some exceptions for developing nations • Later that year an international monetary fund was created to try to lure other countries to sign the agreement
Stratospheric Ozone • Countries signing the agreement would be given grant monies to research CFC alternatives • In 1992 the Montreal Protocol was again amended to set timetables for the reduction and/or elimination of the production of other (e.g., halons) substances that can deplete stratospheric ozone (as late as 2010 for some substances and developing nations) • Currently 197 nations have agreed to one or more provisions of the Montreal Protocol
Stratospheric Ozone • The U.S. agreed (1992) to curtail CFC production at the end of 1995 during the waning days of the Bush (senior) Administration? Why? • The CFC production ban allowed freons produced before the ban started to be used after January 1, 1996 • The CFC ban temporarily produced another Black Market economy! In 1996 the value of illicit CFCs smuggled into the US was second only to the value of illicit cocaine
Stratospheric Ozone • What are the CFC substitutes now being used in new air conditioners, refrigerators and other cooling units? Two types: HFCs - hydrofluorocarbons and HCFCs - hydrochlorofluorocarbons • The slight chemistry differences from CFCs results in substances that are reactive in Earth’s lower atmosphere
Stratospheric Ozone • If the refrigerant gases react in the lower atmosphere there’s a much lesser chance of their Cl and F components ever reaching the stratosphere • Are the substitutes safe? Some evidence suggests that the most commonly used CFC substitute may react to form another type of acid rain detrimental to wetlands and is a potent greenhouse gas!
Stratospheric Ozone • Updates: • In early October 2006 the area of the Antarctic ozone hole was the largest ever measured and correlated to below average stratospheric temperatures • Possible global warming connection? (see figures)
Stratospheric Ozone Source: Science News
Antarctic Ozone 2010: ~21 2010: 118 Source: NASA
Stratospheric Ozone • The figure on the right is from November 7, 2008 • The following link allows you to determine the stratospheric ozone level overhead for anywhere up to two days beforehand • http://toms.gsfc.nasa.gov/teacher/ozone_overhead_v8.html
Stratospheric Ozone Overall, these data (hopefully) suggest that stratospheric ozone levels should begin to show modest increases during the next 30-50 years as the Montreal Protocol provisions lower ozone-depleting chemicals in the upper atmosphere