Transcript Upcoming:
Upcoming: • Reading Quiz 6: Friday November 30: – EFP Chapter 3 • Energy Policy Writings: Due no later than the day of the final at the beginning of the final exam. Global effects • Ozone depletion • Greenhouse effect & Global warming Ozone Depletion • Refers to 2 effect – Slow steady decline of stratospheric ozone of about 4% per decade since the 1970s – Large scale, seasonal decreases in ozone over the poles (also seen at other latitudes)-called Ozone Holes • Here we are talking about stratospheric ozone, as opposed to tropospheric ozone, which we considered a pollutant and a green house gas Ozone formation and regeneration • Oxygen molecules in the stratosphere absorb UV radiation with wavelengths shorter than 240nm and dissociate into 2 oxygen atoms. • Dissociation triggered by the absorption of photons (light) is called photo-dissociation. • The oxygen atoms combine with other oxygen molecules and form ozone (O3). • Ozone absorbs UV radiation with wavelengths between 310 and 200 nm and dissociate into an oxygen molecule and an oxygen atom. • The oxygen atom combines with another oxygen molecule and forms ozone again (called recombination). Need for ozone • Ozone absorbs UV radiation at UV wavelengths between 300 and 210 nm. • UV bands-UVA (400-320nm), UVB (320280nm) and UVC (280-100nm) • UVB is good for you- it stimulates the skin to produce Vitamin D • Also used to treat certain skin conditions Need for Ozone • • Harmful effects of UV radiation UVA –used to be consider the least harmful – – – • UVB –direct DNA damage – – – – – • • • Now known to indirectly contribute to skin DNA damage and skin cancer Radiation produces chemicals which damage DNA Does not cause sunburn-no way for you to determine you have had an over exposure Causes a DNA mutation that results in skin cancers Body has a natural defense-produces melanin (a brown pigment) which traps the UV radiation and dissipates it as heat. Sunscreens also protect against UVB, some also against UVA. However, there is some concern over chemicals in the sunscreens which may be harmful High intensities are hazardous to the eye as well-may lead to cataracts, among other eye disorders Plankton in the ocean are adversely effected by increased levels of UVB-would cause a reduction in the food and oxygen produced by the plankton which would used the marine ecosystem. Increased UVB would produce more ozone in lower levels of the atmosphere-but this is a main ingredient of photochemical smog UVC-highest energy and most dangerous, but pretty much blocked by the atmosphere Materials made of polymers, textiles and paints all can be degraded due to excessive UV radiation So what do we know: • 1985-Scientist measuring ozone concentrations in Antarctica noticed a sudden drop in ozone levels in 1985. • From 1956-1968 levels had been steady at 300 Dobson units (milliatmosphere-centimeter of ozone) • In 1984 it dropped to 200 units, in 1991 to 150. • 2006 showed the worst ever recorded ozone depletion levels, 85DU. • 1978, first satellite Nimbus 1 launched to measure ozone and other atmospheric gases. Multiple satellites, balloons and airplane based experiments have since verified the results. Why is ozone decreasing? • First ask the question what can destroy ozone? • Chlorine can dissociate ozone. • It occurs in a two step reaction, the first step dissociates the chlorine into diatomic oxygen and chlorine monoxide, the second step creates a chlorine atom and diatomic oxygen from an oxygen atom and chlorine monoxide. The resulting chlorine atom can then dissociate more ozone. • One chlorine atom can dissociate 100,000 ozone molecules. Where does the Chlorine come from • Natural sources: – Ocean spray – Swimming pools – Volcanos • Most of this chlorine never reaches the stratosphere. • What about made made sources? • First suggestion of man made sources of ozone depletion was made in 1970 Free radicals? • Free radicals: atoms, molecules or ions with unpaired electrons . These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions. • Hydroxyl (OH) and nitric oxide (NO) are present in the atmosphere and could catalyze the recombination reaction between ozone and oxygen that leads to a dissociation of ozone. • Nitrous oxide is released by bacteria in the soil and can live long enough in the atmosphere to be converted to NO (nitric oxide). In 1970, Paul Curtzen suggested that increased use of fertilizers would increase nitrous oxide production and thus nitric oxide in the stratosphere. Chlorine • In 1970, researchers showed that chlorine was more effective than nitric oxide at catalyzing ozone destruction • 1971, James Lovelock discovered large amounts of chloroflorocarbons (CFC) in the atmosphere. • After hearing about Lovelock’s work, Rowland and Molina proposed that CFS could behave like nitrous oxide-reaching the stratosphere, being dissociated by UV light releasing chlorine which would then participate in the dissociation of ozone. • This became known as the Rowland-Molina hypothesis. What is a CFC? • CFCs-Belonging to a group of chemical compounds called haloalkanes • These are alkanes (like methane) linked to halogens (like chlorine) • Created by substituting a hydrogen atom in the alkane with a halogen. • CFC are a compound of such substitutions substituting florine and chlorine atoms for the H atoms. • Freon is a trade name for a group of CFCs used as a refrigerant. • CFCs are also used for semiconductor device fabrication, foam blowing agents, solvents, aerosol spray propellants, fire extinguishing agents, and chemical reagents. CFCs • Over the next 3 years, experimental data confirmed the RM hypothesis • Along with Crutzen, they were awarded the 1995 Noble Prize in Chemistry for the work on Stratospheric Ozone • Beautiful example of the scientific method at work Ozone Hole • 1985, British scientists reported a sudden large depletion in the Ozone over the south pole • Simultaneous satellite measurements confirmed the decrease, but were rejected by the processing software-the decrease seen was so large it was rejected by the data quality control algorithms. • When the data were re-run through the processing programs without the data quality flags, the ozone hole had been seen all the way back to 1976! Causes • Unique set of conditions in the atmosphere at the south pole. • Polar Stratospheric Clouds (PSC) are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters (50,000– 80,000 ft). • Due to their high altitude and the curvature of the surface of the Earth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn or after dusk. • PSCs form at very low temperatures, below −78 °C. PSCs and Ozone • Ordinarily, most of the chlorine in the stratosphere resides in stable "reservoir" compounds, primarily hydrochloric acid (HCl) and chlorine nitrate (ClONO2). • During the Antarctic winter and spring, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO). • The clouds can also remove NO2 from the atmosphere by converting it to nitric acid, which prevents the newly formed ClO from being converted back into ClONO What is being done • 1976 Report by the National Academy of Science that there was credible scientific evidence for ozone depletion – US and several other countries banned the use of CFC in aerosol spray cans • 1985 Vienna Convention for the Protection of the Ozone Layer signed – Established a framework for the negotiation of international agreements regulating ozone depleting substances • 1987 Montreal Protocol is signed by 43 nations – Froze CFC production at 1986 levels and reduce it to 50% by 1999 – Follow up meeting in London modified it to to phase out CFCs entirely by 2000 – This date was moved up to 1996 in 1992 • CFCs replaced now by HCFCs or HFC (which are greenhouse gases) – Most commonly used in the US is HFC-134a (R-134a) which has replaced CFCs in automobile air conditioners Effect of these initiatives • Atmospheric concentrations have been declining • Antarctic Ozone hole will continue for decades • Expected to see Ozone concentrations increase by 5-10% in the lower stratosphere by 2020 • A return to pre 1980 levels is expected by 20602075. • Eventual complete recovery of the ozone will occur • Delays will be the result of developing countries which employ CFCs