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This lecture will help you understand: • Outdoor air pollution • Chemical reaction in the atmosphere • Acid deposition • Indoor air pollution • Stratospheric ozone depletion Central Case: London’s 1952 “Killer Smog” • In December 1952 a thick smog settled on London, killing 4,000–12,000 people. • It was caused by weather conditions that exacerbated the city’s air pollution from factories. • This and other events led Britain to pass clean air laws. Donora Smog • On October 26, 1948, people in Donora, Pennsylvania woke up to a dense fog that lasted 5 days. – The town has a large steel mill that used high-sulfur coal and a plant that roasted sulfur-containing ores. • People complained of difficulty breathing, stomach pain, headaches, nausea, and choking. – Within 1 month, 70 people died and 6,000 got sick. – Mill owners did not think their mills were responsible. • State and national laws now control pollution. – Culminating in the Clean Air Act (1970) air Clean Air Act impacts Air pollution essentials • Air pollutants: substances in the atmosphere (gases and aerosols) that have harmful effects. • The atmosphere contains many gases. – N2, O2, Ar, CO2, water vapor – 40 trace gases: ozone, helium, hydrogen, nitrogen oxides – Aerosols: microscopic liquid or solid particles (dust, pollen, sea salts, etc.) from land and water • The Industrial Revolution changed the mixture of atmospheric gases and particles. Primary air pollutants Five primary air pollutants Pollutants • Three factors determine the level of air pollution. – The amount of pollutants entering the air – The amount of space into which the pollution is added – Mechanisms that remove pollutants from the air • Troposphere: the lower atmosphere – The site and source of weather, water vapor, clouds – Pollutants are removed within hours or days. – Pollutants in the upper troposphere can persist for days. • Pollutants in the stratosphere are resistant to cleansing. – Ozone-depleting chemicals (chlorine, bromine) Processes producing air pollution • Incomplete combustion of fossil fuels and refuse – Creating gaseous and particulate products • Evaporation: creates gaseous and particulate products • Strong winds: pick up dust and other particles • Primary pollutants: direct products of combustion and evaporation – Particulates, volatile organic compound(VOC)s, CO, NOx, SO2, lead, air toxics • Secondary pollutants: reaction products of primary pollutants in the air – Ozone, peroxyacetyl, nitrates, sulfuric and nitric acids Artificial sources of air pollution Human-caused air pollution includes: • Point sources : specific spots where large amounts of pollution are discharged (factory smokestacks) • Non-point sources : diffuse, often made up of many small sources (charcoal fires from thousands of homes) Primary Pollutants CO CO2 SO2 NO NO2 Most hydrocarbons Secondary Pollutants SO3 HNO3 H2SO4 Most suspended particles H2O2 Most Sources Natural Mobile Stationary – NO3 O3 PANs and SO24– salts Primary pollutants • Power plants: the major source of sulfur dioxide – Industrial plants: particulates – Transportation: carbon monoxide, nitrogen oxides – Burning fossil fuels and wastes: soot, smoke – Unburned fragments of fuel molecules: VOCs • Nitrogen oxides (NOx): nitrogen gas is oxidized to nitric oxide (NO) under high combustion temperatures. – Nitric oxide and oxygen form nitrogen dioxide (NO2: photochemical smog) and nitrogen tetroxide (N2O4) • Coal also contains sulfur and heavy metals. Secondary pollutants • Photochemical oxidants: ozone and other reactive organic compounds formed by nitrogen oxides and VOCs. – Sunlight provides the reaction’s energy • Ozone concentrations in preindustrial times: 10–15 ppb – Unpolluted, summer air in North America: 20–50 ppb – Polluted air: 150 ppb or more (very unhealthy) • Ambient U.S. ozone levels decreased by 20% during 1980–92 – But only a few percent in the 2000s Major air pollutants • Carbon oxides: – CO is a highly toxic gas that forms during the incomplete combustion of carbon-containing materials. – 93% of CO2 in the troposphere occurs as a result of the carbon cycle. – 7% of CO2 in the troposphere occurs as a result of human activities (mostly burning fossil fuels). • Nitrogen oxides and nitric acid: – Nitrogen oxide (NO) forms when nitrogen and oxygen gas in air react at the high-combustion temperatures in automobile engines and coal-burning plants. NO can also form from lightening and certain soil bacteria. • NO reacts with air to form NO2. • NO2 reacts with water vapor in the air to form nitric acid (HNO3) and nitrate salts (NO3-) which are components of acid deposition. • Sulfur dioxide (SO2) and sulfuric acid: – About one-third of SO2 in the troposphere occurs naturally through the sulfur cycle. – Two-thirds come from human sources, mostly combustion (S + O2 SO2) of sulfur-containing coal and from oil refining and smelting of sulfide ores. – SO2 in the atmosphere can be converted to sulfuric acid (H2SO4) and sulfate salts (SO42-) that return to earth as a component of acid deposition. • Suspended particulate matter (SPM): – Consists of a variety of solid particles and liquid droplets small and light enough to remain suspended in the air. – The most harmful forms of SPM are fine particles (PM-10, with an average diameter < 10 micrometers) and ultrafine particles (PM-2.5). – According to the EPA, SPM is responsible for about 60,000 premature deaths a year in the U.S. • Ozone (O3): – Is a highly reactive gas that is a major component of photochemical smog. – It can cause and aggravate respiratory illness and can aggravate heart disease. – Damage plants, rubber in tires, fabrics, and paints. • Volatile organic compounds (VOCs): – Most are hydrocarbons emitted by the leaves of many plants and methane. – About two thirds of global methane emissions comes from human sources. – Other VOCs include industrial solvents such as trichlorethylene (TCE), benzene, and vinyl chloride. – Long-term exposure to benzene can cause cancer, blood disorders, and immune system • Radon (Rn): – Is a naturally occurring radioactive gas found in some types of soil and rock. – It can seep into homes and buildings sitting above such deposits. Emission of air pollutants internal combustion engine from four-cycle Catalytic converter HCs + H2O H2 + CO 2 NO + 2 H2 N2 + 2 H2O 2 CO + O2 2 CO2 HC + 2 O2 CO2 + 2 H2O The overall reaction for the reduction of NO and the oxidation of CO 2 NO + 2 CO Rh, Pt, Pd catalysts N2 + 2 CO2 The oxidation of a typical gasoline hydrocarbon, octane 2 C8H18 + 25 O2 Pt, Pd catalysts 16 CO2 + 18 H2O The fate of atmospheric SO2: acid rain SO2 . HSO3 SO3 . + + OH O2 + H2SO4 (g) + SO3 H2O H2O . HSO3 + H2SO4(g) H2SO4 (aq) SO2 (g) + H2O (aq) KH . HOO H2SO3(aq) = [H2SO3(aq)]/PSO2 (0.1 ppm) [H2SO3(aq)] = PSO2KH = 1.0 x 10-7 atm x 1.0 M/atm = 1.0 x 10-7 M H2SO3(aq) HSO3- (aq) + H+ (aq) - + [HSO3 ] = [H ] + Ka = 1.7 x 10-2 = [HSO3 ][H ] [H2SO3] = - 2 [HSO3 ] 1.0 x 10-7 - [HSO3 ] = 4.1 x 10-5 M So the ratio of HSO3- to H2SO3 is 410:1 (4.1 x 10-5 /1.0 x 10-7). Because [HSO3-] = [H+] = 4.1 x 10-5 M Therefore the pH of the aerosol droplets is 4.4. Hydroxyl radicals • Photochemical breakdown of ozone is a major source of hydroxyl radical. • They remove anthropogenic pollutants from the air. – Highly reactive hydrocarbons are rapidly oxidized. – Nitrogen oxides are oxidized within a day – Less reactive substances (e.g., CO) take months. • Atmospheric levels of hydroxyl radicals are determined by levels of anthropogenic air pollutants. – Hydroxyl’s cleansing power is used up. – Pollutants are able to build up. The hydroxyl radical: Nature’s cleanser Factors influencing levels of outdoor air pollution • Outdoor air pollution can be reduced by: – settling out, precipitation, sea spray, winds, and chemical reactions. • Outdoor air pollution can be increased by: – urban buildings (slow wind dispersal of pollutants), mountains (promote temperature inversions), and high temperatures (promote photochemical reactions). Atmospheric cleansing • Natural air pollutants: volcanoes, fires, dust storms – Plants emit volatile organic compounds – Mechanisms in the biosphere remove, assimilate, and recycle natural pollutants • Hydroxyl radical (OH): a naturally occurring compound – Oxidizes many gaseous pollutants to harmless products brought to land or water by precipitation • Sea salts: a cleansing agent that helps form raindrops – Picked up by wind flowing over oceans • Sunlight: breaks down organic molecules Impacts of air pollutants • We are exposed to a mixture of pollutants that varies over time and place. – Plants may be so stressed from pollution that they become vulnerable to drought or insects • Human health: every one of the primary and secondary air pollutants is a threat to human health. • Acute exposure can be life threatening. – Chronic exposure: long-term exposure that causes gradual deterioration and premature mortality. • Some pollutants contribute to lung cancer. Long-term exposure and chronic effects • Sulfur dioxide: leads to bronchitis (inflammation of the bronchi) • Ozone: leads to inflammation and scarring of the lungs • Carbon monoxide: reduces the oxygen-carrying capacity of the blood and leads to heart disease • Nitrogen oxides: impair lung function and affect the immune system • Particulate matter: respiratory and cardiovascular pathologies • Other factors (diet, exercise, genetics) influence effects Size of selected particulates that produce the greatest lung damage Bronchitis and emphysema Epithelial cell Cilia Nasal cavity Goblet cell (secreting mucus) Oral cavity Pharynx (throat) Mucus Trachea (windpipe) Bronchioles Bronchus Alveolar duct Right lung Bronchioles Alveolar sac (sectioned) Alveoli Chronic obstructive pulmonary disease (COPD) • Chronic obstructive pulmonary disease (COPD) – A slowly progressive lung disease that makes it hard to breathe. – The 4th leading cause of death: affects 18 million in the U.S. – Affects 10% of adults over 40 worldwide. – From smoking and burning wood or dung for fuel. • Involves three diseases: emphysema (destruction of the lung alveoli), bronchitis, and asthma. Asthma • Most sensitive to air pollution: small children, asthmatics, those with chronic pulmonary or heart disease, the elderly • Asthma: an immune disorder – Impaired breathing caused by constricted airways – Is triggered by allergens (dust, mites, mold, pet dander) – Is also triggered by pollution (ozone, particulates, SO2) • Causes 500,000 hospitalizations/year – 1.8 million visits to emergency departments Strong evidence • Studies of thousands of adults show strong evidence of harm caused by fine particulates and sulfur pollution – Asthma, chronic bronchitis, cardiovascular problems, etc. • Higher concentrations of fine particles correlate with increased mortality from cardiopulmonary disease and lung cancer. – The EPA used these studies to regulate fine particles. • Fine particles and ozone exceeded standards – Meeting standards would save $28 billion/year in avoided health costs, missed work, premature deaths, etc. Acute and carcinogenic effects • Air pollution can kill people already suffering from heart or respiratory diseases. – Lethal doses also occur in accidental poisoning. • Moderate air pollution can change cardiac rhythms in people with heart disease. – Triggering fatal heart attacks. • Diesel: a likely human carcinogen. • Benzene: clearly correlated with cancer – In motor fuels, solvents, explosives, smoke, medicines – Linked to leukemia, blood disorders, damaged immunity Smogs and brown clouds • Industrial smog: smoke + fog – An irritating, grayish mix of soot, sulfur compounds, and water vapor – In industrialized, cool areas that use coal – China, India, Korea, eastern European countries • Photochemical smog: in cities with huge freeway systems – A brownish, irritating haze in warm, sunny areas – Arises during the morning traffic – Pollutants from vehicle exhaust are acted on by sunlight – Nitrogen oxides, volatile organic compounds Industrial and photochemical smog Creation of industrial and photochemical smog Industrial smog Photochemical smog Atmospheric brown clouds • Atmospheric brown cloud (ABC): relatively new – 1–3 km blanket of pollution over south/central Asia • Similar to North Temperate Zone’s aerosol pollution – But persists year round and has a different make up. • ABC: black carbon and soot – From burning biomass and fossil fuels (coal, diesel). • Impacts: dimming over large cities, less rainfall, heating of air, decreased reflection of snow and ice – Shrinking glaciers will reduce water sources. – Weaker Indian monsoons, less crops, health effects. Industrial smog Flue gas desulfurization • SO2 scrubbing, or Flue Gas Desulfurization processes can be classified as: Throwaway or Regenerative, depending upon whether the recovered sulfur is discarded or recycled. Wet or Dry, depending upon whether the scrubber is a liquid or a solid. • Flue Gas Desulfurization Processes The major flue gas desulfurization (FGD ) processes are : Limestone Scrubbing Lime Scrubbing Dual Alkali Processes Lime Spray Drying Wellman-Lord Process Limestone scrubbing • Limestone slurry is sprayed on the incoming flue gas. The sulfur dioxide gets absorbed The limestone and the sulfur dioxide react as follows : CaCO3 + H2O + 2SO2 ----> Ca+2 + 2HSO3-+ CO2 CaCO3 + 2HSO3-+ Ca+2 ----> 2CaSO3 + CO2 + H2O Lime scrubbing • The equipment and the processes are similar to those in limestone scrubbing Lime Scrubbing offers better utilization of the reagent. The operation is more flexible. The major disadvantage is the high cost of lime compared to limestone. The reactions occurring during lime scrubbing are : CaO + H2O -----> Ca(OH)2 SO2 + H2O <----> H2SO3 H2SO3 + Ca(OH)2 -----> CaSO3.2 H2O CaSO3.2 H2O + (1/2)O2 -----> CaSO4.2 H2O Dual alkali system Lime and Limestone scrubbing lead to deposits inside spray tower. The deposits can lead to plugging of the nozzles through which the scrubbing slurry is sprayed. The Dual Alkali system uses two regents to remove the sulfur dioxide. Sodium sulfite / Sodium hydroxide are used for the absorption of sulfur dioxide inside the spray chamber. The resulting sodium salts are soluble in water, so no deposits are formed. The spray water is treated with lime or limestone, along with make-up sodium hydroxide or sodium carbonate. The sulfite/sulfate ions are precipitated, and the sodium hydroxide is regenerated. Lime – spray drying – Lime Slurry is sprayed into the chamber – The sulfur dioxide is absorbed by the slurry – The liquid-to-gas ratio is maintained such that the spray dries before it reaches the bottom of the chamber – The dry solids are carried out with the gas, and are collected in fabric filtration unit – This system needs lower maintenance, lower capital costs, and lower energy usage Wellman – Lord process • This process consists of the following subprocesses: – Flue gas pre-treatment. – Sulfur dioxide absorption by sodium sulfite – Purge treatment – Sodium sulfite regeneration. – The concentrated sulfur dioxide processed to a marketable product. stream is The flue gas is pre-treated to remove the particulate. The sodium sulfite neutralizes the sulfur dioxide : Na2SO3 + SO2 + H2O -----> 2NaHSO3 Wellman – Lord process • Some of the Na2SO3 reacts with O2 and the SO3 present in the flue gas to form Na2SO4 and NaHSO3. • Sodium sulfate does not help in the removal of sulfur dioxide, and is removed. Part of the bisulfate stream is chilled to precipitate the remaining bisulfate. The remaining bisulfate stream is evaporated to release the sulfur dioxide, and regenerate the bisulfite. Photochemical smog Production of hydroxyl radicals NO2 + sunlight (less than 320 nm) NO + O photo-dissociation reaction O + O2 O3 + NO O3 O + H2O O3 NO2 + O2 O2 + O . 2 OH (hydroxyl radical) By this pathway one NO2 molecule produces two hydroxyl radicals. Destruction of hydroxyl radicals . . OH + . NO2 OH + HOO . 2 OOH . HNO3 H2O + O2 H2O2 + O2 Secondary air pollutants from HCs Production of peroxyacetyl nitrate (PAN) . . RCHO + OH R CO + H2O . R CO + O2 RCOOO . . . RCOOO + NO2 RCOOONO2 When R = CH3, CH3COOONO2 Development of photochemical smog over the Los Angeles area on a typical warm day • Cold, cloudy weather in a valley surrounded by mountains can trap air pollutants (left). • Areas with sunny climate, light winds, mountains on three sides and an ocean on the other are susceptible to inversions. The environmental damages • Plants are more sensitive than humans to air pollution. – Sulfur dioxide from smelters and power plants killed large areas of vegetation. • Ozone damages crops, orchards, and forests. – Ozone enters plants through stomata (pores) – Symptoms of damage: black flecks, yellow leaves • Crops vary in their susceptibility to ozone. – Soybeans, corn, wheat are damaged at ambient ozone levels. – Countries lose billions of dollars/year in lower yields. Ozone impact on crop yields National ambient air quality standards Indoor air pollution • Indoor air pollution usually is a greater threat to human health than outdoor air pollution. • According to the EPA, the four most dangerous indoor air pollutants in developed countries are: – Tobacco smoke. – Formaldehyde. – Radioactive radon-222 gas. – Very small fine and ultrafine particles. Indoor air pollution More disease from indoor (orange) pollution than outdoor (red) In developing nations, indoor cooking fires are common, and a major health risk. Indoor air pollutants Chloroform Para-dichlorobenzene Tetrachloroethylene 1, 1, 1Trichloroethane Formaldehyde Benzo-a-pyrene Nitrogen Oxides Styrene Tobacco Smoke Asbestos Radon-222 Carbon Monoxide Methylene Chloride Concentrations of common indoor air pollutants compared with outdoor concentrations We can reduce indoor air pollution • In developed countries: – Use low-toxicity materials, limit use of plastics and treated wood, monitor air quality, keep rooms clean – Provide adequate ventilation – Limit exposure to known toxicants – Test homes and offices and use CO detectors • In – – – developing countries: Dry wood before burning Cook outside Use less-polluting fuels (natural gas) Bringing air pollution under control • By the 1960s it was obvious that pollutants were overloading natural cleansing processes. – Unrestricted pollution discharge could not continue • The Clean Air Act of 1970 (CAA): passed by Congress – Amended in 1977 and 1990 – Is administered by the EPA – The foundation of U.S. air-pollution control efforts – Ambient standards: levels protecting human health and the environment National standards We can reduce smog • Regulations require new cars to have catalytic converters • Require cleaner industrial facilities – Close those that can’t improve • Financial incentives to replace aging vehicles – Restricting driving • Vehicle inspection programs (“smog checks”) • Reduce sulfur in diesel; remove lead in gasoline • Electronic pollution indicator boards raise awareness • But increased population and cars can wipe out advances Reducing particulates • Before 1970, major sources of particulates were industrial stacks and open burning of refuse. – The CAA prevents both of these – Disposal of refuse is now through landfilling – Industries installed filters, electrostatic precipitators, etc. to reduce stack emissions • Particulates are still emitted from steel mills, power plants, cement plants, smelters, construction sites, Diesel engines, wood-burning stoves, fires Reducing particulates • The EPA added new ambient air quality standards for particulates (PM2.5) in 1997 – Particulates smaller than 2.5 micrometers (microns) are the most dangerous to the lungs. – Larger particulates are still regulated. • 24-hour primary standards were lowered. – Saving $9–$75 billion/year in health benefits Managing ozone • To control ozone: address the VOCs and NOx that form it • Reducing emissions of VOCs from motor vehicles, point sources (industries), and area sources (dry cleaners, print shops, household products) – Since the CAAA, emissions have declined 24% • The 1997 revised ozone standard was strongly opposed by industry, but the Supreme Court upheld the EPA • Implementation was delayed until 2004 – Health benefits far outweighed compliance costs • Some don’t think the current ozone standard is strict enough. Down with NOx • The EPA implemented regulations to reduce NOx emissions from mobile sources, power plants, industrial boilers, turbines. • Tier 2 Standards: emissions from all SUVs, trucks, passenger vans are held to the same standard as cars. – They are not being met due to older, polluting cars. • NOx Budget Trading Program (NBP): the EPA establishes a market-based “cap and trade” system – Industries have flexibility in attaining the targets – States can come back to compliance Structure of the atmosphere and ozone concentration Dobson spectrophotometer •Dobson spectrophotometer invented in 1924, can infer ozone concentrations from the ground, and helped scientists detect ozone depletion. • 1 DU= 2.7 x 1016 O3 molecules /cm2, layer of ozone that would be 0.001 cm thick at STP(273 K, 1 atm) UV radiation with wavelengths between 0.2 and 0.4 μm Ultraviolet A Ultraviolet B Thin layer of dead cells Squamous Cell Carcinoma Hair Epidermis Squamous cells Basal layer Sweat gland Melanocyte cells Melanoma Basal Cell Carcinoma Dermis Basal cell Blood vessels Squamous Cell Carcinoma Basal Cell Carcinoma Melanoma Radiation and importance of the shield • • • • • Skin cancer (700,000 new cases each year) Premature skin aging Eye damage Cataracts Blindness Ozone formation • The ozone shield: stratospheric ozone that absorbs most (99%) UV radiation – The 1% UVB radiation causes skin cancer, and damages crops and other life forms • Ozone is formed in the stratosphere: – UV radiation splits O2 molecules into free oxygen (O) O2 + UVC → O + O – Some O combines with O2 to form ozone (O3) O + O2 → O3 Total ozone (Dobson units) Ozone cycle 400 October monthly means 350 300 250 200 150 100 19551960196519701975198019851990199520002005 Year Chapman cycle: Steady-State Concentration of Ozone O2 + hv (λ <240 nm) O + O Rate of reaction 1 = K1 [O2] O + O2 + M O3 + M Rate of reaction 2 = K2 [O][O2][M] O3 + hv (λ 240 to 320 nm) Rate of reaction 3 = K3[O3] O3 + O 2 O2 Rate of reaction 4 = K4[O][O3] O2 + O Ozone is produced only in the second reaction. Ozone is destroyed in reactions 3 and 4. The ozone steady state can be expressed as: O3 production O3 destruction Rate of reaction 2 = Rate of reaction 3 + Rate of reaction 4 K2 [O][O2][M] = K3[O3] + K4 [O][O3] 5.1 O production O reduction 2 (rate of 1) + rate 3 = rate of 2 + rate of 4 substituting 2K1[O2] + K3[O3] = K2[O][O2][M] + K4[O][O3] 5.2 subtraction equation 5.1 from equation 5.2 gives 2 K2[O][O2][M] = 2 K1[O2] + 2 K3[O3] 5.3 2 K2[O][O2][M] = 2 K3[O3] when K1[O2] ≪K3[O3] 5.4 rearranging [O] = K3[O3] K2[M][O2] addition of 5.1 and 5.2 gives 5.5 2 K1[O2] = 2 K4[O][O3] 5.6 substitute 5.5 into 5.6 2 K1[O2] = 2 K4K3[O3]2 K2[M][O2] [O3]/[O2] = K1K2[M] K3K4 1/2 = 10-4 (실제 10 ppm 존재) 5.7 → This result suggests that there are additional pathways for ozone destruction. Catalytic destruction of ozone X + O3 XO XO + O X + + O2 O2 The sum of these two reactions is: O3 XO + + O O 2 O2 X + Decomposition reaction O2 X acts as catalysts, OH, NO, Cl-, Br- . Hydroxyl radical cycles • Production reaction . O + H2O → 2 OH . . O + CH4 → OH + CH3 . . H2O + hν → H + OH . • Destruction cycle reaction(40 O3/1 OH) . . OH + O3 → OOH + O2 . . OOH + O → OH + O2 Net reaction: O + O3 → 2O2 NO Cycle N2O + hv N2 + O* Below 30 km, in the stratosphere, the excited state oxygen reacts with the N2O to produce nitric oxide. N2O + O* 2 NO . NO can act as an X catalytic species (105 O3/NOx radical). NO . + . O3 . . NO2 + O2 NO2 + O NO + O2 -------------------------------------Net reaction: O + O3 2O2 Halogens in the atmosphere • Chlorofluorocarbons (CFCs): halogenated hydrocarbons – Nonreactive, nonflammable, nontoxic organic molecules – Chlorine and fluorine atoms replace some hydrogens. – Normally gaseous, but liquefy under some pressure • Used in refrigerators, air conditioners, heat pumps – Production of plastic foams – Cleaning computer parts – Pressuring agent in aerosol cans Reaction of ozone with CFC Ultraviolet light hits a chlorofluorocarbon (CFC) molecule, such as CFCl3, breaking off a chlorine atom and leaving CFCl2. Sun Cl Cl C F Cl UV radiation Once free, the chlorine atom is off to attack another ozone molecule and begin the cycle again. Cl Cl O O The chlorine atom attacks an ozone (O3) molecule, pulling an oxygen atom O off it and leaving O O an oxygen molecule (O2). Cl Summary of Reactions CCl3F + UV Cl. + CCl2F Cl. + O3 ClO. + O2 Repeated ClO. + O Cl. + O2 many times A free oxygen atom pulls the oxygen atom off the chlorine monoxide Cl molecule to form O2. O O Cl The chlorine atom and the O oxygen atom join O to form a chlorine O monoxide molecule (ClO.). Chlorine is regenerated after reacting with O3 • Chlorine catalytic cycle: chlorine is regenerated – Chlorine acts as a catalyst—promoting a chemical reaction without being used up. • Every chlorine molecule lasts 40–100 years in the air. – It can break down 100,000 O3 molecules. • The EPA banned CFC use in aerosol cans in 1978. – Manufacturers switched to nondamaging substitutes. – CFCs were still used in other applications. • Bromine, a soil fumigant and pesticide, depletes ozone – It is 60 times as potent as Cl. Null cycles Prevent catalytic species from taking part in the catalytic cycles NO2 + O3 NO3 + O2 NO3 + hv NO2 + O ---------------------------------------net reaction: O3 + hv O2 + O Some of the NO3 can react to produce N2O5: NO3 . NO2 + + . Cl . NO2 OH + + CH4 N2O5 +M M HNO3 HCl + . + CH3 + M M As the atmospheric concentration of ClO. increases, the concentration of O3 decreases Ozone-depleting substances production and presence in the atmosphere CFCs HCFCs Relative abundances of chlorine and bromine in the stratosphere The ozone “hole” • In 1985, scientists noticed a hole (serious thinning) in the stratospheric ozone layer over the South Pole. – Ozone levels were 50% lower than normal. • In summer: gases (NO2, methane) trap Cl, preventing ozone depletion. • The Antarctic winter (in June) creates a vortex that traps stratospheric gases. • Extremely cold temperatures create stratospheric clouds. – Cloud surfaces allow chemical reactions to release Cl2. The Antarctic’s ozone “hole” • Spring sunlight breaks up the stratospheric clouds. • UV light attacks molecular chlorine. – Initiating the chlorine cycle, which destroys ozone. • Summer (November) breaks up the vortex. – Ozone-rich air returns to the area. • Ozone-poor air has spread over the Southern Hemisphere. – Queensland, Australia: two out of three are expected to develop skin cancer. • The ozone hole is now the size of North America. • The Arctic does have 25% depletion, but no ozone hole. Ozone depletion in Antarctica Antarctic ozone depletion ClONO2 (g) HOCl (g) Cl2 + + + . 2 Cl + . ClO + ClOOCl . HCl (s) Cl2 (g) + HCl (s) Cl2 (g) hv H2O (s) . . . + 2 Cl 2 O3 2 ClO ClO HNO3 (s) + 2 O2 ClOOCl . + hv ClOO . . + Cl ClOO Cl + O2 ----------------------------------------------net reaction: 2 O3 + hv 3 O2 Stratospheric ozone depletion • In 1985, the Antarctica. “ozone hole” was detected over • Ozone levels had declined 40–60% over the previous decade. Ozone loss and the extent of the ozone hole Annual record of ozone hole area Further ozone depletion • Worldwide ozone losses of 3%–6% from 2002 to 2007. • Ozone losses of the 1980s will have caused 12 million people in the U.S. to develop skin cancer. – More UVB radiation than ever reached Earth • In 1987, UN member countries met in Montreal, Canada. – The Montreal Protocol: 194 nations agreed to scale back CFC production 50% by 2000 – Amendments moved the target date for complete phaseout of CFCs to January 1, 1996 – CFCs are still being used. International efforts to reduce stratospheric ozone depletion • In 1987, 196 nations signed the Montreal Protocol, which restricted CFC production in half by 1998 • Follow-up agreements deepened cuts, advanced timetables, and addressed other ozone-depleting chemicals – Industry shifted to safer, inexpensive, and efficient alternatives • Challenges still face us – CFCs will remain in the stratosphere for a long time – Nations can ask for exemptions to the ban • The Montreal Protocol is one of the biggest environmental success stories of our time. • Reasons for success of the Montreal Protocol: • Government and industry cooperated on finding solutions (cheap replacement technologies for CFCs, CF3CH2F), so battles typical to environmental debates were minimized. • Protocol was implemented with “adaptive management”—ability to fine-tune actions as time goes on, in response to new data or conditions. Protecting the ozone layer International agreements reduced ozone-depleting substances The hole in the ozone has stopped growing Final thoughts • The ozone story is a remarkable episode in human history. • The world has shown that it can respond collectively and effectively to a clearly perceived threat. • The scientific community played a crucial role. – Alerting the world – Researching the threat • Skeptics stressed uncertainties but scientific consensus convinced politicians to act.