1. © 2011 Pearson Education, Inc. This lecture will help you understand: The Earth’s atmosphere Weather, climate, and atmospheric conditions Outdoor pollution and solutions Stratospheric ozone depletion Acidic deposition and consequences Indoor air pollution and solutions

2. © 2011 Pearson Education, Inc. Central Case: L.A. and its sister cities struggle for a breath of clean air Vehicles caused smog in Los Angeles from 1970s to 1990s Policies and technologies improved its air qualities -But its “sister cities” are not as clean 3,600/month die in Tehran from air pollution -Old cars use cheap gas -Topography, immigration, etc.

3. © 2011 Pearson Education, Inc. The atmosphere Atmosphere = the thin layer of gases around Earth -Provides oxygen -Absorbs radiation and moderates climate -Transports and recycles water and nutrients -78% N 2, 21% O 2 Minute concentrations of permanent (remain at stable concentrations) gases -Variable gases = varying concentrations across time and place Human activity is changing the amount of some gases -CO 2, methane (CH 4 ), ozone (O 3 )

4. © 2011 Pearson Education, Inc. QUESTION: Review The major component of Earth’s atmosphere is: a)Nitrogen gas b)Oxygen gas c)Argon gas d)Water vapor

5. © 2011 Pearson Education, Inc. The atmosphere’s composition

6. © 2011 Pearson Education, Inc. The first two layers of the atmosphere Troposphere = bottommost layer (up to16 km) (how many miles? -Air for breathing, weather -The air gets colder with altitude -Tropopause = The upper boundary of the troposphere. Limits mixing between troposphere and the layer above it Stratosphere =(16-50km) above sea level -Drier and less dense, with little vertical mixing -Becomes warmer with altitude (Why?) -Contains UV radiation-blocking ozone (O 3 ), 17–30 km above sea level

7. © 2011 Pearson Education, Inc. The two highest levels of the atmosphere Mesosphere = 50–80 km (31–56 mi) above sea level -Extremely low air pressure -Temperatures decrease with altitude Thermosphere = atmosphere’s top layer -Extends upward to 500 m (300 mi) -Very high temperatures for individual molecules (up to 1750 0 C!) But the extremely low pressure and relatively few molecules mean that it would still feel cold. (Why?)

8. © 2011 Pearson Education, Inc. The atmosphere’s four layers Atmospheric layers have different -Temperatures -Densities -Composition KNOW THIS! 

9. © 2011 Pearson Education, Inc. QUESTION: Interpreting Graphs and Data What does this graph show about the stratosphere? a)It contains the most ozone. b)It is a very thin layer. c)Temperature decreases with increasing altitude. d)Temperature is not affected with increasing altitude.

10. © 2011 Pearson Education, Inc. Atmospheric properties Atmospheric pressure = the force per unit area produced by a column of air Relative humidity = the ratio of water vapor air contains to the amount it could contain at a given temperature -High humidity makes it feel hotter than it really is Temperature = varies with location and time Atmospheric pressure decreases with altitude

11. © 2011 Pearson Education, Inc. Solar energy heats the atmosphere The spatial relationship between the Earth and sun determines how much solar energy strikes the Earth Microclimate = a localized pattern of weather conditions Energy from the sun: -Heats and moves air -Creates seasons -Influences weather and climate Solar radiation is highest near the equator

12. © 2011 Pearson Education, Inc. Solar energy creates seasons Because the Earth is tilted, each hemisphere tilts toward the sun for half the year -Results in a change of seasons Equatorial regions are unaffected by this tilt, so days average 12 hours throughout the year

13. © 2011 Pearson Education, Inc. QUESTION: Review With convective circulation: a)Less dense, cooler air rises b)Denser, warmer air rises c)Less dense, warmer air rises d)Denser, cooler air rises

14. © 2011 Pearson Education, Inc. Solar energy causes air to circulate Air near Earth’s surface is warm and moist Convective circulation = less dense, warmer air rises -Creating vertical currents -Rising air expands and cools -Cool air descends and becomes denser -Replacing rising warm air Convection influences weather and climate

15. © 2011 Pearson Education, Inc. The atmosphere drives weather and climate Weather and climate involve the physical properties of the troposphere -Temperature, pressure, humidity, cloudiness, wind Weather = specifies atmospheric conditions over short time periods and within small geographic areas Climate = patterns of atmospheric conditions across large geographic regions over long periods of time Mark Twain said, “Climate is what we expect; weather is what we get”

16. © 2011 Pearson Education, Inc. Air masses produce weather Front = the boundary between air masses that differ in temperature, moisture, and density Warm front = boundary where warm, moist air replaces colder, drier air Cold front = where colder, drier air displaces warmer, moister air Warm fronts produce light rain Cold fronts produce thunderstorms

17. © 2011 Pearson Education, Inc. Air masses have different pressures High-pressure system = air that descends because it is cool -It spreads outward as it nears the ground -Brings fair weather Low-pressure system = warm air rises and draws air inward toward the center of low pressure -Rising air expands and cools -It brings clouds and precipitation

18. © 2011 Pearson Education, Inc. Thermal (temperature) inversion Thermal inversion = a layer of cool air occurs beneath warm air Inversion layer = the band of air where temperature rises with altitude -Denser, cooler air at the bottom of the layer resists mixing Inversions trap pollutants in cities surrounded by mountains Air temperature decreases as altitude increases - Warm air rises, causing vertical mixing

19. © 2011 Pearson Education, Inc. Circulation systems produce climate patterns Convective currents contribute to climatic patterns Hadley cells = convective cells near the equator -Surface air warms, rises, and expands -Causing heavy rainfall near the equator -Giving rise to tropical rainforests Currents heading north and south are dry -Giving rise to deserts at 30 degrees Ferrel cells and polar cells = lift air and create precipitation at 60 degrees latitude north and south -Conditions at the poles are dry

20. © 2011 Pearson Education, Inc. Global wind patterns Atmospheric cells interact with Earth’s rotation to produce global wind patterns -As Earth rotates, equatorial regions spin faster Coriolis effect = the apparent north-south deflection of air currents of the convective cells -Results in curving global wind patterns called the doldrums, trade winds, and westerlies

21. © 2011 Pearson Education, Inc. Climate patterns and moisture distribution

22. © 2011 Pearson Education, Inc. Global wind patterns Doldrums = a region near the equator with few winds Trade winds = between the equator and 30 degrees -Blow from east to west -Weaken periodically, leading to El Niño conditions Westerlies = from 30 to 60 degrees latitude -Blow from west to east People used these winds to sail across the ocean Wind and convective circulation in ocean water maintain ocean currents -And can create violent storms

23. © 2011 Pearson Education, Inc. QUESTION: Review If you were on a sailing ship going from the United States to Europe, you would want to be in the area of the _____. a)Doldrums b)Trade winds c)Westerlies d)Polar cell

24. © 2011 Pearson Education, Inc. Storms pose hazards Atmospheric conditions can produce dangerous storms Hurricanes = form when winds rush into areas of low pressure -Warm, moist air over the topical oceans rises Typhoons (cyclones) = winds turn counterclockwise in the Northern Hemisphere -Drawing up huge amounts of water vapor -Which falls as heavy rains Tornadoes = form when warm air meets cold air -Quickly rising warm air forms a powerful convective current (spinning funnel)

25. © 2011 Pearson Education, Inc. Hurricanes and tornadoes Understanding how the atmosphere works helps us to: -Predict violent storms and protect people -Comprehend how pollution affects climate, ecosystems, and human health

26. © 2011 Pearson Education, Inc. Outdoor air pollution Air pollutants = gases and particulate material added to the atmosphere -Can affect climate or harm people or other organisms Air pollution = the release of pollutants Outdoor (ambient) air pollution = pollution outside -Has recently decreased due to government policy and improved technologies in developed countries -Developing countries and urban areas still have significant problems

27. © 2011 Pearson Education, Inc. Natural sources pollute: volcanoes Release particulate matter, sulfur dioxide, and other gases -Can remain for months or years Aerosols = fine droplets of sulfur dioxide, water, oxygen -Reflect sunlight back to space -Cool the atmosphere and surface Volcanoes are one source of natural air pollution, as shown by the Mount Saint Helens eruption in 1980

28. © 2011 Pearson Education, Inc. Natural sources pollute: fires Fires pollute the atmosphere with soot and gases Over 60 million ha (150 million acres) of forests and grasslands burn per year Human influence makes fires worse -Fuel buildup from fire suppression, development in fire-prone areas, “slash-and-burn” agriculture -Climate change will increase drought and fires In 1997, unprecedented forest fires sickened 20 million and caused a plane to crash

29. © 2011 Pearson Education, Inc. Wind over arid land sends huge amounts of dust aloft -Even across oceans Businesses, schools, and governments close Unsustainable farming and grazing promote: -Erosion -Desertification Natural sources pollute: dust storms

30. © 2011 Pearson Education, Inc. We create outdoor air pollution Air pollution comes from mobile or stationary sources Point sources = specific spots where large quantities of pollutants are discharged (power plants and factories) Non-point sources = more diffuse, consisting of many small sources (automobiles) Primary pollutants = directly harmful and can react to form harmful substances (soot and carbon monoxide) Secondary pollutants = form when primary pollutants interact or react with components of the atmosphere -Tropospheric ozone and sulfuric acid

31. © 2011 Pearson Education, Inc. Pollutants exert local and global effects Residence time = the time a pollutant stays in the atmosphere Pollutants with brief residence times exert localized impacts over short time periods -Particulate matter, automobile exhaust Pollutants with long residence times exert regional or global impacts -Pollutants causing climate change or -ozone depletion

32. © 2011 Pearson Education, Inc. Legislation addresses pollution Air Pollution Control Act (1963) funded research and encouraged emissions standards The Clean Air Act of 1970 -Set standards for air quality, limits on emissions -Provided funds for pollution-control research -Allowed citizens to sue parties violating the standards The Clean Air Act of 1990 strengthened regulations for auto emissions, toxic air pollutants, acidic deposition, stratospheric ozone depletion -Introduced emissions trading for sulfur dioxide

33. © 2011 Pearson Education, Inc. The EPA sets standards The EPA sets nationwide standards for emissions and concentrations of toxic pollutants States monitor air quality -They develop, implement, and enforce regulations -They submit plans to the EPA for approval The EPA takes over enforcement if plans are inadequate Criteria pollutants = pollutants that pose especially great threats to human health -Carbon monoxide, sulfur dioxide, nitrogen dioxide, tropospheric ozone, particulate matter, lead

34. © 2011 Pearson Education, Inc. Criteria pollutants: CO and SO 2 Carbon monoxide (CO) = colorless, odorless gas -Produced primarily by incomplete combustion of fuel -From vehicles and engines, industry, waste combustion, residential wood burning -Poses risk to humans and animals, even in small concentrations Sulfur dioxide (SO 2 ) = colorless gas with a strong odor -Coal emissions from electricity generation, industry -Can form acid precipitation

35. © 2011 Pearson Education, Inc. Criteria pollutants: NO 2 Nitrogen dioxide (NO 2 ) = a highly reactive, foul- smelling reddish brown gas -Nitrogen oxides (NO x ) = formed when nitrogen and oxygen react at high temperatures in engines -Vehicles, industrial combustion, electrical utilities -Contribute to smog and acid precipitation

36. © 2011 Pearson Education, Inc. Criteria pollutants: tropospheric ozone Tropospheric ozone (O 3 ) = a colorless gas with a strong odor -Results from interactions of sunlight, heat, nitrogen oxides, and volatile carbon-containing chemicals -A secondary pollutant -A major component of smog -Participates in reactions that harm tissues and cause respiratory problems -The pollutant that most frequently exceeds EPA standards

37. © 2011 Pearson Education, Inc. Criteria pollutants: particulate matter and lead Particulate matter = suspended solid or liquid particles -Primary pollutants: dust and soot -Secondary pollutants: sulfates and nitrates -Damages respiratory tissue when inhaled -From dust and combustion processes Lead = in gasoline and industrial metal smelting -Bioaccumulates and damages the nervous system -Banned in gasoline in developed, but not in developing, countries

38. © 2011 Pearson Education, Inc. Areas in the U.S. fail air quality standards Many Americans live in areas with unhealthy levels of criteria pollutants

39. © 2011 Pearson Education, Inc. QUESTION: Review Ozone in the _________ is a pollutant, but in the ______ is vital for life. a)Stratosphere, troposphere b)Troposphere, stratosphere c)Troposphere, tropopause d)Stratosphere, thermosphere

40. © 2011 Pearson Education, Inc. Agencies monitor emissions State and local agencies monitor, calculate, and report to the EPA the emissions of these pollutants: -Carbon monoxide, sulfur dioxide, particulate matter, lead, and all nitrogen oxides Tropospheric ozone has no emissions to monitor -It is a secondary pollutant Agencies monitor volatile organic compounds (VOCs) = carbon-containing chemicals -Used and emitted by engines and industrial processes -VOCs can react to produce ozone

41. © 2011 Pearson Education, Inc. U.S. air pollution In 2008, the U.S. emitted 123 million tons of the six monitored pollutants The average U.S. driver emits 6 metric tons of CO 2 /yr as well as other pollutants!

42. © 2011 Pearson Education, Inc. We have reduced air pollution Total emissions of the six monitored pollutants have declined 60% since the Clean Air Act of 1970 -Despite increased population, energy consumption, miles traveled, and gross domestic product

43. © 2011 Pearson Education, Inc. We reduced emissions and improved the economy Technology and federal policies Cleaner-burning engines and catalytic converters Permit-trading programs and clean coal technologies reduce SO 2 emissions Scrubbers = chemically convert or physically remove pollutants before they leave smokestacks Phaseout of leaded gasoline

44. © 2011 Pearson Education, Inc. Toxic substances pose health risks Toxic air pollutants = substances that cause: -Cancer, reproductive defects -Neurological, developmental, immune system, or respiratory problems The EPA regulates 188 toxic air pollutants from metal smelting, sewage treatment, industry, etc. Include heavy metals, VOCs, diesel, urban hazards Clean Air Act regulations helped reduce emissions by more than 35% since 1990

45. © 2011 Pearson Education, Inc. U.S. health risks vary geographically Nationwide cancer risks Non-cancerous respiratory ailments

46. © 2011 Pearson Education, Inc. Industrializing nations suffer increasing pollution Outdoor pollution is getting worse in developing nations Factories and power plants pollute -Governments emphasize economic growth, not pollution control People burn traditional fuels (wood and charcoal) -And more own cars China has the world’s worst air pollution -Coal burning, more cars, power plants, factories -Causing over 300,000 premature deaths/year

47. © 2011 Pearson Education, Inc. Pollution in developing nations is high More people own cars Smog in Beijing surrounds an Olympic stadium

48. © 2011 Pearson Education, Inc. Air pollution in China The government is trying to decrease pollution -Shutting down heavily polluting factories and mines -Phasing out some subsidies for polluting industries -Installing pollution controls in factories -Encouraging renewable and nuclear energy -Mandating cleaner burning fuels Air is improving in Beijing but not in other places Asian (Atmospheric) Brown Cloud = a 2-mile-thick layer of pollution over southern Asia -Decreased plant productivity, increased flooding, etc.

49. © 2011 Pearson Education, Inc. Air quality is a rural issue, too Airborne pesticides from farms Industrial pollutants from cities, factories, and power plants Feedlots, where cattle, hogs, or chickens are raised in dense concentrations -Voluminous amounts of dust, methane, hydrogen sulfide, and ammonia -Also create objectionable odors -People living or working nearby have high rates of respiratory illness

50. © 2011 Pearson Education, Inc. Smog: our most common air quality problem Smog = an unhealthy mixture of air pollutants over urban areas Sulfur in burned coal combines with oxygen to form sulfuric acid Industrial (gray air) smog = industries burn coal or oil -Regulations in developed countries reduced smog Coal-burning industrializing countries face health risks -Coal and lax pollution control Smog in Donora killed 21 people and sickened 6,000

51. © 2011 Pearson Education, Inc. Photochemical (brown air) smog Produced by a series of reactions -Formed in hot, sunny cities surrounded by mountains Light-driven reactions of primary pollutants and atmospheric compounds -Morning traffic releases NO and VOCs -Irritates eyes, noses, and throats Los Angeles smog kills 3,900/year and costs $28 billion/year High levels of NO 2 cause photochemical smog to form a brown haze over cities

52. © 2011 Pearson Education, Inc. Industrial smogPhotochemical smog Creation of industrial and photochemical smog

53. © 2011 Pearson Education, Inc. 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

54. © 2011 Pearson Education, Inc. Synthetic chemicals deplete stratospheric ozone Ozone layer = ozone in the lower stratosphere -Blocks incoming ultraviolet (UV) radiation -Protecting life from radiation’s damaging effects Ozone-depleting substances = human-made chemicals that destroy ozone by splitting its molecules apart Halocarbons = human-made compounds made from hydrocarbons with added chlorine, bromine, or fluorine Chlorofluorocarbons (CFCs) = a halocarbon used as refrigerants, in fire extinguishers, in aerosol cans, etc. -Releases chlorine atoms that split ozone

55. © 2011 Pearson Education, Inc. CFCs destroy ozone CFCs are inert (don’t react) CFCs remain in the stratosphere for a century UV radiation breaks CFCs into chlorine and carbon atoms The chlorine atom splits ozone Ozone hole = decreased ozone levels over Antarctica One chlorine atom can destroy 100,000 ozone molecules

56. © 2011 Pearson Education, Inc. The Antarctic ozone hole High-altitude polar stratospheric clouds form during the dark, frigid winter Nitric acid in clouds splits chlorine off of CFCs -A polar vortex (swirling winds) traps chlorine -UV radiation in September (spring) sunshine dissipates the clouds and releases the chlorine -The chlorine destroys the ozone -December’s warmer air shuts down the polar vortex -Ozone-poor air diffuses, while ozone-rich air enters

57. © 2011 Pearson Education, Inc. The Montreal Protocol Montreal Protocol = 196 nations agreed to cut 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

58. © 2011 Pearson Education, Inc. The Montreal Protocol is a success It is considered our biggest environmental success story Research developed rapidly, along with technology Policymakers included industry in helping solve the problem Implementation of the plan allowed an adaptive management strategy -Strategies responded to new scientific data, technological advances, and economic figures The Montreal Protocol can serve as a model for international environmental cooperation

59. © 2011 Pearson Education, Inc. Protecting the ozone layer International agreements reduced ozone-depleting substances The hole in the ozone has stopped growing

60. © 2011 Pearson Education, Inc. Acid deposition Acid deposition is another transboundary issue Acidic deposition = the deposition of acid, or acid- forming pollutants from the atmosphere onto Earth’s surface Acid rain = precipitation containing acid -Rain, snow, sleet, hail Atmospheric deposition = the wet or dry deposition on land of pollutants (mercury, nitrates, organochlorines) -From automobiles, electric utilities, industrial facilities

61. © 2011 Pearson Education, Inc. Burning fossil fuels produces acid rain Burning fossil fuels releases sulfur dioxide and nitrogen oxides -These compounds react with water, oxygen, and oxidants to form sulfuric and nitric acids

62. © 2011 Pearson Education, Inc. Impacts of acid deposition Nutrients are leached from topsoil Soil chemistry is changed Metal ions (aluminum, zinc, etc.) are converted into soluble forms that pollute water Affects surface water and kills fish Damages agricultural crops Erodes stone buildings, corrodes cars, erases writing on tombstones

63. © 2011 Pearson Education, Inc. pH of precipitation in the U.S. Many regions of acidification are downwind of major sources of pollution The acid-neutralizing capacity of soil, rock, or water impacts the severity of acid rain’s effects

64. © 2011 Pearson Education, Inc. We have begun to address acid deposition Reducing acid deposition involves reducing the pollution that contributes to it The Clear Air Act of 1990 established an emissions trading program for sulfur dioxide -Benefits outweighed costs 40:1 New technologies such as scrubbers have helped Acid deposition is worse in the developing world -Especially in China, which burns coal in factories lacking pollution control equipment

65. © 2011 Pearson Education, Inc. Indoor air pollution Indoor air pollution = in workplaces, schools, and homes -Health effects are greater than from outdoor pollution The average U.S. citizen spends 90% of the time indoors -Exposed to synthetic materials that have not been comprehensively tested Being environmentally prudent can make it worse -To reduce heat loss and improve efficiency, ventilation systems were sealed off -Windows do not open, trapping pollutants inside

66. © 2011 Pearson Education, Inc. Indoor air pollution in the developing world Stems from burning wood, charcoal, dung, crop wastes with little to no ventilation Fuel burning pollution causes 1.6 million deaths/year -Soot and carbon monoxide -Pneumonia, bronchitis, lung cancer, allergies, cataracts, asthma, heart disease, etc.

67. © 2011 Pearson Education, Inc. Tobacco smoke and radon The most dangerous indoor pollutants in developed nations Secondhand smoke from cigarettes is very dangerous -Contains over 4,000 chemical compounds -Causes eye, nose, and throat irritation -Smoking has declined in developed nations Radon causes 21,000 deaths a year in the U.S. -A radioactive gas resulting from natural decay of rock, soil, or water that can seep into buildings -New homes are being built that are radon resistant

68. © 2011 Pearson Education, Inc. Radon risk across the U.S.

69. © 2011 Pearson Education, Inc. VOCs pollute indoor air The most diverse group of indoor air pollutants -Released by everything from plastics and oils to perfumes and paints -Most VOCs are released in very small amounts Unclear health implications due to low concentrations Formaldehyde leaking from pressed wood and insulation irritates mucous membranes and induces skin allergies Pesticides seep through floors and walls -Are brought in on shoe soles

70. © 2011 Pearson Education, Inc. Sources of indoor air pollution

71. © 2011 Pearson Education, Inc. Living organisms can pollute indoors Dust mites and animal dander worsen asthma Fungi, mold, mildew, airborne bacteria cause allergies, asthma, other respiratory ailments, and diseases Building-related illness = a sickness produced by indoor pollution Sick building syndrome = a sickness produced by indoor pollution with general and nonspecific symptoms -Reduced by using low-toxicity building materials and good ventilation

72. © 2011 Pearson Education, Inc. 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)

73. © 2011 Pearson Education, Inc. Conclusion Indoor air pollution is a potentially serious health threat -We can significantly minimize risks Outdoor air pollution has been addressed by government legislation and regulation in developed countries Reduction in outdoor air pollution represents some of the greatest strides in environmental protection -There is still room for improvement, especially in developing countries

74. © 2011 Pearson Education, Inc. QUESTION: Review The Clean Air Act does all of the following, EXCEPT: a)Forbid emissions trading b)Provide funds for pollution-control research c)Allow citizens to sue violators d)Set standards for air quality

75. © 2011 Pearson Education, Inc. QUESTION: Review Which criteria pollutant is highly reactive, foul smelling, and has a reddish brown color? a)Sulfur dioxide b)Nitrogen dioxide c)Tropospheric ozone d)Carbon monoxide

76. © 2011 Pearson Education, Inc. QUESTION: Review Why is the Montreal Protocol considered our greatest environmental success story? a)It has stopped global warming. b)It decreased criteria pollutants. c)It successfully stopped ozone depletion. d)It slowed acid deposition.

77. © 2011 Pearson Education, Inc. QUESTION: Viewpoints Think of a major city near you. Do you think drivers should have to pay to drive downtown? a)Yes, if mass transit is available. b)Yes, but only charge people who do not live in the downtown area. c)No, it’s my right to drive wherever I want to. d)I don’t care, because I don’t own a car.

78. © 2011 Pearson Education, Inc. QUESTION: Viewpoints Should the government be able to force industries to put pollution-control devices on their factories? a)Yes, I don’t want to be exposed to pollution. b)Yes, only if the people in the area agree. c)No, let the factory owner decide. d)No, in these tough economic times, we need to leave businesses alone.

79. © 2011 Pearson Education, Inc. QUESTION: Interpreting Graphs and Data Which conclusion can you draw from this graph? a)Even though population and consumption increased, emissions have decreased. b)Emissions have decreased but population has increased. c)People have increased emissions, but only slightly. d)The United States no longer needs the Clean Air Act.