Transcript Slide 1
AIR CHEMISTRY “This most excellent canopy, the air” – fades with height while the airless Moon has an abrupt edge. Without the proper mix of gases in air, we would perish in seconds. http://spaceflight.nasa.gov/gallery/images/station/crew-2/hires/iss002e9767.jpg Gases in the Dry Atmosphere (molecules of gas per million air molecules) Gas Formula Volume (ppm) Nitrogen N2 780,840 Oxygen O2 209,460 Argon Ar 9,340 Carbon Dioxide CO2 Neon Ne 18.18 Helium He 5.24 383 Methane CH4 1.745 Krypton Kr 1.14 Hydrogen H2 0.55 Nitrous Oxide N 2O Xenon Xe 0.09 Ozone O3 0.04 Nitrogen Dioxide NO2 0.02 Iodine I 0.01 0.3 The dull data gives nary a hint of the wondrous content of our lifebreathing (O2 rich) atmosphere. Most gases in the air have fixed proportions – air is well mixed. The atmosphere’s two most variable gases are water vapor (H2O) and ozone (O3). Ozone concentrations are very tiny but have a major impact on life. O3 is 10 more abundant in the stratosphere than in the troposphere (the layer from the ground to about 7-17 km). Water vapor concentrations run from almost 0 in frigid air near the poles and at high altitudes to about 3% in hot air near sea level in summer and the tropics. Other Atmospheres: The Sun Venus Transits across the Sun 08 June 2004 Gas Hydrogen Helium Oxygen Carbon Iron Sulfur Neon Nitrogen Silicon Magnesium Formula ppm H 734600 He 248500 O 7700 C 2900 Fe 1600 S 1200 Ne 1200 Ne 900 Si 700 Mg 500 The chemical composition of the Sun is like that rest of the Universe. Earth and its atmosphere are very different. We will see how truly unique and extraordinary Earth’s Atmosphere is! Atmospheric Composition and Escape Velocity When rockets move fast enough they can escape to space. Escape velocity from sea level on Earth is about 11.1 km/s or 25,000 mph. Molecules or atoms can escape from the atmosphere when they have this velocity. Molecule Velocity (m/s) Most molecules move much slower than this, but the speed of molecules increases with T. Also, at any T, light molecules move faster than more massive molecules. UV radiation speeds the molecules and atoms in the upper atmosphere (the thermosphere), raising T above 1000C. This enables the fastest hydrogen atoms and molecules and helium atoms to exceed escape velocity. Over the long course of Earth History, almost all the Hydrogen and Helium Earth 18000 has collect has escaped to space, v(H) Above this Temperature 16000 v(H2) and this also happened on the Hydrogen atoms will escape v(He) 14000 other planets near the Sun v(O) (Mercury, Venus, and Mars). But 12000 v(O2) Escape Velocity V esc the giant outer planets (Jupiter, 10000 Saturn, Uranus, and Neptune) are 8000 so far from the Sun and hence so 6000 cold that even Hydrogen and 4000 Helium move too slowly to escape, 2000 so these planets were able to 0 collect and hold their vast 100 300 500 700 900 1100 1300 reservoirs of H and He. 2 Temperature (Kelvin) Atmospheres of Selected Planets and Moons Gas Pressure Temperature Formula Vol (ppm) Venus Mars Jupiter *Earth* rel to E 1 92.4 0.0075 >>> C 289 462 -50 Nitrogen Oxygen Argon Carbon Dioxide Neon Helium Methane Krypton Hydrogen Nitrous Oxide Xenon Ozone Nitrogen Dioxide Sulfur Dioxide W ater Vapor Carbon Monoxide Formaldehyde N2 O2 Ar CO2 Ne He CH4 Kr H2 N2O Xe O3 NO2 SO2 CO 780,840 35000 27000 101 209,460 1300 773 9,340 70 16000 3.5 383 965,000 953200 327 18.18 7 2.5 111 5.24 12 88725 1.745 0.013 1.14 0.3 0.00015 0.55 910000 0.3 0.09 0.08 0.0002 0.04 0.03 0.02 150 20 300 17 700 0.13 Titan 1.46 -179 984000 16000 Jupiter’s swirling atmosphere No O2 Earth vs. Titan No O2 Venus: A Cowering Inferno No O2 Mars Where even ice is dry No O2 Mars A thin, dusty atmosphere with an ice cap of water ice and dry ice (CO2). Sometimes the surface of Mars is obscured by dust storms. But once upon a time… Mars had Rivers! CO2 : H2O : O2 : Photosynthesis : Life The miracle, life-breathing gas O2 is a highly corrosive molecule that readily combines with other molecules or atoms (i.e. oxidizes them). This means that O2 will quickly disappear from the atmosphere and oceans unless some active process keeps producing it. On Earth that process is PHOTOSYNTHESIS, which uses the energy of sunshine to convert water and carbon dioxide to glucose and oxygen following the reaction, Sunlight 6CO2 12 H 2O green plants C6 H12O6 6O2 6 H 2O This reaction needs 2,816,000 Joules of sunshine to make 180 g (1 mole) of glucose or 3397 kilocalories per pound. Average solar irradiance on Earth = 342 W m-2 = 29,550,000 Joules m-2/day. But photosynthesis is very inefficient. Its maximum theoretical efficiency is 25% but since it cannot use all waves of sunlight, such as Infrared (IR), and does not work at high or low T, its efficiency is only 3%. Even then, only a small fraction (< 10%) of each plant is edible such as the seeds of wheat, and some seeds must be saved to plant for the next year. This means that plants convert less than 105 J m-2/day to food. So, since we need to eat about 2000 Kcal or almost 107 J per day, we need an absolute minimum of 100 m2 and probably more like 1000 to 3000 m2 of ground to grow the food each person needs. This sets a limit on world population because Earth only has an area of 1.451014 m2 of which perhaps half is fit for agriculture. Using 2000 m2 as the minimum area per person yields an absolute maximum sustainable world population of 40 billion! The Corrupted Air: Atmospheric Pollution Air pollution occurs when abnormally large concentrations of aerosol particles or reactive gases accumulate in the atmosphere. By this definition, air pollution is as old as the atmosphere. Long before we appeared on the scene to take dominion over the Earth, nature provided enough pollution to turn the skies over the Blue Ridge Mountains hazy and sometimes redden or blacken the skies with more dramatic events such as huge volcanic eruptions or forest fires. We now add greatly to atmospheric pollution. The main sources are burning of wood and fossil fuels (coal, oil, and gas), agriculture, and construction. The chemical industry contributes a range of toxic substances to the atmosphere including lead and the freons that have reduced ozone worldwide and are responsible for the Ozone Hole over Antarctica. Recognition of the damage we have caused our atmosphere (and ourselves) has led to several effective regulations and restrictions on national and even global scales. The severity of automotive smog and acid rain (due mainly to sulfur released when burning coal) has been considerably reduced over the past 30 years in the United States and the banning of freons has begun to replenish the lost Ozone. But the growth of the world’s population and economy mean that places like China, India, and Africa are becoming major polluters and that on the global scale, pollution is still increasing. James Abbott London McNeill and Whistler. London Bridge. 1885. Air Pollution Freer Gallery of Art, DC,increasingly USA. As the forests around London were cutWashington, down, coal was used as a dirty substitute fuel. As early as 1285, laws that restricted the burning of coal in London were passed and ignored. By 1661, London’s air pollution problem had become so serious that John Evelyn was commissioned to conduct a study. His report sounds surprisingly modern. “While these smokestacks are belching smoke from their sooty jaws London resembles rather the face of Mt. Etna or the suburbs of hell than an assembly of rational creatures.This acrimonious soot carries away multitudes by languishing and deep consumptions, as the bills of mortality do weekly inform us.” The pollution problem really worsened as the Industrial Revolution got underway. Painters in the 19th century responded to the almost continuous pall over London, showing what Londoners had to live and breathe in. Sometimes, weather combined with the normal output of pollutants to produce disastrous pollution outbreaks. The worst occurred from December 5-9, 1952, when high pressure combined with clear skies and calm conditions. After the air cooled at night, soot-laden fog (smog) formed. This reflected sunlight, cooling the air, which trapped the pollution and making people burn even more coal in their stoves. Visibility fell to a few meters. People inhaled the sulfuric acid laden soot making it painful for everyone to breathe. All in all, some 4000 people died. Olga’s Gallery: http://www.abcgallery.com/W/whistler/whistler72.html This led to England’s Clean Air Act of 1956 that restricted coal burning. ph of Acid Rain - Some Regulations Actually Work The Corrupted Air: Aerosols Aerosols are solid or liquid particles suspended in the atmosphere. Many are natural but humans add many particles. In some regions and weather conditions, concentrations rise to intolerably high levels and produce thick haze or smog. Particle concentrations vary from 100 per cubic cm (cm-3) over calm oceans far from land to more than 10000 cm-3 over polluted cities. In this photo taken over the Ganges River Valley of Bangladesh, India, and Nepal, thick, light gray haze (from human and natural activity) blurs the land below. The snowcovered Himalaya Mountains protrude above the haze, prevent its dispersal aloft, and block its northward progress. Impacts of Air Pollutants Condensation Nuclei Salt particles that derive mainly from bursting bubbles in the ocean (like fizz in a glass of soda) serve as nuclei that facilitate the formation and growth of cloud droplets and raindrops. Freezing Nuclei Clay particles that derive from windblown soils (and farming) serve as nuclei that facilitate the formation and growth of ice crystals and snowflakes. Climate All aerosols absorb and scatter light. In general, aerosols act to cool the climate because they reflect sunlight. An example can be seen in the previous slide. Health Acid rain burns the eyes and scars the lungs while many aerosols may cause or aggravate emphysema and asthma. Also, many viruses and bacteria are airborne. Architecture Many structures (particularly limestone) are harmed by acid rain. Much of the degredation of the Science Building at CCNY is due to disintegration of the cement due to acid rain. Visibility and Atmospheric Optics Pollutants generally reduce visibility and make the sky brighter and less blue particularly near the sun. They can make sunrises and sunsets brighter. Ozone and Ultraviolet Radiation Freons decrease Stratospheric O3 and increase UV reaching the ground while nitrogen oxide exhausts increase O3 near the ground. A sandstorm approaching Al Asad, Iraq, just before nightfall on April 27 2005. Mineral Dust Dust (mineral aerosols) diameter size: 2-300 µm main material: sand, silt, clay includes essential trace metals such as Fe Size Range of the Various Types of Aerosols 1 103 Diameter (micrometers top, nanometers bottom) 109 nm = 106 mm = 103 mm = 102 cm = 1 m 106 Aerosol Modes Aitken mode – 0.01-0.1 mm Accumulation mode – 0.1-1 mm Coarse mode - >1 mm nucleation mode <0.01 um 103 mm = 106 nm http://www.esf.edu/chemistry/dibble/pre sentations/IX_Aerosol.ppt#260,7,The Aerosol Modes http://www.esf.edu/chemistry/dibble/fch 511.htm The general increase is due to Human activity. The wiggles show the annual cycle, which is due to growth and decay of the North Hemisphere forests. When forests grow in spring and summer, they inhale CO2, removing it from the atmosphere. When the leaves decay in fall and winter, the forests essentially exhale CO2, restoring it to the atmosphere Increased burning of fossil fuels and forests have loaded the atmosphere with CO2. And the rate of increase of CO2 itself continues to increase as global economies expand. Fires from BP Oil Rig Catastrophe and Subsequent Oil Spill April, 2010 This major catastrophe is only a drop in the bucket of the carbon humans burn each year And, if you still have any doubt about the human impact on CO2 and other greenhouse gases, look at the next slide. Max CO2 content over past Million years Reservoir Storage (1012 kg) Where Carbon is and Where it Goes: Atmosphere 800 The Carbon Cycle Plants 700 The carbon cycle is illustrated by the figure in the next slide Soil 2000 Oceans 40000 Limestone 60,000,000 Organic Sediments 10,000,000 Coal, Oil, Gas Methane Hydrates Reservoirs AtmosphereBiosphere Biosphere+Fuel↔Atmosphere Atmosphere↔Ocean 5000 10000 The natural part of the Carbon Cycle is much larger than the human part. That has allowed doubters to deny our impact on increasing CO2. But we have caused an imbalance and it is always the extra 5 or 10% of calories that fattens us. Flux (1012 kg yr-1) 113.5 10 5 Burial to Sediments 0.28 Weathering 0.13 Volcanism 0.22 1 Trillion = 1012 = 1,000,000,000,000 Earth’s Ozone (O3) Shield Ozone is another minor gas of the atmosphere that has a major impact on life and climate, and that humans have had a major impact on. http://www.atm.ch.cam.ac.uk/tour/index.html 1 DOBSON UNIT (DU) = 0.01 mm O3 Production l 0.3 mm and Destruction l 0.2 mm UV of Sunlight Around 50 km above High Frequency UV z = 50 km O drifts down to 25 km. It then l 0.3 mm Enters Atmosphere sea level, Short Wave splits O2 O + O O2 + hn O + O collides with O2 to form O3. O2 + O O3 z = 25 km O3 + hn O2 + O O3 + hn O2 + O Longer UV splits O3 and is extinguished. This saves our DNA. O3, Atmospheric Thickness and Penetration of UV Radiation As the number and size of obstacles and the length of the path of light increase, less light is able to penetrate. Because the atmosphere is so thin, the lower the Sun in the sky the longer the path and the less light penetrates to the ground. E217LAB_22_O3_MODEL_SOL.xls Low Sun •Long Path •Little UV High Sun •Short Path •Much UV 80000 S ituation for Y oung E arth: Situation forthe Earth Now: O xygenRich P oorAtmosphere Atmos phere AnAn Oxygen 70000 At great heights there is so little O2 that little O3 is produced. O3 production and absorption of UV are maximum in the stratosphere, 25 km above the surface. Little UV penetrates to the ground, safeguarding life. But early in Earth History, there was less O2, hence less O3, so lethal UV reached the ground. Height (m) 60000 50000 40000 P enetrating S olar UV 30000 O z one P roduc tion R ate 20000 10000 0 0 0.2 0.4 0.6 Relative Rates 0.8 1 UV ENERGY Destructive Efficiency Now, humans are changing the Air. For years we released bad freons (CCl2F2) into the atmosphere. They disintegrate and free chlorine (Cl) when they reach the stratosphere. The Cl destroys life-saving O3 creating the Ozone Hole every September over Antarctica. Because we finally stopped releasing the bad freons, the Ozone Hole will slowly fill over the next 50 years and finally disappear. The Ozone Hole showing O3 vs Height over the South Pole The blue area shows the O3 concentration vs height over the South Pole. Once the Sun rises in September almost all O3 between 15 and 20 km is destroyed because clouds form in the stratosphere where T < -80C. As T warms later in October the clouds evaporate and the Ozone hole gradually refills. Click to see O3 disappear. If we tamper too much with the atmosphere, it may be good-night for us all! http://antwrp.gsfc.nasa.gov/apod/archivepix.html