Topic 28

1. Topic 28

2. Table of Contents Topic 28 Topic 28: Environmental Chemistry Basic Concepts Additional Concepts

3. Environmental Chemistry: Basic Concepts Topic 28 Earth’s Atmosphere • To the best of our knowledge, Earth is the only planet capable of supporting life as we know it. • One glance at the photo of Earth helps explain why. See those wispy clouds?

4. Environmental Chemistry: Basic Concepts • They are part of a protective envelope, the atmosphere, that blankets Earth and plays a key role in maintaining life. Topic 28 Earth’s Atmosphere

5. Environmental Chemistry: Basic Concepts Topic 28 A Balanced Atmosphere • The atmosphere extends from Earth’s surface to hundreds of kilometers into space. • A largely gaseous zone, the atmosphere contains the air we breathe, the clouds overhead, and the all-important substances that protect Earth and its inhabitants from the Sun’s most powerful radiation.

6. Environmental Chemistry: Basic Concepts Topic 28 A Balanced Atmosphere • Chemical reactions that occur in the atmosphere help maintain a balance among the different atmospheric gases, but human activities, such as burning fossil fuels, can change this balance.

7. Environmental Chemistry: Basic Concepts Topic 28 A Balanced Atmosphere • Earth’s atmosphere has five layers that vary in composition, temperature, altitude, and pressure.

8. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • Earth’s atmosphere is divided into five layers based on altitude and temperature variation. • The lowest layer—the troposphere— extends from Earth’s surface to a height of approximately 15 km.

9. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • Temperatures in the troposphere generally decrease with increasing altitude, reaching a minimum of –58°C at 12 km.

10. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • Rain, snow, wind, and other weather phenomena occur in this layer. • We live our entire lives within the troposphere. Only astronauts in spacecraft go beyond its reach.

11. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • Above the troposphere, temperatures increase with altitude, reaching a maximum of nearly 2°C at about 50 km. • This region of the atmosphere is called the stratosphere.

12. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • The stratosphere contains a layer of ozone, a gas that helps shield Earth’s surface from the Sun’s harmful ultraviolet radiation. • Ozone protects Earth by absorbing solar radiation, which raises the temperature of the stratosphere in the process.

13. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • Beyond the stratosphere lie the mesosphere and the thermosphere. • Temperatures in the mesospheredecrease with altitude because there is little ozone in the air to absorb solar radiation.

14. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • The thermosphere is a region of rapidly increasing temperatures. • This is because the relatively few gas molecules in this region have extremely high kinetic energies. • At an altitude of about 200 km, temperatures can reach 1000°C.

15. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • The outermost layer of the atmosphere is the exosphere. • Extending from about 500 km outward, the exosphere marks the transition from Earth’s atmosphere to outer space.

16. Environmental Chemistry: Basic Concepts Topic 28 Structure of Earth’s Atmosphere • There is no clear boundary between the two, however. There are simply fewer and fewer molecules of gas at increasingly higher altitudes. • Eventually, there are so few molecules that, for all practical purposes, Earth’s atmosphere has ended.

17. Environmental Chemistry: Basic Concepts Topic 28 Composition of Earth’s Atmosphere • Just as the temperature of the atmosphere varies by altitude, so does its composition. • Roughly 75% of the mass of all atmospheric gases is found in the troposphere. • Nitrogen and oxygen make up the vast majority of these gases. However, there are a number of minor components.

18. Environmental Chemistry: Basic Concepts Topic 28 Composition of Earth’s Atmosphere • In addition to gases, the troposphere contains solids in the form of dust, salts, and ice. • Dust—tiny particles from Earth’s surface, ash, soot, and plant pollen—enters the atmosphere when it is lifted from Earth’s surface and carried by wind.

19. Environmental Chemistry: Basic Concepts Topic 28 Composition of Earth’s Atmosphere • Salts are picked up from ocean spray. • Ice is present in the form of snowflakes and hailstones.

20. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Outer Atmosphere • Earth is constantly being bombarded with radiation and high-energy particles from outer space.

21. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Outer Atmosphere • The short-wavelength, high-energy ultraviolet (UV) radiation is the most damaging to living things.

22. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Outer Atmosphere • Because this radiation is capable of breaking the bonds in DNA molecules, it can cause cancer and genetic mutations.

23. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Outer Atmosphere • Life as we know it is possible primarily because two processes, which occur in the thermosphere and the exosphere, shield us from most of this radiation.

24. Environmental Chemistry: Basic Concepts Topic 28 Photodissociation • Photodissociation is a process in which high-energy ultraviolet solar radiation is absorbed by molecules, causing their chemical bonds to break.

25. Environmental Chemistry: Basic Concepts Topic 28 Photodissociation • In the upper atmosphere, the photodissociation of oxygen absorbs much of the high-energy UV radiation and produces atomic oxygen. • The amount of atomic oxygen in the atmosphere increases with increasing altitude.

26. Environmental Chemistry: Basic Concepts Topic 28 Photoionization • The second process that absorbs high-energy solar radiation is photoionization, which occurs when a molecule or atom absorbs sufficient energy to remove an electron. • Molecular nitrogen and oxygen, as well as atomic oxygen, undergo photoionization in the upper atmosphere.

27. Environmental Chemistry: Basic Concepts Topic 28 Photoionization • Note that a positively charged particle is produced for every negatively charged electron in the atmosphere, so neutrality of charge is maintained.

28. Environmental Chemistry: Basic Concepts Topic 28 Photoionization • Ultraviolet radiation with the very highest energy is absorbed during photo-dissociation and photoionization in the upper atmosphere. • Because most of this harmful radiation does not reach Earth’s surface, life can exist.

29. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Stratosphere • In addition to light gases, the upper atmosphere—more specifically, the stratosphere—contains a substance called ozone.

30. Environmental Chemistry: Basic Concepts Topic 28 Formation of ozone • Although the UV radiation with the very highest energy has been absorbed by photoionization reactions in the outer atmosphere, much of the UV radiation that has sufficient energy to cause photodissociation still reaches the stratosphere.

31. Environmental Chemistry: Basic Concepts Topic 28 Formation of ozone • In the stratosphere, these ultraviolet waves are absorbed by O2 molecules, which are more plentiful here than in the upper atmosphere.

32. Environmental Chemistry: Basic Concepts Topic 28 Formation of ozone • The O2 molecules are split into two atoms of oxygen. • These highly reactive atoms immediately collide with other O2 molecules, forming ozone (O3). • The O3 molecule that forms is highly unstable because its bonds contain excess energy that was gained from the UV radiation.

33. Environmental Chemistry: Basic Concepts Topic 28 Formation of ozone • To achieve stability, the energized O3 molecule must lose this excess energy by colliding with another atom or molecule, as molecule X, and transferring energy to it. • Usually, N2 or O2 molecules are most abundant and serve as energy-absorbing molecules for the reaction.

34. Environmental Chemistry: Basic Concepts Topic 28 Formation of ozone • Ozone molecules are formed in the stratosphere. • An oxygen molecule forms two oxygen atoms by photodissociation. • An oxygen atom combines with an oxygen molecule to form an energized ozone molecule (O3*).

35. Environmental Chemistry: Basic Concepts Topic 28 Formation of ozone • The energized ozone molecule collides with molecule X. Excess energy is transferred to X, producing ozone and an energized X molecule (X*). • The oxygen molecule that forms when ozone photodissociates is available to start the ozone cycle anew.

36. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Troposphere—Photochemical smog • In large cities such as Los Angeles, Denver, and Mexico City, a hazy, brown blanket of smog is created when sunlight reacts with pollutants in the air. • Because the smog forms with the aid of light, it is called photochemical smog. • The smog-producing pollutants enter the troposphere when fossil fuels such as coal, natural gas, and gasoline are burned.

37. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Troposphere—Photochemical smog • The burning of fossil fuels in internal combustion engines causes nitrogen and oxygen to react, forming nitrogen oxides such as NO and NO2.

38. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Troposphere—Photochemical smog • The NO2, in turn, photodissociates in the presence of high energy UV that penetrates through the upper atmosphere to form atomic oxygen, which combines with O2 to form ozone.

39. Environmental Chemistry: Basic Concepts Topic 28 Chemistry in the Troposphere—Photochemical smog • You have learned in that the ozone in the stratosphere protects you from ultraviolet radiation. • In the troposphere, ozone can irritate your eyes and lungs and increase your susceptibility to asthma and pneumonia.

40. Environmental Chemistry: Basic Concepts Topic 28 Acid rain • Sulfur-containing compounds are normally present in small quantities in the troposphere. • However, human activities have greatly increased the concentration of these compounds in the air. • Sulfur dioxide (SO2) is the most harmful of the sulfur-containing compounds.

41. Environmental Chemistry: Basic Concepts Topic 28 Acid rain • Most of the sulfur dioxide in the troposphere is produced when coal and oil that contain high concentrations of sulfur are burned in power plants. • The sulfur dioxide that forms is oxidized to sulfur trioxide (SO3) when it combines with either O2 or O3 in the atmosphere.

42. Environmental Chemistry: Basic Concepts Topic 28 Acid rain • When SO3 reacts with moisture in the air, sulfuric acid is formed. • Acidic air pollution is created also when nitrogen oxides from car exhausts combine with atmospheric moisture to form nitric acid. • In either case, when this acidic moisture falls to Earth as rain or snow, it is known as acid rain.

43. Environmental Chemistry: Basic Concepts Topic 28 Earth’s Water—The Hydrosphere • Water is the most abundant substance in the human body and the most common substance on Earth, covering approximately 72% of the surface of this planet. • All the water found in and on Earth’s surface and in the atmosphere is collectively referred to as the hydrosphere.

44. Environmental Chemistry: Basic Concepts Topic 28 Earth’s Water—The Hydrosphere • More than 97% of this surface water is located in the oceans. • Another 2.1% is frozen in glaciers and polar ice caps. • That leaves a meager 0.6% available as liquid freshwater.

45. Environmental Chemistry: Basic Concepts Topic 28 The Water Cycle • Both seawater and freshwater move through Earth’s atmosphere, its surface, and below its surface in a process known as the water cycle. You may also see the water cycle referred to as the hydrologic cycle. • In this cycle, water continually moves through the environment by the processes of evaporation, condensation, and precipitation. • The Sun provides the energy for these processes.

46. Environmental Chemistry: Basic Concepts Topic 28 The Water Cycle • Solar radiation causes liquid water to evaporate into a gaseous state. • The resulting water vapor rises in the atmosphere and cools. • As it cools, the water vapor again becomes a liquid when it condenses on dust particles in the air, forming clouds.

47. Environmental Chemistry: Basic Concepts Topic 28 The Water Cycle • Clouds are made of millions of tiny water droplets that collide with each other to form larger drops.

48. Environmental Chemistry: Basic Concepts Topic 28 The Water Cycle • When the drops grow so large that they can no longer stay suspended in the clouds, they fall to Earth in the form of precipitation—rain, snow, sleet, or hail.

49. Environmental Chemistry: Basic Concepts Topic 28 The Water Cycle • Most of the falling precipitation soaks into the ground and becomes part of groundwater, the underground water that collects in small spaces between soil and rock particles.

50. Environmental Chemistry: Basic Concepts Topic 28 The Water Cycle • If the soil becomes saturated with water, the excess water flows along Earth’s surface and into lakes and streams. This is called runoff.