Gases and the atmosphere

Transcription

1 Gases and the atmosphere Chemical reactions in the atmosphere Ozone cycle Ozone depletion Greenhouse effect Global Warming Pollution in the troposphere 1

2 Gases and the atmosphere Review Composition of the atmosphere Use mole fraction and ppm Pressure profile of atmosphere Low pressure means few molecular collisions Chemical reactions occur less frequently Temperature profile of atmosphere High temperature means avg. KE of molecules is high! 2

3 Composition of the atmosphere mole fraction: x i = moles of i total moles N 2 and O 2 represent > 99% of atm 3

4 Composition of the atmosphere For other components use parts per million (ppm). ppm = x i 10 6 Eg. Neon x Ne = Ne concentration = ppm If you know the barometric pressure, you can determine the partial pressure. P BAR = atm= P TOT P Ne = x Ne P TOT = ( atm) = atm 4

5 Pressure decreases exponentially with altitude. WHY? Pressure at any altitude depends on weight of gas above it. What is pressure? 5

6 Pressure and altitude Altitude P collision frequency (km) (atm) (sec -1 ) Low pressure means fewer collisions When collision frequency is low, chemical reactions occur less frequently 6

7 Regions of Atmosphere Based on Temperature Profile Thermosphere: High energy radiation is absorbed Ions formed. Mesosphere: Density of gases is small. Stratosphere: Warming caused by ozone cycle Ultraviolet light absorbed Troposphere: Life: where we live! weather planes Present life forms could not survive (above ground) if all solar radiation reaches earth. 7

8 The plot below shows the distribution of solar radiation, i.e., the intensity and wavelength of radiation outside the atmosphere and at sea level. Much of the solar radiation is absorbed by the atmosphere. HOW???? (How does light interact with matter???) 8

9 Penetration of Solar Radiation 9

10 An introduction to reactions in the atmosphere Photochemical Reactions: reactions that involve the absorption or emission of a photon. Electronic excitation of molecules NO 2 (g) + hν NO 2* (g) Photodissociation bond broken by absorption of a photon (light) O 2 + hν O + O E = hν Photon needs sufficient energy to break bond (Bond energy for O 2 = 495 kj/mol) λν = c λ < 242 nm Photoionization removal of an outer (valence electron) from a molecule by absorption of a photon N 2 + hν N e 10

11 Thermosphere (Ionosphere) Photoionization occurs in the thermosphere. 11

12 Absorption of uv light occurs in the stratosphere. This is responsible for the warming of the stratosphere. Ozone concentration peaks in stratosphere ~25km, concentration 10ppm Presence of ozone causes temperature increase in stratosphere. 12

13 Ozone O 3 O O O O O O Structure 2 resonance forms: bent, bond angle = 117 bond length = 1.28Å (O 2 is 1.21Å) (in agreement with VSEPR predictions) Properties: Light blue gas Pungent odor (smell near electrical discharges) ΔH f = 142kJ/mol (reactive less stable than O 2 ) 13

14 formation of O 3 Why is ozone so important? Ozone cycle O 2 + hν 2O λ < 242nm O + O 2 O 3 uv blocking by O 3 O 3 + hν O 2 + O λ < 320nm Solar radiation (ultraviolet) is converted to heat The (small) amount of O 3 in stratosphere reflects the delicate balance between creation and destruction. 14

15 Problem: Ozone depletion What causes it? Chlorofluorocarbons (CFCs) Examples: CFCl 3 CF 2 Cl 2 freon-11 freon-12 Properties relatively inert easily liquefied Non-combustible volatile evaporate and diffuse upward easily Uses aerosol propellants coolants for refrig. solvents for cleaning polymer manufacturing 15

16 CFC destruction by ozone CF 2 Cl 2 + hν CF 2 Cl + Cl (λ<240nm) Cl + O 3 ClO + O 2 ClO + O Cl + O 2 NET: O 3 + O 2O 2 Cl atom from CFC catalyzes O 3 destruction (speeds up reaction, but is not used up). 1 Cl atom destroys ~ 2 million O 3 molecules Global concentration of O 3 has been on the decline since

17 Rest of the radiation from the sun (mostly uv and visible) reaches the earth s surface: causes warming. Energy comes to earth from the sun. Some is reflected by atmosphere Some is absorbed Earth radiates heat back out. mainly infrared energy 17

18 The earth s energy balance Distribution of Radiation Solar radiation warms the earth Earth radiates infrared light back 18

19 Greenhouse effect is a good thing! H 2 O and CO 2 thermally regulate the atmosphere. Absorption of infrared radiation: Activates stretching and bending modes keeps heat from being released back into space Without the greenhouse effect, Earth would be ~33 colder (water ice) 19

20 What is Global Warming? Over the past century atmospheric CO 2 has increased ~25% Mean global temperature has increased ~1 Increased quantities of CO 2 may be causing global warming. 20

21 How serious is the problem? Need to know about Carbon cycle: interconversion of C in the environment 21

22 The Carbon Cycle Sources of atmospheric CO 2 Natural volcanic eruption sources decaying organic materials respiration Man-made fuel combustion sources deforestation cement manufacturing Sinks for CO 2 Formation of rock Dissolves in oceans Photosynthesis 22

23 Summary ~200 billion metric tons (bmt) of C in the form of CO 2 is removed each year from the atmosphere. ~110 bmt removed by photosynthesis ~ 90 bmt removed by absorption in ocean (seashells, coral, limestone ) ~203 bmt of C are released into the atmosphere respiration decomposition deforestation ~3.5% of CO 2 released fossil fuel use 23

24 Summary Projections CO 2 levels are expected to double in 25 years change in global T (?) 1-5m rise in ocean levels (but then more CO 2 dissolves!) shift in climate and crop production See National Geographic s Sept Edition 24