Chemistry in the Troposphere

1
Chemistry in the Troposphere

• Sections 18.4

2
Objectives

• Examine the constituents of the troposphere that
  affect air quality and the acidity of rainwater.
• Analyze the effects of carbon dioxide on warming
  of the atmosphere and climatic changes.

3
Key Terms

• Acid rain
• Hemoglobin
• Photochemical smog

4
Troposphere

• Although the troposphere is made up almost
  entirely of nitrogen and oxygen, other gases
  present in relatively small amounts still have a
  profound effect on the troposphere.

5
Sulfur and Acid Rain

• Sulfur compounds exist in natural, unpolluted air
  (volcanic gases, bacterial decay)
• Sulfur release 23 natural 77 human activity
• SO2 is one of most unpleasant/harmful pollutants
• 0.08 ppm or higher in typical urban areas (Table
  18.4)
• Most serious health hazard among pollutants

6
Sulfur and Acid Rain (cont.)

• Sulfur dioxide is a by-product of the burning of
  coal or oil (source of 80 of total SO2 in US)
• Coal and oil vary in their sulfur concentration
• SO2 oxidizes to SO3 which reacts with moisture in
  the air to form sulfuric acid
• Results in acid rain (pH of 4)
• Uncontaminated rain is pH of 5.6 naturally acid
  due to CO2 (forms H2CO3 in water)

7
pH Values from Freshwater in US

• pH of natural waters is 6.5-8.5
• At pH below 4, all vertebrates, most
  invertebrates, and many microorganisms are
  destroyed

8
Other Sulfur Damage

• Acid reacts with metal carbonates
• High acidity in rainfall causes corrosion in
  building materials.
• Marble and limestone (calcium carbonate) react
  with the acid structures made from them erode.

9
Reducing Sulfur

• To reduce sulfur quantity remove it from
  coal/oil before burning
• SO2 can be removed by injecting powdered
  limestone (CaCO3) which is converted to CaO
• The CaO reacts with SO2 to form a precipitate of
  calcium sulfite.
• EXPENSIVE!

CaCO3 ? CaO CO2 CaO SO2 ? CaSO3
10
Carbon Monoxide

• CO is formed by incomplete combustion of
  carbon-containing materials (fossil fuels)
• Most abundant of all pollutant gases (0.05 ppm
  66 from automobiles)
• No direct threat to vegetation/materials. BUT it
  affects humans
• Binds to iron in red blood cells (hemoglobin)
  responsible for transporting O2 to blood
• Exposure to significant amount of CO can lower O2
  levels to the point that loss of consciousness
  and death can result.

11
Carbon Monoxide

• A person breathing air that is only 0.1 CO for
  just a few hours can reduce the bloods normal
  oxygen capacity to 60.

12
Carbon Monoxide

• Products that can produce carbon monoxide must
  contain warning labels.
• Carbon monoxide is colorless and odorless, so
  detectors are a good idea.

13
Nitrogen Oxides

• What we recognize as smog, that brownish gas that
  hangs above large cities like Los Angeles, is
  primarily nitrogen dioxide, NO2.
• It forms from the oxidation of nitric oxide, NO,
  a component of car exhaust.

14
Reactions forming Photochemical Smog

• N2 O2 ? 2NO (reaction in engines)
• 2NO O2 ? 2NO2 (in the air)
• NO2 hv ? NO O (photodissociation by sunlight
  of wavelength 393 nm)
• O O2 M ? O3 M (formation of ozone- in the
  troposhpere!)

15
Ozone in Troposphere

• Undesirable pollutant
• Very reactive and toxic
• Two ozone problems in our atmosphere
• 1) Excessive amount in urban environments (where
  it is harmful)
• 2) Depletion in the stratosphere (where it is
  vital)

16
Other Components of Photochemical Smog

• Carbon monoxide and hydrocarbons also contribute
  to air pollution that causes severe respiratory
  problems in many people.
• Hydrocarbons are emitted from gasoline in engines

17
Photochemical Smog

• As a result, government emission standards for
  automobile exhaust have become continually more
  stringent.

18
Water Vapor and Carbon Dioxide

• Gases in the atmosphere form an insulating
  blanket that causes the Earths thermal
  consistency (temperature)
• Two of the most important such gases are carbon
  dioxide and water vapor.

19
Water Vapor and Carbon Dioxide

• Earth radiates energy into space at a rate equal
  to the rate it absorbs energy from the Sun
  (thermal balance)
• Suns surface temp 6000 K From outer space,
  Earth is 254 Kwhy do we appear so much colder
  than our surface actually is?
• Answer H2O(g) and CO2 in troposphere absorb
  outgoing infrared radiation from Earths surface,
  hold it in, and we feel the heat
• This blanketing effect is the greenhouse
  effect.

20
Water Vapor

• Water vapor, high specific heat, is a major
  factor in greenhouse effect.
• Major role in maintain temperature at night when
  Earth does not receive energy from the Sun
• Ex Desert areas (low H2O (g) concentration) have
  very hot days and very cool nights

21
Carbon Dioxide

• CO2 plays a secondary, but important role in
  maintaining surface temperature
• Combustion of fossil fuels (coal and oil) have
  increased CO2 levels (Figure 18.12)
• This may be causing an unnatural increase in
  atmospheric temperatures.

22
Figure 18.12

• Steady increase of CO2 (as high as 375 ppm)
• May be responsible for global air temperature
  increase of 0.3 to 0.6 ?C over the past century
• CO2 level is expected to double from sometime
  between 2050-2100 (increase of 1-3 ?C)
• Could result in major changes to global climate

23
Homework

• 18.23-18.30 on page 798
• 18.25 (a ONLY)
• 18.30 (a ONLY)

24
The World Ocean and Fresh Water

• Section 18.5-18.6

25
Objectives

• Examine the many salts that participate in the
  global cycling of the elements and nutrients.
• Understand and analyze how humans may eventually
  use the worlds oceans for a source of fresh
  water through the process of desalination,
  distillation, and reverse osmosis.
• Examine the treatment of freshwater sources to
  render them usable

26
Key Terms

• Salinity
• Desalination
• Reverse osmosis
• Biodegradable
• Hard water
• Lime-soda process
• Ion exchange

27
Water

• Most common liquid on Earth
• 72 of Earths surface
• Our bodies are 65 water by mass
• Unusually high melting point and boiling point,
  and a high heat capacity
• Ability to dissolve many ionic and polar covalent
  substances

28
Seawater

• Constant composition and connected
• 97.2 of the water on Earth is the world ocean
• 2.1 is in ice caps and glaciers
• 0.6 is fresh water (lakes, rivers, groundwater)
• 0.1 is brackish water (like Great Salt Lake)

29
Salinity

• Salinity mass (g) of dry salt in 1 kg of
  seawater
• World ocean is about 35 (or 3.5 dissolved salts
  by mass)

30
Ocean

• The vast ocean contains many important compounds
  and minerals.
• Table 18.6 11 most common constituents of
  seawater

31
Ocean

• Salinity, density, and temperature vary as a
  function of depth
• Sunlight penetrates only to 200 m
• 200-1000 m? twilight zone
• Below 1000 m ? pitch black and cold (4?C)

32
Ocean

• Rarely used as a source for raw materials
• Cost of extraction is too high
• Only 3 substances are obtained from seawater
  sodium chloride, bromide, and magnesium

33
Carbon Dioxide in the Ocean

• Ocean absorbs CO2? large role in global climate
• CO2 H2O ? H2CO3
• More carbon dioxide, more carbonic acid
• Carbon mostly exists as HCO3- and CO32- in the
  ocean these ions buffer the pH between 8.0-8.3
• Buffer is predicted to decrease as CO2
  concentration increases
• More calcium carbonate precipitate

34
Desalination

• Seawater has too high a concentration of NaCl for
  human consumption.
• Municipal water supplies restricted to no more
  than 500 ppm
• It can be desalinated through reverse osmosis
  (large scale distillation is not efficient)

35
Reverse Osmosis

• Osmosis ?net movement of solvent molecules, but
  not solute molecules, through a semipermeable
  membrane
• Solvent passes from the more dilute solution into
  the more concentrated one
• If high pressure is applied through reverse
  osmosis, the water can be forced in the opposite
  direction.
• Solvent passes from more concentrated into more
  dilute solution

36
Fresh Water

• US fresh water reserve 1.7 x 1015 liters
• Renewed by rainfall
• 9 x 1011 liters of fresh water used daily in US
• Used in agriculture, power, industry, household
  needs, drinking water
• One American uses about 300L a day
• Population and environmental pollutants are
  increasing? more expensive to maintain supply

37
Dissolved Oxygen and Water Quality

• Amount of dissolved oxygen is an important
  indicator of water quality
• Cold water fish require at least 5 ppm
• Aerobic bacteria consume dissolved oxygen to
  oxidize organic (biodegradable) material

38
Biodegradable Material

• Also known as oxygen-demanding wastes
• Sewage, industrial wastes, liquid wastes
  (meatpacking plants)
• Excessive amounts will deplete the oxygen in the
  water

39
Biodegradable Material (cont)

• In oxygen biodegradable material becomes CO2,
  HCO3-, H2O, NO3-, SO42-, phosphates
• Too much of these can reduce amt of dissolved
  oxygen aerobic bacteria die
• Anaerobic bacteria takes over ? forms CH4, NH3,
  H2S, PH3
• Foul odor of polluted waters
• P and N are plant nutrients? lead to excessive
  aquatic plants

40
Treatment of Municipal Water

• The water for daily use comes from fresh water,
  underground sources, or reservoirs.
• Most water in municipal systems is used? been
  through sewage or industrial plants
• Must be treated with 5 steps
• Coarse filtration
• Sedimentation
• Sand filtration
• Aeration
• Sterilization

41
Water Purification

• CaO and Al2(SO4)3 are added to aid in the removal
  of very small particles (sedimentation step)

42
Homework

• 18.32-34
• 18.37, 38, 39, 43, 44