Chemistry in the Troposphere

Sections 18.4

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.

Key Terms

• Acid rain• Hemoglobin• Photochemical smog

TroposphereAlthough 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.

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

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)

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

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.

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

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.

Carbon Monoxide

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

Carbon Monoxide

• Products that can produce carbon monoxide must contain warning labels.

• Carbon monoxide is colorless and odorless, so detectors are a good idea.

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.

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!)

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)

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

Photochemical Smog

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

Water Vapor and Carbon Dioxide• Gases in the atmosphere form an insulating

blanket that causes the Earth’s thermal consistency (temperature)

• Two of the most important such gases are carbon dioxide and water vapor.

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)

• Sun’s surface temp: 6000 K; From outer space, Earth is 254 K…why do we appear so much colder than our surface actually is?

• Answer: H2O(g) and CO2 in troposphere absorb outgoing infrared radiation from Earth’s surface, hold it in, and we feel the heat

• This blanketing effect is the “greenhouse effect.”

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

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.

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

Homework

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

The World Ocean and Fresh Water

Section 18.5-18.6

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 world’s 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

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

Water

• Most common liquid on Earth– 72% of Earth’s 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

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)

Salinity

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

by mass)

Ocean

• The vast ocean contains many important compounds and minerals.

• Table 18.6: 11 most common constituents of seawater

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 (4C)

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

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

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)

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

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

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

Biodegradable Material

• Also known as oxygen-demanding wastes• Sewage, industrial wastes, liquid wastes

(meatpacking plants)• Excessive amounts will deplete the oxygen in

the water

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

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:

1. Coarse filtration2. Sedimentation3. Sand filtration4. Aeration5. Sterilization

Water Purification

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

Homework

• 18.32-34• 18.37, 38, 39, 43, 44