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CHAPTER 8 AIR AND THE ATMOSPHERE From Green Chemistry and the Ten Commandments of Sustainability , Stanley E. Manahan, ChemChar Research, Inc., 2006 [email protected] 8.1. MORE THAN JUST AIR TO BREATHE A Sea of Gas

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From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, ChemChar Research, Inc., 2006

[email protected]

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A Sea of Gas

We live and breathe in the atmosphere, a sea of gas consisting primarily of elemental O2 and N2.

Gas molecules are in constant, rapid motion, which explains

• Pressure • Temperature • Diffusion

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The Gas Laws

Gas laws

•Pressure in atmospheres (atm) • Temperature absolute (˚C + 273)

Avogadro’s law: At constant temperature and pressure the volume of a gas is directly proportional to the number of moles.

Charles’ law: At constant pressure the volume of a fixed number of moles of gas is directly proportional to the absolute temperature.

Boyle’s law: At constant temperature the volume of a fixed number of moles of gas is inversely proportional to the pressure.

General gas law relating volume (V), pressure (P), number of moles (n), absolute temperature (T), a constant (R)

• PV = nRT (8.1.1)

Gas law calculations of volumes based upon

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Gas Law Calculations

Calculate the volume of a fixed number of moles of gas initially occupying 12.0 liters when the temperature is changed from 10˚C to 90˚C at constant pressure.

T1 = 10˚ + 273˚ = 283˚, and T2 = 90˚ + 273˚ = 363˚

P and n are constant and cancel

Calculate V2 at constant T when P on a volume of gas occupying initially 11.4 L is changed from 1.16 atm to 0.858 atm

Remember that an increase in temperature increases the volume and an increase in pressure decreases the volume and vice versa

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The Protective Atmosphere

Keeps Earth’s surface warm by delaying outgoing infrared radiation

Absorbs very short wavelength ultraviolet radiation from the sun

Nature of air

In the troposphere within a few kilometers of Earth’s surface, a mixture of gases of generally uniform composition

• On a dry basis, 78.1% (by volume) nitrogen, 21.0% oxygen, 0.9% argon, and 0.04% carbon dioxide

• Water vapor 1-3% of the atmosphere by volume

• Trace gases below 0.002% including ammonia, carbon monoxide, helium, hydrogen, krypton, methane, neon, nitrogen dioxide, nitrous oxide, ozone, sulfur dioxide, and xenon.

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The Protective Atmosphere (Cont.)

If Earth is represented as a globe, the relative thickness of the atmosphere would be about that of the paint on the globe surface.

The atmosphere is so thin that in an aircraft that suddenly loses pressure cruising at 35,000 feet (about 6.6 miles or 10.7 kilometers), the pilot has only about 15 seconds to grab an oxygen mask before losing consciousness.

• Earth’s diameter is almost 13,000 km.

Aircraft cruise at the upper limit of the troposphere (next slide)

• Average T of about 15˚ C at sea level

• Average T of -56˚ C at 11 km

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The Stratosphere

-2˚ C at 50 km altitude

Virtually no water vapor in the stratosphere

Contains ozone, O3, and O atoms as the result of ultraviolet radiation acting upon stratospheric O2

The ozone in the stratosphere absorbs damaging ultraviolet radiation and is essential for protecting life on Earth

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Atmospheric chemistry refers to chemical processes that occur in the atmosphere.

Atmospheric chemistry occurs in the gas phase where molecules are relatively far apart.

A second major aspect of atmospheric chemistry is the occurrence of photochemical reactions.

• Initiated when a photon of ultraviolet radiation is absorbed by a molecule

• The energy of a photon, E, is given by E = h where h is Planck’s constant and  is the frequency of the radiation.

• Electromagnetic radiation of a sufficiently short wavelength can cause chemical bonds to break in molecules

• This can lead to the formation of reactive species that can participate in reaction sequences called chain reactions.

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Photochemical and Chain Reactions

Example of dichlorodifluoromethane, CCl2F2, which was used in automobile air conditioners

CCl2F2 + hCCl2F2 + Cl(Stratosphere)(8.2.1)

Species with unpaired electrons such as Cl are very reactive and are called free radicals.

Reaction of Clwith stratospheric ozone and the O atoms required for ozone formation

Cl+ O3O2 + ClO (8.2.2)

ClO+ OO2 + Cl (8.2.3)

Net reaction: O3 + OO2 + O2

One Cl atom can bring about the destruction of as many as 10,000 ozone molecules!

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Atmospheric Particles

Very small particles of the size of a micrometer or less called aerosols are important in atmospheric chemical processes.

• Particle surfaces can act to catalyze (bring about) atmospheric chemical reactions.

• Solution chemical reactions can occur inside water droplets.

• Condensation nuclei, such as small particles of NaCl formed from sea spray, act to form water droplets from atmospheric moisture.

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The flux of energy reaching Earth’s atmosphere is 1,340 watts/m2.

This enormous amount of energy is redistributed around Earth’s surface and eventually radiated back out to space as electromagnetic radiation.

Energy received from the sun is distributed away from the Equator largely by convection in moving masses of air.

• Sensible heat from the kinetic energy of rapidly moving air molecules

•Latent heat in the form of water vapor

•Heat of vaporization of water is very large 2,259 joules per gram (J/g)

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The movement of air masses, cloud formation, and precipitation in the atmosphere are covered by the science of meteorology.

Meteorologic phenomena have a strong effect upon atmospheric chemistry by processes such as

• Movement of air pollutants from one place to another

• Conditions under which stagnant pollutant air masses remain in place so that secondary pollutants, such as photochemical smog, can form

• Precipitation, which can carry acidic compounds from the atmosphere to Earth’s surface in the form of acid rain.

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Weather refers to relatively short term variations in the state of the atmosphere as expressed by temperature, cloud cover, precipitation, relative humidity, atmospheric pressure, and wind.

• Weather is driven by redistribution of energy and water vapor around Earth’s surface.

•Clouds consisting of droplets of liquid water

• Wind and air currents may influence air pollution

• Long term trends in weather are expressed by climate.

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Temperature Inversions

Temperature inversion in which warmer air masses overlay cooler ones influence air pollution phenomena.

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N2 and O2 are by far the most abundant gases in the atmosphere.

Crucial importance of the stratospheric layer of ozone, O3

Oxygen reacts with atmospheric chemical species.

• Through action of intermediate species, particularly hydroxyl radical, HO

• SO2 is converted to H2SO4

• CO is converted to CO2

Atmospheric oxygen comes from photosynthesis

CO2 + H2O + h {CH2O} + O2 (8.4.2)

where {CH2O} is a generic formula representing biomass

Nitrogen in the atmosphere

Atmospheric N2 is very unreactive

Most important reaction of N-containing species in the atmosphere

NO2 + h NO + O (8.4.3)

Reactive O atom initiates many tropospheric photochemical reactions

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Green Oxygen and Nitrogen from the Air

Elemental oxygen and nitrogen are obtained by distilling cold liquid air, a process that can also produce noble gas neon, krypton, and xenon, if desired.

Essentially pure oxygen is used in a number of applications, such as for steel making, breathing, and many other applications.

Pure nitrogen provides inert atmospheres free of oxygen and is used as the very cold liquid in cryogenics.

Oxygen and nitrogen can be separated from air at room temperature based upon their different adsorption characteristics on solids or variable permeability through membranes.

• One common such process is called pressure swing adsorption

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Emergency Oxygen

Chlorate candle on aircraft

2NaClO3 2NaCl + 3O2 (8.4.4)

Heat generated by

4Fe + 3O2 2Fe2O3 (8.4.4)

ValuJet crash over the Florida Everglades in 1997 from a fire of tires fed by chlorate candles improperly shipped in the cargo compartment

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Dispersion aerosols formed by grinding solids, dispersing dusts, or atomizing liquids

Condensation aerosols produced when gases or vapors, often formed as the result of atmospheric chemical processes, condense

Mists include raindrops, fog, cloud droplets, and droplets of sulfuric acid produced when atmospheric SO2 is oxidized.

Fly ash is the mineral residue from fuel combustion.

Health effects of atmospheric particles

• Allergen pollen • Acidic particles

• Heavy metals, such as lead, mercury, beryllium

Radioactive radon including 222Rn (half-life 3.8 days) and 220Rn (half-life 54.5 seconds), alpha emitters that decay to radioactive 218Po and 216Po

Particles have both direct effects (reduction of visibility) and indirect effects (reaction sites) in the atmosphere.

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Limiting Emissions of Atmospheric Particles

Devices for limiting particle emissions include

• Sedimentation • Inertial mechanisms • Scrubbers

• Fabric filters in baghouses • Electrostatic precipitators

An electrostatic pre-cipitator (right)

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Carbon Monoxide

Toxic to humans by binding to blood hemoglobin and preventing the hemoglobin from transporting oxygen from the lungs to other tissues.

Catalytic destruction in auto exhausts:

2CO + O2 2CO2 (8.6.1)

Modern automobile engines use computerized control of engine operating parameters along with exhaust catalysts to control carbon monoxide emissions.

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Pollutant Gaseous Oxides (Cont.)

Sulfur Dioxide

From several natural and pollutant sources

Direct effects

• On people with respiratory problems • On plants

Most important indirect effect is atmospheric sulfuric acid formation

2SO2 + O2 + 2H2O  2H2SO4 (8.6.2)

Avoiding sulfur dioxide pollution by not using sulfur-containing fuels (coal)

Fluidized bed combustion in a granular medium of CaO that absorbs SO2

CaO + SO2 CaSO3 (8.6.3)

Scrubbing with substances that absorb sulfur dioxide from stack gas

Ca(OH)2 + SO2CaSO3 + H2O (8.6.4)

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Green Chemistry and Sulfur Dioxide

Sulfur is a valuable raw material required in the manufacture of sulfuric acid, one of the largest volume chemicals made.

Hydrogen sulfide, H2S, can be used to make sulfur dioxide.

In the Kalundborg, Denmark, industrial ecosystem, sulfur dioxide scrubbed from stack gas is oxidized

CaSO3 + 1/2O2 + 2H2O CaSO4.2H2O (8.6.5)

and used to make gypsum for wallboard.

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Nitrogen Oxides in the Atmosphere

Nitrous oxide (N2O), colorless, odorless, nitric oxide (NO), and pungent-smelling, red-brown nitrogen dioxide (NO2) occur in the atmosphere.

Nitrous oxide generated by bacteria

In the stratosphere: N2O + h N2 + O (8.6.6)

Both NO and NO2, collectively designated as NOx, are produced from natural sources, such as lightning and biological processes, and from pollutant sources.

Pollutant concentrations can become too high locally and regionally.

In the internal combustion engine,

N2 + O2 2NO (8.6.7)

Combustion of fuels that contain organically bound nitrogen also produces NO.

Atmospheric chemical reactions convert some of the NO emitted to NO2.

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NO2 in the Atmosphere

Electromagnetic radiation below 398 nm causes

NO2 + h NO + O (8.6.8)

• Produces highly reactive O atoms

• O atoms can participate in a series of chain reactions through which NO is converted back to NO2, which can undergo photodissociation again to start the whole cycle over.

NO2 more toxic than NO

• Exposure to 100-500 ppm of NO2 causes a lung condition called bronchiolitis fibrosa obliterans

•Exposed plants may suffer decreased photosynthesis, leaf spotting, and breakdown of plant tissue.

Reducing release of NO from combustion sources

• Limiting excess air so that there is not enough excess oxygen to produce NO

• Exhaust catalytic converters reduce NOx emissions from automobile exhausts.

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Acid rain from H2SO4, HNO3, HCl

Acid deposition, refers to the effects of atmospheric strong acids, acidic gases (SO2), and acidic salts (NH4NO3 and NH4HSO4)

Acidic precipitation is a regional air pollution problem

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Acid Precipitation (Cont.)

Adverse effects of acidic precipitation

• Direct effects of reduced and distorted visibility from and particles of acidic salts, such as NH4HSO4

• Direct phytotoxicity (toxicity to plants) and destruction of sensitive forests

• Indirect phytotoxicity from release of Al3+ ion by the action of acidic rainfall on soil

• Direct respiratory effects on humans and other animals

• Effects upon plants and fish in acidified lake water

• Damage to materials, especially acid-soluble limestone and marble

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Ammonia, NH3, from industrial pollution, coke manufacture, bacterial sources, decay of animal wastes, accidental releases from liquid anhydrous ammonia used as an agricultural nitrogen fertilizer

Ammonia dissolved in water droplets

Acts as base to produce corrosive salts

NH3 + H2SO4 NH4HSO4 (8.8.1)

NH3 + HNO3 NH4NO3 (8.8.2)

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Halogen Gases in the Atmosphere

Gaseous chlorine, fluorine, and volatile fluorides are uncommon air pollutants, but very serious where they occur.

Elemental chlorine, Cl2, is widely produced and distributed as a water disinfectant, bleach, and industrial chemical.

Accidental releases of Cl2 have killed people

Hydrogen chloride, HCl, from accidental releases and by reaction of reactive chlorine-containing chemicals, such as SiCl4,

SiCl4 + 2H2O  SiO2 + 4HCl(8.8.3)

HCl gas from combustion of polyvinylchloride (PVC) plastic

Exists as droplets of hydrochloric acid

Elemental fluorine (F2) and hydrogen fluoride, both highly toxic, are rarely released to the atmosphere.

Gaseous silicon tetrafluoride, SiF4, can be released when fluorspar (CaF2) reacts with sand (SiO2):

2CaF2 + 3SiO2 2CaSiO3 + SiF4 (8.8.4)

Sulfur hexafluoride, SF6, is astoundingly unreactive and a powerful greenhouse warming gas

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Hydrogen Sulfide, H2S

Hydrogen sulfide, H2S is as toxic as hydrogen cyanide.

From geothermal sources, the microbial decay of organic sulfur compounds, and the microbial conversion of sulfate, SO42-, to H2S when sulfate acts as an oxidizing agent in the absence of O2

Wood pulping processes can release hydrogen sulfide.

H2S is a common contaminant of petroleum and natural gas.

Poza Rica, Mexico, incident in 1950 killed 22 people

H2S is phytotoxic (harms or kills plants)

H2S forms a black coating of copper sulfide, CuS, on copper roofing which weathers to CuSO43Cu(OH)2.

H2Soxidizes toSO2.

COS and CS2, occur in the atmosphere

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Carbon dioxide, CO2, is a normal essential constituent of the atmosphere.

Levels now about 380 parts per million by volume and increasing by at least 1 ppm/year

Potential greenhouse effect

Evidence of warming during 1980s, 1990s, early 2000s

Other gases such as N2O and CH4 can cause greenhouse warming

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Adverse Effects of Greenhouse Warming

Adverse effects of greenhouse warming

• Predictions of average global temperature increase of 1.5–5˚ C, as much again as since the last ice age

• Would greatly affect climate and rainfall

• Melting of the polar and Greenland ice caps along with expansion of warmer ocean water would raise sea levels by 0.5–1.5 meters

• Decreased rainfall and increased water evaporation would contribute to severe drought and water shortages

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Can Green Chemistry Help Deal With Global Warming?

Provide means to prevent global warming from taking place

Coping with global warming, if it occurs.

Avoid release of carbon dioxide by using biomass as fuel or raw material for the manufacture of various products

Carbon sequestration in which carbon dioxide is produced, but is bound in a form such that it is not released to the atmosphere

• Convert carbon in coal to concentrated carbon dioxide that is pumped underground or into oceans

2C + O2 + 2H2O  2CO2 + 2H2 (8.9.1)

Alternative methods of energy production

• More efficient photovoltaic cells

• Devices for direct photochemical dissociation of water to produce elemental hydrogen and oxygen, which could be used in fuel cells

• Plants with much higher efficiencies for photosynthesis

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Dealing with Global Warming

Prevent release of greenhouse gases other than carbon

• Replacement of very persistent chlorofluorocarbons (Freons) with compounds readily destroyed in the troposphere

• Limit emissions of methane, CH4.

Green chemistry, biochemistry, and biology can be used to deal with global warming when it occurs.

• Crops, fertilizers, and pesticides can be developed that enable plants to grow under the drought conditions that would follow global warming

• Development of salt-tolerant crops that can be grown on soil irrigated with saline water, where fresh water supplies are limited.

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Photochemical smog occurs in dry, stagnant air masses, usually stabilized by a temperature inversion, that are subjected to intense sunlight.

A smoggy atmosphere contains ozone, O3, organic oxidants, N oxides, aldehydes, and other noxious species, as well as a haze of fine particles.

The chemical ingredients of smog are nitrogen oxides and organic compounds, both released from the automobile, as well as from other sources.

The driving energy force behind smog formation is electromagnetic radiation with a wavelength at around 400 nm or less, in the ultraviolet region, just shorter than the limit for visible light.

• Formation of active species starting photochemical reactions.

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Reactions Leading to Photochemical Smog

Absorption of a photon of electromagnetic radiation with a wavelength less than398 nm by a molecule of nitrogen dioxide,

NO2 + h NO + O (8.10.2)

CH4 + O  H3C + HO (8.10.3)

Methyl radical, H3C, and a hydroxyl radical, HO, where the dot shows a single unpaired electron

A chemical species with such a single electron is a free radical.

The hydroxyl radical is especially important in the formation of smog and in a wide variety of other kinds of photochemical reactions.

The methyl radical can react with an oxygen molecule,

H3C + O2 H3COO (8.10.4)

to produce a methylperoxyl radical, H3COO, a strongly oxidizing, reactive species

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Reactions Leading to Photochemical Smog

Important reaction is oxidation of NO back to photochemically active NO2

NO + H3COO NO2 + H3CO (8.10.5)

Literally hundreds of other reactions can occur, leading eventually to oxidized organic matter that produces the small particulate matter characteristic of smog.

Numerous noxious intermediates are generated.

• Ozone, O3, is the single species most characteristic of smog, toxic to plants and animals.

• Oxygen-rich organic compounds containing nitrogen that are potent oxidizers, of which peroxyacetyl nitrate, PAN, is the most common example.

• Aldehydes, which are irritants to eyes and the respiratory tract

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Harmful Effects of Smog

Adverse effects upon human health and comfort, plants, materials, and atmospheric quality

Ozone is generally regarded as being most harmful to humans, plants, and materials

• People exposed to 0.15 parts per million of ozone in air experience irritation to the respiratory mucous tissues accompanied by coughing, wheezing, and bronchial constriction.

• Especially pronounced for people exercising

Plants are harmed by exposure to nitrogen oxides, ozone, and peroxyacetyl nitrate (PAN, see above)

• PAN is the most harmful of these constituents, damaging younger plant leaves, especially.

• Ozone exposure causes formation of yellow spots on leaves, a condition called chlorotic stippling (below):

(Chlorotic stippling is manifested by yellow spots on a green leaf)

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Harmful Effects of Smog (Cont.)

Materials are attacked by oxidants

• Natural rubber is attacked by ozone; the hardening and cracking of natural rubber has been used as a test for the presence of ozone in the atmosphere.

Visibility-reducing aerosol particles

In general, quality of life and esthetics are harmed by photochemical smog.

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Smog and Green Chemistry: Can it Help?

A basic premise of green chemistry is to avoid the generation and release of chemical species with the potential to harm the environment.

The best way to avoid formation of smog is to avoid the release of nitrogen oxides and organic vapors that enable smog to form.

At an even more fundamental level, measures can be taken to avoid the use of technologies likely to release such substances, for example, by using alternatives to polluting automobiles for transportation.

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Solutions to the Smog Problem

The evolution of automotive pollution control devices to reduce smog provide an example of how green chemistry can be used to reduce pollution.

Initially command-and-control and “end-of-pipe” measures which often led to

• Poor performance • Very bad fuel economy

Now the automobile engine is a highly sophisticated computer-controlled machine that generally performs well, emits few air pollutants, and is highly efficient.

Has required an integrated approach involving reformulation of gasoline

• Elimination of tetraethyllead

• Reduction in smog-forming hydrocarbons

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Sustainable Measures with the Automobile

Electric automobiles that do not burn gasoline

• Limited range

Hybrid automobiles using a small gasoline or diesel engine that provides electricity to drive electric motors propelling the automobile and to recharge relatively smaller batteries.

Fuel cells that can produce electricity directly from the catalytic combination of elemental hydrogen and oxygen yielding water exhaust

• Best for fleet vehicles that can be refueled frequently

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Other Measures to Reduce Smog-Forming Emissions

Green chemistry applied to devices and processes other than automobiles to reduce smog-forming emissions

Organic solvents used for parts cleaning and other industrial operations, vapors of which are often released to the atmosphere contribute to photochemical smog.

• Substitution of water with proper additives or use of supercritical carbon dioxide fluid can eliminate such emissions.