CHAPTER 8 AIR AND THE ATMOSPHERE From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, ChemChar Research, Inc., 2006 [email protected].

CHAPTER 8AIR AND THE ATMOSPHERE

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

Inc., [email protected]

8.1. MORE THAN JUST AIR TO BREATHE

A Sea of GasWe 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

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 uponR = P1V1 = P2V2n1T1 n2T2 V2 = V1 × n2T2P1n1T1P2 :Rearanged

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

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

V2 = V1 × T2 = 12.0 L × 363˚ = 15.4 LT1 283˚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

The Protective Atmosphere

Keeps Earth’s surface warm by delaying outgoing infrared radiationAbsorbs very short wavelength ultraviolet radiation from the sunNature of airIn 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.

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

The Troposphere

10-16 km, -56˚C

TemperatureinversionO2, N2, Ar, CO2, trace gasesNO2 + hν → + NO O Photochemical reactions

, Stratosphere upper atmosphereTroposphere

WeatherH2OVaporDropletsParticles Air pollutants

The Stratosphere

-2˚ C at 50 km altitudeVirtually 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

The Stratosphere (Cont.)

10-16 km, -56˚CO2 + hν → + O OO2 + O → O3O3 + hν → O2 + O( -filtration of ultra )violet radiationStratosphere~ 50 , -2˚km C 200-330Ultraviolet between nanometers penetrating to 50 around km altitude

- , -High energy ultraviolet wave 100 ,length less than nanometers 200 penetrating to around kmaltitude 330 -Ultraviolet above nano, , ,meters visible light infrared -penetrating through the strat ’ osphere and to Earth s surface

8.2. ATMOSPHERIC CHEMISTRY AND PHOTOCHEMICAL REACTIONS

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 ν where h is Planck’s

constant and ν 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.

Photochemical and Chain ReactionsExample of dichlorodifluoromethane, CCl2F2, which was used in automobile air conditioners

CCl2F2 + hν 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!

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.

8.3. ENERGY AND MASS TRANSFER IN THE ATMOSPHERE

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)

Redistribution of Energy in the Atmosphere

Some reflected fromatmosphere, clouds,Earth’s surfaceSome absorbedby atmosphereInfrared outInfrared absorbedInfrared re-emittedEnergy redistributedaway from equator

Solar radiationreaching atmosphere

Meteorology

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.

Weather

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.

Temperature Inversions

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

8.4. ATMOSPHERIC OXYGEN AND NITROGENN2 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 + hν {CH2O} + O2 (8.4.2)

where {CH2O} is a generic formula representing biomassNitrogen in the atmosphere

Atmospheric N2 is very unreactiveMost important reaction of N-containing species in the atmosphere

NO2 + hν NO + O (8.4.3)

Reactive O atom initiates many tropospheric photochemical reactions

Green Oxygen and Nitrogen from the AirElemental 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

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

8.5. ATMOSPHERIC POLLUTANT PARTICLESDispersion aerosols formed by grinding solids, dispersing dusts, or atomizing liquidsCondensation aerosols produced when gases or vapors, often formed as the result of atmospheric chemical processes, condenseMists 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, berylliumRadioactive radon including 222Rn (half-life 3.8 days) and 220Rn (half-life 54.5 seconds), alpha emitters that decay to radioactive 218Po and 216PoParticles have both direct effects (reduction of visibility) and indirect effects (reaction sites) in the atmosphere.

Chemical Processes on and in Atmospheric Particles

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)

8.6. POLLUTANT GASEOUS OXIDESCarbon MonoxideToxic 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.

Pollutant Gaseous Oxides (Cont.)Sulfur DioxideFrom several natural and pollutant sourcesDirect effects• On people with respiratory problems • On plantsMost 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)

Green Chemistry and Sulfur DioxideSulfur 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.

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 + hν 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.

NO2 in the AtmosphereElectromagnetic radiation below 398 nm causes

NO2 + hν 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.

8.7. ACID RAIN

Acid rain from H2SO4, HNO3, HClAcid 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 4.54.54.44.44.34.34.25.35.2

5.14.95.14.74.54.44.3Line of pH in precipitation

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

8.8. MISCELLANEOUS GASES IN THE ATMOSPHEREAmmonia, NH3, from industrial pollution, coke manufacture, bacterial sources, decay of animal wastes, accidental releases from liquid anhydrous ammonia used as an agricultural nitrogen fertilizerAmmonia dissolved in water dropletsActs as base to produce corrosive salts

NH3 + H2SO4 NH4HSO4 (8.8.1)

NH3 + HNO3 NH4NO3 (8.8.2)

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 peopleHydrogen 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) plasticExists as droplets of hydrochloric acidElemental 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

Hydrogen Sulfide, H2SHydrogen sulfide, H2S is as toxic as hydrogen cyanide.From geothermal sources, the microbial decay of organic sulfur compounds, and the microbial conversion of sulfate, SO4

2-, 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 CuSO43Cu(OH)2.

H2S oxidizes to SO2.

COS and CS2, occur in the atmosphere

8.9. CO2: THE ULTIMATE AIR POLLUTANT?

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/yearPotential greenhouse effectEvidence of warming during 1980s, 1990s, early 2000s

Other gases such as N2O and CH4 can cause greenhouse warming

Increase in Atmospheric Carbon Dioxide Levels

Global Temperature Trends

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

Can Green Chemistry Help Deal With Global Warming?

Provide means to prevent global warming from taking placeCoping with global warming, if it occurs.Avoid release of carbon dioxide by using biomass as fuel or raw material for the manufacture of various productsCarbon 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

Dealing with Global WarmingPrevent 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.

8.10. PHOTOCHEMICAL SMOGPhotochemical 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.

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 + hν 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 electronA 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

Reactions Leading to Photochemical SmogImportant 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.

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

• 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.

Peroxyacetyl nitrate (PAN)

NO2OC

H

H

H

O

C O

FormaldehydeHCH

C

Harmful Effects of SmogAdverse effects upon human health and comfort, plants, materials, and atmospheric qualityOzone 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 exercisingPlants 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)

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

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.

Solutions to the Smog ProblemThe 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 economyNow 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

Sustainable Measures with the AutomobileElectric automobiles that do not burn gasoline• Limited rangeHybrid 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

Other Measures to Reduce Smog-Forming EmissionsGreen chemistry applied to devices and processes other than automobiles to reduce smog-forming emissionsOrganic 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.