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Chapter 9
THE ATMOSPHERE AND ATMOSPHERIC
CHEMISTRY
Environmental Chemistry, 9th Edition
Stanley E. Manahan
Taylor and Francis/CRC Press
2010
For questions, contact:
Stanley E. Manahan
[email protected]
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엘리뇨 현상
라니뇨 현상
기온역전현상
날씨를 보면 겨울은 건조하고 여름은 습하다고 하는데
습도가 몇%이하가 될 때 건조하다고 하고 몇%이상이 될 때
습하다고 하나요?
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9.1 The Atmosphere and Atmospheric Chemistry
The atmosphere consists of the following (on a dry basis):
• 78.1% N2 • 21.0% O2 • 0.9% Ar • 0.04% CO2
• Low levels of noble gas helium, neon, krypton, xenon
• Trace gases (see Table 9.1)
Most of the mass of the atmosphere is very close to Earth’s
surface relative to Earth’s diameter
• If Earth were a classroom globe, virtually all air would be
in a layer the thickness of the coat of varnish on the globe!
Photochemistry and Some Important Terms
Photochemical reactions occur in the atmosphere when
molecules absorb energy in the form of photons
• Mostly in ultraviolet region of spectrum • E = hn
A chemical species in an excited (energized) state is
designated with an asterisk, *
The photochemical reaction of stratospheric ozone:
• O3 + hn (l < 420nm)  O*+ O2
• The O3 absorbs a photon of energy hn
• The O3 undergoes photodissociation
• The oxygen atom product is excited denoted O*
Free radicals with unpaired electrons shown with a dot, •
• H2O2 + hn  HO• + HO•
Energy-absorbing third body, M, usually N2 molecule
• O + O2 + M  O 3 + M
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Gaseous Oxides in the Atmosphere
Low and variable levels of C, N, and S oxides
• Pollutants at elevated levels
• Discussed in Chapters 11 and 14
Atmospheric methane
• From anoxic bacteria and underground sources
• Significant greenhouse gas
• Influences levels of hydroxyl radical (HO•), ozone,
stratospheric water vapor
Hydrocarbons and photochemical smog
• Hydrocarbons required for photochemical smog
formation (see Chapters 13 and 14)
Particulate matter (see Chapter 10)
Primary pollutants emitted directly
Secondary pollutants formed from reactions of primary
pollutants
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9.2 Importance of the Atmosphere
Protective function
• Filters out harmful radiation
• Stabilizes temperature
Part of hydrologic cycle
Source of CO2 for plant photosynthesis
Source of N for plant growth, industrial chemicals
Variation of pressure and density with altitude
• Pressure and density decrease rapidly with increasing
altitude
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Figure 9.1 Variation of air pressure and
temperature with altitude
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Stratification of the Atmosphere
• Lower atmosphere, the troposphere
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Stratosphere and Upper Atmosphere
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9.4 Energy Transfer in the Atmosphere
Solar Flux
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Earth’s Atmospheric Heat Balance
See detail of Earth’s atmospheric
heat balance in text Figure 9.4
Re-absorption of outbound
infrared stabilizes atmospheric
temperature
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9.5 Atmospheric Mass Transfer, Meteorology,
Weather
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Meteorology is the science of physical atmospheric
phenomena
Weather: Short-term variations in
• Temperature • Clouds • Winds • Humidity • Pressure
• Horizontal visibility • Precipitation type and quantity
Climate: Long-term weather conditions
Atmospheric Water in Energy and Mass Transfer
• Carries energy as latent heat released when water vapor
condenses
Humidity is water content of air
• Relative humidity is % saturation level
Water condenses below dew point
• Condensation nuclei
Clouds are composed of microdroplets of water
• Coalesce to form larger droplets and precipitation
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Distinct air masses in the atmosphere
• Uniform temperature and water vapor content
• Horizontally homogeneous
• Conditions and movement affect pollutant reactions,
effects, and dispersal
• Air masses separated by fronts
• Warm fronts • Cold fronts
• Wind is horizontally moving air
• Air currents are vertically moving air
Topographical effects
• Topography, surface configuration and relief features
strongly affect winds and air currents
Cyclonic storms
• Hurricanes (Atlantic) • Typhoons (Pacific)
Figure 9.5 Circulation of air masses and water, uptake
and release of solar energy as latent heat in water vapor
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Global Weather
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Figure 9.6 Global circulation of air in the northern hemisphere
9.6 Inversions and Air Pollution
Figure 9.7 Pollutants trapped by a temperature
inversion and confining topography
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9.7 Global Climate and Microclimate
Climate
• Characteristic of a particular region
• Varies with season
Example: Alternating monsoons and dry seasons
• Ice age manifested by long-term change in climate
Humans may be modifying climate largely by pumping
carbon dioxide into the atmosphere causing warming
Microclimate
• Highly localized climate
Example: At soil surface shaded by plants
Effects of urbanization on microclimate
• Heat dome over cities
• City atmosphere up to 5˚C warmer
• Counteracting cooling effect from particulate matter
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9.8 Chemical and Photochemical Reactions
Study of atmospheric chemistry is complicated
• Effects of photochemical energy input
• Extreme dilution of species in air
• Container walls complicate laboratory study
Major categories of atmospheric chemical species
• Inorganic oxides (CO, CO2, NO2, SO2)
• Oxidants (O3, H2O2, HO•,HO2•, and ROO• radicals, NO3)
• Reductants (CO, SO2, H2S)
• Organics (such as CH4, most also reductants)
• Oxidized organics (carbonyls, organic nitrates)
• Photochemically active species (NO, formaldehyde)
• Acids (H2SO4), bases (NH3), salts (NH4HSO4)
• Unstable reactive species (NO2*, HO•)
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Fig 9.8 Atmospheric chemical and photochemical 19
processes
Photochemical Processes
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Initiated when a molecule absorbs a photon of
electromagnetic radiation to produce an excited species, *
• NO2 + hn  NO2*
• Usually in ultraviolet region
Loss of excess energy from an excited state may occur by
several processes including
• Dissociation: NO2*  NO + O(유리산소, 활성산소)
• Luminescence: NO2*  NO2 + hn
• Photoionization: N2*  N2+ + e-
Ions in the Atmosphere
Ionosphere above about 50 km
• From photoionization by solar ultraviolet
• Raises at night as ions recombine
Figure 9.9 Van
Allen belts of ions
encircling Earth (in
cross section)
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Free Radicals
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Reactive species with unpaired electrons denoted •
Generally formed by photochemical reactions or reactions
of molecules with other free radicals
Two free radicals may react to form a stable species
Hydroxyl radical in the atmosphere
• HO• is the most important free radical in the atmosphere
(see text Figure 9.10)
• Produced by many reactions such as
CH4 + O (from photodissociation of NO2)  H3C• + HO•
• Removed by many reactions, especially with CO or CH4
Hydroperoxyl radical in the atmosphere
• HOO•
• Less important than HO•, but still significant
Evolution of the Atmosphere
Atmospheric oxygen from photosynthesis
• CO2 + H2O + hn  {CH2O} + O2
• Evidence from iron oxide deposits
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9.9 Acid-Base Reactions in the Atmosphere
Rainwater weakly acidic from CO2
• CO2 + H2O  H+ + HCO3Pollutant SO2 is more acidic than CO2
Strong acid H2SO4, HNO3, and HCl are responsible for
damaging acid rain
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9.10 Reactions of Atmospheric Oxygen, Figure 9.11
Reactions of Atmospheric Nitrogen
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N2 molecule is very stable
• No significant tropospheric chemical or photochemical
reactions of N2
• N2 is the most common energy-absorbing third body, “M”,
in atmospheric chemistry
Fixation of N from atmospheric N2 is an important
environmental phenomenon
• Biochemically by specialized bacteria
• Chemically by NH3 synthesis
N compounds such as NO and NO2 are very active species
in tropospheric chemistry
9.12 Atmospheric Water
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Normal range 1-3% by volume
Vapor responsible for atmospheric temperature stability
Hydrologic cycle
Crucial in atmospheric energy transfer
Tropopause prevents water vapor transfer from troposphere
to stratosphere
Stratospheric water from following several-step process:
• CH4 + 2O2 + hn  CO2 + 2H2O
Stratospheric water produces hydroxyl radical
• H2O + hn  HO• + H
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9.13 Influence of the Anthrosphere
Many air pollutants from the anthrosphere
• Particles affecting visibility
• Acid-forming gases such as SO2
• Nitrogen oxides and hydrocarbons forming
photochemical smog
Two major kinds of species affecting global climate
• Chlorofluorocarbons that deplete stratospheric ozone
• Greenhouse gases that cause global warming
• Primarily CO2
• Other gases such as CH4
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9.14 Chemical Fate and Transport in Atmosphere
Considers the following regarding airborne pollutants
• Sources • Transport • Dispersal • Fluxes
Atmosphere/surface boundary interaction
• Rock/soil • Water • Vegetation
Transport and dispersal
• Movement of air masses • Diffusive and Fickian transport
Long-range movement such as radionuclides from
Chernobyl reactor meltdown
Distillation of semivolatile organic pollutants to polar
regions
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Fig 9.12 Localized atmospheric chemical fate and
transport from a point source