Chuong Hoa Hoc khi quyen (1) potx

9.1 The Atmosphere and Atmospheric ChemistryThe 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, k

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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!

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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 = hν

A chemical species in an excited (energized) state is

designated with an asterisk, *

The photochemical reaction of stratospheric ozone:

• O3+ hν(λ< 420nm) → O*+ O2

•The O3 absorbs a photon of energy hν

• The O3 undergoes photodissociation

• The oxygen atom product is excited denoted O*

Free radicals with unpaired electrons shown with a dot, •

•H2O2+ hν→ HO • + HO •

Energy-absorbing third body , M, usually N2 molecule

•O + O2 + M → O3+ M

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

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

Figure 9.1 Variation of air pressure and temperature with altitude

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Stratification of the Atmosphere

• Lower atmosphere, the troposphere

Stratosphere and Upper Atmosphere

9.4 Energy Transfer in the Atmosphere

Solar Flux

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

9.5 Atmospheric Mass Transfer, Meteorology,

Weather

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

Figure 9.5 Circulation of air masses and water, uptake

and release of solar energy as latent heat in water vapor

Global Weather

Figure 9.6 Global circulation of air in the northern hemisphere

9.6 Inversions and Air Pollution 15

Figure 9.7 Pollutants trapped by a temperature

inversion and confining topography

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

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 • )

Fig 9.8 Atmospheric chemical and photochemical processes

Photochemical Processes

Initiated when a molecule absorbs a photon of

electromagnetic radiation to produce an excited species, *

• NO2+ hν→ NO2*

• Usually in ultraviolet region

Loss of excess energy from an excited state may occur by several processes including

• Dissociation: NO2*→NO+ O

• Luminescence: NO2*→ NO2 + hν

•Photoionization: N2*→N2++ e

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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)

Free Radicals

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+ hν→ {CH2O} + O2

•Evidence from iron oxide deposits

9.9 Acid-Base Reactions in the Atmosphere

Rainwater weakly acidic from CO2

• CO2+ H2O ↔ H+ + HCO3

-Pollutant 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

N2 molecule is very stable

• No significant tropospheric chemical or photochemical reactions of N2

• N2 is the most common energy-absorbing third body, “M”,

9.12 Atmospheric Water

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+ hν→ CO2+ 2H2O

Stratospheric water produces hydroxyl radical

•H2O+ hν→ HO • + H

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

Fig 9.12 Localized atmospheric chemical fate and transport from a point source