Environmental Chemistry

Chapter 14:The Atmosphere

Copyright © 2011 by DBS

Contents

• The Atmosphere

• Importance of the Atmosphere

• Physical Characteristics of the Atmosphere

• Energy Transfer in the Atmosphere Atmospheric Mass Transfer, Meteorology and Weather

• Inversions and Air Pollution

• Global Climate and Microclimate

• Atmospheric Oxygen

• Atmospheric Nitrogen

• Atmospheric Water

The Atmosphere

The Atmosphere

• In a sense, the atmosphere is very thin because virtually all of it is within several km of Earth’s surface

• In another sense, the atmosphere is very thick because it extends for several hundred km above Earth’s surface

– Although extremely rarefied at higher altitudes

– At 10 km altitude and higher, insufficient air for breathing

• On a dry basis, the atmosphere is (by volume) 78.1% N2, 21.0% O2, 0.9% argon, 0.04% CO2, traces of neon, helium, krypton, xenon, and trace gases shown in Table 14.1

• Normally, the atmosphere is 1-3% water vapor by volume

• The atmosphere contains a variety of trace gases at very low levels

– Some natural

– Some pollutant

• The atmosphere contains suspended particles

– Solid, such as dust

– Liquid, especially cloud water droplets, also pollutants e.g. H2SO4

– Aerosol – suspension of solid or liquid droplets in air

The Atmosphere

Dominant, permanent gases Variable, trace gases

The Atmosphere

• The atmosphere is divided into several layers on the basis of temperature and pressure (density)

• Troposphere from Earth’s surface to about 11 km

– T from about 15˚C at surface, about -56˚C at around 11 km

• Stratosphere from about 11 km to about 50 km

– T increases to about -2˚C at top of stratosphere

– T increases in stratosphere because of ultraviolet absorption

• Atmospheric Chemistry

– Characterized by photochemical reactions started by absorption of photons of ultraviolet radiation

– Energy of photons: E = h– Chain reactions involving free radical species with unpaired electrons

Videos:

• Wild Weather (2002) - http://www.youtube.com/watch?v=xux-P2FsLcs

at 1:53 – 5:50

• Earth – Power of the Planet (2007) - http://www.youtube.com/watch?v=J5ViCNJAkHg

At 2:07 – 15:00

The Atmosphere

• Layers of the atmosphere – divided based on temperature– Troposphere 0 – 10 km ~33,000 ft– Stratosphere 10 – 50 km ~164,000 ft ~ 31 mi– Mesosphere 50 – 90 km ~56 mi– Thermosphere 90 – 500 km– Exosphere > 500 km ~310 mi

(1 km = 0.62 miles)

The Atmosphere

Origins of the Atmosphere

• Evidence points to Earth’s atmosphere being a secondary phase

• Originally a reducing atmosphere, no O2 present

• UV, lightening and radioactivity provided energy for chemical reactions which produced first amino acids

• O2 eventually produced by photosynthesis of cyanobacteria

• Evidence of O2 production by a primitive atmosphere lies with large deposits of iron oxides

Fe2+(aq) + O2 + 4H2O 2Fe2O3(s) + 8H+

• An oxygen atmosphere enabled evolution of organisms, particularly animals, that utilize O2 metabolically - eventually forms ozone layer

Earth – Power of the Planet (2007) - http://www.youtube.com/watch?v=J5ViCNJAkHgat 33:00 to 42:00

Importance of the Atmosphere

Importance of the Atmosphere

• Protective blanket against the hostile environment of outer space

• Absorbs damaging radiation

• CO2 for photosynthesis, transmits light for photosynthesis

• O2 for respiration

• Medium for hydrologic cycle which replenishes fresh water

• The atmosphere transmits and absorbs electromagnetic radiation

• Absorbs and protects from cosmic radiation and damaging ultraviolet radiation below 350 nm wavelength

• Transmits near-ultraviolet, visible, and near-infrared radiation (300-2500 nm)

• Transmits radio waves of 0.01-40 m wavelength

• By re-absorbing outbound infrared radiation, the atmosphere stabilizes Earth’s surface temperature

Importance of the Atmosphere

The Atmosphere As a Reservoir of Natural Capital

• Natural capital consists of resources that Earth and its ecosystems provide

• • Materials • Energy • Waste assimilative capacity • Esthetics

• The atmosphere is a huge part of natural capital

1. Protective and regulatory function

Absorbs harmful radiation • Stabilizes temperature

2. Source of essential raw materials

O2 for respiration

CO2 for photosynthesis

Source of N2 to fix for chemicals, biomolecules, cold liquid N2

Source of Ar for industrial applications

3. Conduit of fresh water in the hydrologic cycle

4. Assimilate materials including pollutants

5. Esthetics

Physical Characteristics of the Atmosphere

Physical Characteristics of the Atmosphere

Variation of Pressure and Density with AltitudeFigure 14.1. Variation of temperature and pressure with altitude

99% of air below 30 km

Persistence of normally very reactive species (atomic O, free radicals) at very high altitudes

Physical Characteristics of the Atmosphere

Figure 14.2. Stratification of the Atmosphere

Tropopause layer where water vapor condenses to ice crystals

Physical Characteristics of the Atmosphere

• Upper atmosphere - Presence of electrons and +ve ions

• Called the Ionosphere at Altitudes Above About 50 km

• Formed by ionizing radiation, such as ultraviolet

O + h O+ + e-

• From about 50 km

– Lower limit higher at night as ions recombine and are not produced

• At lower altitudes, molecular ions predominate

• • O2+ • NO+

• Earth’s magnetic field has a strong influence on the ions in the upper atmosphere…

Physical Characteristics of the Atmosphere

Figure 14.3. Van Allen Belts of Ions Around Earth

Physical Characteristics of the Atmosphere

14.4. ENERGY TRANSFER IN THE ATMOSPHERE

Figure 14.4. Solar Flux

Energy Transfer in the Atmosphere: Atmospheric Mass Transfer, Meteorology

and Weather

Energy Transfer in the Atmosphere

• Energy Transport in the Atmosphere

• Conduction of heat between adjacent air molecules (slow)

• Convection by movement of entire air masses

• Sensible heat from kinetic energy of molecules

• Latent heat released when water vapor condenses

• Incoming radiation largely in relatively short wavelength around visible radiation

• Outgoing radiation largely in relatively longer wavelength infrared

• Albedo refers to degree of reflection or absorption of incoming radiation at Earth’s surface

• Outgoing radiation partially re-absorbed by the atmosphere warming it

• Normal greenhouse effect

Energy Transfer in the Atmosphere Atmospheric Mass Transfer, Meteorology

and Weather

Earth’s Radiation Budget Figure 14.5

Atmospheric Mass Transfer, Meteorology and Weather

• Meteorology is the science of atmospheric phenomena

• • Air mass movement • Heat • Wind • Precipitation

• Weather consists of short-term variation in atmosphere’s condition

• Climate is long-term condition of the atmosphere

Atmospheric Mass Transfer, Meteorology and Weather

Atmospheric Water in Energy and Mass Transfer

• Large amounts of heat absorbed to evaporate water

• Large amounts of heat released when water condenses

– Major means of atmospheric energy transport

• Clouds consisting of suspended water droplets in the atmosphere

• Air masses of relatively uniform composition

• Separated by fronts

• Wind is horizontally moving air

• Air currents are vertically moving air

• Movement of air masses and water vapor they contain, uptake and release of solar energy as latent heat in water vapor

1. Redistribution of solar energy

2. Horizontal and vertical movement of air masses with varying moisture contents

3. Evaporation and condensation of water accompanied by uptake and release of heat

Energy Transfer in the Atmosphere: Atmospheric Mass Transfer, Meteorology

and Weather

Figure 14.6. Movement of Air Masses, a Major Factor in Weather

Energy Transfer in the Atmosphere: Atmospheric Mass Transfer, Meteorology

and Weather

Global WeatherCirculation of Air in the Northern Hemisphere

Atmospheric Mass Transfer, Meteorology and Weather

Weather Fronts and Storms

• Front is the interface of two air masses that differ in temperature, pressure, density, and water content

• Cold front when cold air displaces warm air

• Warm front when warm air displaces cooler air

• As warm, moist air rises when it contacts a cold front

• • Water vapor condenses

• • Heat is released

• • Warmed air rises, creating air currents and wind

• • Cyclonic storms with rotating air masses may result

Inversions and Air Pollution

Inversions and Air Pollution

Figure 14.8. Illustration of Air Pollutants Trapped by a Temperature Inversion

Inversions and Air Pollution

• Increasing T with altitude

• Limits vertical movement of air, result in stagnation and trapping of air pollutants

Demo: http://academics.rmu.edu/faculty/short/envs1160/envs1160-demos/Demo-Temp-Inversion.mov

Global Climate and Microclimate

Global Climate and Microclimate

• Climate is long-term weather patterns over large geographic areas

– Monsoons with heavy rainfall

– Chronic drought

– Global warming

– Phenomena such as El Niño

– Humans may be modifying climate

• Effects of Urbanization on Microclimate

– Paving and structures affect albedo

– Cities overlain with a heat dome

– As much as 5˚C warmer than surrounding countryside

Atmospheric Oxygen

Atmospheric Oxygen

14.8. ATMOSPHERIC OXYGEN

Figure 14.9. Interchange of various species of oxygen among the spheres of the environment

Atmospheric Oxygen

• High in the thermosphere atomic oxygen (ground state) predominates

O2 + h O + O

• O may be present as excited species, O*, from photolysis of O3

• O* causes airglow from emissions at 636, 630, and 558 nm

• O forms ions in the ionosphere

O + h O++ e-

Atmospheric Oxygen

• Most important stratospheric reaction of O2 is formation of stratospheric ozone

O2 + h O + O

O + O2 + M (third body molecule) O3 + M

• Ozone absorbs ultraviolet radiation strongly in the 220-330 nm region

– Crucial protective function

– Generates heat causing temperature maximum at top of stratosphere

Atmospheric Nitrogen

Atmospheric Nitrogen

Nitrogen Cycle

• N is interchanged among the atmosphere, OM, and inorganic compounds

• MO’s mediate reactions

Atmospheric Nitrogen

• Atmosphere is predominantly nitrogen

• Nitrogen cycle and fixation discussed in Chapter 11

• Atmospheric nitrogen is mostly N2 with little photochemical dissociation to N atoms below 100 km altitude

• Atmospheric NO is involved with stratospheric ozone removal (catalytic cycle)

• NO2 is very important in tropospheric chemistry

NO2 + h NO + O

– regenerates NO and produces reactive O atoms in troposphere

– O involved with important tropospheric processes, ozone/smog formation

Atmospheric Water

Atmospheric Water

• Normal range 1-3%, extremes of 0.1-5%

• Decreases rapidly with increasing altitude

• Circulates through the atmosphere in hydrologic cycle

• Water vapor re-absorbs outgoing infrared radiation warming the atmosphere

• Heat absorbed and released as water evaporates and condenses in the atmosphere

– Important in atmospheric energy transfer

– Big factor in weather and climate

• Tropospheric water may carry corrosive salts and acids

• Stratospheric water formed from methane

– CH4 + 2O2 + h CO2 + 2H2O

– Forms stratospheric hydroxyl radical, HO•: H2O + h HO• + H•