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Global warming is caused by• Reabsorption of outgoing infrared by CO2, H2O vapor, CH4, other species in the atmosphere Global warming is generally a good thing, but bad if it happens in

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Chapter 14THE ENDANGERED GLOBAL ATMOSPHERE

Environmental Chemistry, 9th Edition

Stanley E Manahan Taylor and Francis/CRC Press

2010

14.1 Climate Change and Anthropogenic Effects

Lovelock’s Gaia Hypothesis : Organisms have established and maintained the

atmospheric O2/CO2 balance that has maintained Earth’s climate and moist conditions conducive to life

• Human activities must avoid upsetting this balance

Earth’s present oxygen-rich atmosphere was established by photosynthetic bacteria

• CO2 + H2O+ hν→ { CH2O} +O2

• Evidence of O2 production from iron oxide deposits

• 4Fe2+ + O2 + 4H2O→ 2Fe2O3+8H +

Accumulation of O2 in the atmosphere enabled development of oxic organisms

Establishment of stratospheric ozone layer that absorbs ultraviolet radiation enables terrestrial life to exist

Organisms regulate atmospheric CO2 and maintain climate

Human activities strongly influence the atmosphere particularly through emissions that

1 Warm the atmosphere by absorbing outgoing infrared

2 Pollutant particles that scatter and reflect sunlight

3 Photochemically reactive species such as NO2

4 Emissions that catalyze destruction of stratospheric ozone

In 1957 Revelle and Suess called human effects on the atmosphere a “massive

geophysical experiment”

Changes in Climate

Evidence of massive changes in climate in past

Currently in 10,000-year interglacial period called holocene

Concern over positive feedback mechanisms that could result in massive climate

change

•Example: Warming melts ice cover that leads to greater absorption of light and more warming

14.2 Global Warming

Figure 14.1 Global temperature trends since 1880

Global warming is caused by

• Reabsorption of outgoing infrared by CO2, H2O vapor, CH4, other species in the atmosphere

Global warming is generally a good thing, but bad if it happens in excess

Concern over global warming from record hot years during the last two decades

Doubling of atmospheric CO2 levels from pre-industrial levels by 2100 is projected to raise Earth’s means surface temperature 1.5-4.5˚C, which could have some very bad effects on climate

Figure 14.2 Increasing atmospheric CO2 levels with annual variation in the northern hemisphere from photosynthesis

Figure 14.3 CO2 Emissions per person annually

Methane and Other Greenhouse Gases

Greenhouse gases other than CO2 contribute to global warming

• Halocarbons • Hydrohalocarbons • N2O

Methane, CH4, is especially important

• Now at 1.8 ppm • Increasing about 0.02 ppm/year

Methane from several important sources

• Natural gas leakage • Coal mine emissions

• Petroleum recovery • Burning savannas, tropical forests

• From methane entrapped in thawing permafrost

Much methane from biogenic sources including

• Degradation of organic matter such as in landfills

• Anoxic biodegradation in rice paddies

• Anoxic bacteria in ruminant animals

Radiative forcing of methane

• Absorption of infrared per molecule

• About 25 times that of CO2

Particles and Global Warming

Effects of particles are complicated making their modelling difficult

Both cooling and warming effects

• Incoming visible radiation scattered, cooling effect

• Incoming radiation absorbed, warming effect

• Absorption of outbound infrared has warming effect

Sulfuric acid and sulfates from oxidation of SO2 produces droplets containing water which has a generally cooling effect

The Outlook for Global Warming and Associated Effects

CO2 levels to double to around 600 ppm by 2100 tending to cause global warming

• Affect cloud cover, both warming and cooling

• Drought

• Increased water evaporation and transpiration (from plants)

Crop-destroying insects, weeds, diseases, and rodents will be more troublesome as atmosphere warms

Pollutant sulfur dioxide may have a counteracting effect due to formation of reflecting cloud cover

light-14.3 Green Science and Technology to Alleviate Global Warming

1 Minimization of CO2 emissions

• Energy from sources other than carbon-based fuels

• Electricity from renewable and electrical sources

• Light-reflecting aerosols into atmosphere

• Probably not very practical

3 Adaptation

• Will be necessary

• Drought will require more efficient water utilization

• Fresh water from wastewater and saline water

• Abundant, renewable, non-carbon energy is key

14.4 Acid Rain

Acids stronger than CO2 in aqueous precipitation

• H2SO4 (most common) • H2SO3 • HNO3 • HCl

• Rain • Mist • Sleet • Fog (may be especially acidic)

• Acid rime (frozen cloudwater on surfaces)

Acid deposition is a more general term

• Aqueous acids • Acid gases (SO2)

• Acidic salts, such as NH4HSO4

Most acids in acid precipitation are secondary pollutants from oxidation

• SO2→H2SO4 • NOX→HNO3

Acid precipitation is a regional air pollution problem often spreading over several

hundred kilometers

• Photochemical smog is a local air pollution problem typically over several tens of km

• Stratospheric ozone depletion is a global air pollution problem

Figure 14.4 Isopleths of pH showing typical acid precipitation pattern

Harmful Effects of Acid Precipitation

• Direct phytotoxicity to plants from strong acids

• Phytotoxicity from acid-forming gases (SO2, NOx)

• Acidification of water bodies

• Acidification of soil

• Indirect phytotoxicity, especially from Al3+ in soil

• Destruction of sensitive forests

• Respiratory effects on humans and other animals

• Corrosion of metals, such as in electrical relays

• Deterioration of stone

2H+ + CaCO3(limestone)→Ca2+ + CO2(g) + H2O

• Associated effects

• Visibility reduction and increased haziness from sulfate aerosols

• Physical and optical properties of clouds

• Potential reduction in greenhouse warming from increased cloud cover

14.5 Stratospheric Ozone Destruction

Ozone is produced in the stratosphere

• O2+ hν→O + O (λ< 242.4 nm)

• O + O2 + M→O3+ M (energy-absorbing N2 or O2)

Stratospheric ozone is destroyed by absorption of UV

The Stratospheric Ozone Layer

From about 15 to about 35 km in altitude

• Below 15 km energetic UV required to split O2 has been filtered out

• Above 35 km there is very little oxygen and most is atomic O

If all stratospheric ozone were in a pure layer at 25˚C and 1 atm pressure, the ozone layer would be only 3 mm thick!

The total amount of ozone in the atmosphere above a particular point is measured in Dobson units, DU

• 1 DU = 0.001 atm-cm

• Average ozone layer thickness is 300 DU

Shielding Effect of the Ozone Layer

Ozone absorbs ultraviolet radiation very strongly in the range 220-330 nm

Effectively absorbs very damaging UV-B radiation in the range 290-320 nm

• Essential protection for life on Earth’s surface

Partial destruction of the stratospheric ozone layer could have adverse effects

• Increased skin cancer in humans

• Increased incidence of cataracts

• Damage to food crop plants

• Lowered productivity of marine phytoplankton

Ozone Layer Destruction

1974 discovery of chlorofluorocarbon destruction of O3

• Nobel Prize to Molina, Rowland, and Crutzen

Stratospheric reactions of chlorofluorocarbons

• O3 + O→O2 + O2 (Net reaction up to 10,000 times per Cl •)

An important reaction sequence involves formation and subsequent destruction of the dimer from ClO• to give Cl• that catalyzes O3 destruction

• See reactions in text

Antarctic ozone hole discovered in 1985

• Occurs in southern hemisphere springtime (September/ October when sunlight reaches stratospheric clouds in Antarctica

• Active forms of Cl are stored in (largely crystalline ice) stratospheric Antarctic clouds during winter

• ClONO2 • Cl2 • HOCl

• Reaction of ClONO2with HCl produces photochemically reactive Cl2

• Starting in September ultraviolet solar radiation reaches these clouds producing active Cl•that catalyzes ozone destruction

• Cl2 + hν→Cl • +Cl •

• HOCl+ hν→HO • + Cl •

Figure 14.5 The area and ozone density of the Antarctic ozone hole at its maximum extent in September and October

Green Chemistry Solutions to Stratospheric Ozone Depletion

Restrictions from 1986 Montreal Protocol on Substances that Deplete the Ozone Layer

• Ban on production of chlorofluorocarbons (CFCs)

• Now having noticeable effects on levels of ozone-depleting substances

• Carbon dioxide substitute for CFCs to make plastic foams

• Insulation • Food containers • Packing

14.6 Atmospheric Brown Clouds

A layer of brown-hued air about 3 km thick extending from the Arabian Peninsula across China and the western Pacific

Caused by inefficient burning practices

• Coal-fired power plants • Slash-and-burn agriculture

• Cooking fires with wood or dung fuel

Adverse effects

• Darkening megacities such as Bejing, New Dehli

• Melting glaciers • Decreased agricultural productivity

• Adverse health effects

Yellow Dust

Huge masses of windblown dust and sand

• Begin with winds over Mongolia and China spreading eastward

• Mix with brown cloud

• Associated with desertification and deforestation aggravated by global warming Adverse economic effects

• Bad for clean manufacturing processes

Adverse health effects

• Asthma • Lung disease • Perhaps immune disease

The “good yellow dust”

• Dust from Sahara blown to Amazon

• Fertilization of ocean, especially with iron

• Increased phytoplankton growth (base of ocean food chain)

14.7 Atmospheric Damage by Photochemical Smog

Urban atmosphere acting as a chemical reactor for

• Hydrocarbons • NOx • O2 • Sulfur compounds

Driven by sunlight to produce

• Ozone • Organic oxidants • Aldehydes

• Organic particles • Nitrates • Sulfates • Other

Photochemical smog is a secondary air pollutant

Forms in an air parcel about 1000 meters thick

Produces urban aerosol of particles

• Urban brown cloud

• Condensation aerosol particles that are quite small

• Most likely to scatter light • Most respirable

Potentially carcinogenic aromatic hydrocarbon particles such as benzo(a)pyrene in smog

Types of Damage from Photochemical Smog

Adverse health effects

• Largely from ozone and other oxidants

• Corrosion, especially from H2SO4 and HNO3 produced by oxidation of acid gases

Photochemical smog in lightly populated industrial areas

• Burning of savanna grasses produces NOxand reactive hydrocarbons that produce smog

14.8 Nuclear Winter

Nuclear winter refers to catastrophic climate change that could follow full-scale nuclear attack

• Huge amounts of sooty particulate matter carried to stratospheric altitudes

• Possible dramatic cooling effects

• Reflection back to space of solar radiation

• Less re-absorption of outgoing infrared radiation

• Less water vapor in the atmosphere to re-absorb outgoing infrared radiation Closest thing to nuclear winter experienced by humankind has come from huge volcanic eruptions

• Massive Tambora, Indonesia, volcanic eruption in 1815

• Followed in 1816 by “year without a summer”

Doomsday Visitors from Space

Impact of asteroids several km in diameter have caused major extinction events in the past due to resulting climate cooling

• Dinosaurs and most other animals extinct 66 million years ago from asteroid impact

• About 90% of all living species extinct 251 million years ago from asteroid impact

• Evidence of a less catastrophic event followed by 1,300 years of a “little ice age” about 10,000 years ago

• Extinction of mastodon, short-face bear, American Lion

Damage to the climate from

• Use of low-quality, high-sulfur, fossil fuels

• Slash-and-burn agriculture

• Deforestation

• Destructive agricultural practices

• Desertification

Three Approaches to the Global Warming Problem

1 Minimization by reducing greenhouse gas emissions

• Alternate energy sources •Energy conservation

• Carbon tax •Reforestation • Grassland restoration

2 Counteracting measures

• Light-reflecting particles to upper atmosphere

• Unlikely to be practical

3 Adaptation (will be required)

• Especially increased efficiency and flexibility in the distribution and use of water

• Water desalination

• Complete restoration and purification of wastewater to drinking water standards

• Aquifer recharge

• Reforestation •Restoration of desert lands

“Tie-in strategy” with measures that are sensible even if global warming turns out to

be less severe than is now expected