Chapter 18 Chemistry of the Environment

• Earth‟s Atmosphere

• Outer Regions of the Atmosphere

• Ozone in the Upper Atmosphere

• Chemistry of the Troposphere

• You will need to read the Course Pack to compliment the Textbook it is

available at the UVic Bookstore and includes,

• Ozone hole over Antarctica Acid Rain

• Global Warming and The Greenhouse Effect

The temperature of the atmosphere varies in a complex manner as altitude

increases

18.1 Earth’s Atmosphere

The atmosphere is divided

into 4 regions

At the boundary between

regions (suffix -

pause), temperature

extremes are observed

Gases mix slowly at these

boundaries

The pressure of the atmosphere decreases in a regular manner as altitude

increases

The atmospheric pressure (100kPa) at Sea-

level, decreases to 0.3Pa at 100km.

The sun bombards us with radiation and energetic particles

The atmosphere is our first line of defense!

The aurora borealis (Northern

Lights) is caused by collisions

of high speed electrons and

protons from the Sun with air

molecules

The charged particles are

channeled towards the polar

regions by Earth‟s magnetic

field

Because of the great variation in atmospheric conditions, the composition of

gases in the atmosphere is not uniform.

Composition of the Atmosphere

Parts per million (ppm) are commonly used when referring to the trace

constituents

For substances in aqueous solution 1ppm is equivalent to 1 mg in 1 kg

Parts per million

Sample Exercise 18.1

PV = n RT so Volume is proportional to n

So 1 mole in 1 million moles of gas = 1 ppm

The Sun emits radiant energy over a wide range of wavelengths

18.2 Outer Regions of the Atmosphere

As the radiation and high

energy particles pass

through the outer

atmosphere it causes two

types of chemical change:

Photons of sufficient energy are absorbed by a molecule causing Homolytic

splitting

Photodissociation

99% of the oxygen molecules in the upper atmosphere are dissociated

Very few Nitrogen molecules are dissociated

Nitrogen has a very high bond enthalpy and does not readily absorb

photons

Molecules absorb radiation (high energy photons) causing a valence electron

to be lost, (ejected)

Photoionization

Ozone absorbs much of the radiation in the 240 -310nm wavelength range

(UV)

18.3 Ozone in the Upper Atmosphere

The rate at which ozone forms depends on the factors that vary in opposite

directions as the altitude increases

The cycle of ozone formation and dissociation is responsible for the

temperature peak observed at the stratosphere boundary

Ozone cycle

•Used as coolant gases and propellants

•Un-reactive and insoluble so they are able to survive extended periods in

the atmosphere

Depletion of the Ozone Layer

It has been known for 25 years that the ozone layer can be depleted due to

reaction with chlorine from Chlorofluorocarbons (CFCs)

The C-Cl bond is easily broken as the molecule diffuses to higher altitudes and

absorbs photons with wavelengths between 190 and 225nm

The dark area represents

the area of lowest O3

concentration - The „Hole‟

centered over Antarctica

The Chlorine monoxide generated is photo-dissociated back to Cl(g) and

O(g), this generates more Cl(g) that can react with more ozone!

Overall these 3 equations can be combined (eliminate the like species) to give

the following reaction:

In this catalytic mechanism, 1Cl destroys 50 O3 on a spring day over Antarctica

* Course Pack!

99% of the Troposphere is made up of N2 and O2, however minor constituent

gases can have a profound effect on the Troposphere

18.4 Chemistry of the Troposphere

Many of the minor constituents are present naturally

However Human activity, primarily combustion, can increase the

concentration of these constituents causing adverse effects.

Oxides of Sulfur and Nitrogen cause acidification of rain

Acid Rain

Sulfur Compounds

Sulfur Compounds occur naturally due to decay of organic matter and volcanic

activity, however Human activity accounts for an approximate three fold

increase in Sulfur compounds, primarily sulfur dioxide (SO2)

Nitrogen Compounds

Nitrogen Compounds, primarily Nitrogen monoxide (NO), also occur naturally,

being formed due to lightening. However NO(g) can also be formed during

hydrocarbon combustion (transportation), and in Industrial processes involving

fossil fuel combustion.

The high temperatures associated with these processes results in NO(g)

Rainwater is naturally slightly acidic, due to the formation of carbonic acid

(H2CO3) from carbon dioxide and water

Effects of Acid Rain

The pH of freshwater

sites around the US

Don‟t think the West

coast is OK: Rain fall

and fog near LA has

been measured to have

a pH less than 2 !

Acid Rain causes the acidification of freshwater

Acid Rains attacks metals and stone (calcium carbonate)

This stone statue shows evidence of

acid rain damage

Acid rain also damages forests

acid rain causes forest soils to lose

valuable nutrients such as calcium and

magnesium. It also increases the

concentration of aluminum in the

soil, which interferes with the uptake of

nutrients by the trees

Sulfur

Powdered limestone (CaCO3) can be used to remove SO2 from the gases

formed by combustion of coal and oil

Carbon Monoxide

CO is relatively unreactive and has little

effect on vegetation and materials.

Carbon monoxide (CO) is formed by the

incomplete combustion carbon

compounds and is present in unpolluted

air at levels of 0.5ppm.

Photochemical Smog

What we recognize as smog, the brownish haze that hangs above

major urban areas is largely NO2, nitrogen dioxide

NO2(g) is formed by oxidation of the NO(g)

produced by the reaction of nitrogen and oxygen

at the very high temperatures in combustion

engines.

Nitrogen oxides and ozone are just some of the components of photochemical

smog

Catalytic Convertors use precious metals Rh and Pt to catalyse the conversion

of NO and CO to CO2 and N2

Global warming and the Greenhouse effect

The greenhouse effect describes the trapping of infra-red (IR) radiation

(thermal energy) emitted from the earth, by gases, primarily carbon dioxide

and water vapor in the atmosphere. This trapped heat is re-emitted in all

directions, some towards the earth. The effect gives the earth thermal

consistency.

IR is only absorbed by molecules with polar bonds

All molecules vibrate with a specific energy, the

vibrations of molecules possessing polar bonds

causes the dipole moment to change during the

course of vibration

The excited molecules lose the excess energy through

collisions with other molecules

Although the level of water vapor in our atmosphere has

not changed appreciably the levels of other greenhouse

gases, especially CO2 have increased since the 1900‟s.

Greenhouse gases include all molecules with polar

bonds, CH4, NOx, CFCs etc

Carbon Dioxide

Carbon dioxide is naturally present in the atmosphere, the carbon

cycle, describes the mechanism of how carbon is transferred to and from the

atmosphere. Details in the course pack

Increase in Greenhouse gas emissions disrupt the earth‟s delicate thermal

balance and causes “Global Warming”

US sources of CO2 electricity production

industry

transport

residential heating

Global Warming

Increases in greenhouse gases have caused the earths surface temperature to

increase abnormally over the past 150 years

Signs of global warming

This graph (from Wiki) plots the

temperature change during this time period

The small increase may seem

insignificant but has had very

noticeable effect on global weather

patterns with related adverse

consequences.

9 out of the 10 hottest years recorded

occurred during 1994 to 2004

*More extreme weather

Total annual rainfall increased in the last century, however, typically dry areas

north and south of the equator became even drier, while cooler climates

became wetter, hurricanes and storms have become more frequent

Shorter Winters

Over the last 3 decades winters have become 11 days shorter (on average) and

warmer, there are fewer “frost” days

Melting Ice cover

Antarctica OK? Contains 90%

of the worlds ice.

Glaciers, Arctic and Greenland

all in danger, changing fragile

eco-systems

Warmer BC winters are failing to control

the population of the mountain pine beetle

Mosquito borne diseases such as malaria,

dengue fever and West Nile virus are becoming

prevalent in areas where they were previously

not viable

Warmer oceans and rising sea levels

A warmer ocean is the dominant factor

in rising sea levels. Water is most

dense at 4oC. Sea levels are predicted

to rise between 15 and 95cm over the

next 100 years.

The Pacific island of Tuvalu is already

falling victim to rising sea levels

Warmer oceans are also killing coral and threatening sea-life

A quarter of the worlds coral has disappeared

and another 30% is expected to be gone in 30

years

Warmer water contains less dissolved oxygen

stressing fish populations and adversely

effecting marine life