Modeling!UV!photo/chemistry!and!clouds!in!the! atmosphere ...
Chapter 18 Chemistry of the Environmentweb.uvic.ca/~chem102/LEE/atmosphere.pdfChapter 18 Chemistry...
Transcript of Chapter 18 Chemistry of the Environmentweb.uvic.ca/~chem102/LEE/atmosphere.pdfChapter 18 Chemistry...
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