Week 8, Lesson 1
Chapter 18 – The Atmosphere
The Atmosphere Supports Life
• The small fraction of the Earth’s environment that
supports life is known as the biosphere.
• The atmosphere consists of a mixture of gases
extending about 100km above the Earth’s surface.
• The troposphere is the layer closest to the Earth.
• The interactions that occur in the upper layers of the
atmosphere are mostly influenced by the high-energy
radiation from the Sun.
• The chemistry in the troposphere, however, is
significantly affected by human activity on Earth such
as deforestation and the release of millions of tonnes
of waste gases from industrial processes.
• The various layers of the atmosphere have very specific
roles in maintaining life in our environment.
The Atmosphere & Radiation
• High-energy UV radiation from the Sun is damaging to
living organisms.
• Ozone gas can absorb some radiation and so acts as a
protective filter.
• In the stratosphere, within a band 10-30km above the
Earth’s surface, ozone occurs at concentrations
significantly higher than in the lower regions of the
atmosphere.
• This region is known as the ozone layer.
• Ultraviolet radiation is classified as UV-A, UV-B and UVC.
• Ozone absorbs all UV-C, most UV-B but little UV-A.
The Atmosphere & Radiation cont…
• In the ozone layer, high energy UV radiation from
the sun causes the double bond of O2 to break to
form highly reactive oxygen atoms.
• Some of the reactive oxygen atoms can then
combine with other oxygen molecules to form
ozone molecules.
• Overall, with the effect of human activity, the rate
of formation of ozone is about the same as its
rate of depletion, so the concentration of ozone
in the stratosphere remains fairly constant.
The Atmosphere & Climate
• The Earth has maintained its average
temperature over time because of the balance
between the radiation received for the Sun
and that reflected back into space.
• Radiation of shorter wavelength – visible and
UV radiation – can pass through the
atmosphere from the Sun to the Earth’s
surface.
• The Earth then radiates back lower energy,
longer wavelength infrared radiation.
The Atmosphere & Climate cont…
• Gases in the troposphere, such as CO2, H2O,
and CH4, absorb some of the reflected infrared
radiation before it can reach space and
reradiate some of it back towards Earth as
heat.
• This process is known as the greenhouse
effect because it warms the Earth’s surface in
the same way the Sun’s energy is used to heat
a greenhouse.
The Atmosphere & the Water Cycle
• The Earth’s weather patterns are mainly
determined by the amount of water vapour in
the air.
• If it were not for the evaporation of water and
its constant cycling as rain and snow back over
the land masses of the world, we would have
no fresh water supply to sustain life of Earth.
The Atmosphere & Essential Gases
• Nitrogen and oxygen are the two most
abundant gases in the atmosphere, making up
99% of clean dry air at sea level.
• Both of these as well as much less abundant
CO2, are essential in maintaining life on Earth.
Oxygen and Carbon Dioxide
• Oxygen which makes up almost 21% of the
atmosphere is required for respiration in
plants and animals.
• In respiration, glucose reacts with oxygen to
produce carbon dioxide and water.
• Respiration is the major energy producing
process of most living things.
Oxygen and Carbon Dioxide cont…
• Carbon dioxide makes up only 0.035%, by
volume, of the atmosphere.
• However, this small amount provides, either
directly or indirectly, all food used by plants
and animals.
• In photosynthesis, plants combine carbon
dioxide and water, in the presence of green
pigment chlorophyll and sunlight to produce
glucose and oxygen.
Oxygen and Carbon Dioxide cont…
• Through the process of photosynthesis and
respiration, both oxygen and carbon dioxide
are cycled in nature.
• The process, called carbon-oxygen cycle.
• Photosynthesis in plants is the most significant
process that consumes carbon dioxide, which
is then released into the atmosphere during
respiration in plants and animals.
Carbon-Oxygen Cycle
Oxygen and Carbon Dioxide cont…
• The cycling of both oxygen and carbon dioxide
in the processes of photosynthesis and
respiration is quite rapid.
• Carbon dioxide is also returned to the
atmosphere when an organism’s wastes are
decomposed or when it dies.
• This is a much slower recycling process.
Oxygen and Carbon Dioxide cont…
• Other processes that contribute to the
carbon-oxygen cycle include:
– Volcanic action and and the combustion of fossil
fuels, which add carbon dioxide to the
atmosphere.
– The slow production of coal, oil and natural gas
over a period of several million years from the
remains of dead organisms, and the formation of
calcium carbonate from dead coral and the
skeletons of marine animals in the sea. This
process removes carbon from the cycle.
Oxygen and Carbon Dioxide cont…
• The amount of carbon present at any
particular stage of this cycle depends on the
rate at which it enters and leaves that stage.
• At present, atmospheric carbon dioxide levels
are increasing dur to various factors including
the rate at which we are burning fossil fuels.
Nitrogen
• All living creatures need a source of nitrogen in order
to produce essential large biomolecules such as
proteins and DNA.
• Most living things cannot use it in nitrogen gas form.
• Some microorganisms break down the triple covalent
bond within the nitrogen molecule to form simple
nitrogen containing ions such as ammonium or
nitrate.
• These can be used by plants to make more complex
nitrogen containing molecules.
• Animals rely on these complex molecules as their
source of nitrogen.
Nitrogen cont…
• A small amount of nitrogen fixation occurs when lightening
discharges convert nitrogen gas to nitrogen (II) oxide gas (NO).
• This is released by volcanic action and is present in industrial
and automobile emissions.
• It reacts with oxygen to form nitrogen(IV) oxide gas, or
nitrogen dioxide
• Nitrogen dioxide reacts with rain water to form a mixture of
nitrous acid and nitric acid.
• The rain becomes as mildly acidic fertiliser.
• Nitrogen is returned to the atmosphere when bacteria and
other organisms in the soil cause organic matter to decay.
• Some stages of the nitrogen cycle have been significantly
affected by industrial and agricultural activities.
Nitrogen-Oxygen Cycle
Human Activities and the
Atmosphere
• The rate of change in composition of the
atmosphere has increased dramatically in the
past 200 years as a result of human activity.
• Contributing factors include:
– Clearing of forests to make way for agriculture and so reducing the
amount of carbon dioxide taken up by photosynthesis.
– Burning of coal to produce electricity, which converts sulfur on coal to
sulfur dioxide, which is released into the atmosphere.
– Using fossil fuels in transport, which results in significant emissions of
carbon and nitrogen oxides and unburnt hydrocarbons.
– Using propellants in aerosols and foaming agents, which interfere with
the balance of atmospheric gases.