The composition of the atmosphere

1. What is the importance of the atmosphere as a life support system?
2. What are the main gases and aerosols of the lower atmosphere?
3. What are the main processes associated with such gases and aerosols which influence life on Earth?

10 reasons why we need it;
1. Gives us oxygen (O
2
) to breathe!
2. Provides Nitrogen (N
2) which are essential to make proteins and DNA
3. CO
2 for photosynthesis
4. Water vapour released as precipitation (Fresh water supply – Hydrological Cycle)
5. Protects us from meteorites – most burn up in thermosphere
6. Absorbs harmful UV radiation from the sun (Ozone Layer – O
3
)
7. Absorbs heat which regulates temperature allowing Earth to be habitable
8. Creates climates by the circulation of gases and particles
9. Circulation of hot and cold air to regulate different climate zones – this is important as different species of flora and fauna require certain climatic conditions to survive
10. …….. Can you think of any others?
quite important?

The Earth’s atmosphere
• A mixture of transparent, odourless gases and aerosols held to Earth by gravity
• Three main types of constituents
– Permanent gases – e.g. nitrogen, oxygen, argon
– Variable gases (greenhouse gases and reactive gas species)
– e.g. carbon dioxide, methane, water vapour
– Aerosols (non-gaseous components of atmosphere) – e.g. sea salt, soot particles, volcanic ash

The composition of the atmosphere
Permanent gases
• Mostly passive in weather-related processes
• N
2
, O
2 and Ar constitute 99.9% of atmosphere
Variable gases and aerosols
• Occur as a result of both natural processes (e.g. carbon cycle) and human activities (e.g. burning coal)
• Affect transmission of radiation through atmosphere
Gas
Nitrogen
% by volume
78.08
Parts per million (ppm)
780 840.0
Symbol
N
2
Importance
Biologically inert
– remains stable. Needed to make proteins and DNA
Oxygen 20.95
209 460.0
O
2
Needed for aerobic respiration
Argon
Carbon dioxide
0.93
0.03
9 340.0
350.0
Ar
CO
2
Inert
Major contribution to the Greenhouse
Effect but essential for
Photosynthesis

Gas % by
Volume
Parts per
Million
(ppm)
Symbol Importance
Neon
Helium
Methane
Krypton
Nitrous Oxide
Hydrogen
Xenon
Ozone
0.0018
0.00052
0.00014
0.00010
0.00005
0.00005
0.000009
0.000007
18.0
5.2
1.4
1.0
0.5
0.5
0.09
0.07
Ne
He
CH
4
Kr
N
2
O
H
2
Xe
O
3
Inert
Inert
Major contribution to the Greenhouse
Effect and Acid Rain
Inert
Major contribution to the Greenhouse
Effect
None
Inert
Stratospheric – filters UV radiation
Tropospheric – health hazard

Sources, sinks and atmospheric residence times
• Source = point from which a gas or aerosol is released into atmosphere
• Sink = point where a gas or aerosol is removed from atmosphere
– by chemical reaction
– by absorption into other components of earth-atmosphere system (e.g. oceans, ice sheets)
– Photosynthesis (Plankton blooms)
• Atmospheric residence time = average period of time a molecule or particle resides within atmosphere
• Determined by relative rates of emission
(from source) and removal (via sink)

The Greenhouse Gases
• Naturally occurring greenhouse gases include carbon dioxide (CO methane (CH
4
), nitrous oxide (N
2
O), ozone (O
2
),
3
) and water vapour (H
2
O)
• Play an important role in regulating atmosphere’s energy budget by absorbing some of outgoing terrestrial radiation
– Increases global mean surface temperature
–
Process = natural greenhouse effect
• Atmospheric concentrations of almost all these gases have increased since 1800s (Industrial Revolution) due to:
–
Combustion of fossil fuels
– Deforestation and agricultural change (livestock/food production increase)
• Anthropogenic greenhouse gases include chlorofluorocarbons, methyl bromide (pesticides), solvents, halons
– Released into atmosphere since 1930s
–
Consequence = enhanced Greenhouse Effect

Changes in atmospheric concentrations of greenhouse gases
Gas Symbol Conc. in
1750*
Conc. In
2011
278 ppmv 390.4 ppmv Carbon dioxide
Methane
Nitrous oxide
CFC-11
CFC-12
CO
2
CH
4
N
2
O
CFC-11
CFC-12
0.72 ppmv 1.80 ppmv
270 ppbv 324 ppbv
0
0
0.24 ppbv
0.53 ppbv ppmv = parts per million by volume ppbv = parts per billion by volume
*
Figures for 1750 based on proxy records ( ice core samples)

Volcanic eruption
Industrial pollution
Carbon dioxide (CO
2
)
• Most important greenhouse gas after water vapour –involved in complex ‘carbon cycle’
• Natural sources
– Respiration
– Decomposition of organic matter
–
Forest fires
– Volcanic eruptions
– Evaporation from oceans
• Anthropogenic sources
– Fossil fuel combustion
–
Deforestation & biomass burning
–
Manufacture of cement
• Sinks (removal from atmosphere)
–
Plant photosynthesis
– Absorption by oceans

The global carbon cycle

Why might CO2 levels increase in
N hem spring and reduce in N hem autumn?
Atmospheric CO
2 concentrations at Mauna Loa, Hawaii – 1958-2006 AD
• world’s longest instrumental record of atmospheric carbon dioxide

Atmospheric CO
2 concentrations
– Ice Core Data, 1850-1950 AD
– Mauna Loa, Hawaii, 1958-present
AD
• Antarctic and Greenland ice sheets preserve a record of past atmospheric composition
• Ice sheets are cored and studied by extracting air bubbles trapped in layers of ice

Methane (CH
4
)
• Primary natural source
– natural anerobic (i.e. oxygendeficient) processes in wetlands & rice paddyfields (c.40% of total)
Paddyfields
Landfill site
• Other natural sources
– Animal digestive processes, cows can produce up to 200 litres a day!
– Termites
– Forest fires
• Anthropogenic sources
– Fossil fuel combustion
–
Biomass burning
– Disposal of refuse in landfill sites
• Sinks
– Oxidized to CO
2 and H
2
O by complex photochemical reactions in troposphere

Fossil fuel combustion
Nitrous oxide (N
2
O)
•
Primary natural source
– Microbial activity (nitrification) in soils and oceans
• Anthropogenic sources
– Fossil fuel combustion
– Biomass burning
– Nylon, nitric acid & nitrogen fertilizer production
• Sinks
– Oxidized to nitrogen oxides
(NO
X
) by photochemical reactions in stratosphere

CFCs
• Chlorofluorocarbons (11,12)
Anthropogenic uses include:
• Used as coolants in refrigeration
• Propellants in aerosols
• Expanders in foam products
Concentrations:
• Distinct increase in concentrations between 1960 -
1990
Impacts:
• Montreal Protocol – led to a huge decline

3
• Produced by a series of complex chemical reactions
• Including pollutants from car exhausts (nitrous oxide and
Hydrocarbons reacting with sunlight)
• More vehicles = greater concentration thus increasing the ‘greenhouse effect’
• Can damage human health and vegetation
• Key ingredient of ‘urban smog’

Greenhouse gases in
Greenland ice cores
• Atmospheric concentrations of four greenhouse gases over the last 1,000 years as preserved within ice cores
• SO
4
, CH
4
, N
2
0 and CO
2
– All show fairly constant or very slowly increasing atmospheric concentrations before c.1800 AD
–
Increasingly rapid rise in atmospheric concentrations since c.1800 AD
–
Close correlation between atmospheric concentrations of greenhouse gases and world population

Nitrogen Neon Nitrous Oxide CFC – 11
Oxygen Helium Hydrogen Helium
Argon Methane Xenon Radon
C0
2
Krypton Ozone Chlorine

1. Look at the cards with gases and aerosols on which may or may not be present in the lower atmosphere
2. In pairs – categorise the gases
3. You must choose your own categories (try to have no more than 5)

• Read the article
• Highlight key facts or information throughout the passage
• Answer the questions
(take note of the mark allocation for each question)