QUESTIONS
1. Is the general atmospheric circulation stronger (i.e., are the
winds faster) in the winter or in the summer hemisphere?
2. Concentrations of CO2, krypton-85, and other gases emitted mainly in the
northern hemisphere DECREASE with altitude in the northern hemisphere
but INCREASE with altitude in the southern hemisphere. Explain.
3. Kuwaiti oil fires during the Persian Gulf war produced large
clouds of soot a few km above the Earth's surface. Soot absorbs solar
radiation. How would such soot clouds affect atmospheric
stability?
4. Where in the world would you expect the atmosphere to be most
unstable?
5. Consider a pollutant emitted from the surface at a constant rate. Is its
surface air concentration higher in the day or at night? In summer or winter?
CHAPTER 6: GEOCHEMICAL CYCLES
THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS
•
Most abundant elements: oxygen (in solid earth!), iron (core),
silicon (mantle), hydrogen (oceans), nitrogen, carbon, sulfur…
•
The elemental composition of the Earth has remained essentially
unchanged over its 4.5 Gyr history
– Extraterrestrial inputs (e.g., from meteorites, cometary
material) have been relatively unimportant
– Escape to space has been restricted by gravity
•
Biogeochemical cycling of these elements between the different
reservoirs of the Earth system determines the composition of the
Earth’s atmosphere and oceans, and the evolution of life
BIOGEOCHEMICAL CYCLING OF ELEMENTS:
examples of major processes
Physical exchange, redox chemistry, biochemistry are involved
Surface
reservoirs
HISTORY OF EARTH’S ATMOSPHERE
N2
CO2
H2O
O2
oceans CO2
form
dissolves
Outgassing
4.5 Gy
B.P
Life forms
in oceans
4 Gy
B.P.
O2 reaches
current levels;
life invades
continents
Onset of
photosynthesis
3.5 Gy
B.P.
0.4 Gy
B.P.
present
COMPARING THE ATMOSPHERES
OF EARTH, VENUS, AND MARS
Venus
Earth
Mars
6100
6400
3400
91
1
0.007
0.96
3x10-4
0.95
N2 (mol/mol)
3.4x10-2
0.78
2.7x10-2
O2 (mol/mol)
6.9x10-5
0.21
1.3x10-3
3x10-3
1x10-2
3x10-4
Radius (km)
Surface pressure (atm)
CO2 (mol/mol)
H2O (atm, mol/mol)
EVOLUTION OF O2 AND O3 IN EARTH’S ATMOSPHERE
Atmospheric
Composition
(average)
1 ppm= 1x10-6
red = increased by
human activity
¶
Gas
Mole fraction
Nitrogen (N2)
0.78
Oxygen (O2)
0.21
Water (H2O)
0.04 to < 5x10-3; 4x10-6 -strat
Argon (Ar)
0.0093
Carbon Dioxide (CO2)
370x10-6 (date: 2000)
Neon (Ne)
18.2x10-6
Ozone (O3) ¶
0.02x10-6 to 10x10 –6
Helium (He)
5.2x10-6
Methane (CH4)
1.7x10-6
Krypton (Kr)
1.1x10-6
Hydrogen (H2)
0.55x10-6
Nitrous Oxide (N2O)
0.32x10-6
Carbon Monoxide (CO)
0.03x10-6 to 0.3x10-6
Chlorofluorocarbons
3.0x10-9
Carbonyl Sulfide (COS)
0.1x10-9
Ozone has increased in the troposphere, but decreased in the stratosphere.
NOAA GREENHOUSE GAS RECORD
OXIDATION STATES OF NITROGEN
N has 5 electrons in valence shell a9 oxidation states from –3 to +5
Increasing oxidation number (oxidation reactions)
-3
0
+1
+2
+3
NH3
Ammonia
NH4+
Ammonium
R1N(R2)R3
Organic N
N2
N2O
Nitrous
oxide
NO
Nitric
oxide
HONO
NO2
Nitrous acid Nitrogen
dioxide
NO2Nitrite
free radical
+4
+5
HNO3
Nitric acid
NO3Nitrate
N2O5
Nitrogen
free radical pentoxide
Decreasing oxidation number (reduction reactions)
THE NITROGEN CYCLE: MAJOR PROCESSES
ATMOSPHERE
N2
combustion
lightning
free radical
NO
oxidation
HNO3
biofixation
orgN
BIOSPHERE
burial
denitrification
deposition
decay
assimilation
NH3/NH4+
nitrification
NO3weathering
LITHOSPHERE
"fixed" or "odd" N is less stable globally=> N2
BOX MODEL OF THE NITROGEN CYCLE
Inventories in Tg N
Flows in Tg N yr-1
N2O: LOW-YIELD PRODUCT OF BACTERIAL
NITRIFICATION AND DENITRIFICATION
Important as
• source of NOx radicals in stratosphere
• greenhouse gas
IPCC
[2007]
PRESENT-DAY GLOBAL BUDGET
OF ATMOSPHERIC N2O
SOURCES (Tg N yr-1)
Natural
18 (7 – 37)
10 (5 – 16)
Ocean
3 (1 - 5)
Tropical soils
4 (3 – 6)
Temperate soils
2 (1 – 4)
Anthropogenic
8 (2 – 21)
Agricultural soils
4 (1 – 15)
Livestock
2 (1 – 3)
Industrial
1 (1 – 2)
SINK (Tg N yr-1)
Photolysis and oxidation in
stratosphere
12 (9 – 16)
ACCUMULATION (Tg N yr-1)
4 (3 – 5)
IPCC
[2001]
Although a closed budget can be constructed, uncertainties in sources are large!
(N2O atm mass = 5.13 1018 kg x 3.25 10-7 x28/29 = 1575 Tg )
BOX MODEL OF THE N2O CYCLE
12
1.57 103 N2O
6
Inventories in Tg N
Flows in Tg N yr-1
8
3
OXIDATION STATES OF SULFUR
S has 6 electrons in valence shell a
oxidation states from –2 to +6
Increasing oxidation number (oxidation reactions)
-2
+4
+6
FeS2
Pyrite
H2S
Hydrogen sulfide
(CH3)2S
Dimethylsulfide
(DMS)
CS2
Carbon disulfide
COS
Carbonyl sulfide
SO2
Sulfur dioxide
H2SO4
Sulfuric acid
SO42Sulfate
Decreasing oxidation number (reduction reactions)
THE GLOBAL SULFUR CYCLE
(sources in Tg S y-1)
SO2
H2S
10
SO42SO2
CS2
60
deposition
runoff
COS
(CH3)2S
ATMOSPHERE
2.8x1012 g S
t = 1 week
20
plankton
volcanoes
uplift
coal combustion
oil refining
smelters
SO42-
OCEAN
1.3x1021 g S
t = 107 years
microbes
vents
FeS2
SEDIMENTS
7x1021 g S
t = 108 years
FAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHERE
• Source of O2: photosynthesis
nCO2 + nH2O g (CH2O)n + nO2
• Sink: respiration/decay
(CH2O)n + nO2 g nCO2 + nH2O
O2 lifetime: 5000 years
CO2
Photosynthesis
less respiration
O2
orgC
litter
orgC
decay
…however, abundance of organic carbon in
biosphere/soil/ocean reservoirs is too small to control
atmospheric O2 levels
SLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERE
O2 lifetime: 3 million years
O2: 1.2x106 Pg O
O2
OCEAN
Photosynthesis
decay
CO2
Fe2O3runoff weathering
H2SO4
FeS2 orgC
CONTINENT
orgC
Uplift
burial
CO2
orgC: 1x107 Pg C
FeS2: 5x106 Pg S
microbes
SEDIMENTS
CO2
O2
orgC
FeS2
Compression
subduction
ATMOSPHERIC CARBON
Unreactive Carbon:
CO2: GHG (more to follow…) (tATM~20 yrs)
Reactive Carbon:
CH4: GHG, important in oxidant chemistry (tATM~9 yrs)
CO: important in oxidant chemistry (later…) (tATM~2 mos)
NMHCs: source of CO, oxidant chemistry (tATM~sec-mos)
Black Carbon: radiatively important (tATM~days)
CH4 (C= -IV)
CO (C= +II)
Oxidation
CO2 (C= +IV)
SOURCES OF ATMOSPHERIC METHANE
WETLANDS
180
BIOMASS
BURNING ANIMALS
90
20
LANDFILLS
50
GLOBAL METHANE
SOURCES (Tg CH4 yr-1)
TERMITES
25
RICE
85
GAS
60
COAL
40
SINKS OF ATMOSPHERIC METHANE
I. Transport to the Stratosphere
Only a few percent, rapidly destroyed  BUT the most
important source of water vapour in the dry stratosphere
II. Oxidation
O2
CH4 + OH  CH3O2CO + other products
Lifetime ~ 9 years
ATMOSPHERIC CH4: PAST TRENDS, FUTURE PREDICTIONS
Variations of CH4 Concentration (ppbv)
Over the Past 1000 years
[Etheridge et al., 1998]
IPCC [2001] Projections of Future
CH4 Emissions (Tg CH4) to 2050
Scenarios
900
1600
1400
800
1200
700
1000
800
1000
A1B
A1T
A1F1
A2
B1
B2
IS92a
600
1500
Year
2000
2000
2020
Year
2040
Recent slowdown in
growth rate
Pinatubo
Fires
Tropical
wetlands
Low T / precip