This Week
READING: Chapter 6 of text
Announcements
Problem Set 1 due Fri Oct 12.
Problem Set 2 due Tuesday Oct 16.
Why N2, O2, etc? (Mars and Venus aren’t)
Atmospheric Composition and Biogeochemical Cycles
• The atmosphere as part of the Earth System
• Global Biogeochemical Cycles (Box-Model Heaven)
•N2
•O2
•CO2
Planetary Atmospheres
Planet
Earth
Venus
Mars
Radius (km)
6400
6100
3400
Tavg (K)
250
700
200
Ps (atm)
1
91
6x10-3
N2
0.78
.03
0.027
O2
0.21
0.007
0.0015
CO2
4x10-6
0.96
0.95
Today: Earth System and N Cycle
Oxidizing Atmosphere
Earth System Surface Reservoirs
N2 Cycling—does it do anything?
The Atmosphere: An Oxidizing Medium
Gas phase radical chemistry
Oxidation
Reduced gas
EARTH
SURFACE
Oxidized gas/
aerosol
Cloud Chemistry
Uptake Deposition
Emission
Reduction
Geological or Biological
Surface Reservoirs of the Earth System
Atmosphere
air-sea
exchange
photosynthesis
decay
Biosphere
Hydrosphere
erosion
runoff
decay
assimilation
Soils
Lithosphere
What are the time scales of exchange between the
various reservoirs of the Earth System?
Oxidation States of Nitrogen
N has 5 electrons in valence shell
 9 oxidation states from –3 to +5
Increasing oxidation number (oxidation reactions)
-3
0
+1
+2
+3
+4
+5
NH3
--Ammonia
NH4+
--Ammonium
R1N(R2)R3
--Organic N
N2
N2O
--Nitrous
oxide
NO
--Nitric
oxide
HONO
--Nitrous
acid
NO2--Nitrite
NO2
--Nitrogen
dioxide
HNO3
--Nitric
acid
NO3--Nitrate
Decreasing oxidation number (reduction reactions)
Nitrogen Cycle: Major Processes
ATMOSPHERE
N2
biofixation
orgN
BIOSPHERE
burial
LITHOSPHERE
combustion
lightning
NO
oxidation
HNO3
denitrification
deposition
decay
assimilation
NH3/NH4+
nitrification
NO3weathering
Box Model of the Nitrogen Cycle
Atmospheric N2
3x109
Combustion, biomass
burning, lightning
Agricult. biofixation
80
150
Land biota
1x104
2530
2300
Inventories in Tg N,
1Tg = 1x1012 g
Flows in Tg N yr-1
From Jaffe, 1992;
Jacob text--modified
40
denitrification
Tropospheric Fixed N
(non-N2O) 5
rain
90
150
80
(NH3)
Soil
1x105
40
rain
30
denitribiofixation fication
40
Ocean biota
1x103
1650
1640
Deep ocean
1x106
10
weathering
10 burial
Lithosphere
2x109
40
Questions
1.
If denitrification shuts off, while fixation continues, how long will it
take for atmospheric N2 to be depleted?
2.
How many times does an N atom cycle between atmospheric N2 and
oceanic N before being transferred to the lithosphere?
3.
Combustion and fertilizer use increase the rate of transfer of N2
from the atmosphere to the soil. Assume that these human
activities have been in place and constant for the past 100 years,
and prior to that they were negligible. By how much have humans
increased the nitrogen contents of the total land reservoir (soil +
land biota) and contributed to a global fertilization of the
biosphere?
N2O
Very important byproduct of nitrification/denitrification
• source of reactive nitrogen in stratosphere
• greenhouse gas
IPCC
[2001]
Fast Oxygen Cycle: Atmosphere--Biosphere
• Source of O2: photosynthesis
nCO2 + nH2O g (CH2O)n + nO2
• Sink: respiration/decay
(CH2O)n + nO2 g nCO2 + nH2O
CO2
Photosynthesis
- respiration
O2
orgC
O2 lifetime: ~ 5000 years
litter
orgC
decay
Fast O2 Cycle: Atmosphere-Biosphere
Can photosynthesis/decay control O2 levels?
I.e., if photosynthesis stopped, by how much would O2
decrease due to complete decay of all biomass?
(figure from DJJ)
Slow Oxygen Cycle: Atmosphere-Lithosphere
0.4 Pg O/yr
O2 in atmosphere: 1.2x106 Pg O
weathering
O2
OCEAN
Photosynthesis
decay
CO2
Fe2O3runoff
H2SO4
FeS2
CO2
orgC
CONTINENT
orgC
Uplift
burial
CO2
orgC: 1x107 Pg C
FeS2: 5x106 Pg S
microbes
SEDIMENTS
O2
orgC
FeS2
Compression
subduction
Question
1.
Does atmospheric oxygen have a seasonal cycle? If so,
when would it maximize?
2.
Do you think humans are increasing or decreasing
atmospheric O2, why?
Recent Growth in Atmospheric CO2
Notice:
• atmospheric increase is ~50% of fossil fuel emissions
• large inter-annual variability
Where is rest of
CO2 going?
IPCC
2001
Arrows indicate
El Nino events
Uptake of CO2 by Oceans
ATMOSPHERE
CO2(g)
KH = 3x10-2 M atm-1
OCEAN
CO2.H2O
K1 = 9x10-7 M CO2.H2O
K2 = 7x10-10 M
HCO3-
HCO3- + H+
CO32- + H+
CO2.H2O
Net uptake:
CO2(g) + CO32-
2HCO3--
HCO3-
CO32-
Equilibrium Partitioning of CO2
Want to know fraction of atmospheric and
oceanic CO2 that is in atmosphere at
equilibrium
atm
nCO

2
PCO2
Psurf
nair
Vocean = 1.4x1018 m3
ocean
nCO
2
F
atm
nCO
2
atm
ocean
nCO

n
CO2
2


K
K
K
H
1
1 2
1 

 Vocean KCO
P

CO
2
2
2
  H (aq )   H   
  ( aq )  
 
PCO2 = 375 x 10-6 atm
pHocean = 8.2
Fcalc = 0.03  97% of CO2 resides in the oceans
This is definitely wrong! It greatly underestimates the fraction
of CO2 that resides in atmosphere (Ftrue~ 70%)…Why? What’s
wrong with this estimate?
CO2 Uptake Limited by Ocean Mixing
Inventories in 1015 m3 water
Flows in 1015 m3 yr-1
Uptake by oceanic mixed layer only (VOC= 3.6x1016 m3) would give f =
94% of added CO2 remains in atmosphere…now estimate is too small…?!
CO2 Uptake also Limited By Ocean Alkalinity
Equilibrium calculation
2.1
2.0
1.9
[CO2.H2O]+[HCO3-]
+[CO32-], 10-3M
1.8
1.6
1.4
4
3
[HCO3-],
10-3M
[CO32-],
10-4 M
8.6
8.2
100
supply of CO32To increase supply of CO32-, CaCO3 in
sediments/deep ocean must dissolve:
CaCO3  Ca2+ + CO32-
2
8.4
auptake of CO2 is limited by the existing
Ocean pH
200 300 400 500
pCO2 , ppm
…which takes place over a time scale of
thousands of years
Questions
1.
Marine biota take in CO2 during photosynthesis to make OrgC. About
10% of this OrgC sinks to the ocean bottom (fecal matter, dead tissue,
etc), and is buried into the sediments. How does this process affect the
equilibrium partitioning of CO2 between the atmosphere and ocean?
2.
Does the growth of corals/shells (Ca2+ + CO32-  CaCO3) cause
atmospheric CO2 to increase or decrease?
3.
A consequence of global warming is melting of the polar ice caps. This
melting decreases deep water formation. Why? Would this effect reduce
or amplify warming caused by anthropogenic CO2 emissions?
Evidence For Land Uptake of CO2
Trends in O2,
1990-2000
Atmosphere--Terrestrial Biosphere C Cycle
790
From DJJ
Inventories in PgC
Flows in PgC yr-1
2000
Time scales are short: ~ 12 yrs w.r.t uptake; ~ 160 yrs w.r.t soil emission
Global Preindustrial Carbon Cycle
Inventories in PgC
Flows in PgC yr-1
(from DJJ)
When we burn fossil fuels, we take C
from the sediments and put it into the
atmosphere as CO2. How long-term is
this perturbation to the carbon cycle?
A Long View of Fossil Fuel Perturbation
It takes a long time for fossil fuel CO2
to completely leave the atmosphere.
Future Atmospheric CO2
Using estimates about future
population growth, energy
needs, etc. project future
CO2 emissions.
Using a climate model with a
carbon cycle, predict CO2
based on projected emissions
and sinks.
2000
2100
2200
2300
CO2 double pre-industrial
value by ~ 2150
Stabilization Scenarios
To make CO2 growth
rate 0, sources must
balance sinks
These calculations
show what our
emissions can be for
different CO2 levels.
Note that sinks are
predicted to get
smaller.
2000
2100
2200
2300
To stop CO2 increase
now, we’d have to cut
our emissions by 50%
Projected Trends in CO2 Sinks
IPCC [2001]
Questions
1. The Kyoto Protocol (heard of it?) aimed to cut emissions
to be 6% lower than the 1990 values. Emissions would be
only slightly less than 7 GtC/yr. Why was this even
considered potentially useful?
2. To keep CO2 constant at its current value 380 ppm, we’d
have to cut emissions by 50% to 4 GtC/yr. This would
match the current sink rate. After a few hundred years,
if we didn’t want CO2 to start increasing again, we’d have
to cut our emissions even lower. Why might this be?
3. Fossil fuel abundance is estimated at ~ 5000 GtC. If we
burn this much eventually, will the terrestrial biosphere
be of much significance as a sink/storage of this carbon?