How ocean CO2 fluxes are estimatedmeasured

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How ocean CO2 fluxes are estimated/measured
Colm Sweeney csweeney_at_ldeo.columbia.edu Princet
on University and Lamont-Doherty Earth Observatory
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Outline

• Concept
• -Ocean carbon chemistry primer
• -The air-sea flux

• II. The air-sea flux measurement
• Covariance
• Gradient technique

• III. Surface measurements
• Measurements of surface pCO2
• Methods for interpolation

• IV. Improving our estimates of air-sea fluxes
• Time-space distribution of pCO2
• Parameterization of gas transfer velocity

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Ocean Carbon Chemistry Primer
CO2(gas)
CO2 H2O ?? H2CO3 H3CO2 ?? H HCO3- HCO3-
?? H CO32-
Carbonic acid
Bicarbonate
Carbonate
TCO2
CO2 CO32- ??2 HCO3-
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Ocean Carbon Chemistry Primer
CO2(gas)
280 matm
560 matm
CO2 H2O ?? H2CO3 H3CO2 ?? H HCO3- HCO3-
?? H CO32-
8 mmol kg-1
15 mmol kg-1
Carbonic acid
1617 mmol kg-1
1850 mmol kg-1
Bicarbonate
268 mmol kg-1
176 mmol kg-1
Carbonate
1893 mmol kg-1
2040 mmol kg-1
TCO2
CO2 CO32- ??2 HCO3-
100 DpCO2? 8 DTCO2
Taken from Feely et al. (2001)
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Concept
Keeling et al.
Eks(pCO2w)
River input 0.6 PgC yr-1 DpCO22 matm
Iks(pCO2a)
k f(u) Sc-n u frictional velocity s
solubility Sc schmit number (v/D) n
0.4 0.67 (high slopelow slope)
Net air-sea gas flux Fgasks(pCO2w-pCO2a)
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Bomb 14C
Semi-infinite Half space
Broecker and Peng (1994)
Transfer velocity kav 22 cm/hr u 7.4 m/s
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Early estimates air-sea CO2 exchange
n14N?14C
Natural 14CO2/12CO2 in gassing
Solve for I
14CO2/12CO2 out gassing
Decay 14C ? 14N e-
Pre-industrial assumption 14CO2 in 14CO2 out
Decay
0.061 mol m-2 yr-1 uatm-1 21.4 cm hr-1
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Early estimates air-sea CO2 exchange
n14N?14C
Natural 14CO2/12CO2 in gassing
Outgassing of Radon
Solve for I
Rn
14CO2/12CO2 out gassing
222Rn ? 218Po 4He
226Raaq ? 222Rngas 4He
Decay 14C ? 14N e-
Rnmixed layer lRn Rnno loss
lRn lgas exchange

Pre-industrial assumption 14CO2 in 14CO2 out
Decay
0.062 mol m-2 yr-1 uatm-1 21.9 cm hr-1
0.061 mol m-2 yr-1 uatm-1 21.4 cm hr-1
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Flux Measurements in the Atmosphere
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Direct covariance technique
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Covariance flux of H2O and CO2Fair-sealtc'w'gt
3-D Sonic Anemometers
H2O/CO2 samples
IR Detector (Sample)
Res
Pump
IR Detector (Motion Detection)
Std
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Gradient Flux Technique
Frictional velocity
McGillis et al. (2001)
Gradient Function -empirically determined based
on Monin Obukhov (MO) similarity theory
Measured Gradient (3-13m)
Covariance intake
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GasEx-98 Comparison-estimates of transfer
velocity
GasEx-2001
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Estimates of gas transfer velocity
Rayleigh Distribution For ocean wind speeds P(u)
Bomb 14C kav22 cm /hr
k- short term
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Estimates of CO2 fluxes from measurements of DpCO2
1. Shipboard measurements of atmospheric and
surface ocean pCO2
2. The ocean pCO2 climatology
3. Flux calculations using the climatology
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Shipboard measurements of atmospheric and surface
ocean pCO2
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Equilibration of air sample
Re-circulation
IR Detector
Air flow
Drain
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Takahashi pCO2 database
1,183,000 measurements - Since 1968
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Monthly distribution of pCO2
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The climatology

• Exclude all El-Nino years.
• dramatic change in annual fluxes have been
  observed
• El-Nino periods based on SIOlt-1.5 and SST changes.

• 2. Normalize pCO2 single reference year (1995)
• In warm waters (lat. lt45) DpCO2 remains constant

• 3. Interpolate data on to 4ox 5ox 365 day grid
• finite differencing algorithm is used with a 2-D
  transport model from Toggwieler et al. (1989) to
  propagate the influence of observed data at one
  day time steps. Distribution is solved iteratively

pCO2
Time
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The pCO2 Climatology
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Global CO2 flux
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Test of interpolation
DpCO2
3.5
DT 0.28 C 0.8 PgC
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Sampling resolution
250K samples (Takahashi 97)
500K samples (Takahashi 99)
940K samples (Takahashi 02)
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Change in fluxes with increases in samples
PgC yr-1
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Gas Transfer Velocity and Fluxes
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Estimates using different gas exchange-wind speed
relationships
Feely et al., 2001
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Long vs. short term winds
PgC yr-1
41 Year average Monthly
NCEP(1995)
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Sources of uncertainty

• Seasonal distribution of pCO2 (0.8 PgC)
• Estimate of skin temperature (-0.6 to 0.1 PgC)
• Estimates of the transfer velocity (20-40)
• Estimates of windspeed (2 m/s)

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How can we do better?
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Factors influencing CO2 flux estimates
Wind
Wind Waves
Biology
Surface Film
k
DpCO2
SST
Bubbles
Near Surface Turbulence
Transport
Air-Sea CO2Flux
Bock et al. (1999)
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Better spatial-temporal coverage
1. Deployment of ships and moorings
2. Predictions using synoptic data sets
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Space and time coverage of ocean carbon observing
networks
time
centuries
Repeat Trans-basin Sections
decadal
Shipboard Time-Series
Inter-annual
Moored Time-Series
VOS surface pCO2
Remote sensing
seasonal
daily
Process Studies
hourly
space
Ocean Basin
1 m2
1 km2
Globe
Regional (106 km2)
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Factors influencing surface water pCO2
Variable Range Relation Effect
Temperature (C) -2 30 (?ln pCO2/?T)
0.0423oC-1 400
TCO2(mmol kg-1) 1900-2200 (?ln pCO2/?Tln TCO2)
10 400
Salinity(mmol kg-1) 33.5-37 (?ln pCO2/?Tln S)
0.94 10
Alkalinity(mmol kg-1) 2150-2350 (?ln pCO2/?Tln
TALK) -9.4 -200
Alkalinity and salinity are proportional and can
be accounted for
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Temperature correlations
Spring
Winter
Summer
Fall
Stephens et al., 1996
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Prediction of DpCO2
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Bermuda
100 uatm
4.23 C-1
9.5 C
160 uatm Due to temperature

Courtesy of Nick Bates
TCO233 mmol/kg
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Temp vs. Biology
Takahashi et al. (2002)
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CO2H2O ?? O2CH2O
Upwelling
Palmer Sta.
Temp. (C)
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MODIS
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Predicting pCO2
SST
NPP
Zmix
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Estimates of gas transfer velocity
Wind
Wind Waves
Surface Film
k
Bubbles
Near Surface Turbulence
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Gas exchange vs. rain rate (MP distribution)
Ho et al. 1997
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Summary

• I. The air-sea flux measurement
• Provide true short-term (1 hr) measurements of
  flux which can be associated with wind speeds
  measured on that same time scale.
• Are limited to areas of high DpCO2

• II. Estimates using surface DpCO2
• Provide us with estimates of fluxes on a monthly
  basis based climatology adjusted for a single
  non-El Nino year
• Errors in flux estimates occur due to lack of
  direct pCO2, wind speed and understanding of the
  gas transfer velocity

• III. Improving our estimates of air-sea fluxes
• Time-space distribution of pCO2
• Deployment of ships and buoys
• Use of satellite measurements to calculate
  change in TCO2
• Parameterization of gas transfer velocity
• micro-scale measurements

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Inventory methods

• Estimates of integrated change in carbon
  inventory
• 1) Time series approach
• Comparing measurements made between two time
  intervals
• Compare residuals of multiple parameter
  regressions using T, S, TALK and nutrients
• 2) C Method
• Estimate of the total inventory of anthropogenic
  carbon in any given region

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Hydrographic samplisg stations
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C Method (Gruber et al.)
DC
O2sat-O2 O2meas 0
Ceq280
pCO2(i)280
Cdiseq
170O2 116CO2
CaCO3
?Cbio
sT
Ca2CO32-
170 O2
16 NO32-
Soft tissue
Carbonate
?CbiorCODO2 ½(rNODO2DCO32-)
Cant Cm ?Cbio Ceq280 Cdiseq ?C -
?Cdiseq
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Anthropogenic CO2
(mmol kg-1)
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Pre-industrial CO2
(mmol kg-1)
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International CLIVAR/CO2 Lines (including US)
CO2 Clivar Repeat Hydro.