Atmospheric Carbon Observations

Britton StephensNCAR Atmospheric Technology Division

Existing measurements:- Absolute and relative

- In situ and flask

Future capabilities:- Increased vertical profiles

- Continental sensor arrays

- Upward looking FTIR

- Satellite measurements

- Additional species

Atmospheric signals are smallRates of change in vertical column abundance for specific CO2 sources and sinks

Source Assumptions ppm/day

Los Angeles Basin 12 106 people, 4,000 km2,1100 mol C / person / day

+10

Netherlands 16 106 people, 40,000 km2,500 mol C / person / day

+0.6

Germany 83 106 people, 350,000 km2,580 mol C / person / day

+0.4

Photosynthetic Uptake Harvard Forest, July -1.2

U.S. Carbon Sink 1 GtCyr-1, constant in time,uniform over the lower 48 states

-0.08

Southern Oceans pCO2 = -30 atm, wind 15 m/s -0.06

Eastern Equatorial Pacific pCO2 = 100 atm, wind 7 m/s +0.04

Absolute Measurement Techniques: Manometric and Gravimetric

NOAA/CMDL Manometer:

Reproducibility of 0.06 ppm for dry mole fraction of CO2

(C. Zhao et al., 1997)

Relative measurement techniques:Infrared Absorption

CMDL Flask Analysis System

LiCor, Inc. CO2 Analyzer

Intra- and Inter-laboratory agreement still not better than 0.2 ppm

[NOAA/CMDL]

13CO2/12CO2 and O2/N2 Ratios

Independent constraints on the land-ocean partitioning of CO2 fluxes

[NOAA/CMDL]

[R. Keeling, SIO]

CO2 Observational Platforms

Expected from fossil fuel emissions

Observations

TransCom1 FF Gradients

What do existing flask measurements tell us?

Longitudinal separation of continental sources

• North America versus Eurasia

• South America versus Africa

What don’t they tell us?

Regional fluxes on scales relevant to the underlying processes

Vertical distributions to improve flux constraints and to reject flawed models

TURC/NDVI Biosphere Takahashi Ocean EDGAR Fossil Fuel

[U. Karstens and M. Heimann, 2001]

Continental mixed-layer CO2 is highly variable

[LSCOP, 2002]

Efforts coordinated by NOAA/CMDL

SBIRAtmospheric Observing

Systems

• 0.1 ppm in one minute• Deployable for 6 months• Towers, buoys, ships, planes• Approximate cost: $20,000

Automated Flask Sampling

Robust, Precise, CO2 Analyzer for Unattended Field Use

Prototype Inexpensive/Autonomous CO2 System

Research items:

• Stability of CO2 in aluminum LPG cylinders

• Correction for zero drift between calibrations

Goals:

• 1-2 year service schedule

• Total installation ~ $3000

• 0.3 ppm accuracy

RMT, Ltd., Russia

Towers over 650 feet AGL in U.S. and proximity

Upward Looking FTIR Spectrometry

• Sun following spectrometer• Measure near infrared

absorption of CO2 and O2

• Demonstrated precision in U.S. and Russia to ~ 1.5 ppm in 30 minutes

Could validate satellite measurements

Should be validated by airborne measurements

Kitt Peak Observatory

Advantages: Dramatic increase in CO2 data, consistent global coverage, total column abundances, comparable to other datasets

Disadvantages: Potential for biases due to aerosols, clouds, land surface type, viewing angle, or sun angle, expensive

Existing or planned Techniques:• Thermal Infrared Emission – TOVS, AIRS, IASI (2005)

Available, but primarily mid to upper troposphere

• Reflected Near Infrared – SCIAMACHY, OCO (ESSP Phase 2, 2006)

Satellite CO2 Measurements

• Targeted precision of better than 1 ppm for 4 x 5 degrees in 16 days

Flask (+) and in situ (-) measurements from COBRA-2000, made during a descent into Boston, MA.

Additional measurement species

Flux footprint, in ppm(GtCyr-1)-1, for a 106 km2 chaparral region in the U.S. Southwest (Gloor et al., 1999).

Using high frequency data makes signals bigger, but the annual-mean signals are still very small:

To measure 0.2 GtCyr-1 regional source or sink to +/- 25%, need to measure annual mean surface gradients to around +/- 0.2 ppm and column gradients to better than 0.1 ppm

Representativeness

360 m

120 m

800 m

S

grid size [km]to

tal re

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rro

r [p

pm

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 100 200 300 400 500

Total representativity error of mixed layer averaged CO2 mixing ratios (combined observational error and representativity error) plotted against the horizontal dimension of the region. Vertical bars indicate the 5-95% range.

[Gerbig et al., submitted to JGR]

COBRA-2000 Daytime Profiles

Conclusions

• Existing atmospheric measurements alone constrain fluxes of broad latitudinal zones

• Room for improvement by assimilating multiple existing data types

• Most significant advances will be from new measurement types and their assimilation

• Systematic biases in and representativeness of data must be considered carefully