Regional Needs and Instrumentation for CO 2 Observations Britton Stephens, NCARASP Colloquium, June 11, 2007

Outline Background –Regional Needs CO 2 Measurement Techniques –AIRCOA Example –Sources of potential bias Regional Applications of in situ CO 2 Observations

Carbon cycle science as a field began with the careful observational work of Dave Keeling Keeling, C.D., Rewards and penalties of monitoring the earth, Annu. Rev. Energy Environ., 23, 25-82, 1998.

Pg C / yr Background CO 2 measurements define global trends and loosely constrain continental-scale fluxes Expected from fossil fuel emissions TransCom 3 Study Fluxes Billion $

Regional scale is critical for linking to underlying processes (NRCS/USDA, 1997) (SeaWIFS, 2002) CHLOROPHYLL TEMPERATURE (C) (IPCC, 2001)

TURC/NDVI Biosphere Takahashi Ocean EDGAR Fossil Fuel [U. Karstens and M. Heimann, 2001] Continental mixed-layer CO 2 is highly variable NWR Mean Diurnal Cycle and Hourly Variability June 2006 Rocky RACCOON CO 2 Concentrations April 2007

Flux footprint, in ppm(GtCyr -1 ) -1, for a 10 6 km 2 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 source/sink to +/- 25% need to measure regional annual mean gradients to ppm

[ Recommendations of the 13 th WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracers Measurement Techniques, ] 1 instrument over time in the field 1 instrument over time in the lab 1 instrument over several seconds 2 instruments over time in the field Important Definitions

Absolute Measurement Techniques: Manometric and Gravimetric NOAA/CMDL Manometer: Reproducibility of 0.06 ppm (C. Zhao et al., 1997) Keeling manometer has run since ’50s

Relative Measurement Techniques: Non-dispersive Infrared (NDIR) Spectroscopy (from Broadband IR radiation filtered for 4.26 um Cooled emitter and detector Pulsed emitter or chopper wheel 1 or 2 detection cells Advantages: Robust, precise, affordable Disadvantages: Non-linear; sensitivity to pressure, temperature, and optical conditions; pressure broadening

LiCor, Inc. CO 2 Analyzer CMDL Flask Analysis System

Cavity Ringdown Spectroscopy (CRDS) Advantages: Precise, extremely stable Disadvantages: Somewhat more expensive, relatively new

Relative measurements require calibration gases tied to a common scale NOAA/CMDL scheme for propagation of WMO CO 2 scale: For NDIR, generally 4 points needed for 0.1 ppm comparability Recalibration needed ~ every 3 years due to drift

NCAR CO 2 and O 2 /N 2 Calibration Facility

Autonomous, Inexpensive, Robust CO 2 Analyzer (AIRCOA) 0.1 ppm precision and 0.1 ppm comparability on a 2.5 min. measurement $10K in parts and operates autonomously for months at a time

Potential source of biasAIRCOA solution Relating to WMO CO 2 ScaleDedicated CO 2 and O 2 calibration transfer facility Short-term IRGA noiseAverage for 2 minutes to get better than 0.1 ppm precision Drift in IRGA sensitivity4-hourly 4-point calibrations and 30-minute 1-point calibrations IRGA pressure sensitivityAutomated 4-hourly pressure sensitivity measurements IRGA temperature sensitivity30-minute 1-point calibrations, temperature control at some sites Incomplete drying of airSlow enough flow (100 sccm), two 96” Nafion driers, downstream humidity sensor to verify performance Drying system altering CO 2 Continuous flows and pressures through Nafions and run surveillance gas through entire system Incomplete flushing of cell and dead volumes Fast enough flow (100 sccm), alternate calibration sequence low ‑ to- high / high-to-low to look for effects Leaks through fittings, solenoid valves, and pumps Automated 8-hourly positive pressure leak-down and 4-hourly ambient pressure leak-up checks Pressure broadening without ArUse calibration gases made with real air Fossil CO 2 in calibration gases and different field and lab 13 C sensitivities Laboratory tests limit current effect to 0.05 ppm, long-term plans to use cylinders with natural CO 2 Regulator temperature effectsTests suggest effect is negligible, but could be regulator dependent Regulator flushing effectsRepeat calibration tests suggest the effect is negligible Whole-system diagnostics and comparability verification Long-term surveillance tank analyzed every 8 hours, co-location with other programs, rotating cylinders, and laboratory comparisons Delay in diagnosis of errorsNear real-time data acquisition, processing, and dissemination

Automated web-based output

The NOAA flasks have a mean offset and standard deviation relative to our measurements of 0.01 ppm +/ ppm. Field Surveillance Tanks NOAA GMD Flask Comparisons Mean offsets (and standard deviations) of these measurements from the laboratory-assigned values were: ‑ 0.08 (+/- 0.11), (+/- 0.11), and (+/- 0.07) ppm at the three sites.

Map of existing North American continuous well-calibrated CO 2 measurements. Colors denote measuring group, with Rocky RACCOON sites in Red. Courtesy of S. Richardson ( Map showing existing Rocky RACCOON sites (red), proposed new sites (purple), and potential future sites (gray). Continuous, well-calibrated CO 2 measurements across North America Towers over 650 feet AGL in U.S.

NWR – SPL = -2.1 ppm CO 2 HYSPLIT back-trajectory calculation for the afternoon of June 17, 2006 (6 PM LT), over topography (GoogleEarth Pro). The EDAS 40 km meteorology used for this simulation predicts that air which passed near SPL at 2 PM LT reached NWR at 6 PM LT. Using the observed CO 2 difference and the model transit time of 260 minutes, the model boundary-layer heights of 2000 m at SPL and 1878 m at NWR, an average atmospheric pressure of 60 kPa over this column, and a simple Lagrangian box model, we obtain a first- order flux estimate of gC m ‑ 2 hr ‑ 1. This value compares reasonably well with the average flux for late afternoon in June from the Niwot Ridge AmeriFlux Site of -0.2 gC m ‑ 2 hr ‑ 1 (S. Burns, personal communication). Point-to-Point Flux Estimates

Bakwin, P.S., K.J. Davis, C. Yi, S.C. Wofsy, J.W. Munger, L. Haszpra, and Z. Barcza (2004), Regional carbon dioxide fluxes from mixing ratio data, Tellus, 56B, Helliker, B.R. et al. (2004), Estimates of net CO2 flux by application of equilibrium boundary layer concepts to CO2 and water vapor measurements from a tall tower (DOI /2004JD004532). Journal of Geophysical Research, 109, D Styles, J.M., P.S Bakwin, K. Davis, B.E. Law (2007), A simple daytime atmospheric boundary layer budget validated with tall tower CO2 concentration and flux measurements, in press. Monthly-mean Boundary-layer Budgeting Monthly mean filtered CO 2 concentrations at the 4 Rocky RACCOON sites and differences from marine boundary layer concentrations interpolated to the same latitude. With estimates of boundary-layer depth can estimate fluxes, following on:

Inversion / Data-assimilation on Finer Scales

Influence-function Optimization of Simple Ecosystem Models Figures from: Matross, D. M., A. Andrews. M. Pathmathevan, C. Gerbig, J.C. Lin, S.C. Wofsy, B.C. Daube, E.W. Gottlieb, V.Y. Chow, J.T. Lee, C. Zhao, P.S. Bakwin, J.W. Munger, and D.Y. Hollinger (2006). Estimating regional carbon exchange in New England and Quebec by combining atmospheric, ground-based and satellite data, Tellus, 58B, Also stay tuned for: Richardson, S.J., N.L. Miles, K.J. Davis, M. Uliasz, A.S. Denning, A.R. Desai, and B.B. Stephens (2007), Demonstration of a high-precision, high-accuracy CO2 concentration measurement network for regional atmospheric inversions, J. Atmos. Oceanic Technol., to be submitted.

wind direction Day 1: late PM Day 2: early AM Day 2: late PM Mixed layer top Regional Scale Lagrangian Experiment “Moving columns” [ Figure courtesy of John Lin ]

CO 2 CO upstream downstream North Dakota Observations [ Figure courtesy of John Lin ]

Airborne Carbon in the Mountains Experiment (ACME-07) Flight June 7, 2007 Figure produced by S. Aulenbach, A. Desai, and D. Moore

Dry Mole Fraction of CO 2 BrazilBoulder 370 molecules of CO molecules of N molecules of O molecules of Ar molecules of H 2 O 5000 molecules of H 2 O Mole Fraction = ppm Mole Fraction = ppm Dry Mole Fraction = ppm For ± 0.1 ppm consistency, need to stabilize to ± 300 ppm H 2 O (or dry to dewpoint of -25 ºC)

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

Automated Flask Sampling (from

Light Aircraft Profile Carr, Colorado, USA (from

13 CO 2 / 12 CO 2 and O 2 /N 2 Ratios  Independent constraints on the land-ocean partitioning of CO 2 fluxes [NOAA/CMDL] [R. Keeling, SIO]

Advantages: Dramatic increase in CO 2 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 Thermal Infrared Emission: Available, but primarily mid to upper troposphere Reflected Near Infrared: Satellite CO 2 Measurements

Representativeness 360 m 120 m 800 m S Total representativity error of mixed layer averaged CO 2 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