Calculating CO2 and H2O eddy covariance fluxes from an enclosed gas analyzer using an instantaneous mixing ratio

Eddy covariance flux research has relied on open- or closed-path gas analyzers for producing estimates of net ecosystem exchange of carbon dioxide (CO2) and water vapor (H2O). The two instruments have had different challenges that have led to development of an enclosed design that is intended to maximize strengths and minimize weaknesses of both traditional designs. Similar to the closed-path analyzer, the enclosed design leads to minimal data loss during precipitation events and icing, and it does not have surface heating issues. Similar to the open-path design, the enclosed design has good frequency response due to small flux attenuation loss in the short intake tube, does not need frequent calibration, has minimal maintenance requirements, and can be used in a very low power configuration. Another important feature of such a design is the ability to output instantaneous mixing ratio, or dry mole fraction, so that instantaneous thermal and pressure-related expansion and contraction, and water dilution of the sampled air have been accounted for. Thus, no density corrections should be required to compute fluxes during postprocessing. Calculations of CO2 and H2O fluxes via instantaneous mixing ratio from the new enclosed CO2/H2O gas analyzer were tested in nine field experiments during 2009–2010 in a wide range of ecosystems and setups. Fluxes computed via a mixing ratio output from the instrument without applying density corrections were compared to those computed the traditional way using density corrections. The results suggest that with proper temperature, water vapor, and pressure measurements in the cell, gas fluxes can be computed confidently from raw covariance of mixing ratio and vertical wind speed, multiplied by a frequency response correction. This has important implications for future flux measurements, because avoiding hourly density corrections could have the advantages of increasing flux measurement quality and temporal resolution, reducing the magnitude of minimum detectable flux, unifying data processing steps, and assuring better intercomparison between different sites and networks.

[1]  Comment on “An Alternative Approach for CO2 Flux Correction Caused by Heat and Water Vapour Transfer” by Liu , 2006 .

[2]  George Burba,et al.  Comparison of net CO 2 fl uxes measured with open-and closed-path infrared gas analyzers in an urban complex environment , 2009 .

[3]  Ray Leuning,et al.  The relative merits of open‐ and closed‐path analysers for measurement of eddy fluxes , 1996 .

[4]  W. Massman,et al.  A Perspective on Thirty Years of the Webb, Pearman and Leuning Density Corrections , 2011 .

[5]  Charles Sayward,et al.  An Alternative Approach , 1996 .

[6]  J. Heilman,et al.  Experimental Test of Density and Energy‐Balance Corrections on Carbon Dioxide Flux as Measured Using Open‐Path Eddy Covariance , 2003 .

[7]  J. Kaimal,et al.  Spectral Characteristics of Surface-Layer Turbulence , 1972 .

[8]  Thomas Foken,et al.  Post-Field Data Quality Control , 2004 .

[9]  W. Massman.  Concerning the Measurement of Atmospheric Trace Gas Fluxes with Open- and Closed-Path Eddy Covariance System: The WPL Terms and Spectral Attenuation , 2004 .

[10]  Stephen Wright,et al.  An alternative approach. , 2010, Nursing standard (Royal College of Nursing (Great Britain) : 1987).

[11]  J. William Munger,et al.  Measurements of carbon sequestration by long‐term eddy covariance: methods and a critical evaluation of accuracy , 1996 .

[12]  A. Kowalski,et al.  On the relationship between the eddy covariance, the turbulent flux, and surface exchange for a trace gas such as CO2 , 2007 .

[13]  John Moncrieff,et al.  Eddy-covariance CO2 flux measurements using open- and closed-path CO2 analysers: Corrections for analyser water vapour sensitivity and damping of fluctuations in air sampling tubes , 1990 .

[14]  A. Grelle,et al.  Improved trace gas flux estimation through IRGA sampling optimization , 2009 .

[15]  William J. Massman,et al.  Handbook of micrometeorology : a guide for surface flux measurement and analysis , 2004 .

[16]  Y. Harazono,et al.  Importance of mixing ratio for a long-term CO2 flux measurement with a closed-path system , 2011 .

[17]  A re-examination of density effects in eddy covariance measurements of CO2 fluxes , 2006 .

[18]  A. Miyata,et al.  Apparent downward CO2 flux observed with open-path eddy covariance over a non-vegetated surface , 2008 .

[19]  D. Billesbach.  Estimating uncertainties in individual eddy covariance flux measurements: A comparison of methods and a proposed new method , 2011 .

[20]  G. Burba,et al.  Novel design of an enclosed CO2/H2O gas analyser for eddy covariance flux measurements , 2010 .

[21]  近藤 文義,et al.  Evaluation of Webb Correction on CO2 Flux by Eddy Covariance Technique Using Open-Path Gas Analyzer over Asphalt Surface , 2008 .

[22]  Donald H. Lenschow,et al.  The attenuation of fluctuations in scalar concentrations through sampling tubes , 1991 .

[23]  S. Larsen,et al.  On the use of the Webb–Pearman–Leuning theory for closed-path eddy correlation measurements , 2007 .

[24]  E. K. Webb,et al.  Correction of flux measurements for density effects due to heat and water vapour transfer , 1980 .

[25]  W. Oechel,et al.  Energy balance closure at FLUXNET sites , 2002 .

[26]  A. Ibrom,et al.  Attenuation of concentration fluctuations of water vapor and other trace gases in turbulent tube flow , 2008 .

[27]  Heping Liu.  An Alternative Approach for CO2 Flux Correction Caused by Heat and Water Vapour Transfer , 2005 .

[28]  X. Lee,et al.  Pressure correction to the long-term measurement of carbon dioxide flux , 2011 .

[29]  Heping Liu.  Reply to the Comment by Kowalski on “An Alternative Approach for CO2 Flux Correction Caused by Heat and Water Vapour Transfer” , 2006 .

[30]  Eddy-covariance CO2 flux measurements using open- and closed-path CO2 analysers: Corrections for analyser water vapour sensitivity and damping of fluctuations in air sampling tubes , 1991 .

[31]  George Burba,et al.  Fine-wire thermometer to correct CO2 fluxes by open-path analyzers for artificial density fluctuations , 2007 .

[32]  S. Verma,et al.  Eddy correlation measurement of CO2 flux using a closed-path sensor: Theory and field tests against an open-path sensor , 1993 .

[33]  Ü. Rannik,et al.  Estimates of the annual net carbon and water exchange of forests: the EUROFLUX methodology , 2000 .

[34]  W. Massman.  A simple method for estimating frequency response corrections for eddy covariance systems , 2000 .

[35]  Comment on “the storage term in eddy flux calculations” , 2008 .

[36]  B. Law,et al.  Handbook of Micrometeorology , 2005 .

[37]  A. Grelle,et al.  Addressing the influence of instrument surface heat exchange on the measurements of CO2 flux from open‐path gas analyzers , 2008 .

[38]  W. Massman.  Reply to comment by Rannik on “A simple method for estimating frequency response corrections for eddy covariance systems” , 2001 .

[39]  Ray Leuning,et al.  The correct form of the Webb, Pearman and Leuning equation for eddy fluxes of trace gases in steady and non-steady state, horizontally homogeneous flows , 2007 .

[40]  W. Massman,et al.  Eddy covariance flux corrections and uncertainties in long-term studies of carbon and energy exchanges , 2002 .

[41]  William J. Massman,et al.  The attenuation of concentration fluctuations in turbulent flow through a tube , 1991 .

[42]  C. J. Moore.  Frequency response corrections for eddy correlation systems , 1986 .

[43]  K. Davis,et al.  A multi-site analysis of random error in tower-based measurements of carbon and energy fluxes , 2006 .

[44]  J. Finnigan.  The storage term in eddy flux calculations , 2006 .