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

Abstract

Eddy covariance flux research has relied on open-or closed-path gas analyzers for producing estimates of net ecosystem exchange of carbon dioxide (CO 2 ) and water vapor (H 2 O). 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 CO 2 and H 2 O fluxes via instantaneous mixing ratio from the new enclosed CO 2 /H 2 O 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.

Field intercomparison of four methane gas analyzers suitable for eddy covariance flux measurements

“…Gas concentration measurements are also affected by air density fluctuations (Webb et al, 1980;Massman and Tuovinen, 2006) and these effects can be corrected by adding the Webb-Pearman-Leuning (WPL) density terms to the measured flux. In these, the effect of atmospheric pressure fluctuations on measured density is assumed negligible, however during recent years the validity of this assumption has been questioned (Lee and Massman, 2011;Nakai et al, 2011;Zhang et al, 2011;Burba et al, 2012). Results from these studies are inconsistent with each other, but generally concur that the small effects of the pressure fluctuations are not negligible in all cases.…”

“…and ρ cm , and ρ a are uncorrected methane and air mass density, respectively, κ is a dimensionless correction factor which is a function of temperature and equivalent pressure P e (Burch et al, 1962), κ P e and κ T are the partial derivatives of κ with respect to pressure and temperature, computed at T = T and P e = P e , respectively. µ is molar mass of air divided with molar mass of water, σ is mean water vapor density divided with mean air density and ξ v is mole fraction of water vapor (mol water /mol air ).…”

The influence of tillage on N<sub>2</sub>O fluxes from an intensively managed grazed grassland in Scotland

“…Corrections for the frequency response of the system, both high and lowfrequency losses, were made using the method of Moncrieff et al (1997). Corrections for density fluctuations were applied on a half-hourly basis using the method of (Burba et al, 2012). The quality control scheme of Foken et al (2005), was used to remove poor-quality flux measurements (their category 2).…”

Estimating surface fluxes using eddy covariance and numerical ogive optimization

“…For the sonic anemometers (Gill R2, Gill R3, Gill Windmaster Pro and METEK USA-1) these include the following: an empirical angle of attack correction (Nakai and Shimoyama, 2012), and humidity and crosswind corrections (Liu et al, 2001;Schotanus et al, 1983). We convert all observations to mixing ratios (Burba et al, 2012) using the Webb-PearmanLeuning correction when necessary (Sahlee et al, 2008;Webb et al, 1980) as recommended by Ibrom et al (2007). The need for instrument heating corrections (Burba et al, 2008) associated with operation of the open path LI-7500 in a cold environment (Daneborg, POLYI and ICEI) is alleviated by using the newer LI-7500A with a "cold" setting correcting observations down to −25 • (Burba et al, 2011).…”