Closed-path System Research Articles

Biases in open-path carbon dioxide flux measurements: Roles of instrument surface heat exchange and analyzer temperature sensitivity

Eddy covariance (EC) measurements of ecosystem-atmosphere carbon dioxide (CO2) exchange provide the most direct assessment of the terrestrial carbon cycle. Measurement biases for open-path (OP) CO2 concentration and flux measurements have been reported for over 30 years, but their origin and appropriate correction approach remain unresolved. Here, we quantify the impacts of OP biases on carbon and radiative forcing budgets for a sub-boreal wetland. Comparison with a reference closed-path (CP) system indicates that a systematic OP flux bias (0.54μmolm−2s−1) persists for all seasons leading to a 110% overestimate of the ecosystem CO2 sink (cumulative error of 78 gC m−2). Two potential OP bias sources are considered: Sensor-path heat exchange (SPHE) and analyzer temperature sensitivity. We examined potential OP correction approaches including: i) Fast temperature measurements within the measurement path and sensor surfaces; ii) Previously published parameterizations; and iii) Optimization algorithms. The measurements revealed year-round average temperature and heat flux gradients of 2.9 °C and 16 W m−2 between the bottom sensor surfaces and atmosphere, indicating SPHE-induced OP bias. However, measured SPHE correlated poorly with the observed differences between OP and CP CO2 fluxes. While previously proposed nominally universal corrections for SPHE reduced the cumulative OP bias, they led to either systematic under-correction (by 38.1 gC m−2) or to systematic over-correction (by 17-37 gC m−2). The resulting budget errors exceeded CP random uncertainty and change the sign of the overall carbon and radiative forcing budgets. Analysis of OP calibration residuals as a function of temperature revealed a sensitivity of 5μmolm−3K−1. This temperature sensitivity causes CO2 calibration errors proportional to sample air fluctuations that can offset the observed growing season flux bias by 50%. Consequently, we call for a new OP correction framework that characterizes SPHE- and temperature-induced CO2 measurement errors.

Temporal and spatial variations of energy balance closure across FLUXNET research sites

There is always a discrepancy between available energy and output energy in the surface energy budget of FLUXNET research sites. Using the daily data retrieved from the FLUXNET database, the energy balance closure (EBC) of around 150 sites covering nine land covers and five Köppen climate zones [i.e. tropical area (A), dry area (B), mild temperate area (C), snow area (D) and polar area (E)] was analyzed. The temporal and spatial variations of EBC in different land covers and climate zones were summarized, and the relationships between EBC and environmental variables were explored. The possible differences in the EBCs of sites with open path (OP) and closed path (CP) gas analyzers were also examined at different precipitation levels in various climate zones. The results showed that EBC was positively related to air temperature (Ta) and vapor pressure deficit (VPD) below certain thresholds. Better EBCs were observed in land covers with stable evaporative fraction (i.e. the ratio of latent heat flux to the sum of latent and sensible heat fluxes). For land covers with seasonal varying evaporative fraction, the larger evaporative fraction in summer corresponded with better EBCs. OP systems resulted in better EBCs at various precipitation levels, and EBCs decreased with increasing precipitation for both OP and CP systems. In addition, the relationship between EBC and CO2 fluxes was different among the different land covers. There was a positive relationship for most land covers but not for savannah, shrub land, and evergreen broadleaf forest. The relationships between EBC and CO2 as well as EBC and other environmental variables are cross-influenced, which could be related to the stomata aperture and metabolism of the vegetation. Overall, this study summarized patterns of EBCs that could be used to correct eddy covariance data and energy balance closure related models. It further enhanced our understanding of the potential link between EBC and vegetation physiology that could facilitate the modeling and prediction of biophysical processes related to water, energy, and carbon fluxes from the leaf to ecosystem levels.

An eddy-covariance system with an innovative vortex intake for measuring carbon dioxide and water fluxes of ecosystems

Abstract. Closed-path eddy-covariance (EC) systems are used to monitor exchanges of trace gases (e.g., carbon dioxide [CO2], water vapor [H2O], nitrous oxide and methane) between the atmosphere and biosphere. Traditional EC-intake systems are equipped with inline filters to prevent airborne dust particulate from contaminating the optical windows of the sample cell which causes measurement degradation. The inline filter should have a fine pore size (1 to 20 µm is common) to adequately protect the optics and a large filtration surface area to extend the time before it clogs. However, the filter must also have minimal internal volume to preserve good frequency response. This paper reports test results of the field performance of an EC system (EC155, Campbell Scientific, Inc., Logan Utah, USA) with a prototype vortex intake replacing the inline filter of a traditional EC system. The vortex-intake design is based on fluid dynamics theory. An air sample is drawn into the vortex chamber, where it spins in a vortex flow. The initially homogenous flow is separated when particle momentum forces heavier particles to the periphery of the chamber, leaving a much cleaner airstream at the center. Clean air (75 % of total flow) is drawn from the center of the vortex chamber, through a tube, to the sample cell where it is exposed to the optical windows of the gas analyzer. The remaining 25 % of the flow carries the heavier dust particles away through a separate bypass tube. An EC155 system measured CO2 and H2O fluxes in two urban-forest ecosystems in the megalopolis of Beijing, China. These sites present a challenge for EC measurements because of the generally poor air quality which has high concentrations of suspended particulate. The closed-path EC system with vortex intake significantly reduced maintenance requirements by preserving optical signal strength and sample-cell pressure within acceptable ranges for much longer periods. The system with vortex intake also maintained an excellent frequency response. For example, at the Badaling site, the amount of system downtime attributed solely to clogged filters was reduced from 26 % with traditional inline filters to 0 % with the prototype vortex intake. The use of a vortex intake could extend the geographical applicability of the EC technique in ecology and allow investigators to acquire more accurate and continuous measurements of trace-gas fluxes in a wider range of ecosystems.

Measurements of methane emissions from a beef cattle feedlot using the eddy covariance technique

The eddy covariance (EC) technique has been extensively used in several sites around the world to measure energy fluxes and CO2 exchange at the ecosystem scale. Recent advances in optical sensors have allowed the use of the EC approach to measure other trace gases (e.g. CH4, NH3 and N2O), which has expanded the use of eddy covariance for other applications, including measuring gas emissions from livestock production systems. The main objectives of this study were to assess the performance of a closed-path EC system for measuring CH4, CO2, and H2O fluxes in a beef cattle feedlot and to investigate the spatial variability of eddy covariance fluxes measured above the surface of a feedlot using an analytical flux footprint analysis. A closed-path EC system was used to measure CH4, CO2, and H2O fluxes. To evaluate the performance of this closed-path system, an open-path EC system was also deployed on the flux tower to measure CO2 and H2O exchange. The performance assessment of the closed-path EC system showed that this system was suitable for EC measurements. The frequency attenuations, observed for the closed-path system CO2 and CH4 cospectra in this study, are in agreement with results from previous instrument comparison studies. For the water vapor closed-path cospectra, larger attenuations were likely caused by water vapor molecule interaction with the sampling tube walls. Values of R2 for the relationship between H2O and CO2 fluxes, measured by open-path and closed-path systems, were 0.94–0.98, respectively. The closed-path EC system overestimated the CO2 by approximately 5% and underestimated the latent heat fluxes by about 10% when compared with the open-path system measurements. Measured CH4 and CO2 fluxes during the study period from the feedlot averaged 2.63μmolm−2s−1 and 103.8μmolm−2s−1, respectively. Flux values were quite variable during the field experiment and the footprint analysis was useful to interpret flux temporal and spatial variation. This study shows indication that consideration of atmospheric stability condition, wind direction and animal movement are important to improve estimates of CH4 emissions per pen surface or per head of cattle.

Impact of different eddy covariance sensors, site set-up, and maintenance on the annual balance of CO2 and CH4 in the harsh Arctic environment

Improving year-round data coverage for CO2 and CH4 fluxes in the Arctic is critical for refining the global C budget but continuous measurements are very sparse due to the remote location limiting instrument maintenance, to low power availability, and to extreme weather conditions. The need for tailoring instrumentation, site set up, and maintenance at different sites can add uncertainty to estimates of annual C budgets from different ecosystems. In this study, we investigated the influence of different sensor combinations on fluxes of sensible heat, CO2, latent heat (LE), and CH4, and assessed the differences in annual CO2 and CH4 fluxes estimated with different instrumentation at the same sites. Using data from four sites across the North Slope of Alaska, we found that annual CO2 fluxes estimated with heated (7.5±1.4gCm−2yr−1) and non-heated (7.9±1.3gCm−2yr−1) anemometers were within uncertainty bounds. Similarly, despite elevated noise in 30-min flux data, we found that summer CO2 fluxes from open (−17.0±1.1gCm−2yr−1) and close-path (−14.2±1.7gCm−2yr−1) gas analyzers were not significantly different. Annual CH4 fluxes were also within uncertainty bounds when comparing both open (4.5±0.31gCm−2yr−1) and closed-path (4.9±0.27gCm−2yr−1) gas analyzers as well as heated (3.7±0.26gCm−2yr−1) and non-heated (3.7±0.28gCm−2yr−1) anemometers. A continuously heated anemometer increased data coverage (64%) relative to non-heated anemometers (47–52%). However, sensible heat fluxes were over-estimated by 12%, on average, with the heated anemometer, contributing to the overestimation of CO2, CH4, and LE fluxes (mean biases of −0.03μmolm−2s−1, −0.05mgCm−2h−1, and −3.77Wm−2, respectively). To circumvent this potential bias and reduce power consumption, we implemented an intermittent heating strategy whereby activation only occurred when ice or snow blockage of the transducers was detected. This resulted in comparable coverage (50%) during winter to the continuously heated anemometer (46%), while avoiding flux over-estimation. Closed and open-path analyzers showed good agreement, but data coverage was generally greater when using closed-path, especially during winter. Winter data coverage of 26–32% was obtained with closed-path devices, vs 10–14% for the open-path devices with unheated anemometers or up to 46% and 35% using closed and open-path analyzers, respectively with heated anemometers. Accurate estimation of LE remains difficult in the Arctic due to strong attenuation in closed-path systems, even when intake tubes are heated, and due to poor data coverage from open-path sensors in such a harsh environment.

Open-path cavity ring-down spectroscopy for trace gas measurements in ambient air.

The present work used a near-infrared methane cavity ring-down spectroscopy (CRDS) sensor to examine performance and limitations of open-path CRDS for atmospheric measurements. A simple purge-enclosure was developed to maintain high mirror reflectivity and allowed >100 hours of operation with mirror reflectivity above 0.99996. We characterized effects of aerosols on ring-down decay signals and found the dominant effect to be fluctuations by large super-micron particles. Simple software filtering approaches were developed to combat these fluctuations allowing noise-equivalent sensitivity of ~6x10-10 cm-1HJ Hz-1/2 within a factor of ~3 of closed-path systems (based on stability of the absorption baseline). Sensor measurements were validated against known methane concentrations in a closed-path configuration, while open-path validation was performed by side-by-side comparison with a commercial closed-path system.

Advances in Air–Sea $$\hbox {CO}_2$$ CO 2 Flux Measurement by Eddy Correlation

Eddy-correlation measurements of the oceanic \(\hbox {CO}_2\) flux are useful for the development and validation of air–sea gas exchange models and for analysis of the marine carbon cycle. Results from more than a decade of published work and from two recent field programs illustrate the principal interferences from water vapour and motion, demonstrating experimental approaches for improving measurement precision and accuracy. Water vapour cross-sensitivity is the greatest source of error for \(\hbox {CO}_2\) flux measurements using infrared gas analyzers, often leading to a ten-fold bias in the measured \(\hbox {CO}_2\) flux. Much of this error is not related to optical contamination, as previously supposed. While various correction schemes have been demonstrated, the use of an air dryer and closed-path analyzer is the most effective way to eliminate this interference. This approach also obviates density corrections described by Webb et al. (Q J R Meteorol 106:85–100, 1980). Signal lag and frequency response are a concern with closed-path systems, but periodic gas pulses at the inlet tip provide for precise determination of lag time and frequency attenuation. Flux attenuation corrections are shown to be \(<\)5 % for a cavity ring-down analyzer (CRDS) and dryer with a 60-m inlet line. The estimated flux detection limit for the CRDS analyzer and dryer is a factor of ten better than for IRGAs sampling moist air. While ship-motion interference is apparent with all analyzers tested in this study, decorrelation or regression methods are effective in removing most of this bias from IRGA measurements and may also be applicable to the CRDS.

Eddy covariance measurements with a new fast-response, enclosed-path analyzer: Spectral characteristics and cross-system comparisons

A new class of enclosed path gas analyzers suitable for eddy covariance applications combines the advantages of traditional closed-path systems (small density corrections, good performance in poor weather) and open-path systems (good spectral response, low power requirements), and permits estimates of instantaneous gas mixing ratio. Here, the extent to which these advantages are realized in field deployment is assessed, with a focus on the suitability of such an analyzer (the EC155, manufactured by Campbell Scientific) for long-term flux measurements in a new flux monitoring site in the southern Appalachians (NC, USA). The scalar-vertical velocity co-spectra for CO2 fluxes measured with the EC155 were similar to those measured with a co-located open-path system. When humidity was high, attenuation of the EC155 water vapor fluxes for non-dimensional frequencies greater than ∼2 was noted, though results from an ogive analysis suggest that eddies operating on these time scales contributed <2% of the total turbulent flux in this tall forest ecosystem. Inertial sub-range decay of the vertical velocity-scalar co-spectra generally conformed to a -7/3 power law during near-neutral atmospheric stability conditions, supporting the use of an analytical spectral correction approach to the raw measured fluxes. The EC155 fluxes computed directly from instantaneous mixing ratio agreed with will those calculated from mass–density concentration measurements, provided density terms for temperature, water vapor, and pressure were applied. Biases were observed when the EC155 flux records were compared to those measured with the open-path system. These differences were related to wind angle of attack and to an estimate of apparent fluxes related to instrument self-heating, and the biases were minimized after the application of a friction velocity filter. Finally, the EC155 considerably outperformed open-path analyzers during adverse weather conditions favorable to fog development, which occur frequently in the study site.

Relative humidity effects on water vapour fluxes measured with closed-path eddy-covariance systems with short sampling lines

It has been formerly recognised that increasing relative humidity in the sampling line of closed-path eddy-covariance systems leads to increasing attenuation of water vapour turbulent fluctuations, resulting in strong latent heat flux losses. This occurrence has been analyzed for very long (50m) and long (7m) sampling lines. To date, only a few analytical or in situ analyses have been proposed to quantify and correct such effects, among which the comprehensive method by Ibrom et al. (2007) was proved effective for the very long sampling line of a forest eddy-covariance setup.Here we analyze data from eddy-covariance systems featuring short (4m) and very short (1m) sampling lines running at the same clover field and show that relative humidity effects persist also for these setups, and should not be neglected. Starting from the work of Ibrom and co-workers, we propose a mixed method, a composite of two existing approaches, for correcting eddy-covariance fluxes. By means of a comparison with parallel open-path measurements, we show that the mixed method leads to an improved estimation of latent heat fluxes, with respect to the method described by Ibrom et al. (2007). The quantification and correction method proposed here is deemed applicable to closed-path systems featuring a broad range of sampling lines, and indeed applicable also to passive gases as a special case. The methods described in this paper are incorporated, as processing options, in the free and open-source eddy-covariance software packages ECO2S and EddyPro.

ECOSYSTEM WATER USE EFFICIENCY OF NORWAY SPRUCE MONOCULTURE FROM EDDY-COVARIANCE AND ECOPHYSIOLOGICAL MEASUREMENTS

Water release and biomass production of young Norway spruce stand were investigated by different approaches during 2004-2009. Closed-path eddy-covariance system (LI-7000) was found to be suitable for accurate net ecosystem production evaluation (NEP). NEP was in a good correspondence with the biomass inventory data. In contrary, water vapour eddy-covariance fluxes were found underestimating a real water release of the stand. Global radiation and air water vapour pressure deficit at 75% of the stand height were main micrometeorological control drivers for stand transpiration (T) and evapotranspiration (ET). Thus, missing or bad quality data of eddy-covariance ET can be simulated according to these parameters. Water use efficiency (WUE) values calculated from net and gross primary production data highly differed in absolute values as well as in seasonal course mainly due to respiration. Annual WUE values from eddy-covariance and from ecophysiological measurements of T and stand biomass increment were unfortunately incomparable. Therefore, precise research of eddy-covariance H2O fluxes and WUE evaluation need to continue.

Attenuation Correction Procedures for Water Vapour Fluxes from Closed-Path Eddy-Covariance Systems

Evapotranspiration is a source of water vapour to the atmosphere, and as a crucial indicator of landscape behaviour its accurate measurement has widespread implications. Here we investigate errors that are prevalent and systematic in the closed-path eddy-covariance measurement of latent heat flux: the attenuation of fluxes through dampened cospectral power at high frequencies. This process is especially pronounced during periods of high relative humidity through the adsorption and desorption of water vapour along the tube walls. These effects are additionally amplified during lower air temperature conditions. Here, we quantify the underestimation of evapotranspiration by a closed-path system by comparing its flux estimate to simultaneous and adjacent measurements from an open-path sensor. We apply models relating flux loss to relative humidity itself, to the lag time of the cross-correlation peak between the water vapour and vertical wind velocity signals, and to models of cospectral attenuation relative to the cospectral power of simultaneous sensible heat-flux measurements. We find that including the role of temperature in modifying the attenuation–humidity relationship is essential for unbiased flux correction, and that physically based cospectral attenuation methods are effective characterizers of closed-path instrument signal loss relative to the unattenuated flux value.

Influences of various calculation options on heat, water and carbon fluxes determined by open- and closed-path eddy covariance methods

ABSTRACTSynthesis studies using multiple-site datasets for eddy covariance potentially contain uncertainties originating from the use of different flux calculation options, because the choice of the process for calculating half-hourly fluxes from raw time series data is left to individual researchers. In this study, we quantified the uncertainties associated with different flux calculation methods at seven sites. The differences in the half-hourly fluxes were small, generally of the order less than a few percentiles, but they were substantial for the annual fluxes. After the standardisation under current recommendations in the FLUXNET communities, we estimated the uncertainties in the annual fluxes associated with the flux calculations to be 2.6±2.7 W m−2 (the mean 90% ± confidence interval) for the sensible heat flux, 72±37 g C m−2 yr−1 for net ecosystem exchange (NEE), 12±6% for evapotranspiration, 12±6% for gross primary productivity and 16±10% for ecosystem respiration. The self-heating correction strongly influenced the annual carbon balance (143±93 g C m−2 yr−1), not only for cold sites but also for warm sites, but did not fully account for differences between the open- and closed-path systems (413±189 g C m−2 yr−1).

Importance of mixing ratio for a long-term CO2flux measurement with a closed-path system

Importance of mixing ratio for a long-term CO₂flux measurement with a closed-path system

Comparing laser-based open- and closed-path gas analyzers to measure methane fluxes using the eddy covariance method

Closed- and open-path methane gas analyzers are used in eddy covariance systems to compare three potential methane emitting ecosystems in the Sacramento-San Joaquin Delta (CA, USA): a rice field, a peatland pasture and a restored wetland. The study points out similarities and differences of the systems in field experiments and data processing. The closed-path system, despite a less intrusive placement with the sonic anemometer, required more care and power. In contrast, the open-path system appears more versatile for a remote and unattended experimental site. Overall, the two systems have comparable minimum detectable limits, but synchronization between wind speed and methane data, air density corrections and spectral losses have different impacts on the computed flux covariances. For the closed-path analyzer, air density effects are less important, but the synchronization and spectral losses may represent a problem when fluxes are small or when an undersized pump is used. For the open-path analyzer air density corrections are greater, due to spectroscopy effects and the classic Webb–Pearman–Leuning correction. Comparison between the 30-min fluxes reveals good agreement in terms of magnitudes between open-path and closed-path flux systems. However, the scatter is large, as consequence of the intensive data processing which both systems require.

Importance of mixing ratio for a long-term CO2 flux measurement with a closed-path system

The new type closed-path CO2/H2O infrared gas analyser enables us to calculate CO2 fluxes from both the mixing ratio (FMR c ) and mass density of CO2 (FWPLc ). After the WPL correction was applied, FMR c and FWPLc were almost in accord with each other. However, FWPLc tended to be slightly larger than FMR c , which resulted in a significant difference in cumulative CO2 fluxes.We found that this difference was explained by the pressure covariance term, which is normally omitted in the WPL correction. Therefore, ignoring the pressure covariance term in the WPL correction can cause a serious error in estimation of annual net ecosystem exchange. To reduce uncertainties in calculations, we recommend using the mixing ratio for calculation of CO2 fluxes when using the new type closed-path infrared gas analyser.

Eddy covariance methane measurements at a Ponderosa pine plantation in California

Abstract. Long term methane flux measurements have been mostly performed with plant or soil enclosure techniques on specific components of an ecosystem. New fast response methane analyzers make it possible to use the eddy covariance (EC) technique instead. The EC technique is advantageous because it allows continuous flux measurements integrating over a larger and more representative area including the complete ecosystem, and allows fluxes to be observed as environmental conditions change naturally without disturbance. We deployed the closed-path Fast Methane analyzer (FMA) from Los Gatos Research Ltd and demonstrate its performance for EC measurements at a Ponderosa pine plantation at the Blodgett Forest site in central California. The fluctuations of the CH4 concentration measured at 10 Hz appear to be small and their standard deviation is comparable to the magnitude of the signal noise (±5 ppbv). Consequently, the power spectra typically have a white noise signature at the high frequency end (a slope of +1). Nevertheless, in the frequency range important for turbulent exchange, the cospectra of CH4 compare very well with all other scalar cospectra confirming the quality of the FMA measurements are good for the EC technique. We furthermore evaluate the complications of combined open and closed-path measurements when applying the Webb-Pearman-Leuning (WPL) corrections (Webb et al., 1980) and the consequences of a phase lag between the water vapor and methane signal inside the closed path system. The results of diurnal variations of CH4 concentrations and fluxes are summarized and compared to the monthly results of process-based model calculations.

Open-path vs. closed-path eddy covariance measurements of the net ecosystem carbon dioxide and water vapour exchange: A long-term perspective

The differential design, deployment and data post-processing of open-path (OP) and closed-path (CP) eddy covariance systems is a potential source of bias for ongoing global flux synthesis activities. Here we use a unique 6-year data set of concurrent CP and OP carbon dioxide (CO 2) and water vapour (H 2O) eddy covariance flux measurements above a temperate mountain grassland in Austria to explore the consequences of these differences on a long-term basis. The theoretically based transfer function approach was able to account and correct for the differences in low-pass filtering between the two systems. Corrected CO 2 and H 2O fluxes exhibited excellent 1:1 correspondence, but the CP system tended to underestimate OP H 2O fluxes during conditions of high air temperature, wind speed and global radiation, large sun angles and low relative humidity. Corrections for self-heating of the OP infra-red gas analyser had a very small effect on these relationships. Energy balance closure was slightly more favourable for the OP system. No significant differences were found for the random flux uncertainty of both systems. A larger fraction of OP data had to be excluded because of obstructions of the infra-red path by water and snow. This, however, did not translate into a correspondingly larger fraction of accepted CP flux values, because of a larger percentage of CP flux data failing on the stationarity test. Integrated over the annual cycle, the CP system yielded on average a more positive net ecosystem CO 2 exchange (25 gC m −2 year −1 vs. 0 gC m −2 year −1) and a lower evapotranspiration (465 mm year −1 vs. 549 mm year −1) as compared to the OP system.

Another Simple Method of Spectral Correction to Obtain Robust Eddy-Covariance Results

The eddy-covariance method is the state-of-the-art tool to measure mass and energy fluxes, though many measuring systems (particularly closed-path systems) show strong spectral attenuation that causes significant underestimates of actual mass fluxes. The standard way to correct these underestimates is to use theoretically derived transfer functions. Practical implementation has shown that the attenuation, especially of closed-path systems, is underestimated by this method, even when the additional tube damping is considered. This paper introduces and tests three alternative site-specific and hardware specific correction methods based on spectral analysis, which typically enhance—additionally to the classical correction—the calculated mass fluxes based on closed-path analysers, as their inevitable tubing and the filters used cause additional flow attenuation. Two of the three methods are based on a site-specific and hardware specific transfer functions, the third uses direct comparison of cospectra. Primarily the methods based on transfer functions proved to be easy-to-handle once established for the specific set-up and measurement site. They represent practical and robust methods to correct for spectral attenuation.

Apparent downward CO2 flux observed with open-path eddy covariance over a non-vegetated surface

The open-path eddy covariance (EC) method often shows unlikely downward CO2 fluxes in late winter and early spring over drained paddy fields with few active plants. To understand why, we carried out intensive measurements in a bare paddy field from 9 to 11 April 2003, simultaneously using open- and closed-path EC methods; aerodynamic and dynamic closed-chamber methods were also used. During this period, the open-path EC method showed downward daytime CO2 fluxes ranging from 0 to −5.9 µmol m−2 s−1, even after application of the WPL correction (density correction) and ordinary quality control tests. Because the closed-path EC and aerodynamic methods showed upward CO2 fluxes, the downward CO2 fluxes observed with open-path EC appear not to represent true CO2 transport. Diurnal variations in the downward daytime CO2 fluxes appeared to be correlated with increases in solar radiation during the day, and also with increases in sensible heat flux in weak winds but not under strongly windy conditions. The daytime 10-Hz time series data of vertical wind and CO2 mixing ratio demonstrated that updrafts were CO2 depleted in the open-path system, whereas the same updrafts were CO2 enriched in the closed-path system. Careful examination of the discrepancies between the open- and the closed-path EC measurements revealed that the amplitudes of the 10-Hz temperature signals from the sonic anemometer and the resultant sensible heat fluxes were too small to compensate for the discrepancies observed during the daytime. The open-path EC method with the conventional application of the WPL correction is not necessarily appropriate for measuring small magnitudes of CO2 flux (≤5 µmol m−2 s−1) under such surface and atmospheric conditions that the magnitude of the WPL correction is as great as that of the uncorrected CO2 flux itself.

Seasonal and interannual variations in carbon dioxide exchange of a temperate larch forest

This paper reports 3-year measurements (2001–2003) of the net ecosystem CO 2 exchange (NEE) over a northern Japan larch plantation forest, using an eddy covariance technique with both open- and closed-path systems. The study evaluates interannual variations in the seasonal patterns of NEE, the annual NEE, clarifying the factors controlling the carbon balance. NEE for both open- and closed-path systems look equivalent from the half-hourly mean values; however, there is a small systematic bias in the annual cumulative totals. In this study, we used closed-path data because open-path data during winter are doubtful and the annual NEE from closed-path data was closer to the biometric NEP than that from open-path data. The 3-year averages of annual NEE, gross primary production (GPP), and ecosystem respiration (RE) were −212 ± 43 (standard deviation), −1673 ± 59, and 1462 ± 42 gC m −2 y −1, respectively. The larch plantation forest absorbed large amounts of CO 2 during just 1 month, June. During that period, photosynthetic photon flux density (PPFD) was large while the temperature was not high. These meteorological conditions were suitable for CO 2 absorption before the leaf area index (LAI) reached its peak. Maximum GPP at light saturation ( P max) was also large. During the 3-year measurement period, PPFD in summer, and temperatures in spring and summer engendered remarkable interannual differences of GPP, RE, and NEE. In spring, snowmelt and larch foliation occurred about 2 weeks earlier in 2002 than in either 2001 or 2003 because of higher temperatures; consequently, the ecosystem began to photosynthesize earlier. In July, NEE was more negative in 2003 than in other years. The GPP became larger because of higher PPFD, larger P max; while the RE became smaller because of lower temperatures. Compared with similar forests, the amounts of GPP and RE measured here were large, but NEE was similar. The reason is that the photosynthetic capacity of larch is naturally large: the ample PPFD, and the lack of environmental stresses from dry air or soil, consequently allows high photosynthesis rates to be maintained. Moreover, it can be inferred that RE values are enhanced by the high photosynthetic activity of larch forest during the growing season.