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1.
Methods of calibrating infrared CO2 analysers for sensitivity to CO2 and water vapour are described. Equations to correct eddy covariance CO2 flux measurements are presented for: (i) analyser cross-sensitivity to water vapour and the effects of density fluctuations arising from atmospheric fluxes of water vapour and sensible heat, (ii) flux losses caused by signal processing and limited instrument frequency response for open- and closed-path CO2 analysers, and (iii) flux losses resulting from damping of concentration fluctuations in a tube used to sample air for closed-path CO2 analysers. Examples of flux corrections required for typical instruments are presented.  相似文献   

2.
We have examined the potential of using a closed-path sensor to accurately measure eddy fluxes of CO2. Five inlet tubeflow configurations were employed in the experimental setup. The fluxes of CO2 were compared against those measured with an open-path sensor. Sampling air through an intake tube causes a loss of flux, due to the attenuation of CO2 density fluctuations. Adjustments need to be made to correct for this loss and to account for density effects due to the simultaneous transfer of heat and water vapor. Theory quantifying these effects is discussed.The raw CO2 flux measured with the closed-path sensor was smaller than that measured with the open-path sensor by about 15% (on average) for the turbulent tubeflow configurations with a short (3 m) intake tube, by 31% for turbulent tubeflow with a longer (6 m) intake tube and by 24% for laminar tubeflow. The difference was, in part, caused by tube attenuation of the CO2 density fluctuations and inadequate sensor time response. The elimination of the flux adjustment for the simultaneous transfer of sensible heat (i.e., the attenuation of ambient temperature fluctuations in the intake tube) generally accounted for the rest of this difference.The raw flux measured with the closed-path sensor was corrected for frequency response and density effects. Except in the case of laminar tubeflow, the corrected closed-path flux agreed consistently with the corrected open-path flux within a few percent (<5%). These results suggest that closed-path sensors, with appropriate corrections, can be used to measure CO2 flux accurately. Recommendations are included on selecting an optimum flow configuration to minimize the effect of sampling air through a tube.Published as Paper No. 9938, Journal Series, Nebraska Agricultural Research Division.  相似文献   

3.
An eddy covariance system using a closed-path CO2analyser was constructed for long-term CO2flux measurements above a forest, and its total frequency response was valuated experimentally. The amplitude and phase responses of the system wereexamined through a preliminary test, in which a prescribed pattern of CO2fluctuation was input to the system. The result showed that the amplitude of the output from the system was attenuated as frequency increased, with a half-power frequency of 0.3 Hz. The phase was delayed by the air sampling through a long tube, but the delay in phase decreased asfrequency increased. We then presented a new technique for the correction of flux loss due to the inadequate system response for the eddy covariance measurements of CO2 flux. Using the present system and the correction technique, diurnal variations in CO2 flux were measured over a temperate deciduous forest on three days in 1997. The results were compared with the CO2fluxes measured with a fast response open-path gas analyser. The CO2fluxes from the closed-path system agreed with those from the open-path system after the Webb, Pearman and Leuning correction was made for the latter. In the present test over a forest, the contribution of the frequency-response correction to the CO2fluxes was small and its averaged percentage was only 3%in the daytime. However, the percentage would likely increase, if the system were applied to a shorter vegetation site where high frequency components are more important. The comparison confirmed that we can obtain correct measurements of CO2flux using the present closed-path system and the correction technique.  相似文献   

4.
Comparison was made of the flux measurements of a closed-path CO2/H2O analyzer and an open-path H2O analyzer above a clover field and the forest floor of a Douglas-fir stand. The attenuation of the gas concentration fluctuations caused by the sampling tube of the closed-path analyzer resulted in underestimation of the H20 flux above both surfaces. The degree of underestimation above the clover field depended on wind speed, but was smaller than that calculated from the transfer function for laminar flow in a circular tube and the scalar cospectrum in the neutral and unstable surface layer. Above the forest floor CO2 fluctuations led those of H2O by 0.7s. The implications of this are discussed regarding the determination of the time delay caused by the sampling tube of the closedpath analyzer. The day-time CO2 efflux from the forest floor, averaged over three days, was 0.043 mg/(m2s).  相似文献   

5.
A modified infrared CO2 gas analyzer, a small thermocouple assembly, a heated-thermocouple anemometer for horizontal wind, and a propeller-type vertical wind sensor were used to measure the eddy fluxes of heat and CO2 above a corn crop. Experimental results of these fluxes are discussed. The main sources of errors of the eddy fluxes using these instruments were estimated:
  1. Sensors with a time constant of 0.5 s appear to be fast enough to detect most of the vertical CO2 transfer as long as the sensors are located at least one meter above the crop surface.
  2. The deviation from steady-state conditions for 10-min periods was found to have a significant effect on the eddy flux estimates.
  3. Temperature fluctuations of the air sample passing through the CO2 infrared gas analyzer were found to be non-negligible but could be easily corrected.
  4. A 1° misalignment of the vertical anemometer affected these eddy fluxes by less than 10% under all circumstances studied.
  相似文献   

6.
Components of the surface energy balance of a mature boreal jack pine forest and a jack pine clearcut were analysed to determine the causes of the imbalance that is commonly observed in micrometeorological measurements. At the clearcut site (HJP02), a significant portion of the imbalance was caused by: (i) the overestimation of net radiation (R n ) due to the inclusion of the tower in the field of view of the downward facing radiometers, and (ii) the underestimation of the latent heat flux (λE) due to the damping of high frequency fluctuations in the water vapour mixing ratio by the sample tube of the closed-path infrared gas analyzer. Loss of low-frequency covariance induced by insufficient averaging time as well as systematic advection of fluxes away from the eddy-covariance (EC) tower were discounted as significant issues. Spatial and temporal distributions of the total surface-layer heat flux (T), i.e. the sum of sensible heat flux (H) and λE, were well behaved and differences between the relative magnitudes of the turbulent fluxes for several investigated energy balance closure (C) classes were observed. Therefore, it can be assumed that micrometeorological processes that affected all turbulent fluxes similarly did not cause the variation in C. Turbulent fluxes measured at the clearcut site should not be forced to close the energy balance. However, at the mature forest site (OJP), loss of low-frequency covariance contributed significantly to the systematic imbalance when a 30-min averaging time was used, but the application of averaging times that were long enough to capture all of the low-frequency covariance was inadequate to resolve all of the high-frequency covariance. Although we found qualitative similarity between T and the net ecosystem exchange (NEE) of carbon dioxide (CO2), forcing T to closure while retaining the Bowen ratio and applying the same factor to CO2 fluxes (F C ) cannot be generally recommended since it remains uncertain to what extent long wavelength contributions affect the relationship between T, F C and C.  相似文献   

7.
An infrared device designed to measure simultaneous fluctuations of atmospheric CO2 and water vapor concentrations is described. The measuring frequency is 30 Hz. The sensing path length is 20 cm. It is compatible with the path length of the standard type of a sonic anemometer. The noise level of the device is equivalent to fluctuations of about 0.8 ppm peak-to-peak for CO2 and 0.02 g kg-1 peak-to-peak for water vapor. Field tests have showed that the device is suitable for simultaneous measurement of turbulent fluxes of CO2 and water vapor in conjunction with a sonic anemometer.  相似文献   

8.
We present an approach for assessing the impact of systematic biases in measured energy fluxes on CO2 flux estimates obtained from open-path eddy-covariance systems. In our analysis, we present equations to analyse the propagation of errors through the Webb, Pearman, and Leuning (WPL) algorithm [Quart. J. Roy. Meteorol. Soc. 106, 85–100, 1980] that is widely used to account for density fluctuations on CO2 flux measurements. Our results suggest that incomplete energy balance closure does not necessarily lead to an underestimation of CO2 fluxes despite the existence of surface energy imbalance; either an overestimation or underestimation of CO2 fluxes is possible depending on local atmospheric conditions and measurement errors in the sensible heat, latent heat, and CO2 fluxes. We use open-path eddy-covariance fluxes measured over a black spruce forest in interior Alaska to explore several energy imbalance scenarios and their consequences for CO2 fluxes.  相似文献   

9.
Sonic anemometers are capable of measuring the wind speed in all three dimensions at high frequencies (10–50 Hz), and are relied upon to estimate eddy-covariance-based fluxes of mass and energy over a wide variety of surfaces and ecosystems. In this study, wind-velocity measurement errors from a three-dimensional sonic anemometer with a non-orthogonal transducer orientation were estimated for over 100 combinations of angle-of-attack and wind direction using a novel technique to measure the true angle-of-attack and wind speed within the turbulent atmospheric surface layer. Corrections to the vertical wind speed varied from −5 to 37% for all angles-of-attack and wind directions examined. When applied to eddy-covariance data from three NOAA flux sites, the wind-velocity corrections increased the magnitude of CO2 fluxes, sensible heat fluxes, and latent heat fluxes by ≈11%, with the actual magnitude of flux corrections dependent upon sonic anemometer, surface type, and scalar. A sonic anemometer that uses vertically aligned transducers to measure the vertical wind speed was also tested at four angles-of-attack, and corrections to the vertical wind speed measured using this anemometer were within ±1% of zero. Sensible heat fluxes over a forest canopy measured using this anemometer were 15% greater than sensible heat fluxes measured using a sonic anemometer with a non-orthogonal transducer orientation. These results indicate that sensors with a non-orthogonal transducer orientation, which includes the majority of the research-grade three-dimensional sonic anemometers currently in use, should be redesigned to minimize sine errors by measuring the vertical wind speed using one pair of vertically aligned transducers.  相似文献   

10.
A cospectral correction model for measurement of turbulent NO2 flux   总被引：1，自引：1，他引：0  
A correction model for eddy correlation flux measurements is developed and applied to nitrogen dioxide flux measurements obtained from a SOLENT sonic anemometer and a Scintrex Luminox LMA-3 analyser for NO2. Four field campaigns were carried out near the village of Merenschwand in Central Switzerland from which two were selected for further analysis in this paper. The need for the correction of measured eddy covariance fluxes arises due to the damping loss of the NO2 analyser at high frequencies. This damping loss is described by an analogy to inductance in an electronical alternating current circuit. The independent variables in the correction model are:z (measuring height above zero-plane displacement), (mean horizontal wind speed), (Monin-Obukhov stability parameter),f (natural frequency) and inductanceL. The value for inductanceL can be derived from spectral and cospectral analysis. The theoretical cospectrum of an ideal measurement is taken from Kaimalet al. (1972) and extended with a damping term in order to describe the real measurements of the cospectrum. The inductanceL of the LMA-3 with a 0.6 cm teflon aspiration tube of 5 m length lies in the order of 0.30 to 0.35 for the dataset from Merenschwand. With this inductance, a correction factor of 1.17 in August/September 1992 and of 1.18 in May 1993 was determined for the NO2 flux maxima during daytime. The range of the correction factor is 1.05 to 1.31 for the mean daily cycles of both datasets.  相似文献   

11.
Energy and CO2 fluxes are commonly measured above plant canopies using an eddy covariance system that consists of a three-dimensional sonic anemometer and an H2O/CO2 infrared gas analyzer. By assuming that the dry air is conserved and inducing mean vertical velocity, Webb et al. (Quart. J. Roy. Meteorol. Soc. 106, 85-100, 1980) obtained two equations to account for density effects due to heat and water vapour transfer on H2O/CO2 fluxes. In this paper, directly starting with physical consideration of air-parcel expansion/compression, we derive two alternative equations to correct for these effects that do not require the assumption that dry air is conserved and the use of the mean vertical velocity. We then applied these equations to eddy flux observations from a black spruce forest in interior Alaska during the summer of 2002. In this ecosystem, the equations developed here led to increased estimates of CO2 uptake by the vegetation during the day (up to about 20%), and decreased estimates of CO2 respiration by the ecosystem during the night (approximately 4%) as compared with estimates obtained using the Webb et al. approach.  相似文献   

12.
Model predictions of CO2 concentrations downwind from a line source were calibrated using experimental data. Agreement between the model and experimental data was improved by adjusting for wind direction meander and cup anemometer overshoot. The model predictions showed that by using a negative exponential wind speed profile within the crop canopy, predictions were closer to observed CO2 concentration profiles than when experimentally-observed wind speed profiles, which were constant with height in the lower canopy, were used. This finding suggests that much of the lower canopy airflow was not direct mass flow in the downwind direction. Eddy diffusivity profiles which showed a within-canopy local minimum resulted in arestriction in the predicted loss of CO2 out of the canopy system. Two-dimensional plots of predicted null vertical flux and CO2 concentration portrayed vividly the turbulent diffusion and mass flow transport of CO2 from the line source.  相似文献   

13.
Eddy correlation equipment was used to measure mass and energy fluxes over a soybean crop. A rapid response CO2 sensor, a drag anemometer, a Lyman-alpha hygrometer and a fine wire thermocouple were used to sense the fluctuating quantities.Diurnal fluxes of sensible heat, latent heat and CO2 were calculated from these data. Energy budget closure was obtained by summing the sensible and latent heat fluxes determined by eddy correlation which balanced the sum of net radiation and soil heat flux. Peak daytime CO2 fluxes were near 1.0 mg m–2 (ground area) s–1.The eddy correlation technique was also employed in this study to measure nocturnal CO2 fluxes caused by respiration from plants, soil, and roots. These CO2 fluxes ranged from - 0.1 to - 0.25 mg m–2s–1.From the data collected over mature soybeans, a relationship between CO2 flux and photosynthetically active radiation (PAR) was developed. The crop did not appear to be light-saturated at PAR flux densities < 1800 Ei m–2 s–1. The light compensation point was found to be about 160 Ei m–2 s–1.Published as Paper No. 7402, Journal Series, Nebraska Agricultural Experiment Station. The work reported here was conducted under Nebraska Agricultural Experiment Station Project 27-003 and Regional Research Project 11–33.Post-doctoral Research Associate, Professor and Professor, respectively. Center for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0728.  相似文献   

14.
Observations of vegetation–atmosphere exchange of carbon dioxide (CO2) by the eddy covariance (EC) technique are limited by difficult conditions such as nighttime and heterogeneous terrain. Thus, advective flux components are included into the net ecosystem exchange (NEE) budget. However, advection measurements are experimentally challenging and do not always help to solve the night flux problem of the EC technique. This study investigates alternative methods for the observation of horizontal advection, in particular horizontal concentration gradients, as well as different approaches to coordinate rotation and vertical advection. Continuous high-frequency measurements of the horizontal CO2 concentration field are employed and compared to the often used discontinuous sequential sampling. Significant differences were found in the case of 30-min mean concentration values between the conventional discontinuous sampling approach and the complete observation of the time series by continuous sampling. Estimates of vertical advection rely on accurate estimates of vertical wind velocity ( $\emph{w}$ ). Therefore, different approaches to the planar fit coordinate rotation have been investigated. Sector-wise rotation was able to eliminate directional dependencies of mean $\emph{w}$ . Furthermore, the effect of the data set length used for rotation (window length) was investigated and was found to have significant impact on estimates of vertical advection, with larger window lengths yielding about 50% larger vertical advection. A sequential planar fit with controlled window length is proposed to give reproducible results. The different approaches to the measurement and calculation of horizontal and vertical advection presented are applied to data obtained during the exchange processes in mountainous region experiment at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay). Estimates of NEE including advection are compared to NEE from turbulent and storage flux alone without advection. NEE including vertical advection with sector-wise planar fit rotation and controlled window length and including horizontal advection from continuous gradient measurements, which were comprehensively bias corrected by a new approach, did compare well with the expected night flux error, with meteorological drivers of the fluxes and with soil chamber measurements. Unrealistically large and noisy values of horizontal advection from the conventional discontinuous sampling approach, which lead to unrealistic values of NEE, could be eliminated by the alternative approaches presented. We therefore suggest the further testing of those approaches at other sites in order to improve the accuracy of advection measurements and, subsequently, estimates of NEE.  相似文献   

15.
The theoretical correction of CO2 fluxes for high frequency attenuation in closed-path systems was re-summarized and its applicability examined using both measurements obtained at an Asiaflux forest site and empirical transfer functions used in previous studies. For our measurement system, the theoretical transfer function was applicable to high frequency correction, even when condensation occurred in the sampling line. Further, in respect to some measurement systems described in previous studies, it was found that the theoretical function was potentially applicable along with the empirical functions used. Meanwhile, in some systems significant errors could not be resolved by re-estimation of the theories. In these systems, because of undefined buffering effects, the actual response lag time decided by the maximum covariance method or by measurement of the system response time using tracer gas was significantly different from the lag time calculated from the tube dimensions and the measured flow rate. If the average flow rate calculated by the actual lag time was used to determine the theoretical function, the theoretical function became closer to, and sometimes agreed with, the empirical function. Any remaining deviation from each function might be associated with pressure fluctuations, but this problem was unable to be examined here. The results suggested that an empirical formulation for each site is considered applicable rather than a theoretical approach, although the theories are being developed to practical application.  相似文献   

16.
In order to provide high quality data for climate change studies, the data quality of turbulent flux measurements at the station of SACOL (Semi-Arid Climate & Environment Observatory of Lanzhou University), which is located on a semi-arid grassland over the Loess Plateau in China, has been analyzed in detail. The effects of different procedures of the flux corrections on CO2, momentum, and latent and sensible heat fluxes were assessed. The result showed that coordinate rotation has a great influence on the momentum flux but little on scalar fluxes. For coordinate rotation using the planar fit method, different regression planes should be determined for different wind direction sectors due to the heterogeneous nature of the ground surface. Sonic temperature correction decreased the sensible heat flux by about 9%, while WPL correction (correction for density fluctuations) increased the latent heat flux by about 10%. WPL correction is also particularly important for CO2 fluxes. Other procedures of flux corrections, such as the time delay correction and frequency response correction, do not significantly influence the turbulent fluxes. Furthermore, quality tests on stationarity and turbulence development conditions were discussed. Parameterizations of integral turbulent characteristics (ITC) were tested and a specific parameterization scheme was provided for SACOL. The ITC test on turbulence development conditions was suggested to be applied only for the vertical velocity. The combined results of the quality tests showed that about 62%–65% of the total data were of high quality for the latent heat flux and CO2 flux, and as much as about 76% for the sensible heat flux. For the momentum flux, however, only about 35% of the data were of high quality.  相似文献   

17.
When density fluctuations of scalars such as CO2 are measured with open-path gas analyzers, the measured vertical turbulent flux must be adjusted to take into account fluctuations induced by ‘external effects’ such as temperature and water vapour. These adjustments are needed to separate the effects of surface fluxes responsible for ‘natural’ fluctuations in CO2 concentration from these external effects. Analogous to vertical fluxes, simplified expressions for separating the ‘external effects’ from higher-order scalar density turbulence statistics are derived. The level of complexity in terms of input to these expressions are analogous to that of the Webb–Pearman–Leuning (WPL), and are shown to be consistent with the conservation of dry air. It is demonstrated that both higher-order turbulent moments such as the scalar variances, the mixed velocity-scalar covariances, and the two-scalar covariance require significant adjustments due to ‘external effects’. The impact of these adjustments on the turbulent CO2 spectra, probability density function, and dimensionless similarity functions derived from flux-variance relationships are also discussed.  相似文献   

18.
Vertical turbulent fluxes of water vapour, carbon dioxide, and sensible heat were measured from 16 August to the 28 September 2006 near the city centre of Münster in north-west Germany. In comparison to results of measurements above homogeneous ecosystem sites, the CO2 fluxes above the urban investigation area showed more peaks and higher variances during the course of a day, probably caused by traffic and other varying, anthropogenic sources. The main goal of this study is the introduction and establishment of a new gap filling procedure using radial basis function (RBF) neural networks, which is also applicable under complex environmental conditions. We applied adapted RBF neural networks within a combined modular expert system of neural networks as an innovative approach to fill data gaps in micrometeorological flux time series. We found that RBF networks are superior to multi-layer perceptron (MLP) neural networks in the reproduction of the highly variable turbulent fluxes. In addition, we enhanced the methodology in the field of quality assessment for eddy covariance data. An RBF neural network mapping system was used to identify conditions of a turbulence regime that allows reliable quantification of turbulent fluxes through finding an acceptable minimum of the friction velocity. For the data analysed in this study, the minimum acceptable friction velocity was found to be 0.15 m s−1. The obtained CO2 fluxes, measured on a tower at 65 m a.g.l., reached average values of 12 μmol m−2 s−1 and fell to nighttime minimum values of 3 μmol m −2 s−1. Mean daily CO2 emissions of 21 g CO2 m−2d −1 were obtained during our 6-week experiment. Hence, the city centre of Münster appeared to be a significant source of CO2. The half-hourly average values of water vapour fluxes ranged between 0.062 and 0.989 mmol m−2 s−1and showed lower variances than the simultaneously measured fluxes of CO2.  相似文献   

19.
In the context of CO2 surface exchange estimation, an analysis combining the basic principles of diffusion and scalar conservation shows that the mixing ratio is the appropriate variable both for defining the (eddy covariance) turbulent flux and also for expressing the relationship between the turbulent flux and surface exchange in boundary-layer budget equations. Other scalar intensity variables sometimes chosen, both the CO2 density and molar fraction, are susceptible to the influence of surface exchange of heat and water vapour. The application of a hypsometric analysis to the boundary-layer “control volume” below the tower measurement height reveals flaws in previously applied approaches: (a) incompressibility cannot be assumed to simplify mass conservation (the budget in terms of CO2 density); (b) compressibility alone makes the analysis of mass conservation vulnerable to uncertainties associated with resultant non-zero vertical velocities too small to measure or model over real terrain; and (c) the WPL (Webb et al. (1980) Quart J Roy Meteorol Soc 106:85–100) “zero dry air flux” assumption is invalidated except at the surface boundary. Nevertheless, the definition and removal of the WPL terms do not hinge upon this last assumption, and so the turbulent CO2 flux can be accurately determined by eddy covariance using gas analysers of either open- or closed-path design. An appendix discusses the necessary assumptions and appropriate interpretations for deriving the WPL terms.  相似文献   

20.
A 1-year set of measurements of CO2 and energy turbulent fluxes above and within a 25-m pine forest in southern Brazil is analyzed. The study focuses on the coupling state between two levels and its impact on flux determination by the eddy-covariance method. The turbulent series are split in their typical temporal scales using the multiresolution decomposition, a method that allows proper identification of the time scales of the turbulent events. Initially, four case studies are presented: a continually turbulent, a continually calm, a calm then turbulent, and an intermittent night. During transitions from calm to turbulent, large scalar fluxes of opposing signs occur at both levels, suggesting the transference of air accumulated in the canopy during the stagnant period both upwards and downwards. Average fluxes are shown for the entire period as a function of turbulence intensity and a canopy Richardson number, used as an indicator of the canopy coupling state. Above the canopy, CO2 and sensible heat fluxes decrease in magnitude both at the neutral and at the very stable limit, while below the canopy they increase monotonically with the canopy Richardson number. Latent heat fluxes decrease at both levels as the canopy air becomes more stable. The average temporal scales of the turbulent fluxes at both levels approach each other in neutral conditions, indicating that the levels are coupled in that case. Average CO2 fluxes during turbulent periods that succeed very calm ones are appreciably larger than the overall average above the canopy and smaller than the average or negative within the canopy, indicating that the transfer of air accumulated during calm portions at later turbulent intervals affects the flux average. The implications of this process for mean flux determination are discussed.  相似文献   

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