Closed-path Infrared Gas Analyzer Research Articles

Dried, closed-path eddy covariance method for measuring carbon dioxide flux over sea ice

Abstract. The Arctic marine environment plays an important role in the global carbon cycle. However, there remain large uncertainties in how sea ice affects air–sea fluxes of carbon dioxide (CO2), partially due to disagreement between the two main methods (enclosure and eddy covariance) for measuring CO2 flux (FCO2). The enclosure method has appeared to produce more credible FCO2 than eddy covariance (EC), but is not suited for collecting long-term, ecosystem-scale flux datasets in such remote regions. Here we describe the design and performance of an EC system to measure FCO2 over landfast sea ice that addresses the shortcomings of previous EC systems. The system was installed on a 10 m tower on Qikirtaarjuk Island – a small rock outcrop in Dease Strait located roughly 35 km west of Cambridge Bay, Nunavut, in the Canadian Arctic Archipelago. The system incorporates recent developments in the field of air–sea gas exchange by measuring atmospheric CO2 using a closed-path infrared gas analyzer (IRGA) with a dried sample airstream, thus avoiding the known water vapor issues associated with using open-path IRGAs in low-flux environments. A description of the methods and the results from 4 months of continuous flux measurements from May through August 2017 are presented, highlighting the winter to summer transition from ice cover to open water. We show that the dried, closed-path EC system greatly reduces the magnitude of measured FCO2 compared to simultaneous open-path EC measurements, and for the first time reconciles EC and enclosure flux measurements over sea ice. This novel EC installation is capable of operating year-round on solar and wind power, and therefore promises to deliver new insights into the magnitude of CO2 fluxes and their driving processes through the annual sea ice cycle.

Measuring turbulent CO<sub>2</sub> fluxes with a closed-path gas analyzer in a marine environment

Abstract. In this study, we introduce new observations of sea–air fluxes of carbon dioxide using the eddy covariance method. The measurements took place at the Utö Atmospheric and Marine Research Station on the island of Utö in the Baltic Sea in July–October 2017. The flux measurement system is based on a closed-path infrared gas analyzer (LI-7000, LI-COR) requiring only occasional maintenance, making the station capable of continuous monitoring. However, such infrared gas analyzers are prone to significant water vapor interference in a marine environment, where CO2 fluxes are small. Two LI-7000 analyzers were run in parallel to test the effect of a sample air drier which dampens water vapor fluctuations and a virtual impactor, included to remove liquid sea spray, both of which were attached to the sample air tubing of one of the analyzers. The systems showed closely similar (R2=0.99) sea–air CO2 fluxes when the latent heat flux was low, which proved that neither the drier nor the virtual impactor perturbed the CO2 flux measurement. However, the undried measurement had a positive bias that increased with increasing latent heat flux, suggesting water vapor interference. For both systems, cospectral densities between vertical wind speed and CO2 molar fraction were distributed within the expected frequency range, with a moderate attenuation of high-frequency fluctuations. While the setup equipped with a drier and a virtual impactor generated a slightly higher flux loss, we opt for this alternative for its reduced water vapor cross-sensitivity and better protection against sea spray. The integral turbulence characteristics were found to agree with the universal stability dependence observed over land. Nonstationary conditions caused unphysical results, which resulted in a high percentage (65 %) of discarded measurements. After removing the nonstationary cases, the direction of the sea–air CO2 fluxes was in good accordance with independently measured CO2 partial pressure difference between the sea and the atmosphere. Atmospheric CO2 concentration changes larger than 2 ppm during a 30 min averaging period were found to be associated with the nonstationarity of CO2 fluxes.

Eddy covariance carbonyl sulphide flux measurements with a quantum cascade laser absorption spectrometer.

The trace gas carbonyl sulphide (COS) has lately received growing interest in the eddy covariance (EC) community due to its potential to serve as an independent approach for constraining gross primary production and canopy stomatal conductance. Thanks to recent developments of fast-response high-precision trace gas analysers (e.g. quantum cascade laser absorption spectrometers (QCLAS)), a handful of EC COS flux measurements have been published since 2013. To date, however, a thorough methodological characterisation of QCLAS with regard to the requirements of the EC technique and the necessary processing steps has not been conducted. The objective of this study is to present a detailed characterization of the COS measurement with the Aerodyne QCLAS in the context of the EC technique, and to recommend best EC processing practices for those measurements. Data were collected from May to October 2015 at a temperate mountain grassland in Tyrol, Austria. Analysis of the Allan variance of high-frequency concentration measurements revealed sensor drift to occur under field conditions after an averaging time of around 50 s. We thus explored the use of two high-pass filtering approaches (linear detrending and recursive filtering) as opposed to block averaging and linear interpolation of regular background measurements for covariance computation. Experimental low-pass filtering correction factors were derived from a detailed cospectral analysis. The CO2 and H2O flux measurements obtained with the QCLAS were compared against those obtained with a closed-path infrared gas analyser. Overall, our results suggest small, but systematic differences between the various high-pass filtering scenarios with regard to the fraction of data retained in the quality control and flux magnitudes. When COS and CO2 fluxes are combined in the so-called ecosystem relative uptake rate, systematic differences between the high-pass filtering scenarios largely cancel out, suggesting that this relative metric represents a robust key parameter comparable between studies relying on different post-processing schemes.

Interannual variability of net ecosystem production for a broadleaf deciduous forest in Sapporo, northern Japan

To estimate net ecosystem production (NEP), ecosystem respiration (RE), and gross primary production (GPP), and to elucidate the interannual variability of NEP in a cool temperate broadleaf deciduous forest in Sapporo, northern Japan, we measured net ecosystem exchange (NEE) using an eddy covariance technique with a closed-path infrared gas analyzer from 2000 to 2003. NEP, RE, and GPP were derived from NEE, and data gaps were filled using empirical regression models with meteorological variables such as photosynthetic active radiation and soil temperature. In general, NEP was positive (CO2 uptake) from May to September, either positive or negative in October, and negative (CO2 release) from November to the following April. NEP rapidly increased during leaf expansion in May and reached its maximum in June or July. The four-year averages (± standard deviation) of annual NEP, GPP, and RE were 443 ± 45, 1,374 ± 39, and 931 ± 11…−2 year−1, respectively. The lower annual NEP and GPP in 2000 may have been caused by lower solar radiation in the foliated season. During the foliated season, monthly GPP varied from year to year more than monthly RE. Variations in the amount of incoming solar radiation may have caused the interannual variations in the monthly GPP. Additionally, in May, the timing of leaf expansion had a large impact on GPP. Variations in GPP affected the interannual variation in NEP at our site. Thus, interannual variation in NEP was affected by the incoming solar radiation and the timing of leaf expansion.

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.

New Transfer Functions for Correcting Turbulent Water Vapour Fluxes

We address the problem of the high-frequency correction of water vapour fluxes measured by eddy covariance with a closed-path infrared gas analyser (IRGA). Different transfer functions are compared and evaluated at a forested (Vielsalm, Belgium) and an agricultural (Lonzée, Belgium) site. Classical functions, usually applied to correct CO2 fluxes (Gaussian, Lorentzian), are found to be unsuited to water vapour cospectral corrections, being characterised by too sharp a decrease at high frequency. Two other functions characterised by a lower decreasing slope are found to better fit experimental transfer functions. They were calibrated and validated on experimental transfer functions and their dependency on air humidity is parameterised. On this basis, new correction coefficients are estimated. The coefficients are found to be larger than those based on the classical functions, even when the dependency of the latter on air humidity is taken into account. The difference amounts to 10% at the forested site and to 5% larger at the crop site. The study highlights the necessity of characterising the water transfer function shape and taking it into account in the correction factor at each site equipped with a closed path IRGA.

Energy Balance Closure Using Eddy Covariance Above Two Different Land Surfaces and Implications for CO2 Flux Measurements

Componentsofthesurfaceenergybalanceofamatureborealjackpineforestand 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 (Rn) 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 fluc- tuations 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 systematicadvectionoffluxesawayfromtheeddy-covariance(EC)towerwerediscountedas significant issues. Spatial and temporal distributions of the total surface-layer heat flux (T), i.e. the sum of sensible heat flux (H )a ndλE, were well behaved and differences between the relative magnitudes of the turbulentfluxes 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,

다중 수증기/이산화탄소 프로파일 관측 시스템의 원리와 활용

The multi-level profile system is designed to measure the vertical profile of and concentrations in the surface layer to estimate the storage effects within the plant canopy. It is suitable for long-term experiments and can be used also in advection studies for estimating the spatial variability and vertical gradients in concentration. It enables the user to calculate vertical fluxes of water vapor, and other trace gases using the surface layer similarity theory and to infer their sources or sinks. The profile system described in this report includes the following components: sampling system, calibration and flow control system, closed path infrared gas analyzer(IRGA), vacuum pump and a datalogger. The sampling system draws air from 8 inlets into the IRGA in a sequence, so that for 80 seconds air from all levels is measured. The calibration system, controlled by the datalogger, compensates for any deviations in the calibration of the IRGA by using gas sources with known concentrations. The datalogger switches the corresponding valves, measures the linearized voltages from the IRGA, calculates the concentrations for each monitoring level, performs statistical analysis and stores the final data. All critical components are mounted in an environmental enclosure and can operate with little maintenance over long periods of time. This report, as a practical manual, is designed to provide helpful information for those who are interested in using profile system to measure evapotranspiration and net ecosystem exchanges in complex terrain.

Inter-annual variability of carbon budget components in an AsiaFlux forest site estimated by long-term flux measurements

The CO 2 flux has been measured by the aerodynamic method since 1993 and by the eddy covariance method since 1998 over a cool-temperate deciduous forest in Japan, which is one of the AsiaFlux forest sites. In the aerodynamic method, the CO 2 flux is estimated using the vertical gradient of the CO 2 concentration and the diffusion coefficient over the canopy. The eddy fluxes of sensible heat, water vapor, and CO 2 are measured using a sonic anemometer and a closed-path infrared gas analyzer. The CO 2 flux estimated by the eddy covariance method was mainly used to analyze observational errors and uncertainties caused by measurement and data processing. The CO 2 flux determined by the aerodynamic method was validated by comparison with that estimated by the eddy covariance method. The nighttime net ecosystem CO 2 exchange (NEE) was highly sensitive to atmospheric stability because of the complex terrain around the site. The nighttime NEE was estimated with and without correction, depending on the friction velocity. The annual net ecosystem production (NEP) values for 1999, 2000, and 2001 were estimated to be 198, 309, and 290 g C m −2 year −1 with correction and 251, 376, and 342 g C m −2 year −1 without correction, respectively. The nighttime correction is responsible for an uncertainty of approximately 50–70 g C m −2 year −1 in the annual NEP at this site. The NEP has been calculated for 9 years, which is the longest record at any particular site in Asia, as a consequence of the flux measurements. The annual NEP was estimated to be 237 ± 92 g C m −2 year −1 (mean ± S.D.) from 1994 to 2002 with a large year-to-year variability. The annual NEP had a similar year-to-year variation to that for gross primary production (GPP), and the carbon budget components representing productivity had more remarkable year-to-year variability than the respiration (ecosystem respiration and soil respiration). A high annual NEP was observed in 1998 (329 g C m −2 year −1) and in 2002 (346 g C m −2 year −1), mainly due to the high CO 2 uptake observed during the first half of the growth period in both years. The results of the present study suggest that the high GPP in 1998 and 2002 was caused by early leaf emergence related to an unusually warm spring under the influence of the El Niño. The results also suggest that the increase in CO 2 uptake due to the long growth periods in 1998 and 2002 was more marked than the enhancement of respiration in the cool-temperate forest.

Mobile, outdoor continuous‐flow isotope‐ratio mass spectrometer system for automated high‐frequency 13C‐ and 18O‐CO2 analysis for Keeling plot applications

A continuous-flow isotope-ratio mass spectrometer (CF-IRMS, custom-made GasBenchII and Delta(plus)Advantage, ThermoFinnigan) was installed on a grassland site and interfaced with a closed-path infrared gas analyser (IRGA). The CF-IRMS and IRGA were housed in an air-conditioned travel van. Air was sampled at 1.5 m above the 0.07-m tall grassland canopy, drawn through a 17-m long PTFE tube at a rate of 0.25 L s(-1), and fed to the IRGA and CF-IRMS in series. The IRMS was interfaced with the IRGA via a stainless steel capillary inserted 0.5 m into the sample air outlet tube of the IRGA (forming an open split), a gas-tight pump, and a sample loop attached to the eight-port Valco valve of the continuous-flow interface. Air was pumped through the 0.25-mL sample loop at 10 mL s(-1) (a flushing frequency of 40 Hz). Air samples were analysed at intervals of approx. 2.8 min. Whole system precision was tested in the field using air mixed from pure CO2 and CO2-free air by means of mass flow controllers. The standard deviation of repeated single measurements was 0.21-0.07 per thousand for delta13C and 0.34-0.14 per thousand for delta18O of CO2 in air with mixing ratios ranging between 200-800 micromol mol(-1). The CO2 peak area measured by the IRMS was proportional to the CO2 mixing ratio (r2 = 1.00), allowing estimation of sample air CO2 mixing ratio from IRMS data. A 1-day long measurement cycle of CO2, delta13C and delta18O of air sampled above the grassland canopy was used to test the system for Keeling plot applications. Delta18O exhibited a clear diurnal cycle (4 per thousand range), but short-term (1-h interval) variability was small (average SD 0.38 per thousand). Yet, the correlation between delta18O and CO2 mixing ratio was relatively weak, and this was true for both the whole data set and 1-h subsets. Conversely, the delta13C of all 541 samples measured during the 25.2-h interval fitted well the Keeling regression (r2 = 0.99), yielding an intercept of -27.40 per thousand (+/-0.07 per thousand SE). Useful Keeling regressions (r2 > 0.9, average r2 = 0.96) also resulted from data collected over 1-h intervals of the 12-h long twilight and dark period. These indicated that 13C content of ecosystem respiration was approx. constant near -27.6 per thousand. The precision of the present system is similar to that of current techniques used in ecosystem studies which employ flask sampling and a laboratory-based CF-IRMS. Sampling (and measurement) frequency is greatly increased relative to systems based on flask sampling, and sampling time (0.025 s per sample) is decreased. These features increase the probability for sampling the entire CO2 range which occurs in a given time window. The system obviates sample storage problems, greatly minimises handling needs, and allows extended campaigns of high frequency sampling and analysis with minimal attendance.

Spectral Characteristics and Correction of Long-Term Eddy-Covariance Measurements Over Two Mixed Hardwood Forests in Non-Flat Terrain

We present turbulence spectra and cospectra derived from long-term eddy-covariancemeasurements (nearly 40,000 hourly data over three to four years) and the transferfunctions of closed-path infrared gas analyzers over two mixed hardwood forests inthe mid-western U.S.A. The measurement heights ranged from 1.3 to 2.1 times themean tree height, and peak vegetation area index (VAI) was 3.5 to 4.7; the topographyat both sites deviates from ideal flat terrain. The analysis follows the approach ofKaimal et al. (Quart. J. Roy. Meteorol. Soc. 98, 563–589, 1972) whose results were based upon 15 hours of measurements atthree heights in the Kansas experiment over flatter and smoother terrain. Both thespectral and cospectral constants and stability functions for normalizing and collapsingspectra and cospectra in the inertial subrange were found to be different from those ofKaimal et al. In unstable conditions, we found that an appropriate stabilityfunction for the non-dimensional dissipation of turbulent kinetic energy is of the form Φe(ζ) = (1 - b-ζ)-1/4 - c-ζ, where ζ representsthe non-dimensional stability parameter. In stable conditions, a non-linear functionGxy(ζ) = 1 + bxyζc xy (cxy < 1) was found to benecessary to collapse cospectra in the inertial subrange. The empirical cospectralmodels of Kaimal et al. were modified to fit the somewhat more (neutraland unstable) or less (stable) sharply peaked scalar cospectra observed over forestsusing the appropriate cospectral constants and non-linear stability functions. Theempirical coefficients in the stability functions and in the cospectral models varywith measurement height and seasonal changes in VAI. The seasonal differencesare generally larger at the Morgan Monroe State Forest site (greater peak VAI) andcloser to the canopy. The characteristics of transfer functions of the closed-path infrared gas analysersthrough long-tubes for CO2 and water vapour fluxes were studied empirically. This was done by fitting the ratio between normalized cospectra of CO2 or watervapour fluxes and those of sensible heat to the transfer function of a first-order sensor.The characteristic time constant for CO2 is much smaller than that for water vapour. The time constant for water vapour increases greatly with aging tubes. Three methods were used to estimate the flux attenuations and corrections; from June through August, the attenuations of CO2 fluxes are about 3–4% during the daytime and 6–10% at night on average. For the daytime latent heat flux (QE), the attenuations are foundto vary from less than 10% for newer tubes to over 20% for aged tubes. Correctionsto QE led to increases in the ratio (QH + QE)/(Q* - QG) by about 0.05 to0.19 (QH is sensible heat flux, Q* is net radiation and QG is soil heat flux),and thus are expected to have an important impact on the assessment of energy balanceclosure.

Year-long carbon dioxide exchange above a broadleaf deciduous forest in Sapporo, Northern Japan

This paper reports the results of a full year (2000) of measurements of CO2 flux at a successional forest of mature birch and growing oak with Sasa-bamboo in Sapporo, Japan. Eddy covariance fluxes of CO2 were obtained using a closed-path infrared gas analyzer. Changes in CO2 storage under the eddy-flux measurement level were quantified using vertical profiles of the CO2 concentration. Seasonal variations in net CO2 exchange between the forest and the atmosphere are discussed in terms of both phenological developments of the forest canopy and micrometeorological variables. To estimate the annual exchange of CO2, the net CO2 exchange data both during periods of poor turbulence and during periods of missing data were replaced by simple parametric models based on measurements of soil temperatures and photosynthetically active radiation. The corrected annual carbon sequestration estimate was 260 g C m−2. The estimates of annual gross carbon gain and loss at the forest were 1120 and 860 g C m−2.

Energy balance closure at FLUXNET sites

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO 2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated with the eddy covariance technique or from errors in calculating the available energy terms. Although it was not entirely possible to critically evaluate all the possible sources of the imbalance, circumstantial evidence suggested a link between the imbalance and CO 2 fluxes. For a given value of photosynthetically active radiation, the magnitude of CO 2 uptake was less when the energy imbalance was greater. Similarly, respiration (estimated by nocturnal CO 2 release to the atmosphere) was significantly less when the energy imbalance was greater.

Measuring forest floor CO 2 fluxes in a Douglas-fir forest

CO 2 exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system between April and December, 2000. Fluxes were measured every half hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path infrared gas analyzer. Maximum CO 2 fluxes measured varied by a factor of almost 3 between the chamber locations, while the highest daily average fluxes observed at two chamber locations occasionally reached values near 15 μmol C m −2 s −1. Generally, fluxes ranged between 2 and 10 μmol C m −2 s −1 during the measurement period. CO 2 flux from the forest floor was strongly related to soil temperature with the highest correlation found with 5 cm depth temperature. A simple temperature dependent exponential model fit to the nighttime fluxes revealed Q 10 values in the normal range of 2–3 during the warmer parts of the year, but values of 4–5 during cooler periods. Moss photosynthesis was negligible in four of the six chambers, while at the other locations, it reduced daytime half-hourly net CO 2 flux by about 25%. Soil moisture had very little effect on forest floor CO 2 flux. Hysteresis in the annual relationship between chamber fluxes and soil temperatures was observed. Net exchange from the six chambers was estimated to be 1920±530 g C m −2 per year, the higher estimates exceeding measurement of ecosystem respiration using year-round eddy correlation above the canopy at this site. This discrepancy is attributed to the inadequate number of chambers to obtain a reliable estimate of the spatial average soil CO 2 flux at the site and uncertainty in the eddy covariance respiration measurements.

Measurements of CO 2 and energy fluxes over a mixed hardwood forest in the mid-western United States

Results from the first full year of measurements (1998–1999) of above canopy CO 2 and energy fluxes at the AmeriFlux site in the Morgan-Monroe State Forest, IN, USA, are presented. The site is located in an extensive secondary successional broadleaf forest in the maple-beech to oak-hickory transition zone. The minimum fetch is 4 km. Turbulent flux measurements are obtained by an eddy-covariance system at 46 m (1.8 times the canopy height) with a closed-path infrared gas analyzer. Peak vegetation area index (VAI) was determined as 4.7±0.5 and the mean albedo during the vegetative period was 0.15±0.02. The aerodynamic roughness length was estimated as 2.1±1.1 m. It showed little variation with wind direction or season. The seasonal variations of energy partitioning and of net CO 2 exchange are discussed in terms of the phenological development of the forest. To estimate the annual net ecosystem production (NEP) and carbon sequestration, eddy-covariance measurements during periods of poorly developed turbulence at night, and missing data were replaced by a simple parametric model based on measurements of soil temperature and photosynthetically active radiation (PAR). The night-time flux correction reduces the annual sequestration estimate by almost 50%. The corrected estimate of annual NEP for the 1998–1999 season is 2.4 t C ha −1 per year ±10%.

Carbon dioxide and latent heat flux measurements in a windbreak-sheltered orchard

An eddy correlation system for carbon dioxide and latent heat flux measurement comprising a twin-channel, closed-path infra-red gas analyzer and sonic anemometer is described. Its performance was examined by comparing latent heat fluxes measured concurrently with an open-path sensor over windbreak-sheltered kiwifruit ( Actinidia deliciosia var. deliciosia). Measurements were carried out at the height of surrounding windbreaks (6 m). Excellent average agreement was found between latent heat fluxes measured by either system with no evidence of bandwidth limiting by the closed-path components. This agreement gives confidence that carbon dioxide fluxes were also correctly measured. It also provides validation of the density corrections for eddy fluxes which differ significantly for closed- and open-path sensors. Carbon dioxide fluxes were closely related to solar radiation, reaching 800 μg m −2 s −1 towards the canopy early in the afternoon. Water-use efficiency was approximately 7 mg of CO 2 per g of water evaporated and showed little dependence on either radiation or water use. Night-time fluxes were erratic in low wind conditions showing infrequent large excursions which are attributed to the action of occasional gusts sweeping out high levels of carbon dioxide accumulated from canopy and under-storey respiration. During windier conditions, night-time fluxes were typically 200 μg m −2 s −1 away from the canopy. The role of windbreaks in isolating the canopy from the free airstream was evident in the large diurnal variation in ambient carbon dioxide concentrations (640–800 mg m −3) observed 3–4 m above the height of the crop.