Open-path Analyzer Research Articles

Evaluation of open- and closed-path sampling systems for the determination of emission rates of NH3 and CH4 with inverse dispersion modeling

Abstract. The gas emission rates of ammonia (NH3) and methane (CH4) from an artificial source covering a surface area of 254 m2 were determined by inverse dispersion modeling (IDM) from point-sampling and line-integrated concentration measurements with closed- and open-path analyzers. Eight controlled release experiments were conducted with different release rates ranging from 3.8±0.21 to 17.4±0.4 mg s−1 and from 30.7±1.4 to 142.8±2.9 mg s−1 for NH3 and CH4, respectively. The distance between the source and concentration measurement positions ranged from 15 to 60 m. Our study consisted of more than 200 fluxes averaged over intervals of 10 or 15 min. The different releases cover a range of different climate conditions: cold (<5 ∘C), temperate (<13 ∘C), and warm (<18 ∘C). As the average of all releases with all instrument types, the CH4 recovery rate QbLS/Q was 0.95±0.08 (n=19). There was much more variation in the recovery of NH3, with an average of 0.66±0.15 (n=10) for all the releases with the line-integrated system. However, with an improved sampling line placed close to the source an average recovery rate of 0.82±0.05 (n=3) was obtained for NH3. Under comparable conditions, the recovery rate obtained with an open-path analyzer was 0.91±0.07 (n=3). The effects of measurement distance, physical properties of the sampling line, and deposition are discussed.

Real-World Application of Open-Path UV-DOAS, TDL, and FT-IR Spectroscopy for Air Quality Monitoring at Industrial Facilities

Open-path spectroscopy is known for its ability to provide real-time measurements of dozens of compounds over sampling paths of up to 1000 meters in length. Advances in open-path monitoring technology and data processing techniques, coupled with new regulatory requirements, have greatly increased the acceptance and widespread application of spectroscopy-based open-path measurements. Large industrial facilities adjacent to residential communities are a particular application of interest, because traditional fixed-point analyzers lack the spatial coverage of the open-path instruments. This work discusses technical and practical considerations for the installation and operation of more than 120 open-path analyzers that are currently providing continuous data at several oil refineries in California. Open-path analyzers include ultraviolet differential optical absorbance spectroscopy (UV-DOAS), Fourier transform infrared (FT-IR), and tunable diode laser (TDL) technologies. We will discuss lessons learned from these projects, including fundamental approaches to compound identification, target species detectability, interferences, and data management.

Importance of the Webb, Pearman, and Leuning (WPL) correction for the measurement of small CO<sub>2</sub> fluxes

Abstract. The WPL (Webb, Pearman, and Leuning) correction is fully accepted to correct trace gas fluxes like CO2 for density fluctuations due to water vapour and temperature fluctuations for open-path gas analysers. It is known that this additive correction can be on the order of magnitude of the actual flux. However, this is hardly ever included in the analysis of data quality. An example from the Arctic shows the problems, because the size of the correction is a multiple of the actual flux. As a general result, we examined and tabulated the magnitude of the WPL correction for carbon dioxide flux as a function of sensible and latent heat flux. Furthermore, we propose a parameter to better estimate possible deficits in data quality and recommend integrating the quality flag derived with this parameter into the general study of small carbon dioxide fluxes.

An open-path ammonia analyzer for eddy covariance flux measurement

Although biosphere-atmosphere exchanges of ammonia (NH3) are of great environmental importance regarding, e.g., eutrophication and aerosol-related human health problems, their magnitudes are not well constrained due to the challenges in in-situ measurements of NH3 fluxes. In this paper we introduce a portable, low-power and quantum cascade laser-based open-path analyzer for eddy covariance (EC) measurement of NH3 fluxes particularly at sites without grid power. This analyzer minimizes the sampling artifacts due to surface adsorption of NH3 by using an open-path optical Herriott cell configuration, while also increases its portability and convenience in field deployment by assembling most instrumental components in a highly integrated and compact enclosure. Its performance was tested through laboratory experiments and one-week EC measurements at a subtropical rice paddy. Its uncertainty in measuring NH3 concentration is ± 15%, being comparable to those of the currently available high-sensitivity NH3 analyzers. The flux random error averaged at 26 ± 28% during the entire experimental period. The instrumental noise of the analyzer with a sampling frequency of 10 Hz is estimated to be 0.286 and 0.302 nmol mol‒1 (ppbv) under the laboratory and field conditions, respectively. Under the given test conditions, the detection limit of the EC system is 7.1 ± 1.1 μg N m−2 h−1 for half-hourly NH3 flux measurement. Nearly 91% of the half-hourly fluxes observed during the experimental period were larger than the mean detection limit. Such instrumental sensitivity is adequate for quantifying NH3 emissions from most source ecosystems, and also possible to monitor deposition fluxes at ecosystems away from strong sources with low NH3 concentrations. Availability of this open-path NH3 analyzer provides an access to a better understanding of the biosphere-atmosphere NH3 exchanges and to a more reliable estimate of the NH3 inventory at ecosystem or larger scales.

Normal Cyclic Variation in CO2 Concentration in Indoor Chambers Decreases Leaf Gas Exchange and Plant Growth.

Attempts to identify crop genetic material with larger growth stimulation at projected elevated atmospheric CO2 concentrations are becoming more common. The probability of reductions in photosynthesis and yield caused by short-term variation in CO2 concentration within elevated CO2 treatments in the free-air CO2 enrichment plots raises the question of whether similar effects occur in glasshouse or indoor chamber experiments. These experiments were designed to test whether even the normal, modest, cyclic variation in CO2 concentration typical of indoor exposure systems have persistent impacts on photosynthesis and growth, and to explore mechanisms underlying the responses observed. Wheat, cotton, soybeans, and rice were grown from seed in indoor chambers at a mean CO2 concentration of 560 μmol mol−1, with “triangular” cyclic variation with standard deviations of either 4.5 or 18.0 μmol mol−1 measured with 0.1 s sampling periods with an open path analyzer. Photosynthesis, stomatal conductance, and above ground biomass at 20 to 23 days were reduced in all four species by the larger variation in CO2 concentration. Tests of rates of stomatal opening and closing with step changes in light and CO2, and tests of responses to square-wave cycling of CO2 were also conducted on individual leaves of these and three other species, using a leaf gas exchange system. Reduced stomatal conductance due to larger amplitude cycling of CO2 during growth occurred even in soybeans and rice, which had equal rates of opening and closing in response to step changes in CO2. The gas exchange results further indicated that reduced mean stomatal conductance was not the only cause of reduced photosynthesis in variable CO2 conditions.

A practical application for monitoring methane in ambient air at waste disposal facillities using a portable open path analyzer

It is well known that methane (CH4) is a greenhouse gas, and one of the main gases emittedfrom landfills, CH4 is also emitted from biogasification facilities and composting facilities, Anew method to monitor CH4 in ambient air has been developed using a portable open pathmethane analyzer (POMA) that is retailed as an urban gas leak detector. In this study, we haveinvestigated a practical application of a POMA in the field, and the characteristics of CH4emissions at landfill sites, biogasification facilities, and a composting facility,It is clear that this new method is convenient, and can be used to monitor the averageconcentration of CH4 in ambient air at a waste disposal facility, such as a landfill site, acomposting facility, or a biogasification facility, The concentration of CH4 in ambient air atlandfill sites, a composting facility, and biogasification facilities were found to be 2-30, 2-13,and 3-13 ppm, respectively,

Accounting for spectroscopic effects in laser-based open-path eddy covariance flux measurements.

A significant portion of the production and consumption of trace gases (e.g. CO2 , CH4 , N2 O, NH3 , etc.) by world ecosystems occurs in areas without sufficient infrastructure or easily available grid power to run traditional closed-path flux stations. Open-path analyzer design allows such measurements with power consumption 10-150 times below present closed-path technologies, helping to considerably expand the global coverage and improve the estimates of gas emissions and budgets, informing the remote sensing and modeling communities and policy decisions, all the way to IPCC reports. Broad-band nondispersive infrared devices have been used for open-path CO2 and H2 O measurements since the late 1970s, but since recently, a growing number of new narrow-band laser-based instruments are being rapidly developed. The new design comes with its own challenges, specifically: (a) mirror contamination, and (b) uncontrolled air temperature, pressure and humidity, affecting both the gas density and the laser spectroscopy of the measurements. While the contamination can be addressed via automated cleaning, and density effects can be addressed via the Webb-Pearman-Leuning approach, the spectroscopic effects of the in situ temperature, pressure and humidity fluctuations on laser-measured densities remain a standing methodological question. Here we propose a concept accounting for such effects in the same manner as Webb et al. proposed to account for respective density effects. Derivations are provided for a general case of flux of any gas, examined using a specific example of CH4 fluxes from a commercially available analyzer, and then tested using "zero-flux" experiment. The proposed approach helps reduce errors in open-path, enclosed, and temperature- or pressure-uncontrolled closed-path laser-based flux measurements due to the spectroscopic effects from few percents to multiple folds, leading to methodological advancement and geographical expansion of the use of such systems providing reliable and consistent results for process-level studies, remote sensing and Earth modeling applications, and GHG policy decision-making.

Assessing the surface heating effect of the open-path analyzer of an eddy covariance system with fine-wire thermocouples

The open-path CO2/H2O analyzer that is currently widely used in the eddy covariance (EC) system possesses a surface heating effect, which may influence the accuracy of the eddy-flux measurements. Taking the open-path EC system in the temperate deciduous forest of the Maoershan Ecosystem Research Station as a case, we assessed the heating effect of the open-path CO2/H2O ana-lyzer (Li-7500) with fine-wire thermocouples and tested the applicability of Burba's equations. The results indicated that between April 23 and May 28 in 2016, the daytime air temperature in the mid-optical-path of the Li-7500 was on average 0.2 ℃ higher than the ambient air temperature, while the nocturnal temperature was almost equal, with the peak of the heating often occurring in the morning transition period and at noon or in the early afternoon. The diurnal variation in the heating effect on the sensible heat flux (HS,HE) estimated from the simple linear-fitting models in the fourth method by Burba (BurbaLF) was overall a rectangle-like wave, whereas that from the multiple regression models (BurbaMR) showed a weak mono-peak shape. Compared with direct mea-surements (K079) with the K-type fine-wire thermocouples (with a 0.079 mm diameter of single bare wire) and the HS,HE modeled with two fine-wire thermocouples (K079Model), the estimated HS,HE with the BurbaLF and BurbaMR varied little in the daytime with a lower but wider daytime-peak (20 W·m-2), a sharp transition between daytime and nighttime, and a typical nocturnal va-lue of ＜ 5 W·m-2. The peak values of HS,HE estimated from the K079 and K079Model could be higher than 40 W·m-2, and the values at night fluctuated around zero. The sensible heat flux in the optical-path of the Li-7500 was increased by 13.6% during the daytime. The value of the daytime heating effect on the CO2 turbulent flux (Fc,HC) estimated from the K079 and K079Model was averaged about 0.5 mg CO2·m-2·s-1, twice of the value estimated previously. Compared with the direct measurements, the Burba equations underestimated the Fc,HC around the noon due to the inadequate HS,HE, but overestimated the Fc,HC in the morning, the late afternoon and the night because of over-corrections of HS,HE, resulting in an overall overestimate of Fc,HC. These findings validated that both K079 and K079Model methods can be used to estimate the Fc,HC.

Development of an open-path gas analyser for plume detection in security applications

Abstract We present here an open-path analyser, initially intended for security applications, specifically for the detection of gas plumes from illicit improvised explosive device (IED) manufacturing. Subsequently, the analysers were adapted for methane measurement and used to investigate its applicability for leak detection in different scenarios (e.g. unconventional gas extraction sites). Preliminary results showed consistent measurements of gas plumes in the open path.

Intermediate-scale community-level flux of CO2 and CH4 in a Minnesota peatland: putting the SPRUCE project in a global context

Peatland measurements of CO2 and CH4 flux were obtained at scales appropriate to the in situ biological community below the tree layer to demonstrate representativeness of the spruce and peatland responses under climatic and environmental change (SPRUCE) experiment. Surface flux measurements were made using dual open-path analyzers over an area of 1.13 m2 in daylight and dark conditions along with associated peat temperatures, water table height, hummock moisture, atmospheric pressure and incident radiation data. Observations from August 2011 through December 2014 demonstrated seasonal trends correlated with temperature as the dominant apparent driving variable. The S1-Bog for the SPRUCE study was found to be representative of temperate peatlands in terms of CO2 and CH4 flux. Maximum net CO2 flux in midsummer showed similar rates of C uptake and loss: daytime surface uptake was −5 to −6 µmol m−2 s−1 and dark period loss rates were 4–5 µmol m−2 s−1 (positive values are carbon lost to the atmosphere). Maximum midsummer CH4-C flux ranged from 0.4 to 0.5 µmol m−2 s−1 and was a factor of 10 lower than dark CO2–C efflux rates. Midwinter conditions produced near-zero flux for both CO2 and CH4 with frozen surfaces. Integrating temperature-dependent models across annual periods showed dark CO2–C and CH4–C flux to be 894 ± 34 and 16 ± 2 gC m−2 y−1, respectively. Net ecosystem exchange of carbon from the shrub-forb-Sphagnum-microbial community (excluding tree contributions) ranged from −3.1 gCO2–C m−2 y−1 in 2013, to C losses from 21 to 65 gCO2–C m−2 y−1 for the other years.

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.

Optical Open-Path Analyzer with Filters for Measuring Near-Surface Ozone

The distinguishing feature of our optical open-path analyzer for measuring near-surface ozone concentration is the use of interference filters for isolating individual narrow bands in the UV absorption spectrum of ozone plus a “solar-blind” photoelectron multiplier, with the aim of suppressing transmission in the long-wavelength wings of the interference filters and reducing noise in the detection circuit. The estimated random uncertainty in measuring the ozone concentration is no greater than 4 ppbv.

Experimental evaluation of water vapour cross-sensitivity for accurate eddy covariance measurement of CO2 flux using open-path CO2/H2O gas analysers

Non-dispersive infrared CO2/H2O gas analysers produce erroneous CO2 outputs when CO2 is measured in humid air, unless a correction for water vapour cross-sensitivity is applied. Spectroscopic cross-sensitivities arising from direct absorption interference and from the pressure broadening effect are significant in CO2 flux measurements by the eddy covariance technique using open-path gas analysers over the ocean, as opposed to land-surface measurements, where CO2 fluxes are orders of magnitude larger. In this study, a widely used analyser with manufacturer-determined correction coefficients for both cross-sensitivities was tested by laboratory experiments. Our results showed that the correction coefficient for direct absorption interference was not optimised to calculate CO2 flux accurately, and that the correction coefficient for the pressure broadening caused overestimation of the CO2 mixing ratio flux in the same direction as the water vapour flux. Overestimations of open-path eddy covariance measurements of upward CO2 fluxes in previous ocean observations probably resulted from inaccuracies in both of these correction coefficients. We also found that slight changes in spectroscopic cross-sensitivities due to contamination of the analyser's optical windows by sea salt caused a low bias in CO2 outputs with increasing H2O; however, this contamination effect was not always observed in repeated tests under different contamination conditions. We suggest that previously proposed methods for correcting the effect of optical window contamination is of limited value and that measurement of small CO2 fluxes by the open-path eddy covariance technique over the ocean should be performed after confirming the spectroscopic cross-sensitivity and ensuring that the optical windows are as clean as possible.

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.

Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration

The carbon dioxide concentration in free air carbon dioxide enrichment (FACE) systems typically has rapid fluctuations. In our FACE system, power spectral analysis of CO2 concentration measured every second with an open path analyzer indicated peaks in variation with a period of about one minute. I used open-top chambers to expose cotton and wheat plants to either a constant elevated CO2 concentration of 180 &mu;mol mol-1 above that of outside ambient air, or to the same mean CO2 concentration, but with the CO2 enrichment cycling between about 30 and 330 &mu;mol mol-1 above the concentration of outside ambient air, with a period of one minute. Three short-term replicate plantings of cotton were grown in Beltsville, Maryland with these CO2 concentration treatments imposed for 27-day periods over two summers, and one winter wheat crop was grown from sowing to maturity. In cotton, leaf gas-exchange measurements of the continuously elevated treatment and the fluctuating treatment indicated that the fluctuating CO2 concentration treatment consistently resulted in substantial down-regulation of net photosynthetic rate (P N) and stomatal conductance (g s). Total shoot biomass of the vegetative cotton plants in the fluctuating CO2 concentration treatment averaged 30% less than in the constantly elevated CO2 concentration treatment at 27 days after planting. In winter wheat, leaf gas-exchange measurements also indicated that down-regulation of P N and g s occurred in flag leaves in the fluctuating CO2 concentration treatment, but the effect was not as consistent in other leaves, nor as severe as found in cotton. However, wheat grain yields were 12% less in the fluctuating CO2 concentration treatment compared with the constant elevated CO2 concentration treatment. Comparison with wheat yields in chambers without CO2 addition indicated a nonsignificant increase of 5% for the fluctuating elevated CO2 concentration treatment, and a significant increase of 19% for the constant elevated treatment. The results suggest that treatments with fluctuating elevated CO2 concentrations could underestimate plant growth at projected future atmospheric CO2 concentrations.

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.

A Simple Method of Estimating Scalar Fluxes Over Forests

A simple aerodynamic-variance method is proposed to fill gaps in continuous CO2 flux measurements in rainy conditions, when open-path analysers do not function. The method requires turbulent conditions (friction velocity greater than 0.1 ms–1), and uses measurements of mean wind speed, and standard deviations of temperature and CO2 concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible heat flux and CO2 flux. Friction velocity is estimated from the mean wind speed with a flux-gradient relationship modified for the roughness sublayer. Since normalised standard deviations do not follow Monin-Obukhov similarity theory in the roughness sublayer, a simple classification scheme according to the scalar turbulence scale was used. This scheme is shown to produce sensible heat and CO2 flux estimates that are well correlated with the measured values.

Ten-year measurements of gaseous pollutants in urban air by an open-path analyzer

Object of this study is the versatility of a remote-sensing measurement system through experimental survey of the levels of primary and secondary pollutants in urban areas. For the first time a long-range measurement of different pollutant for a long period are reported in the urban area of Rome. Over 10 years of data collection, levels of NO2, O3, SO2, benzene and toluene are measured and discussed by means of a differential optical absorption spectroscopy (DOAS) system installed at ground level.The period is interesting because it represents one of the most important life-changes in Europe and in Italy: in fact, during the 1990s some significant decrees introduced the catalytic pots and limited the amount of benzene and sulfur dioxide in the fuel. The paper will describe how the main primary and secondary pollutants are changed during all the period: particularly, benzene and SO2 are not over a big social problem for the human health whereas the secondary pollutants, ozone and nitrogen dioxide, have the same trends during the period, meaning a no contribution to their evolution from the atmospheric processes.The DOAS technique is resulted very useful and versatile: comparisons with other traditional analyzers (i.e., GC-PID for benzene and toluene investigations) demonstrate how it allows to have information over all the territory saving the qualitative trends and giving a similar quantitative result. In this respect the authors show that the difference is due to the different sampling ways (the traditional analyzers are punctual whereas the DOAS is a remote-sensing technique covering a wide area) and does not influence the atmospheric process interpretation.Finally, it should be considered that the pollutant trends have been interpreted using the concentration measurements of natural radioactivity as tracer of the dynamic properties of the atmospheric boundary layer.

Fine-wire thermometer to correct CO 2 fluxes by open-path analyzers for artificial density fluctuations

A fine-wire thermometer was applied to measure high-frequency air temperature fluctuations averaged over the length of the optical path of an LI-7500 open-path gas analyzer that was used for CO 2 flux measurements over a forest clear-cut in Sweden. Heat fluxes derived by the fine-wire temperature data were used to correct the CO 2 fluxes for density fluctuations within the optical path. As this method incorporates both natural heat fluxes, accounted for in the conventional Webb–Pearman–Leuning term, and heat fluxes caused by analyzer surface heating, it provides a means to minimize or eliminate the bias in open-path fluxes that could lead to apparent CO 2-uptake in ecosystems during the off-season period, and to a general underestimation of carbon losses in other periods. The proposed technique was compared to two other methods: the conventional WPL-correction and a theoretical correction for the instrument surface heating with empirically estimated instrument surface temperatures. All methods were referenced to a closed-path LI-6262 system because high frequency temperature fluctuations in that system were attenuated by the long intake tube. Measurements demonstrated that the fine-wire correction yielded integrated fluxes that were within 4% of the LI-6262-derived fluxes. The theoretical correction with empirically estimated temperatures led to integrated fluxes being within 6% of those measured using the LI-6262. For the same time period, the conventional WPL-correction underestimated CO 2 losses by 19%. During another period with external heating applied to the LI-7500, the conventional WPL-correction led to an underestimation of the CO 2 efflux by 66%, while the PRT-correction yielded fluxes within 8% of the LI-6262. The experiment confirmed that the bias in the open-path CO 2 flux measurements is mostly caused by instrument surface heating, and that such heat should be accounted for in the density corrections. The experiment also demonstrated that a fine-wire thermometer provided a reliable instrumental solution for the open-path heating phenomenon, and could give adequate corrections on a half-hourly and seasonal basis even when external heat was applied to the open-path analyzer.

Estimate of annual carbon balance of a young Siberian larch (Larix sibirica) plantation in Iceland

The aim of present study was to estimate the annual carbon balance of a young Larix sibirica plantation on former heathland pasture in Iceland. The data covered 1 yr of measurements, year 2005. The daily NEE varied from about +7.7 to -22.5 g CO2 m-2 d-1 in October and July, respectively, resulting in annual sequestration of –727 g CO2 m-2 a-1. This annual carbon balance falls within the observed range of published annual NEEs for other plantation forests. Daily net uptake started in middle of April, when thawing of the soil surface began and it increased as the spring progressed. During a cold spell in the middle of May, the air temperature dropped below zero for some days, resulting in a decline of carbon uptake and a slow recovery of the ecosystem. Such freezing incidents are expected to become more frequent at northern latitudes in warmer future climate and their impact should, therefore, be studied further. Moreover, longer time series of measurements on NEE are needed to reveal the effect of weather conditions on long-term carbon balance. Corrections for small errors in winter and night fluxes from open-path gas analysers also need to be developed.