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  • 1.
    Metcalfe, Daniel
    et al.
    Lunds universitet.
    Riccuito, Daniel
    Oak Ridge National Laboratory.
    Palmroth, Sari
    Duke University.
    Campbell, Catherine
    Sveriges Lanbruksuniversitet.
    Hurry, Vaughan
    Sveriges Lanbruksuniversitet.
    Mao, Jiafu
    Oak Ridge National Laboratory.
    Keel, Sonja
    Institute for Sustainability Sciences, Agroscope, Zurich.
    Linder, Sune
    Sveriges Lantbruksuniversitet.
    Shi, Xiaoying
    Oak Ridge National Laboratory.
    Näsholm, Torgny
    Sveriges Lantbruksuniversitet.
    Ohlsson, Anders
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umea SE-901 83, Sweden.
    Blackburn, M.
    Sveriges Lantbruksuniversitet.
    Thornton, Peter
    Oak Ridge National Laboratory.
    Oren, Ram
    Duke University.
    Informing climate models with rapid chamber measurements of forest carbon uptake2017In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 23, no 5, p. 2130-2139Article in journal (Refereed)
    Abstract [en]

    Models predicting ecosystem carbon dioxide (CO2) exchange under future climate change rely on relatively few real-world tests of their assumptions and outputs. Here, we demonstrate a rapid and cost-effective method to estimateCO2exchange from intact vegetation patches under varying atmospheric CO2concentrations.We find that net ecosys-tem CO2uptake (NEE) in a boreal forest rose linearly by 4.7  0.2% of the current ambient rate for every 10 ppmCO2increase, with no detectable influence of foliar biomass, season, or nitrogen (N) fertilization. The lack of any clearshort-term NEE response to fertilization in such an N-limited system is inconsistent with the instantaneous downreg-ulation of photosynthesis formalized in many global models. Incorporating an alternative mechanism with consider-able empirical support – diversion of excess carbon to storage compounds – into an existing earth system modelbrings the model output into closer agreement with our field measurements. A global simulation incorporating thismodified model reduces a long-standing mismatch between the modeled and observed seasonal amplitude of atmo-spheric CO2. Wider application of this chamber approach would provide critical data needed to further improvemodeled projections of biosphere–atmosphere CO2exchange in a changing climate.

  • 2.
    Ohlsson, Anders
    Department of Forest Ecology and Management, SLU, SE-90183 Umeå, Sweden.
    Reduction of bias in static closed chamber measurement of delta C-13 in soil CO2 efflux2010In: Rapid Communications in Mass Spectrometry, ISSN 0951-4198, E-ISSN 1097-0231, Vol. 24, no 2, p. 180-184Article in journal (Refereed)
    Abstract [en]

    The C-13/C-12 ratio of soil CO2 efflux (delta(e)) is an important parameter in studies of ecosystem C dynamics, where the accuracy of estimated C flux rates depends on the measurement uncertainty Of delta(e). The static closed chamber method is frequently used in the determination of delta(e), where the Soil CO2 efflux is accumulated in the headspace of a chamber placed on top of the soil surface. However, it has recently been shown that the estimate of delta(e) obtained by using this method could be significantly biased, which potentially diminish the usefulness of delta(e) for field applications. Here, analytical and numerical models were used to express the bias in delta(e) as mathematical functions of three system parameters: chamber height (H), chamber radius (R-c), and soil air-filled porosity (theta). These expressions allow optimization of chamber size to yield a bias, which is at a level suitable for each particular application of the method. The numerical model was further used to quantify the effects on the delta(e) bias from (i) various designs for sealing of the chamber to ground, and (ii) inclusion of the commonly used purging step for reduction of the initial headspace CO2 concentration. The present modeling work provided insights into the effects on the delta(e) bias from retardation and partial chamber bypass of the Soil CO2 efflux. The results presented here supported the continued use of the static closed chamber method for the determination of delta(e), with improved control of the bias component of its measurement uncertainty.

  • 3.
    Ohlsson, Anders
    Department of Forest Ecology and Management, Swedish university of Agricultural Sciences (SLU), Skogsmarksgränd, SE-901 83 Umeå, Sweden.
    Theoretical model of the abiotic component of soil (CO2)-C-13 tracer efflux in C-13 pulse-labeling experiments on plant-soil systems2011In: Soil Biology and Biochemistry, ISSN 0038-0717, E-ISSN 1879-3428, Vol. 43, no 3, p. 675-681Article in journal (Refereed)
    Abstract [en]

    For measurement of the time lag between photosynthesis and CO2 efflux from soil, the carbon isotope pulse-labeling technique is considered as the most suitable. However, an interference from the abiotic tracer CO2 component is identified as a key difficulty for obtaining accurate results with this technique. Guidelines on how to reduce this interference are therefore urgently needed. The flux of abiotic (CO2)-C-13 tracer into soil during the labeling stage, and its return to atmosphere during the monitoring stage was modeled numerically, and the labeling stage also analytically. The controls of the abiotic interference were investigated using these models. The amount of the abiotic tracer component and the time distribution of its rate of return to the atmosphere, were predicted by these models. The main model parameters were D-m (=the ratio between the soil (CO2)-C-13 diffusivity and the retardation factor), and the (CO2)-C-13 concentration at the soil atmosphere interface during the labeling stage (S-13), while background (CO2)-C-13 soil production parameters were unnecessary. The presented models guide the selection of experimental parameters for minimization of the abiotic interference. With parameterization for a particular case, the present numerical model provides a preliminary order-of-magnitude estimate of the abiotic component, which would indicate if this interference is of significance.

  • 4. Ohlsson, Anders
    Uncertainty budget for multi-elemental analysis of plant nutrients in conifer foliar material using inductively coupled plasma atomic emission spectrometry (ICP-AES)2012In: Accreditation and Quality Assurance, ISSN 0949-1775, E-ISSN 1432-0517, Vol. 17, no 3, p. 301-313Article in journal (Refereed)
    Abstract [en]

    Measurement uncertainties evaluated according to GUM were given in an uncertainty budget for the measurement of mass fractions of 12 elements in conifer tree needle materials. The measurement was performed using ICP-AES, with prior microwave digestion of the dried sample material. The uncertainty budget for Ca as an example showed that correction for a systematic error was the main source of measurement uncertainty. The key to reduced measurement uncertainty therefore lies in identifying the main sources of systematic errors, and reducing the uncertainty associated with their correction. The usefulness of the uncertainty budget was demonstrated in its application to method validation, to the design of a quality control program, and finally for guiding method optimization.

  • 5.
    Ohlsson, Anders K. E.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Östin, Ronny
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Olofsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Accurate and robust measurement of the external convective heat transfer coefficient based on error analysis2016In: Energy and Buildings, ISSN 0378-7788, E-ISSN 1872-6178, Vol. 117, p. 83-90Article in journal (Refereed)
    Abstract [en]

    Accurate measurement of the convective heat transfer coefficient hc at external surfaces, e.g. at building facades and roofs, is of fundamental importance for heat transfer studies of the built environment. There are two basic methods for measurement of hc, the Loveday and Ito methods, which use one and two heated sensor units, respectively. To guide in selection of method and operating conditions, and in design of the sensor, we performed an error analysis. This included estimation of systematic errors, comparison between methods, and to established Nusselt number correlations, sensitivity analysis, and an evaluation of the measurement uncertainty. The main conclusion was that both methods, at forced convection, yielded measurement uncertainties at the 4 % level, provided that the Ito method was operated under the new condition, where one of its sensors remained unheated. However, at natural convection conditions, the Ito method cannot be operated with one of its sensors unheated, since hc is then zero at that sensor surface, which violates the method assumption that hc is the same at both sensors. Sensitivity analysis showed that systematic errors will be reduced by decreasing the sensor surface emissivity. The major source of measurement uncertainty was the conductive heat flux estimate.

  • 6.
    Ohlsson, Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Yang, Bin
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Ekblad, Alf
    Örebro universitet.
    Boman, Christoffer
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Nyström, Robin
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Olofsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Stable carbon isotope labelled carbon dioxide as tracer gas for air change rate measurement in a ventilated single zone2017In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 115, p. 173-181Article in journal (Refereed)
    Abstract [en]

    Carbon dioxide (CO2) has often been used as tracer gas for measurement of the air change rate l (h1 ) in buildings. In such measurements, a correction is required for the presence of indoor CO2, which commonly consists of atmospheric CO2 mixed with human respired CO2. Here, 13C isotope-labelled CO2 was employed as tracer gas, and cavity ring-down spectroscopy (CRDS) was used for simultaneous measurement of the two isotope analogues 12CO2 and 13CO2. This enabled the simultaneous measurement of the 13CO2 tracer gas, with correction for background 13CO2, and the concentration of indoor CO2, allowing for presence of occupants. The background correction procedure assumes that the isotope delta of the background indoor CO2 equals dB ¼ 19‰, based on the prior information that the carbon isotope ratio RB ¼ 13C/12C of all carbon in the bio-geosphere of earth is in the interval 0.010900 < RB < 0.011237. Evidence supported that l could be accurately measured, using the new 13CO2 tracer method, even when the background 13CO2 concentration varied during the measurement time interval, or when the actual dB value differed from the assumed value. The measurement uncertainty for l was estimated at 3%. Uncertainty in l due to uncertainty in RB, uRB(l), was estimated to increase with a decreasing amount of 13CO2 tracer. This indicated that at least 4 ppm tracer must be used, in order to obtain uRB(l)/l < 2%. The temporal resolution of the l measurement was 1.25/l h.

  • 7.
    Ohlsson, K. E. Anders
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-90183 Umeå , Sweden.
    Uncertainty of Blank Correction in Isotope Ratio Measurement2013In: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 85, no 11, p. 5326-5329Article in journal (Refereed)
    Abstract [en]

    Blank correction for isotope ratio measurement on small amounts of substances is often limited by presence of a blank, with an apparent isotopic composition different from that of the sample. For isotope ratios, blank correction is commonly performed either by the regression method, which works without the need for estimation of the blank, or by the subtraction method. With the subtraction method, estimation of the amount and isotope delta of the blank is required, and these estimates could be obtained either by direct, semi-indirect, or indirect measurement. Previously given expressions for the standard uncertainties of indirectly measured blank amounts and blank isotope deltas did not account for covariance between input quantities. In the present work, a previously published data set was re-evaluated, with covariance terms properly included in the calculation of uncertainties. It was shown that covariance effects may yield a significant reduction in uncertainty estimates, both for blank quantities and for blank corrected results. For series measurements on a standard material, it was also shown that the distribution of individual corrected isotope delta values around the average value was approximately normal, with its standard deviation equal to the estimated standard uncertainty of the corrected values. In most cases, it was observed that the regression and subtraction methods yield approximately the same blank corrected average values and uncertainties, regardless of method selected for estimation of blank quantities.

  • 8.
    Ohlsson, K. E. Anders
    et al.
    Swedish University of Agricultural Sciences, Department of Forest Ecology and Management, Umeå, Sweden.
    Olofsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Quantitative infrared thermography imaging of the density of heat flow rate through a building element surface2014In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 134, p. 499-505Article in journal (Refereed)
    Abstract [en]

    Infrared thermography is often used to record an image of the building envelope surface temperature, and thereby acquire qualitative information on its thermal insulation performance. Recently, a thermography method has evolved, which enables quantitative measurement of the 2-dimensional pattern of the density of heat flow rate (q) across the building element surface. However, based on previous estimates of its measurement uncertainty, the capacity of the thermography method to yield accurate results has been questioned. We present here an improved procedure for measurement of q, with an evaluation of measurement errors. The main improvement consists of the simultaneous measurement of surface temperature, surrounding radiative temperature, and air temperature, based on information included in one single thermal camera image. This arrangement allows for accurate measurements of small temperatures differences, and thereby reduced uncertainty in the measurement of q. The measurement bias was evaluated experimentally by a comparison of thermography results against a reference method. Under natural convective conditions, there was a 2.6 W m(-2) constant difference between the two methods. The measurement uncertainty u(q) was estimated as a function of q. Based on this, the lower limit of the measurement working range was determined to be 6 W m(-2), which corresponds to less than 10% relative uncertainty. In the case of forced convection, the thermography method yielded less reliable results. The reason for this was the sensitivity of the results to the choice of model for the convective heat transfer coefficient, and the difficulty to select the position for measurement of the wind speed, which is appropriate for this model.

  • 9.
    Ohlsson, K.E. Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Fransson, Åke
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Olofsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Social wasp nests as source of bioinspired design of building skins: 1-2 October 2018, Bern, Switzerland2018In: Advanced Building Skins, Advanced Building Skins GmbH , 2018Conference paper (Refereed)
  • 10.
    Ohlsson, K.E. Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Östin, Ronny
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Olofsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Sol-air thermometer measurement of heat transfer coefficient at building outdoor surface2018In: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 132, p. 357-362Article in journal (Refereed)
    Abstract [en]

    Heat flow measurement with a heat flow meter is a standardized method (ISO 9869-1) to estimate thermal transmittance (U-value) of a building element. The heat flow meter is a thin plate mounted on top of the surface of the element, and measures the heat flux q through the plate. The measured q is the product of the difference in temperatures between exterior and interior environment, and the U-value. The heat transferred from the element is based on the radiant and the convective heat transfer.

    ISO 9869-1 specifies that the environment temperature Te “is a notional temperature" and it "cannot be measured directly” (section A.3.1). The air temperature Ta is proposed as a reasonable approximation for the indoor environment, while overcast conditions and absence of significant solar radiation are specified conditions for replacing Te with Ta for the exterior environment.

    The sol-air thermometer (SAT) measures the sol-air temperature Tsa, i.e. the equivalent temperature of the convective and the radiative environment. In the absence of solar radiation, Te = Tsa. SAT is a sensor consisting of a thin flat solid plate, of high thermal conductivity. The front side of the sensor is exposed to the environment, whose Tsa is to be measured, and the backside is thermally insulated. The temperature of the SAT-plate equals Tsa.

    In this work we propose introduction of the measured Te in the existing standard (ISO 9869-1). The method for measurement of Tsa, using the SAT, has been demonstrated experimentally for different periods, without solar radiation present and under stable climatic conditions.

  • 11.
    Ohlsson, K.E. Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Östin, Ronny
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Olofsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Sol-air thermometer measurement of heat transfer coefficient at building outdoor surface2018In: Cold Climate HVAC 2018: Sustainable Buildings in Cold Climates, Springer, 2018, p. 329-338Conference paper (Refereed)
    Abstract [en]

    There exists a building energy performance gap between theoretical simulations and the actual energy usage as measured. One potential reason for this gap might be a mismatch between predicted and measured values of the heat flux q through the building envelope. There is therefore a need to develop accurate and more cost-efficient methods for measurement of q. The standard ISO 9869-1 states that, at the outdoor surface, q = ho(Ts − Tenv), where ho is the overall heat transfer coefficient, including both convective and radiative components, Tenv is the environmental temperature, and Ts is the temperature of the building surface. It has previously been shown that the sol-air thermometer (SAT) could be used for convenient measurement of Tenv under dark conditions. In the present work, two SAT units, one heated and the other unheated, were employed for accurate outdoor measurements of ho in cold winter climate. Validation was performed by comparison of results from the new method against measurements, where previously established methodology was used. With current operating conditions, the measurement uncertainty was estimated to be 3.0 and 4.4%, for ho equal to 13 and 29 Wm−2K−1, respectively. The new SAT steady-state method is more cost-effective compared to previous methodology, in that the former involves fewer input quantities (surface emissivity and infrared radiation temperature are unnecessary) to be measured, while giving the same ho results, without any sacrifice in accuracy. SAT methodology thus enables measurement of both Tenv and ho, which characterizes the building thermal environment, and supports estimation of q.

  • 12.
    Ohlsson, K.E. Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Östin, Ronny
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Olofsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Step-transient method for measurement of the heat transfer coefficient at surfaces exposed to simulated building outdoor environments using the sol-air thermometer2018In: Journal of Building Physics, ISSN 1744-2591, E-ISSN 1744-2583, Vol. 42, no 3, p. 373-387Article in journal (Refereed)
  • 13.
    Ohlsson, K.E. Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Östin, Ronny
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Staffan, Grundberg
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Olofsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Dynamic model for measurement of convective heat transfer coefficient at external building surfaces2016In: Journal of Building Engineering, ISSN 2352-7102, Vol. 7, p. 239-245Article in journal (Refereed)
    Abstract [en]

    Uncertainties in current empirical models for the convective heat transfer coefficient (CHTC) have large impact on the accuracy of building energy simulations (BES). These models are often based on measurements of the CHTC, using a heated gradient sensor, where steady-state convective air flow is assumed. If this requirement is not fulfilled there will be a dynamic measurement error. The objectives were to construct a validated dynamic model for the heated gradient sensor, and to use this model to improve accuracy by suggesting changes in sensor design and operating procedure. The linear thermal network model included three state-space variables, selected as the temperatures of the three layers of the heated gradient sensor. Predictions of the major time constant and temperature time evolution were in acceptable agreement with experimental results obtained from step-response experiments. Model simulations and experiments showed that the sensor time constant increases with decreasing CHTC value, which means that the sensor response time is at maximum under free convection conditions. Under free convection, the surface heat transfer resistance is at maximum, which cause enhanced heat loss through the sensor insulation layer. Guidelines are given for selection of sampling frequency, and for evaluation of dynamic measurement errors.

  • 14.
    Olofsson, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Ohlsson, K. E. Anders
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Östin, Ronny
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Measurement of the environmental temperature using the sol-air thermometer2017In: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 132, p. 357-362Article in journal (Refereed)
    Abstract [en]

    Heat flow measurement with a heat flow meter is a standardized method (ISO 9869-1) to estimate thermal transmittance (U-value) of a building element. The heat flow meter is a thin plate mounted on top of the surface of the element, and measures the heat flux q through the plate. The measured q is the product of the difference in temperatures between exterior and interior environment, and the U-value. The heat transferred from the element is based on the radiant and the convective heat transfer.

    ISO 9869-1 specifies that the environment temperature Te “is a notional temperature" and it "cannot be measured directly” (section A.3.1). The air temperature Ta is proposed as a reasonable approximation for the indoor environment, while overcast conditions and absence of significant solar radiation are specified conditions for replacing Te with Ta for the exterior environment.

    The sol-air thermometer (SAT) measures the sol-air temperature Tsa, i.e. the equivalent temperature of the convective and the radiative environment. In the absence of solar radiation, Te = Tsa. SAT is a sensor consisting of a thin flat solid plate, of high thermal conductivity. The front side of the sensor is exposed to the environment, whose Tsa is to be measured, and the backside is thermally insulated. The temperature of the SAT-plate equals Tsa.

    In this work we propose introduction of the measured Te in the existing standard (ISO 9869-1). The method for measurement of Tsa, using the SAT, has been demonstrated experimentally for different periods, without solar radiation present and under stable climatic conditions.

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