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1.
An experimental micrometeorological set-up was established at the CARBOEURO-FLUX site in Tharandt, Germany, to measure all relevant variables for the calculation of the vertical and horizontal advective fluxes of carbon dioxide. The set-up includes two auxiliary towers to measure horizontal and vertical CO 2 and H 2O gradients through the canopy, and to make ultrasonic wind measurements in the trunk space. In combination with the long-term flux tower an approximately even-sided prism with a typical side-length of 50 m was established. It is shown that under stable (nighttime) conditions the mean advective fluxes have magnitudes on the same order as the daily eddy covariance (EC) flux, which implies that they play a significant, but not yet fully understood, role in the carbon budget equation. The two advective fluxes are opposite and seem to cancel each other at night (at least for these measurements). During the day, vertical advection tends to zero, while horizontal advection is still present implying a flow of CO 2 out of the control volume. From our measurements, a mean daily gain of 2.2 gC m –2 d –1 for the horizontal advection and a mean daily loss of 2.5 gC m –2d –1 for the vertical advection is calculated for a period of 20 days. However the large scatter of the advective fluxes has to be further investigated. It is not clear yet whether the large variability is natural or due to measurement errors and conceptual deficiencies of the experiment. Similar results are found in the few comparable studies. 相似文献
2.
A field study of surface-air exchange of carbon, water, and energy was conducted at a mid-latitude, mixed forest on non-flat terrain to investigate how to best interpret biological signals from the eddy flux data that may be subject to advective influences. It is shown that during periods of Southwest winds (sector with mild topography), the eddy fluxes are well-behaved in terms of energy balance closure, the existence of a constant flux layer, consistency with chamber observations and the expected abiotic controls on the fluxes. Advective influences are evident during periods with wind from a steep (15%) slope to the Northeast of the tower. These influences appear more severe on CO 2 flux, particularly in stable air, than on the energy fluxes. Large positive flux of CO 2 (> 23 mol m -2 s -1) occurs frequently at night. The annual sum of the carbon flux is positive, but the issue about whether the forest is a source of atmospheric carbon remains inconclusive.Attempts are made to assess vertical advectionusing the data collected on a single tower. Over the Southwestsector, vertical advection makes a statistically significant but small contribution to the 30-min energy imbalance and CO 2 flux variations. Contributions by horizontal advection may be larger but cannot be verified directly by the current experimental method. 相似文献
3.
Anemometer and CO 2 concentration data from temporary campaigns performed at six CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulent fluxes in nighttime conditions. While storage was observed to be significant only during periods of both low turbulence and low advection, the advective fluxes strongly influence the nocturnal CO 2 balance, with the exception of almost flat and highly homogeneous sites. On the basis of the main factors determining the onset of advective fluxes, the ‘advection velocity’, which takes net radiation and local topography into account, was introduced as a criterion to characterise the conditions of storage enrichment/depletion. Comparative analyses of the six sites showed several common features of the advective fluxes but also some substantial differences. In particular, all sites where advection occurs show the onset of a boundary layer characterised by a downslope flow, negative vertical velocities and negative vertical CO 2 concentration gradients during nighttime. As a consequence, vertical advection was observed to be positive at all sites, which corresponds to a removal of CO 2 from the ecosystem. The main differences between sites are the distance from the ridge, which influences the boundary-layer depth, and the sign of the mean horizontal CO 2 concentration gradients, which is probably determined by the source/sink distribution. As a consequence, both positive and negative horizontal advective fluxes (corresponding respectively to CO 2 removal from the ecosystem and to CO 2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO 2 exchange rates. 相似文献
4.
Eddy correlation equipment was used to measure mass and energy fluxes over a soybean crop. A rapid response CO 2 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 CO 2 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 CO 2 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 CO 2 fluxes caused by respiration from plants, soil, and roots. These CO 2 fluxes ranged from - 0.1 to - 0.25 mg m –2s –1.From the data collected over mature soybeans, a relationship between CO 2 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
5.
利用锡林浩特国家气候观象台开路涡度相关系统、辐射土壤观测系统,测得的长期连续通量观测数据,对锡林浩特草原2009—2011年期间的CO 2通量观测特征进行了分析。结果表明:CO 2通量存在明显的年际、季节和日变化特征。3 a中NEE年际变率达到200 g·m -2,季节变率最大达到460 g·m -2,日变化幅度生长季最大达到0.25 mg·m -2·s -1。通过不同时间尺度碳通量与温度、水分、辐射等环境因子的分析,认为CO 2通量日变化主要受温度和光合有效辐射影响,而季节变化和年变化主要受降水和土壤含水量的影响。降水强度及时间分布是制约牧草CO 2吸收的关键因素,大于15%的土壤含水量有利于促进牧草生长。 相似文献
6.
Ultrasonic wind measurements, sonic temperature and air temperature data at two heights in the advection experiment MORE II
were used to establish a complete budget of sensible heat including vertical advection, horizontal advection and horizontal
turbulent flux divergence. MORE II took place at the long-term Carbo-Europe IP site in Tharandt, Germany. During the growing
period of 2003 three additional towers were established to measure all relevant parameters for an estimation of advective
fluxes, primarily of CO 2. Additionally, in relation to other advection experiments, a calculation of the horizontal turbulent flux divergence is proposed
and the relation of this flux to atmospheric stability and friction velocity is discussed. In order to obtain a complete budget,
different scaling heights for horizontal advection and horizontal turbulent flux divergence are tested. It is shown that neglecting
advective fluxes may lead to incorrect results. If advective fluxes are taken into account, the sensible heat budget based
upon vertical turbulent flux and storage change only, is reduced by approximately 30%. Additional consideration of horizontal
turbulent flux divergence would in turn add 5–10% to this sum (i.e., the sum of vertical turbulent flux plus storage change
plus horizontal and vertical advection). In comparison with available energy horizontal advection is important at night whilst
horizontal turbulent flux divergence is rather insignificant. Obviously, advective fluxes typically improve poor nighttime
energy budget closure and might change ecosystem respiration fluxes considerably. 相似文献
7.
Using a climate model with a sophisticated land surface scheme, simulations were conducted to explore the impact of increases in leaf-level carbon dioxide (CO 2) on evaporation, temperature and other land surface quantities. Fifty-one realizations were run, for each of four Januarys and four Julys for CO 2 concentrations at leaf-level of 280, 375, 500, 650, 840 and 1,000 ppmv. Atmospheric CO 2 concentration was held constant at 375 ppmv in all experiments. Statistically significant decreases in evaporation and increases in temperature occur in specific regions as leaf-level CO 2 is increased from 280 to 375 ppmv. These same areas expand geographically, and the magnitude of the changes increase as leaf-level CO 2 is increased further suggesting that changes are caused by the increase in leaf-level CO 2 and are not internal model variability. As leaf-level CO 2 is increased further, larger areas of the continental surface are affected by increasing amounts and a statistically significant change in precipitation is seen. The increase in leaf-level CO 2 from 280 ppmv to 375 ppmv causes statistically significant changes in the evaporation over 12% of continental surfaces in July. This increases to 25% at 500 ppmv, 35% at 650 ppmv, 41% at 840 ppmv and 47% at 1,000 ppmv. This affects temperature and rainfall by similar amounts, generally in coincident regions. An analysis of these results over key regions shows that the probability density functions of the latent heat flux and temperature are affected non-uniformly. There is a shift in the latent heat flux probability density function to lower values, mainly through the reduction in the upper tail of the distribution. The temperature probability density function shifts to higher values, mainly through an increase in the upper tail of the distribution indicating that the impact is focussed on extremes. Given that there are a suite of well evaluated land surface models that include the biogeochemical effects of increasing CO 2 we suggest that the inclusion of such a model should be a recommended component of climate models used in future assessment reports by the Intergovernmental Panel on Climate Change. 相似文献
8.
Carbon dioxide exchange was measured, using the eddy correlation technique, over a tallgrass prairie in northeastern Kansas, U.S.A., during a six-month period in 1987. The diurnal patterns of daytime and nocturnal CO 2 fluxes are presented on eight selected days. These days were distributed throughout most of the growing season and covered a wide range of meteorological and soil water conditions. The midday CO 2 flux reached a maximum of 1.3 mg m -2 (ground area) s -1 during early July and was near zero during the dry period in late July. The dependence of the daytime carbon dioxide exchange on pertinent controlling variables, particularly photosynthetically active radiation, vapor pressure deficit and soil water content is discussed. The nocturnal CO 2 flux (soil plus plant respiration) averaged -0.4 mg m -2 (ground area) s -1 during early July and was about -0.2 mg m -2 s -1 during the dry period.Published as Paper No. 9061, Journal Series, Agricultural Research Division, University of Nebraska-Lincoln, U.S.A.Research Associate and Professor, respectively. 相似文献
9.
The results of one year’s monitoring in Srednja Bijambarska Cave (Bosnia and Herzegovina) are presented and discussed. Temporal
variations of the carbon dioxide (CO 2) concentration are controlled by the switching between two ventilation regimes driven by outside temperature changes. A regression
model with a simple perfectly mixed volume applied to a cave sector (“Music hall”) resulted in an estimate of ventilation
rates between 0.02 h −1 and 0.54 h −1. Carbon dioxide input per plan surface unit is estimated by the model at around 50 × 10 −6 mh −1 during the winter season and up to more than 1000 × 10 −6 mh −1 during the first temperature falls at the end of summer (0.62 μmoles m −2 s −1 and 12.40 μmoles m −2 s −1 for normal conditions respectively). These values have been found to be related to the cave ventilation rate and dependent
on the availability of CO 2 in the surrounding environment. For airflow close to zero the values of CO 2 input per plan surface have a range in the order of magnitude of a few units × 10 −6 mh −1. Based on two experiments, the anthropogenic contribution from cave visitors has been calculated, at between 0.35 l CO2 min −1 person −1 and 0.45 l CO2min −1person −1. 相似文献
10.
Cussac cave was investigated to assess the cave air temperature variations and to understand its ventilation regime. This cave is located in an active karst system in the south west part of France. It has a single entrance and is considered as a cold air trap. In this study, air mass exchanges were probed. Measurements of temperature and Pco2 with a 30-min frequency were made in several locations close to the cave entrance. Speed of the air flow was also measured at the door of cave entrance. Results show that cave air Pco2 varies from 0.18 to 3.33 %. This cave appears to be a CO2 source with a net mass of 2319 tons blown in 2009. Carbon-stable isotope of CO2 (13Cco2) ranges from −20.6 ‰ in cold season to −23.8 ‰ in warm season. Cave air is interpreted as a result of a mix between external air and an isotopically depleted air, coming from the rock environment. The isotopic value of the light member varies through time, from −23.9 to −22.5 ‰. Furthermore, this study ascertains that the cave never stops in communicating with the external air. The ventilation regime is identified. (1) In cold season, the cave inhales at night and blows a little at the warmest hours. However, in warm season, (2) cave blows at night, but (3) during the day, a convection loop takes place in the entrance area and prevents the external air from entering the cave, confirming the cold air trap. 相似文献
11.
With the aid of eddy correlation instrumentation, the components of the energy budget and CO 2 flux were measured over grain sorghum grown at Mead, Nebraska. Diurnal patterns of sensible heat, latent heat, CO 2 and momentum flux are examined for typical days. On a mostly clear day when the crop leaf area index was 3.7, net radiation reached a mid-day peak of 560 W m -2, while sensible and latent heat fluxes peaked at 50 and 460 W m -2, respectively. The peak CO 2 flux occurring just prior to solar noon was 1.5 mg m -2(ground area) s -1. CO 2 flux (respiration from plants, soil and roots) in the early evening was about -0.28 mg m -2 (ground area) s -1.A relationship between CO 2 flux and photosynthetically active radiation (PAR) was developed. Except during the late stage of growth (growth stage 8.5, toward the initiation of senescence), the crop showed no evidence of saturation up to PAR 1800 Ei m -2s -1. The light compensation point was found to be about 211 Ei m -2s -1. Examination of CO 2 flux-PAR relationships for selected days through the season indicated an aging effect in terms of a decrease in photosynthetic activity of the sorghum canopy. Measurements made on two consecutive days demonstrate the effects of weather conditions on CO 2 flux and carbon-water flux ratio (a measure of water use efficiency of the crop). The occurrence of regional sensible heat advection with concommitant high vapor pressure deficit and air temperature-limited CO 2 exchange reduced the carbon-water flux ratio.Published as Paper No. 7717, Journal Series, Nebraska Agricultural Research Division. The work was conducted under Regional Research Project 11-33 and Nebraska Agricultural Research Division Project 27-003.Formerly Post Doctoral Research Associate (now at the University of Connecticut Department of Renewable Natural Resources Storrs CT) 相似文献
12.
本文基于2007年和2008年生长季内蒙古羊草和大针茅草原湍流观测资料,分析了两种典型草原下垫面生长季的不同土壤水分条件下水汽和二氧化碳通量交换特征及其控制因子。主要结果如下:(1)在植被生长峰值期,日尺度上,干旱条件下土壤湿度是潜热通量的主要控制因子,而土壤水分条件较好时潜热通量主要受净辐射控制。(2)与大针茅草原相比,羊草草原叶面积指数较大,水分条件较好时,其潜热通量平均值更大,CO 2吸收能力更强,吸收CO 2更多;但在土壤水分胁迫出现时,羊草草原叶面的气孔闭合度急剧增加,大针茅草原的潜热通量、和CO 2吸收反而更大,表现出更为耐旱的植被特性。(3)地表导度可以用来解释土壤水分条件对羊草和大针茅草原碳水通量的影响。 相似文献
13.
Several trace sulfur gases that can have a significant influence on atmospheric chemistry are emitted from biological systems. In order to begin to address biological questions on the mechnisms of production of such gases, laboratory-scale experiments have been developed that reproduce such emissions under controlled conditions. Using a flux chamber technique, flats containing soil, or soil plus plants were sampled for the net fluxes of sulfur gases. The major sulfur gas emitted from all the plants tested (corn, alfalfa, and wheat) was dimethyl sulfide (DMS). Alfalfa and wheat also emitted lesser amounts of methanethiol, variable amounls of hydrogen sulfide, and in some experiments wheat emitted carbon disulfide. The use of a plant incubator allowed a systematic study of the effects of variables such as temperature, photon flux, and carbon dioxide levels, on these emissions. Fluxes of all the emitted sulfur gases increased exponentially with increasing air temperature, and increased with increasing photon flux up to a saturation level of \~300 E/m –2 sec -1. Three to four-fold changes in DMS flux were observed during light to dark or dark to light transitions. By varying the CO 2 content of the chamber flush gas, it was shown that the observed sulfur fluxes from corn and alfalfa were not related to the CO 2 concentration. Growing these crop plants through holes in a Teflon soil-covering film allowed a separate determination of soil and foliage emissions and substantiation of the light dependent uptake of COS by growing vegetation observed in previous field studies. 相似文献
14.
Turf-grass lawns are ubiquitous in the United States. However direct measurements of land–atmosphere fluxes using the eddy-covariance
method above lawn ecosystems are challenging due to the typically small dimensions of lawns and the heterogeneity of land
use in an urbanised landscape. Given their typically small patch sizes, there is the potential that CO 2 fluxes measured above turf-grass lawns may be influenced by nearby CO 2 sources such as passing traffic. In this study, we report on two years of eddy-covariance flux measurements above a 1.5 ha
turf-grass lawn in which we assess the contribution of nearby traffic emissions to the measured CO 2 flux. We use winter data when the vegetation was dormant to develop an empirical estimate of the traffic effect on the measured
CO 2 fluxes, based on a parametrised version of a three-dimensional Lagrangian footprint model and continuous traffic count data.
The CO 2 budget of the ecosystem was adjusted by 135gCm −2 in 2007 and by 134gCm −2 in 2008 to determine the natural flux, even though the road crossed the footprint only at its far edge. We show that bottom-up
flux estimates based on CO 2 emission factors of the passing vehicles, combined with the crosswind-integrated footprint at the distance of the road, agreed
very well with the empirical estimate of the traffic contribution that we derived from the eddy-covariance measurements. The
approach we developed may be useful for other sites where investigators plan to make eddy-covariance measurements on small
patches within heterogeneous landscapes where there are significant contrasts in flux rates. However, we caution that the
modelling approach is empirical and will need to be adapted individually to each site. 相似文献
15.
Emissions of N 2O, CH 4, and CO 2 from soils at two sites in the tropical savanna of central Venezuela were determined during the dry season in February 1987. Measured arithmetic mean fluxes of N 2O, CH 4, and CO 2 from undisturbed soil plots to the atmosphere were 2.5×10 9, 4.3×10 10, and 3.0×10 13 molecules cm -2 s -1, respectively. These fluxes were not significantly affected by burning the grass layer. Emissions of N 2O increased fourfold after simulated rainfall, suggesting that production of N 2O in savanna soils during the rainy season may be an important source for atmospheric N 2O. The CH 4 flux measurements indicate that these savanna soils were not a sink, but a small source, for atmospheric methane. Fluxes of CO 2 from savanna soils increased ninefold two hours after simulated rainfall, and remained three times higher than normal after 16 hours. More research is needed to clarify the significance of savannas in the global cycles of N 2O, CH 4, CO 2, and other trace gases, especially during the rainy season. 相似文献
16.
We present an approach for assessing the impact of systematic biases in measured energy fluxes on CO 2 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 CO 2 flux measurements. Our results suggest that incomplete energy balance closure does not necessarily lead to an underestimation of CO 2 fluxes despite the existence of surface energy imbalance; either an overestimation or underestimation of CO 2 fluxes is possible depending on local atmospheric conditions and measurement errors in the sensible heat, latent heat, and CO 2 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 CO 2 fluxes. 相似文献
17.
The most direct method for flux estimation uses eddy covariance, which is also the most commonly used method for land-based measurements of surface fluxes. Moving platforms are frequently used to make measurements over the sea, in which case motion can disturb the measurements. An alternative method for flux estimation should be considered if the effects of platform motion cannot be properly corrected for. Three methods for estimating CO 2 fluxes are studied here: the eddy-covariance, the inertial-dissipation, and the cospectral-peak methods. High-frequency measurements made at the land-based Östergarnsholm marine station in the Baltic Sea and measurements made from a ship during the Galathea 3 expedition are used. The Kolmogorov constant for CO 2, used in the inertial-dissipation method, is estimated to be 0.68 and is determined using direct flux measurements made at the Östergarnsholm site. The cospectral-peak method, originally developed for neutral stratification, is modified to be applicable in all stratifications. With these modifications, the CO 2 fluxes estimated using the three methods agree well. Using data from the Östergarnsholm site, the mean absolute error between the eddy-covariance and inertial-dissipation methods is 0.25 μmol m ?2 s ?1. The corresponding mean absolute error between the eddy-covariance and cospectral-peak methods is 0.26 μmol m ?2 s ?1, while between the inertial-dissipation and cospectral-peak methods it is 0.14 μmol m ?2 s ?1. 相似文献
18.
Methods of calibrating infrared CO 2 analysers for sensitivity to CO 2 and water vapour are described. Equations to correct eddy covariance CO 2 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 CO 2 analysers, and (iii) flux losses resulting from damping of concentration fluctuations in a tube used to sample air for closed-path CO 2 analysers. Examples of flux corrections required for typical instruments are presented. 相似文献
19.
The interpretation of stable isotopes in speleothems in terms of past temperature variability or precipitation rates requires a comprehensive understanding of the climatic factors and processes that influence the δ 18O signal in the way through the atmosphere to the cave, where carbonate precipitates acquiring its final isotopic composition. This study presents for the first time in the Iberia Peninsula an integrated analysis of the isotopic composition of rainfall (δ 18O p) during 2010–2012 years and, through a detailed monitoring survey, the transference of the primary isotopic signal throughout the soil and epikarst into the Molinos cave (Teruel, NE Spain). Both air temperature and amount of precipitation have an important effect on δ 18O p values, clearly imprinting a seasonal variability modulated by an amount effect when rainfall events are more frequent or intense. Air mass history and atmospheric circulation influences are considered through the study of weather types, synoptic-scale climate patterns and large-scale atmospheric circulation indexes (North Atlantic Oscillation and Western Mediterranean Oscillation) revealing a dominant source effect on δ 18O p values in this region where tropical North Atlantic and Western Mediterranean are the two moisture source regions. A delay of 2–3 months occurs between the dripwater oxygen isotopic composition (δ 18O d) respect to δ 18O p values as a consequence of large residence time in the epikarst. Limited calcite precipitates are found from winter to spring when δ 18O d values are less negative and dripwater rates are constant. This study suggests that NE Iberian δ 18O calcite proxy records are best interpreted as reflecting a combination of parameters, not just paleotemperature or paleorainfall and, if extending present-day situation towards the recent past, a biased signal towards winter values should be expected in Molinos speleothem records. 相似文献
20.
We measured CO 2, CO, CH 4, H 2, and NO 2 in air masses polluted by savanna fires over Côte d'Ivoire, western Africa. Elevated concentrations of these trace gases were found in fire plumes and also in extensive haze layers. Trace gas mixing ratios ranged as high as 605 ppmv for CO 2, 14.8 ppmv for CO, 2.7 ppmv for CH 4, 4.2 ppmv for H 2, and 25 ppbv for NO 2. We compare our emission ratios to those obtained in previous field and laboratory studies. The emission ratios, expressed as an average and as a range or as an average only, were: dCO/dCO 2 5.3×10 –2 (3–18×10 –2); dCH 4/dCO 5.3×10 –2; dH 2/dCO 2.4×10 –1 and dNO 2/dCO 2 1.8×10 –4 (1.5–2.2×10 –4). The values found match those found during similar measurements, though our results point to rather vigorous burning in the savanna of western Africa. 相似文献
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