In-canopy turbulence is a required input to study pollutant cycling and chemistry within plant canopies and to link concentrations and sources. Despite the importance of grasslands worldwide, most previous work has focused on forests and crops. Here, turbulence parameters in a mature agricultural grassland canopy were measured with a combination of a small ultrasonic anemometer, hotwire anemometry and a radon (Rn) tracer technique, as part of a measurement to study ammonia (NH3) exchange with grassland. The measurements are used to derive vertical profiles of basic turbulent parameters, for quadrant-hole analysis of the two-parametric frequency distributions of u'−w' and to derive in-canopy eddy diffusivities as input for models of in-canopy tracer transport. The results are in line with previous measurements on taller canopies, but shows increased decoupling between in-canopy flow and above-canopy turbulence. The comparison of sonic anemometry and Rn measurements implies that Lagrangian time-scales must decrease sharply at the ground, with important implications for estimating the magnitude of ground-level and soil emissions from concentration measurements. Atmospheric stability above and within the canopy has little influence on the standard deviation of vertical wind component inside the canopy. Use of the turbulence parameters in an analytical Lagrangian framework, which is here validated for heat transfer, suggests that measured in-canopy profiles of NH3 are consistent with a ground-level source, presumably from senescent plant parts, which is recaptured by the overlying canopy.

Stable Cl isotope ratios (37Cl/35Cl) were measured in groundwater samples from the southwestern flow system of the Great Artesian Basin, Australia to gain a better understanding of the Cl− sources and transport mechanisms. δ37Cl values range from 0‰ to −2.5‰ (SMOC), and are inversely correlated with Cl− concentration along the inferred flow direction. The Cl isotopic compositions, in conjunction with other geochemical parameters, suggest that Cl− in groundwaters is not derived from salt dissolution. Mixing of the recharge water with saline groundwater cannot explain the relationship between δ37Cl and Cl− concentration measured. Marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the Cl− concentrations observed in wells that are close to the recharge area, and in groundwaters sampled along the southern transect. δ37Cl values measured in the leachate of the Bulldog shale suggest that the aquitard is the subsurface source of Cl− for the majority of groundwater samples studied. Diffusion is likely the mechanism through which Cl− is transported from the pore water of the Bulldog shale to the aquifer. However, a more detailed study of the aquitard rocks is required to verify this hypothesis.

Abstract. A one-year time series of atmospheric CO2 measurements from Bern, Switzerland, is presented. O2/N2 and Ar/N2 ratios as well as stable carbon and oxygen isotopes of CO2 and δ29N2, δ34O2 and δ36Ar were measured periodically during a one year period. Additionally, the 222Rn activity was measured during three months in the winter 2004. Using the correlation from short-term fluctuations of CO2 and 222Rn we estimated a mean CO2 flux density between February 2004 and April 2004 in the region of Bern of 95±39 tC km–2month–1. The continuous observations of carbon dioxide and associated tracers shed light on diurnal and seasonal patterns of the carbon cycle in an urban atmosphere. There is considerable variance in nighttime δ13C and δ18O of source CO2 throughout the year, however, with generally lower values in winter compared to summertime. The O2:CO2 oxidation ratio during the nighttime build-up of CO2 varies between –0.96 and –1.69 mol O2/mol CO2. Furthermore, Ar/N2 measurements showed that artifacts like thermal fractionation at the air intake are relevant for high precision measurements of atmospheric O2.

A one-year time series of atmospheric CO2 measurements from Bern, Switzerland, is presented. O-2/N-2 and Ar/N-2 ratios as well as stable carbon and oxygen isotopes of CO2 and delta N-29(2), delta O-34(2) and delta Ar-36 were measured periodically during a one year period. Additionally, the Rn-222 activity was measured during three months in the winter 2004. Using the correlation from short-term fluctuations of CO2 and Rn-222 we estimated a mean CO2 flux density between February 2004 and April 2004 in the region of Bern of 95 +/- 39 tC km(-2) month(-1). The continuous observations of carbon dioxide and associated tracers shed light on diurnal and seasonal patterns of the carbon cycle in an urban atmosphere. There is considerable variance in nighttime delta C-13 and delta O-18 of source CO2 throughout the year, however, with generally lower values in winter compared to summertime. The O-2:CO2 oxidation ratio during the nighttime build-up of CO2 varies between -0.96 and -1.69 mol O-2/mol CO2. Furthermore, Ar/N-2 measurements showed that artifacts like thermal fractionation at the air intake are relevant for high precision measurements of atmospheric O-2.

A one-year time series of atmospheric CO2 measurements from Bern, Switzerland, is presented. O2/N2 and Ar/N2 ratios as well as stable carbon and oxygen isotopes of CO2 and δ29N2, δ34O2 and δ36Ar were measured periodically during a one year period. Additionally, the 222Rn activity was measured during three months in the winter 2004. Using the correlation from short-term fluctuations of CO2 and 222Rn we estimated a mean CO2 flux density between February 2004 and April 2004 in the region of Bern of 95±39 tC km–2month–1. The continuous observations of carbon dioxide and associated tracers shed light on diurnal and seasonal patterns of the carbon cycle in an urban atmosphere. There is considerable variance in nighttime δ13C and δ18O of source CO2 throughout the year, however, with generally lower values in winter compared to summertime. The O2:CO2 oxidation ratio during the nighttime build-up of CO2 varies between –0.96 and –1.69 mol O2/mol CO2. Furthermore, Ar/N2 measurements showed that artifacts like thermal fractionation at the air intake are relevant for high precision measurements of atmospheric O2.

Measurements of radiochlorine (36Cl), radiogenic noble gases (4He and 40Ar), and stable chlorine isotope ratios were obtained to assess the residence time of groundwater in the Nubian Aquifer of the Western Desert of Egypt. Measured 36Cl/Cl ratios yield apparent residence times from ∼0.2 to 1.2 × 106 years in the deep (600–1200 m) groundwater (assuming constant Cl) and ≤0.16 × 106 years in the shallow (<600 m) groundwater. Values of δ37Cl in the groundwater strengthen the application of the 36Cl dating method by constraining Cl sources and identifying groundwater mixing. Dissolved gases were measured in some of the deep groundwater samples. Measured 4He concentrations indicate accumulation of radiogenic 4He that is qualitatively consistent with the age progression indicated by the 36Cl/Cl ratios, but the flux of external 4He from the underlying crust has not been quantified and is not constant throughout the aquifer. Concentrations of 40Ar range from 3.3 to 6.7 × 10−4 ccSTP/g and indicate excess air incorporation at recharge. Measured 40Ar/36Ar ratios do not exceed the atmospheric ratio. A two‐dimensional numerical hydrodynamic transect of the aquifer was modeled from the area of the Uweinat Uplift to the northern Bahariya Oasis. Predicted groundwater velocities in the deep portion of the aquifer are 0.5–3.5 m/yr with groundwater residence times up to 9 × 105 years; residence times up to 1.3 × 106 years are predicted in the confining shale. Aquifer properties are estimated by using the model to fit the measured 36Cl/Cl ratios. Under these conditions, hydrodynamic residence times are within about 30% of those calculated from 36Cl when mixing of Cl− is accounted for in the highest‐Cl− deep groundwaters. By mutually calibrating multiple methods (hydrodynamic, 36Cl, and 4He), a consistent picture of the Nubian Aquifer has emerged in which lateral flow from a southern recharge area dominates the deep horizons, while shallow horizons contain younger, autochthonous recharge.

The present study focuses on the description of the vertical dispersion of trace gases within the Amazon rain forest. A Lagrangian approach is parameterised using in-canopy turbulence measurements made at a site in Rondonia (Reserva Jaru). In contrast to common scaling schemes that solely depend on friction parameters measured above the canopy, a combined scaling that also includes night-time free convective mixing in the lower part of dense vegetation canopies is proposed here. Rn-222 concentration profiles and soil flux measurements made at a second site near Manaus (Reserva Cuieiras) are used to evaluate the derived parameterisation and the uncertainties of the forward (prediction of concentration profiles) and inverse (prediction of vertical source/sink distributions) solution of the transfer equations. Averaged day- and night-time predictions of the forward solution agree with the observations within their uncertainty range. During night-time, a weak, but effective free convective mixing process in the lower canopy ensures a relatively high flushing rate with residence times of <1 h at half canopy height in contradiction to earlier estimates for Amazon rain forest.The inverse solution for Rn-222 source/sink distributions shows a high sensitivity to small measurement errors, especially for daytime conditions, when there is efficient turbulent mixing in the upper canopy and profile gradients are small. The inverse approach is also applied to CO2 and H2O profiles. The predicted net fluxes show a reasonable agreement with Eddy Covariance (EC) measurements made above the forest canopy, although the scatter is large and the day-time solutions for CO2 are very sensitive to measurement errors. However, this is not the case for typical night-time conditions, where the CO2 profile gradients in the upper canopy are large. The inverse approach predicts a mean CO2 emission flux of 7.5 mu mol m(-2) s(-1) for the investigation period. This value is somewhat larger compared to estimates based on EC measurements, which are quite uncertain at night-time and thus reduces the upper bound of the estimated carbon sink strength for Amazonian rain forest. (c) 2005 Elsevier B.V. All rights reserved.

Several important fundamental and applied problems require a quantification of slow rates of groundwater flow. To resolve these problems helium appears to be a promising tracer. In this contribution we discuss a new approach, which gives the helium inventory in a rock – pore water system by using the relevant mineral record, i.e., without extraction and investigation of the porewater samples. Some U- and Th-poor minerals such as quartz (quartz separates from Permo-Carboniferous Formation, sandstone–shale interlayering, Molasses Basin, Northern Switzerland, hereafter PCF, are used in this study) contain excessive helium having migrated into their internal helium-accessible volume (HAV) from the surrounding porewater [I.N. Tolstikhin, B.E. Lehmann, H.H. Loosli, A. Gautschi, Helium and argon isotopes in rocks, minerals and related groundwaters: a case study in Northern Switzerland, Geochim. Cosmochim. Acta 60 (1996) 1497–1514]. These volumes are estimated by using helium as a nano-size penetrating tool, i.e., by saturation of the minerals with helium under controlled pressure–temperature conditions and subsequent measurements of the helium-saturated concentrations. In the quartz separates HAV/total volume ratios vary from 0.017% to 0.16%; along with the measured initial (unsaturated) He concentration the HAV gives the internal helium pressure, the mean value obtained for 7 samples (25 sample aliquots) is P=0.45±0.15 atm (1 σ). The product of helium pressure and solubility (7.35×10−3 cc STP He/cc H2O for the temperature and salinity of PCF aquifers reported in [F.J. Pearson, W. Balderer, H.H. Loosli, B.E. Lehmann, A. Matter, T. Peters, H. Schmassmann, A. Gautschi, Applied Isotope Hydrogeology–A Case Study in Northern Switzerland, Elsevier Amsterdam, 1991, 439 pp.]) is the mineral-derived He concentration in the respective porewater, CPW=0.0035±0.0017 cc He/cc H2O. This value is in full accord with measured He concentrations in PCF aquifers, CPCF, varying from 0.0045 to 0.0016 cc He/cc H2O.

Measurements of 81 Kr/Kr in deep groundwater from the Nubian Aquifer (Egypt) were performed by a new laser‐based atom‐counting method. 81 Kr ages range from ∼2 × 10 5 to ∼1 × 10 6 yr, correlate with 36 Cl/Cl ratios, and are consistent with lateral flow of groundwater from a recharge area near the Uweinat Uplift in SW Egypt. Low δ 2 H values of the 81 Kr‐dated groundwater reveal a recurrent Atlantic moisture source during Pleistocene pluvial periods. These results indicate that the 81 Kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrologic problems.

Initial experiments at the ATLAS facility [Nucl. Instr. and Meth. B 92 (1994) 241] resulted in a clear detection of cosmogenic 39Ar signal at the natural level. The present paper summarizes the recent developments of 39Ar AMS measurements at ATLAS: the use of an electron cyclotron resonance (ECR) positive ion source equipped with a special quartz liner to reduce 39K background, the development of a gas handling system for small volume argon samples, the acceleration of 39Ar8+ ions to 232 MeV, and the final separation of 39Ar from 39K in a gas-filled spectrograph. The first successful AMS measurements of 39Ar in ocean water samples from the Southern Atlantic ventilation experiment (SAVE) are reported.

The problem of He atom movement through channels of the quartz crystalline lattice is investigated. Providing the diameters of the atom and of the channel are of similar size the atom interacts with neighbor constituents of the wall. The conservation of momentum law in local form applied to the ‘helium–constituent’ interaction allows reduction of the problem to a one-dimensional one, which is similar to the movement of a dislocation in the Frenkel–Kontorova (FK) model. Within the framework of this model the activation energy for ‘helium+neighbor constituents’ is expressed by the shear modulus for the channel-forming material and the He polarizability. A metastable helium atom in the triplet state (2 3S1) is able to penetrate through the channel. In contrast, helium atoms in the singlet states, both ground state (1 1S0) and metastable (2 1S0), cannot penetrate.

A closed-circuit analytical system for the (quasi)-continuous measurement of radon fluxes from soil consisting of a static accumulation chamber and two radon detectors operated in series is described. The first detector measures the (220Rn+222Rn) activity, the second one the remaining 222Rn activity after the 220Rn atoms (half-life 56s) have decayed when the air passes a delay volume between the two detectors. A step-motor driven cover closes the chamber at the beginning of a flux measurement interval of e.g. 3h. In between measurements the chamber remains open to maintain soil and vegetation inside the chamber as closely as possible to the conditions outside. A mathematical description of the temporal evolution of Rn activities in the analytical system is presented from which procedures are derived to calculate Rn fluxes (Bq m−2s−1) from the measured activities in the two detectors. Examples from field tests illustrate the performance of the experimental set-up and possible complications due to technical and/or environmental difficulties.

The isotopic ratios 81Kr/Kr and 36Cl/Cl and the 4He concentrations measured in groundwater from four artesian wells in the western part of the Great Artesian Basin (GAB) in Australia are discussed. Based on radioactive decay along a water flow path the 81Kr/Kr ratios are directly converted to groundwater residence times. Results are in a range of 225–400 kyr with error bars in the order of 15% primarily due to counting statistics in the cyclotron accelerator mass spectrometer measurement. Additional uncertainties from subsurface production and/or exchange with stagnant porewaters in the confining shales appear to be of the same order of magnitude. These 81Kr ages are then used to calibrate the 36Cl and the 4He dating methods. Based on elemental analyses of rock samples from the sandstone aquifer as well as from the confining Bulldog shale the in situ flux of thermal neutrons and the corresponding 3He/4He and 36Cl/Cl ratios are calculated. From a comparison of: (i) the 3He/4He ratios measured in the groundwater samples with the calculated in situ ratios in rocks and (ii) the measured δ37Cl ratios with the 4He concentrations measured in groundwater it is concluded that both helium and chloride are most likely added to the aquifer from sources in the stagnant porewaters of the confining shale by diffusion and/or mixing. Based on this ‘working hypothesis’ the 36Cl transport equation in groundwater is solved taking into account: (i) radioactive decay, (ii) subsurface production in the sandstone aquifer (with an in situ 36Cl/Cl ratio of 6×10−15) and (iii) addition of chloride from a source in the confining shale (with a 36Cl/Cl ratio of 13×10−15). Lacking better information it is assumed that the chloride concentration increased linearly with time from an (unknown) initial value Ci to its measured present value C=Ci+Ca, where Ca represents the (unknown) amount of chloride added from subsurface sources. Using the 81Kr ages of the four groundwater samples and a reasonable initial 36Cl/Cl ratio of 125×10−15, which is consistent with other studies in this part of the GAB, it is then possible to determine (Ci,Ca) parameter sets for all four samples and consequently to simulate the Cl and the 36Cl evolution with time. Strong evidence that the whole procedure is adequate comes from: (i) a comparison of Ci with the calculated noble gas recharge temperatures (NGRT) indicating that a higher NGRT is related to higher input chloride concentrations Ci (because of higher evapotranspiration) and (ii) a comparison of Ca with the measured 4He concentration confirming the idea that both chloride and helium are added to the groundwater in parallel. It turns out that the four samples fall into two groups: (i) for two of the samples (Raspberry Creek and Oodnadatta) initial 36Cl concentrations are high and 36Cl dating based on radioactive decay is possible. The 4He accumulation rate for these two samples is low (0.2×10−10 cm3 STP 4He/(cm3 water yr)); (ii) for the other two samples (Duck Hole and Watson Creek) the initial 36Cl concentration is low and therefore subsurface processes dominate resulting in almost constant 36Cl concentrations with time; 36Cl groundwater dating is not possible. The 4He accumulation rate for these two samples is about 10 times higher (1.9×10−10 cm3 STP 4He/(cm3 water yr)). 129I concentrations are interpreted as a simple mixing between an atmospheric and a subsurface source.

[1] Quartz crystals in sandstones at depths of 1200 m-1400 m below the surface appear to reach a solubility equilibrium with the He-4-concentration in the surrounding pore- or groundwater after some time. A rather high He-4-concentration of 4.5 . 10(-3) cc STP He-4/cm(3) of water measured in a groundwater sample would for instance maintain a He pressure of 0.47 atm in a related volume. This value is equal within analytical error to the pressure deduced from the measured helium content of the quartz and its internal helium-accessible volume. To determine this volume, quartz crystals of 0.1 to 1 mm were separated from sandstones and exposed to a helium gas pressure of 32 atm at a temperature of 290degreesC for up to 2 months. By crushing, melting or isothermal heating the helium was then extracted from the helium saturated samples. A volume on the order of 0.1% of the crystal volume is only accessible to helium atoms but not to argon atoms or water molecules. By monitoring the diffusive loss of He from the crystals at 350 C an effective diffusion constant on the order of 10(-9) cm(2)/s is estimated. Extrapolation to the temperature of 70degreesC in the sediments at a depth of 1400 m gives a typical time of about 100 000 years to reach equilibrium between helium in porewaters and the internal He-accessible volume of quartz crystals. In a geologic situation with stagnant pore- or groundwaters in sediments it therefore appears to be possible with this new method to deduce a He-4 depth profile for porewaters in impermeable rocks based on their mineral record.

We demonstrate a new method for determining the 81 Kr/Kr ratio in environmental samples based upon two measurements: the 85 Kr/ 81 Kr ratio measured by Atom Trap Trace Analysis (ATTA) and the 85 Kr/Kr ratio measured by Low‐Level Counting (LLC). This method can be used to determine the mean residence time of groundwater in the range of 10 5 –10 6 a. It requires a sample of 100 μl STP of Kr extracted from approximately two tons of water. With modern atmospheric Kr samples, we demonstrate that the ratios measured by ATTA and LLC are directly proportional to each other within the measurement error of ±10%; we calibrate the 81 Kr/Kr ratio of modern air measured using this method; and we show that the 81 Kr/Kr ratios of samples extracted from air before and after the development of the nuclear industry are identical within the measurement error.

We demonstrate a new method for determining the Kr/Kr ratio in environmental samples based upon two measurements: the Kr/Kr ratio measured by Atom Trap Trace Analysis (ATTA) and the Kr/Kr ratio measured by Low-Level Counting (LLC). This method can be used to determine the mean residence time of groundwater in the range of 10 – 10 a. It requires a sample of 100 μl STP of Kr extracted from approximately two tons of water. With modern atmospheric Kr samples, we demonstrate that the ratios measured by ATTA and LLC are directly proportional to each other within the measurement error of ±10%; we calibrate the Kr/Kr ratio of modern air measured using this method; and we show that the Kr/Kr ratios of samples extracted from air before and after the development of the nuclear industry are identical within the measurement error. Kr (t1/2 = 2.3 ×10 a, Kr/Kr ~ 10) has been proposed as the ideal tracer isotope for dating old water and ice in the age range of 10-10 a (Loosli and Oeschger, 1969). Kr is mainly produced in the upper atmosphere by cosmic-ray induced spallation and neutron activation of stable krypton. Because of the constancy of the cosmic ray flux and the fact that the atmosphere is well-mixed and represents the only significant terrestrial Kr reservoir, the Kr abundance in the atmosphere is expected to be constant on the time scale of its lifetime. Subsurface sources and sinks for Kr other than radioactive decay are most likely negligible (Lehmann et al., 2003). Human activities involving nuclear fission have a negligible effect on the Kr concentration because its direct yield from spontaneous fission of U is small and because the stable Br shields Kr from the decay of other fission products. On the other hand, Kr (t1/2 = 10.8 a, Kr/Kr ~ 10) is a fission product of U and Pu, and is released into the atmosphere primarily by nuclear fuel reprocessing. Its abundance has increased by six orders of magnitude since the 1950's. Kr can be used as a tracer to study air and ocean currents, determine residence time of young groundwater in shallow aquifers, and monitor nuclear-fuel processing activities (Loosli, 1992). For Kr analysis, Low-Level Counting (LLC) is performed routinely in several specialized laboratories around the world (Loosli, 1992). LLC was the first method used to detect Kr and measure its abundance in the atmosphere (Loosli, 1969).