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Reservoirs are considered as important carbon source of the atmosphere, whilst, regional and global reservoir CO 2 quantification is hampered by data limitation and bias in spatial and temporal sampling. By deploying chamber measurements and employing the newly developed model of gas transfer velocity, CO 2 partial pressure ( p CO 2 ) and evasion in the main stem of the Three Gorges Reservoir (TGR) were investigated. The p CO 2 ranged from 429 to 8668 μatm with an average of 2511.6 ± 1721.3 μatm, 6.1-fold higher than the ambient air p CO 2 (mean: 410 μatm). All the samples were net CO 2 sources via water-air interface, displaying pronounced spatial and monthly variability. The CO 2 areal flux averaged 212.5 ± 120.1 mmol/m 2 /d in June, 123.3 ± 78.5 mmol/m 2 /d in July in the lotic TGR main stream, much higher than its lentic system, i.e., 79.6 ± 41.3 mmol/m 2 /d in November, and 76.3 ± 88.1 mmol/m 2 /d in March. Much lower k levels in the lentic reservoir surface resulted in lower CO 2 evasion rates. Furthermore, dam impoundment considerably altered the riverine carbon cycle, as reflected by the changing magnitude of CO 2 efflux and environmental controls of dissolved CO 2 . Precipitation and concurrent soil CO 2 influx exhibited a central role in controlling riverine p CO 2 , and respiration of allochthonous organic carbon was a secondary factor in the TGR lotic system, whilst, both in-stream metabolism and terrestrial inputs played crucial roles in controlling aqueous CO 2 in the TGR lentic system. In comparison, we provided key findings of k model and more reliable CO 2 quantification with a consideration of water level shifts and a complete coverage of spatial sampling. Our higher CO 2 emission (1.47 (1.16–2.13) Tg CO 2 /y) than previous studies called more field measurements to assess the resulting changes in CO 2 flux owing to dam operation and changing environment, and their implications for regional carbon budgets should be warranted. [ABSTRACT FROM AUTHOR]

The amount of atmospheric gas absorbed into a planetary interior during formation depends on the temperature, pressure, composition, and dynamics of its atmosphere, along with the planet mass, its composition, rate of accretion, and the average age of its surface. We develop a boundary layer model for terrestrial planet ingassing during accretion that quantifies the amounts of hydrogen and helium dissolved into a global magma ocean from a hot, gravitationally captured nebular atmosphere in the first few million years of solar system history. Assuming that most of Earth's accretion occurred within the lifetime of its nebular atmosphere, one or more ocean masses of ingassed hydrogen are predicted if the surface pressure is high and the magma surface is young. In addition, Earth would have acquired hundreds of petagrams of helium-3 from the same nebular atmosphere. Our model predicts negligible hydrogen ingassing from a gravitationally captured nebular atmosphere on Mars, but shows that vast amounts of hydrogen ingassing – tens of ocean masses – are possible during the accretion of Super-Earth-sized terrestrial planets. [ABSTRACT FROM AUTHOR]

Microalgal technology is one of the most promising technologies to eliminate the anthropogenic CO 2 and to create a benign environment. Carbon dioxide is bubbled into the microalgae reactors during bio sequestration process. Since CO 2 concentration in the atmosphere is very less, mass transfer occur naturally from the reactor to the atmosphere. The possibility of losses to the atmosphere occurs due to the higher flow rate, lower fixation rate of the species and also due to the wind speed available at the particular locations. The objective of the study is to evaluate the CO 2 loss due to the different wind speeds in an open pond reactor. The wind velocities in the range of 2.7 m/s to 10.5 m/s on the percentage loss of carbon dioxide were studied. The results indicate that higher the wind speed, higher the CO 2 loss to the atmosphere. The gas transfer velocity was evaluated at each wind speed. The empirical relation was determined relating the wind speed and gas transfer velocity. The developed correlation was verified satisfactorily for the percentage loss of carbon dioxide in the open air. Based on the wind speed available at any location, the rate of CO 2 loss could be predicted from the correlation. [ABSTRACT FROM AUTHOR]

Air–sea gas transfer velocity for O 2 is calculated using directly measured fluxes with the eddy covariance technique. It is a direct method and is frequently used to determine fluxes of heat, humidity, and CO 2 , but has not previously been used to estimate transfer velocities for O 2 , using atmospheric eddy covariance data. The measured O 2 fluxes are upward directed, in agreement with the measured air–sea gradient of the O 2 concentration, and opposite to the direction of the simultaneously measured CO 2 fluxes. The transfer velocities estimated from measurements are compared with prominent wind speed parameterizations of the transfer velocity for CO 2 and O 2 , previously established from various measurement techniques. Our result indicates stronger wind speed dependence for the transfer velocity of O 2 compared to CO 2 starting at intermediate wind speeds. This stronger wind speed dependence appears to coincide with the onset of whitecap formation in the flux footprint and the strong curvature of a cubic wind-dependent function for the transfer velocity provides the best fit to the data. Additional data using the measured O 2 flux and an indirect method (based on the Photosynthetic Quotient) to estimate oxygen concentration in water, support the stronger wind dependence for the transfer velocity of O 2 compared to CO 2 . [ABSTRACT FROM AUTHOR]

Abstract: A dual tracer experiment was carried out during the SAGE experiment using the inert tracers SF6 and 3He, in order to determine the gas transfer velocity, k, at high wind speeds in the Southern Ocean. Wind speed/gas exchange parameterization is characterised by significant variability and we examine the major measurement uncertainties that contribute to that scatter. Correction for the airflow distortion over the research vessel, as determined by computational fluid dynamics (CFD) modelling, had the effect of increasing the calculated value of k by 30%. On the short time scales of such experiments, the spatial variability of the wind field resulted in differences between ship and satellite QuikSCAT winds, which produced significant differences in transfer velocity. With such variability between wind estimates, the comparison between gas exchange parameterizations from diverse experiments should clearly be made on the basis of the same wind product. Uncertainty in mixed layer depth of ∼10% arose from mixed layer deepening at high wind speed and limited resolution of vertical sampling. However the assumption of equal mixing of the two tracers is borne out by the experiment. Two dual tracer releases were carried out during SAGE, and showed no significant difference in transfer velocities using QuikSCAT winds, despite the differences in wind history. In the SAGE experiment, duration limitation on the development of waves was shown to be an important factor for Southern Ocean waves, despite the presence of long fetches. [Copyright &y& Elsevier]

• Differences in methods in published eddy covariance studies make results difficult to compare. • Including ancillary datasets and their uncertainties will help to clarify study conclusions. • Heterogeneous sea-ice environments should be a major focus for future eddy covariance studies. Sparse in situ measurements and poor understanding of the impact of sea ice on air-sea gas exchange introduce large uncertainties to models of polar oceanic carbon uptake. The eddy covariance technique can be used to produce insightful air-sea gas exchange datasets in the presence of sea ice, but results differ between studies. We present a critical review of historical polar eddy covariance studies and can identify only five that present comparable flux datasets. Assessment of ancillary datasets, including sea-ice coverage and type and air-sea concentration gradient of carbon dioxide, used to interpret flux datasets (with a specific focus on their role in estimating and interpreting sea ice zone gas transfer velocities) identifies that standardised methodologies to characterise the flux footprint would be beneficial. In heterogeneous ice environments both ancillary data uncertainties and controls on gas exchange are notably complex. To address the poor understanding, we highlight how future efforts should focus on the collection of robust gas flux datasets within heterogeneous sea ice regions during key seasonal processes alongside consistent ancillary data with a full characterisation of their associated uncertainties. [ABSTRACT FROM AUTHOR]

Abstract: The relationship between the gas transfer velocity and turbulent lengthscales is investigated experimentally in a grid-stirred turbulent flow. The horizontal velocity field at the water surface is measured using particle image velocimetry (PIV). The gas transfer velocity for oxygen is obtained through reaeration experiments. In addition, the gas transfer process by surface-renewal eddies is visualized using laser-induced fluorescence (LIF) technique, in which carbon dioxide is used as the tracer gas. The definition of the Taylor microscale holds that the root-mean-square (RMS) of the surface divergence is expressed by the square root of the turbulent kinetic energy divided by the Taylor microscale. Experimentally obtained data support this scaling. They show the gas transfer velocity to be proportional to the square root of the RMS of the surface divergence. These experimental results imply that the Taylor microscale is an important parameter for gas transfer velocity at the air–water interface. These relations indicate that a nondimensional gas transfer velocity is proportional to the −1/4 power of a turbulent-macroscale Reynolds number, which is similar to a small-eddy model, assuming that turbulent eddies with the Kolmogorov scale control the gas transfer process. However, this Reynolds number dependence does not necessarily mean the superiority of turbulent eddies with the Kolmogorov scale in the gas transfer. The LIF visualizations in horizontal and vertical planes close to the air–water interface indicate that the horizontal CO2-concentration field has a fine spatial pattern, which resembles that of the surface divergence field, and that surface-renewal motions observed in the vertical plane have a larger lengthscale than the Kolmogorov scale. We infer from both PIV and LIF results that the Taylor microscale is an important lengthscale for air–water gas transfer. [Copyright &y& Elsevier]

Polluted urban river systems might be a strong source of atmospheric methane (CH 4) and nitrous oxide (N 2 O), but so far only a few urban river systems have been quantified with regard to their source strength for greenhouse gases (GHGs). In this study, we measured loads of dissolved inorganic nitrogen and organic carbon, dissolved oxygen (DO) concentrations, and fluxes of CH 4 and N 2 O from an urban river in Beijing, China during the course of an entire year. Fluxes calculated using the floating chamber approach or via the diffusion method with measurements of river water GHG concentrations showed comparable temporal variations. However, the flux magnitude based on the diffusion method was found to strongly depend on the underlying parameterization of the gas transfer velocity. In view of the large differences while applying different methodologies to estimate surface water GHG fluxes further studies are still needed to prove and eventually quantify the systematic errors which are likely caused by either the chamber technique or the approaches of individual diffusion models. For both the floating chamber and the diffusion-based flux estimates, strong seasonal variations in CH 4 and N 2 O fluxes from the river surface were observed, with fluxes ranging from 3 to 8374 μg C m−2 h−1 for CH 4 and 1–3986 μg N m−2 h−1 for N 2 O. The CH 4 fluxes were strongly negatively correlated with the DO concentration (P < 0.01). The highest N 2 O fluxes were observed at times with low CH 4 fluxes (i.e., in spring and autumn). Annual CH 4 and N 2 O fluxes totaled 19.3–79.4 and 17.4–44.8 kg C (N) ha−1 yr−1, respectively. These high fluxes are in agreement with estimates from the few other studies carried out for urban river systems to date and indicate that urban polluted river systems are a significant regional source of atmospheric GHGs. Image 1 • The polluted Xiaoyue River is a strong source of atmospheric CH 4 and N 2 O. • Dissolved oxygen was the primary factor controlling the river CH 4 fluxes. • River CH 4 and N 2 O concentrations and fluxes were inversely correlated seasonally. • Existing diffusion models contribute to the uncertainty in flux estimates. The polluted Xiaoyue River is a strong source of atmospheric CH 4 and N 2 O, whose fluxes do vary substantially depending on different methods used. [ABSTRACT FROM AUTHOR]