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Wave breaking induced bubbles contribute a significant part of air‐sea gas fluxes. Recent modeling of the sea state dependent CO2 flux found that bubbles contribute up to ∼40% of the total CO2 air‐sea fluxes (Reichl & Deike, 2020, https://doi.org/10.1029/2020gl087267). In this study, we implement the sea state dependent bubble gas transfer formulation of Deike and Melville (2018, https://doi.org/10.1029/2018gl078758) into a spectral wave model (WAVEWATCH III) incorporating the spectral modeling of the wave breaking distribution from Romero (2019, https://doi.org/10.1029/2019gl083408). We evaluate the accuracy of the sea state dependent gas transfer parameterization against available measurements of CO2 gas transfer velocity from 9 data sets (11 research cruises, see Yang et al. (2022, https://doi.org/10.3389/fmars.2022.826421)). The sea state dependent parameterization for CO2 gas transfer velocity is consistent with observations, while the traditional wind‐only parameterization used in most global models slightly underestimates the observations of gas transfer velocity. We produce a climatology of the sea state dependent gas transfer velocity using reanalysis wind and wave data spanning 1980–2017. The climatology shows that the enhanced gas transfer velocity occurs frequently in regions with developed sea states (with strong wave breaking and high significant wave height). The present study provides a general sea state dependent parameterization for gas transfer, which can be implemented in global coupled models. Plain Language Summary: The atmosphere‐ocean gas exchange influences Earth's climate, including local and global sinks and sources of CO2. We implement a physics based bubble mediated gas exchange model using state of the art gas transfer velocity models dependent on wind and waves, together with the various gases physicochemical parameters to better constrain the trend of the ocean‐atmosphere gas fluxes, validated against extensive field measurements of CO2 fluxes. In current climate models, the CO2 air‐sea fluxes are parameterized solely based on wind speed, and we discuss the high frequency variability induced by the waves. The climatology of gas transfer velocity for both a simple bulk formula and a complete sea state models are provided for the community and can be used in global ocean or climate models. Key Points: The sea state dependent CO2 gas transfer velocity models that are evaluated against existing data setsTwo models are examined that are consistent with the observations, with differences at low wind speed due to the representation of swellA database of both sea state dependent gas transfer velocities for the period 1980–2017 are archived for future CO2 studies [ABSTRACT FROM AUTHOR]

Copyright of Revista de la Academia Colombiana de Ciencias Exactas, Físicas y Naturales is the property of Academia Colombiana de Ciencias Exactas, Fisicas y Naturales and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract Wave breaking induced bubbles contribute a significant part of air‐sea gas fluxes. Recent modeling of the sea state dependent CO2 flux found that bubbles contribute up to ∼40% of the total CO2 air‐sea fluxes (Reichl & Deike, 2020, https://doi.org/10.1029/2020gl087267). In this study, we implement the sea state dependent bubble gas transfer formulation of Deike and Melville (2018, https://doi.org/10.1029/2018gl078758) into a spectral wave model (WAVEWATCH III) incorporating the spectral modeling of the wave breaking distribution from Romero (2019, https://doi.org/10.1029/2019gl083408). We evaluate the accuracy of the sea state dependent gas transfer parameterization against available measurements of CO2 gas transfer velocity from 9 data sets (11 research cruises, see Yang et al. (2022, https://doi.org/10.3389/fmars.2022.826421)). The sea state dependent parameterization for CO2 gas transfer velocity is consistent with observations, while the traditional wind‐only parameterization used in most global models slightly underestimates the observations of gas transfer velocity. We produce a climatology of the sea state dependent gas transfer velocity using reanalysis wind and wave data spanning 1980–2017. The climatology shows that the enhanced gas transfer velocity occurs frequently in regions with developed sea states (with strong wave breaking and high significant wave height). The present study provides a general sea state dependent parameterization for gas transfer, which can be implemented in global coupled models.

Copyright of Natura Sloveniae: Revija za Terensko Biologijo / Journal of Field Biology is the property of Natura Sloveniae and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Southern Indian Ocean is a major reservoir for rapid carbon exchange with the atmosphere, plays a key role in the world’s carbon cycle. To understand the importance of anthropogenic CO2 uptake in the Southern Indian Ocean, a variety of methods have been used to quantify the magnitude of the CO2 flux between air and sea. The basic approach is based on the bulk formula—the air-sea CO2 flux is commonly calculated by the difference in the CO2 partial pressure between the ocean and the atmosphere, the gas transfer velocity, the surface wind speed, and the CO2 solubility in seawater. However, relying solely on wind speed to measure the gas transfer velocity at the sea surface increases the uncertainty of CO2 flux estimation. Recent studies have shown that the generation and breaking of ocean waves also significantly affect the gas transfer process at the air-sea interface. In this study, we highlight the impact of windseas on the process of air-sea CO2 exchange and address its important role in CO2 uptake in the Southern Indian Ocean. We run the WAVEWATCH III model to simulate surface waves in this region over the period from January 1st 2002 to December 31st 2021. Then, we use the spectral partitioning method to isolate windseas and swells from total wave fields. Finally, we calculate the CO2 flux based on the new semiempirical equation for gas transfer velocity considering only windseas. We found that after considering windseas’ impact, the seasonal mean zonal flux (mmol/m2·d) increased approximately 10%-20% compared with that calculated solely on wind speed in all seasons. Evolution of air-sea net carbon flux (PgC) increased around 5.87%-32.12% in the latest 5 years with the most significant seasonal improvement appeared in summer. Long-term trend analysis also indicated that the CO2 absorption capacity of the whole Southern Indian Ocean gradually increased during the past 20 years. These findings extend the understanding of the roles of the Southern Indian Ocean in the global carbon cycle and are useful for making management policies associated with marine environmental protection and global climatic change mitigation.

The past decade has seen significant technological advance in the observation of trace gas fluxes over the open ocean, most notably CO2, but also an impressive list of other gases. Here we will emphasize flux observations from the air-side of the interface including both turbulent covariance (direct) and surface-layer similarity-based (indirect) bulk transfer velocity methods. Most applications of direct covariance observations have been from ships but recently work has intensified on buoy-based implementation. The principal use of direct methods is to quantify empirical coefficients in bulk estimates of the gas transfer velocity. Advances in direct measurements and some recent field programs that capture a considerable range of conditions with wind speeds exceeding 20 ms-1 are discussed. We use coincident direct flux measurements of CO2 and dimethylsulfide (DMS) to infer the scaling of interfacial viscous and bubble-mediated (whitecap driven) gas transfer mechanisms. This analysis suggests modest chemical enhancement of CO2 flux at low wind speed. We include some updates to the theoretical structure of bulk parameterizations (including chemical enhancement) as framed in the COAREG gas transfer algorithm.

High global uncertainties remain in water‐air CO2, CH4, and N2O fluxes from estuaries due to spatial and temporal variability and the poor predictability of the gas transfer velocity (k600). This is the first study that directly compares k600 of CO2, CH4, and N2O in an estuary with the aim to evaluate the accuracy of using a uniform k600 value for estimating water‐air fluxes. We calculated 155 k600 values from CO2, CH4, and N2O fluxes over spatial (across, along) and temporal (tidal cycle) surveys in the subtropical Maroochy estuary using the floating chamber method. Combined k600 values showed a large range over the entire estuary (0.1–198.6 cm h−1) with slightly lower k600 in the lower compared to the upper estuary. Overall, temporal variability was greater than spatial variability of k600. We found the highest variability of k600 between gas species in the lower estuary, whereas the variability was less distinct in the upper estuary. In the Maroochy estuary, k600CO2 (mean 26.4 ± 37.3 cm h−1) was mostly higher than k600CH4 (mean 10.9 ± 10.6 cm h−1) and k600N2O (mean 9.9 ± 12.3 cm h−1), likely due to chemical and enzymatic enhancements and/or microbial activity in the surface microlayer. We demonstrate that empirical k600 models intended for CO2 may not accurately predict CH4 and N2O fluxes in estuaries. Our tested k600 models predicted the measured fluxes within an uncertainty range of 5%–40% (over or underestimation), but precise flux estimates should be based on in situk600 of all three gases. Plain Language Summary: Greenhouse gas emissions from estuaries are an important component of the coastal ocean carbon cycle. However, carbon dioxide, methane, and nitrous oxide fluxes from estuaries remain uncertain at the global scale, partially because of the gas transfer velocity, a relevant parameter of the flux computation. This study is the first study, to our knowledge, that directly compares gas transfer velocities of the three greenhouse gases with each other and over spatial and temporal gradients in a subtropical estuary in Australia. We find that gas transfer velocities were gas‐specific and differed in upper, mid, and lower estuary regions. The non‐uniformity of the gas transfer velocity in heterogeneous estuaries is likely due to different chemical, physical and biological processes that drive gas transfer of the three gases at the water‐air interface. This is important to consider when estimating estuary fluxes because models for one specific gas may not accurately predict the gas transfer of the other two greenhouse gases. Key Points: k600 of CO2, CH4, and N2O were non‐uniform over spatial and temporal estuary gradients and k600CO2 mostly greater than k600CH4 and k600N2OVariability of k600 between the gas species was greater at the estuary mouth compared to the mid and upper estuary sectionsEmpirical k600CO2 models may not accurately predict CH4 and N2O fluxes because they do not account for gas‐specific non‐diffusive processes [ABSTRACT FROM AUTHOR]

The gas transfer velocity, k, modulates gas fluxes across air‐water interfaces in rivers. While the theory postulates a local scaling law between k and the turbulent kinetic energy dissipation rate ε, empirical studies usually interpret this relation at the reach‐scale. Here, we investigate how local k(ε) laws can be integrated along heterogeneous reaches exploiting a simple hydrodynamic model, which links stage and velocity to the local slope. The model is used to quantify the relative difference between the gas transfer velocity of a heterogeneous stream and that of an equivalent homogeneous system. We show that this aggregation bias depends on the exponent of the local scaling law, b, and internal slope variations. In high‐energy streams, where b>1, spatial heterogeneity of ε significantly enhances reach‐scale values of k as compared to homogeneous settings. We conclude that small‐scale hydro‐morphological traits bear a profound impact on gas evasion from inland waters. Plain Language Summary: Gas emissions from rivers and streams are modulated by the gas transfer velocity at the water‐air interface, k, which is physically related to the energy dissipated by the flow field, ε. Here, we study how a local relation between gas transfer rate and energy dissipation can be spatially averaged in presence of heterogeneous flow fields induced by changes in the local slope. Our results indicate that reach‐scale relations between k and ε in general differ from the corresponding local scaling laws. In particular, we show that high‐energy heterogeneous streams are characterized by a gas transfer velocity significantly higher than that of an equivalent homogeneous stream. These results offer a clue for the interpretation of empirical data about stream outgassing in river networks. Key Points: We analyze local and reach‐wise relations between gas transfer velocity (k) and energy dissipation rate (ε)Reach‐scale k(ε) laws depend on the exponent that modulates the local relation between k and ε and the heterogeneity of the reachReach‐scale k(ε) laws in high‐energy heterogeneous streams are affected by a positive aggregation bias [ABSTRACT FROM AUTHOR]

Streams in high‐elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO2) to the atmosphere by transforming terrestrial carbon to gaseous CO2. Studies of these environments are scarce, and estimates of CO2 fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO2 evasion and other biogeochemical processes. The first method, kinematic k600 (k600‐K), is derived from an empirical relationship between temperature‐adjusted k (k600) and the physical characteristics of the stream. The second method, measured k600 (k600‐M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO2 (pCO2) and CO2 evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5‐week period during the wet season of a peatland‐stream transition within a páramo ecosystem located above 4000 m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250‐m reach on five occasions, and both methods showed a wide range of variability in k600 at small spatial scales. Values of k600‐K ranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values of k600‐M ranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability in k600 was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L/s). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates of k600 values in a high‐elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co‐regulating gas transfer velocities in páramo streams. [ABSTRACT FROM AUTHOR]

Abstract Streams in high‐elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO2) to the atmosphere by transforming terrestrial carbon to gaseous CO2. Studies of these environments are scarce, and estimates of CO2 fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO2 evasion and other biogeochemical processes. The first method, kinematic k600 (k600‐K), is derived from an empirical relationship between temperature‐adjusted k (k600) and the physical characteristics of the stream. The second method, measured k600 (k600‐M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO2 (pCO2) and CO2 evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5‐week period during the wet season of a peatland‐stream transition within a páramo ecosystem located above 4000 m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250‐m reach on five occasions, and both methods showed a wide range of variability in k600 at small spatial scales. Values of k600‐K ranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values of k600‐M ranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability in k600 was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L/s). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates of k600 values in a high‐elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co‐regulating gas transfer velocities in páramo streams.

The patterns of spatial and temporal variability in CO2 and CH4 emission from reservoirs are still poorly studied, especially in tropical regions where hydropower is growing. We performed spatially resolved measurements of dissolved CO2 and CH4 surface water concentrations and their gas‐exchange coefficients (k) to compute diffusive carbon flux from four contrasting tropical reservoirs across Brazil during different hydrological seasons. We used an online equilibration system to measure dissolved CO2 and CH4 concentrations; we estimated k from floating chamber deployments in conjunction with discrete CO2 and CH4 water concentration measurements. Diffusive CO2 emissions were higher during dry season than during rainy season, whereas there were no consistent seasonal patterns for diffusive CH4 emissions. Our results reveal that the magnitude and the spatial within‐reservoir patterns of diffusive CO2 and CH4 flux varied strongly among hydrological seasons. River inflow areas were often characterized by high seasonality in diffusive flux. Areas close to the dam generally showed low seasonal variability in diffusive CH4 flux but high variability in CO2 flux. Overall, we found that reservoir areas exhibiting highest emission rates ("hotspots") shifted substantially across hydrological seasons. Estimates of total diffusive carbon emission from the reservoir surfaces differed between hydrological seasons by a factor up to 7 in Chapéu D'Úvas, up to 13 in Curuá‐Una, up to 4 in Furnas, and up to 1.8 in Funil, indicating that spatially resolved measurements of CO2 and CH4 concentrations and k need to be performed at different hydrological seasons in order to constrain annual diffusive carbon emission. Plain Language Summary: Reservoirs are key for flood control, water supply, and hydropower generation. However, reservoirs are usually not carbon neutral. Studies worldwide point to reservoirs as important net sources of anthropogenic carbon emission to the atmosphere. Carbon emission from reservoirs derives from the decomposition of organic matter. Although carbon emission from reservoirs has been increasingly studied over the past two decades, most studies do not sufficiently describe emissions across space and time. Our study applies highly resolved spatial coverage of dissolved surface water concentrations and gas‐exchange coefficients of CO2 and CH4 to compute rates of CO2 and CH4 diffusion to the atmosphere across distinct hydrological seasons in four contrasting tropical reservoirs. We found that emissions varied substantially over both space and time. More specifically, we found that reservoir areas exhibiting highest emission rates ("hotspots") shifted substantially between dry and rainy seasons. Overlooking the spatial within‐reservoir variability across seasons may result in serious underestimation or overestimation of total diffusive carbon emission from reservoirs, depending on the time and space that studies focus their sampling on. Our work may support scientists in adopting more comprehensive sampling strategies relevant for better constrained upscaling, and, consequently, support informed policy decisions and management actions. Key Points: Reservoir regions displaying highest emission rates shifted substantially between sampling occasions at hydrologically different seasonsRiver inflow areas were often characterized by high seasonality in diffusive carbon fluxSpatially resolved measurements need to be conducted at different hydrological seasons in order to constrain diffusive emission [ABSTRACT FROM AUTHOR]

The air‐water exchange of trace gases such as CO2 is usually parameterized in terms of a gas transfer velocity, which can be derived from direct measurements of the air‐sea gas flux. The transfer velocity of poorly soluble gases is driven by near‐surface ocean turbulence, which may be enhanced or suppressed by the presence of sea ice. A lack of measurements means that air‐sea fluxes in polar regions, where the oceanic sink of CO2 is poorly known, are generally estimated using open‐ocean transfer velocities scaled by ice fraction. Here, we describe direct determinations of CO2 gas transfer velocity from eddy covariance flux measurements from a mast fixed to ice adjacent to a sea‐ice lead during the summer‐autumn transition in the central Arctic Ocean. Lead water CO2 uptake is determined using flux footprint analysis of water‐atmosphere and ice‐atmosphere flux measurements made under conditions (low humidity and high CO2 signal) that minimize errors due to humidity cross‐talk. The mean gas transfer velocity is found to have a quadratic dependence on wind speed: k660 = 0.179 U102, which is 30% lower than commonly used open‐ocean parameterizations. As such, current estimates of polar ocean carbon uptake likely overestimate gas exchange rates in typical summertime conditions of weak convective turbulence. Depending on the footprint model chosen, the gas transfer velocities also exhibit a dependence on the dimension of the lead, via its impact on fetch length and hence sea state. Scaling transfer velocity parameterizations for regional gas exchange estimates may therefore require incorporating lead width data. Plain Language Summary: Polar oceans absorb large amounts of carbon dioxide from the atmosphere, but there is a lot of uncertainty over exactly how much is taken up. The amount the oceans absorb depends on both the concentration of carbon dioxide in the water, and the rate of gas exchange between the ocean and the atmosphere. This rate itself depends mostly on wind speed. In sea ice‐regions, which even in winter have some areas of open water, the gas exchange rate is often estimated by using an ocean gas exchange rate, multiplied by the fraction of the sea‐ice area, that is, open water. However, there are very few measurements of the gas exchange rate in sea‐ice areas, and there is an on‐going debate about whether the sea ice itself increases or decreases the exchange rate. Here, direct measurements of the gas exchange rate were made in an area of water surrounded by sea ice in the Arctic during summer and the beginning of autumn. The measured gas exchange rate was lower than typical ocean rates, suggesting that the absorption of carbon dioxide by polar oceans has previously been overestimated. Key Points: CO2 uptake in lead waters is determined using water‐atmosphere and ice‐atmosphere flux measurements from an on‐ice mastThe wind‐speed dependent gas transfer velocity in the lead is suppressed by 30% relative to the open oceanDependent on choice of footprint model, k varies with fetch, hence polar ocean carbon uptake estimates should incorporate lead width data [ABSTRACT FROM AUTHOR]

The nonlinear dependence of gas transfer velocity on wind speed typically results in the best fit of observational data; however, gas transfer velocity is dimensionally inconsistent with the nonlinear wind speeds. The objective of the current study was to show that, in the case of wind waves, gas transfer velocity with a consistent dimension should be determined by turbulent viscosity instead of by the viscosity of water when parameterised using the renewal model. Turbulent kinetic energy (TKE) near the air–sea interface is significantly intensified by breaking wind waves. By analyzing various models, we found that the TKE dissipation rate was explicitly linearly related to wind speed and dependent on wave age, with powers ranging from –2.36 to 4.0. Various models show that wave energy dissipation (WED) due to wind wave breaking explicitly increases with the cube of the wind speed and weakly depends on wave age. Assuming a balance between WED and total TKE dissipation in a constant dissipation layer, the depth of this layer was shown to be comparable to the wave height. Using the traditional renewal model with wind wave turbulent viscosity and TKE dissipation rate at the sea surface, we found that the gas transfer velocity was explicitly linearly related to wind speed in a dimensionally consistent manner, and depended simultaneously on the wave age and drag coefficient. These results are consistent with observational data obtained using the eddy correlation method. We emphasise that the linear dependence on wind speed is only valid when the wave age and drag coefficient are fixed; thus, this finding cannot be directly confirmed by currently available observational data due to a lack of wave state information.

The Arctic Ocean (AO) is an important basin for global oceanic carbon dioxide (CO2) uptake, but the mechanisms controlling air–sea gas fluxes are not fully understood, especially over short and long timescales. The oceanic sink of CO2 is an important part of the global carbon budget. Previous studies have shown that in the AO differences in the partial pressure of CO2 (ΔpCO2) and gas transfer velocity (k) both contribute significantly to interannual air–sea CO2 flux variability, but that k is unimportant for multidecadal variability. This study combined Earth Observation (EO) data collected in 2010 with the in situ pCO2 dataset from Takahashi et al. (2009) (T09) using a recently developed software toolbox called FluxEngine to determine the importance of k and ΔpCO2 on CO2 budgets in two regions of the AO – the Greenland Sea (GS) and the Barents Sea (BS) with their continental margins. Results from the study indicate that the variability in wind speed and, hence, the gas transfer velocity, generally play a major role in determining the temporal variability of CO2 uptake, while variability in monthly ΔpCO2 plays a major role spatially, with some exceptions.

Wind, chemical enhancement, phytoplankton activity, and surfactants are potential factors driving the air-sea gas exchange of carbon dioxide (CO2). We investigated their effects on the gas transfer velocity of CO2 in a large annular wind-wave tank filled with natural seawater from the North Atlantic Ocean. Experiments were run under 11 different wind speed conditions (ranging from 1.5 ms−1 to 22.8 ms−1), and we increased the water pCO2 concentration twice by more than 950 μatm for two of the seven experimental days. We develop a conceptual box model that incorporated the thermodynamics of the marine CO2 system. Surfactant concentrations in the sea surface microlayer (SML) ranged from 301 to 1015 μgL−1 (as Triton X-100 equivalents) with enrichments ranged from 1.0 to 5.7 in comparison to the samples from the underlying bulk water. With wind speeds up to 8.5 ms−1, surfactants in the SML can reduce the gas transfer velocity by 54%. Wind-wave tank experiments in combination with modeling are useful tools for obtaining a better understanding of the gas transfer velocities of CO2 across the air-sea boundary. The tank allowed for measuring the gas exchange velocity under extreme low and high wind speeds; in contrast, most previous parametrizations have fallen short because measurements of gas exchange velocities in the field are challenging, especially at low wind conditions. High variability in the CO2 transfer velocities suggests that gas exchange is a complex process not solely controlled by wind forces, especially in low wind conditions.

The significance of the water-side gas transfer velocity for air–sea CO2 gas exchange (k) and its non-linear dependence on wind speed (U) is well accepted. What remains a subject of inquiry are biases associated with the form of the non-linear relation linking k to U (hereafter labeled as f(U), where f(.) stands for an arbitrary function of U), the distributional properties of U (treated as a random variable) along with other external factors influencing k, and the time-averaging period used to determine k from U. To address the latter issue, a Taylor series expansion is applied to separate f(U) into a term derived from time-averaging wind speed (labeled as ⟨U⟩, where ⟨.⟩ indicates averaging over a monthly time scale) as currently employed in climate models and additive bias corrections that vary with the statistics of U. The method was explored for nine widely used f(U) parameterizations based on remotely-sensed 6-hourly global wind products at 10 m above the sea-surface. The bias in k of monthly estimates compared to the reference 6-hourly product was shown to be mainly associated with wind variability captured by the standard deviation σσU around ⟨U⟩ or, more preferably, a dimensionless coefficient of variation Iu= σσU/⟨U⟩. The proposed correction outperforms previous methodologies that adjusted k when using ⟨U⟩ only. An unexpected outcome was that upon setting Iu2 = 0.15 to correct biases when using monthly wind speed averages, the new model produced superior results at the global and regional scale compared to prior correction methodologies. Finally, an equation relating Iu2 to the time-averaging interval (spanning from 6 h to a month) is presented to enable other sub-monthly averaging periods to be used. While the focus here is on CO2, the theoretical tactic employed can be applied to other slightly soluble gases. As monthly and climatological wind data are often used in climate models for gas transfer estimates, the proposed approach provides a robust scheme that can be readily implemented in current climate models.

Estuarine CO2 emissions are important components of regional and global carbon budgets, but assessments of this flux are plagued by uncertainties associated with gas transfer velocity (k) parameterization. We combined direct eddy covariance measurements of CO2 flux with waterside pCO2 determinations to generate more reliable k parameterizations for use in small estuaries. When all data were aggregated, k was described well by a linear relationship with wind speed (U10), in a manner consistent with prior open ocean and estuarine k parameterizations. However, k was significantly greater at night and under low wind speed, and nighttime k was best predicted by a parabolic, rather than linear, relationship with U10. We explored the effect of waterside thermal convection but found only a weak correlation between convective scale and k. Hence, while convective forcing may be important at times, it appears that factors besides waterside thermal convection were likely responsible for the bulk of the observed nighttime enhancement in k. Regardless of source, we show that these day‐night differences in k should be accounted for when CO2 emissions are assessed over short time scales or when pCO2 is constant and U10 varies. On the other hand, when temporal variability in pCO2 is large, it exerts greater control over CO2 fluxes than does k parameterization. In these cases, the use of a single k value or a simple linear relationship with U10 is often sufficient. This study provides important guidance for k parameterization in shallow or microtidal estuaries, especially when diel processes are considered. Key Points: When assessed over annual scales, short‐term pCO2 variation, rather than k parameterization, overwhelms uncertainty in calculated CO2 fluxCO2 fluxes generated from bulk transfer equations were significantly greater than those measured by eddy covarianceDay‐night differences in the wind speed versus k relationship can cause parameterized CO2 fluxes to diverge when pCO2 is relatively constant [ABSTRACT FROM AUTHOR]

The nonlinear dependence of gas transfer velocity on wind speed typically results in the best fit of observational data; however, gas transfer velocity is dimensionally inconsistent with the nonlinear wind speeds. The objective of the current study was to show that, in the case of wind waves, gas transfer velocity with a consistent dimension should be determined by turbulent viscosity instead of by the viscosity of water when parameterised using the renewal model. Turbulent kinetic energy (TKE) near the air-sea interface is significantly intensified by breaking wind waves. By analyzing various models, we found that the TKE dissipation rate was explicitly linearly related to wind speed and dependent on wave age, with powers ranging from -2.36 to 4.0. Various models show that wave energy dissipation (WED) due to wind wave breaking explicitly increases with the cube of the wind speed and weakly depends on wave age. Assuming a balance between WED and total TKE dissipation in a constant dissipation layer, the depth of this layer was shown to be comparable to the wave height. Using the traditional renewal model with wind wave turbulent viscosity and TKE dissipation rate at the sea surface, we found that the gas transfer velocity was explicitly linearly related to wind speed in a dimensionally consistent manner, and depended simultaneously on the wave age and drag coefficient. These results are consistent with observational data obtained using the eddy correlation method. We emphasise that the linear dependence on wind speed is only valid when the wave age and drag coefficient are fixed; thus, this finding cannot be directly confirmed by currently available observational data due to a lack of wave state information. [ABSTRACT FROM AUTHOR]

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]

Inland waters are significant sources of atmospheric greenhouse gas (GHG) emissions. The thin boundary layer (TBL) model is often employed as a means of estimating GHG diffusion in inland waters based on gas transfer velocity (k) at the air-water interface, with k being subject to regulation by near-surface turbulence that is primarily driven by wind speed in many cases. This wind speed-based estimation of k (wind- k), however, can introduce substantial uncertainty for turbulent waterways where wind speed does not accurately represent overall turbulence. In this study, GHG diffusion in the Beijing-Hangzhou Grand Canal (China), the first and longest man-made canal in the world, was estimated using the TBL model, revealing that this model substantially underestimated GHG diffusion when relying on wind- k. Strikingly, the carbon dioxide, methane, and nitrous oxide diffusions were respectively underestimated by 159%, 162%, and 124% when using this model. These findings are significant for developing more reliable approaches to evaluate GHG emissions from inland waterways. [Display omitted] [ABSTRACT FROM AUTHOR]

Wave breaking is known to cause air entrainment and enhancement of the near-surface turbulence. Thus, it intensifies the gas exchange across the air–sea interface. Based on the combination of the vertical distribution of the turbulence in the wave-affected layer and the breaking wave-energy dissipation rate in the wave-breaking layer, we proposed a composite model for the gas transfer velocity in the presence of wave breaking. The gas transfer velocity was calculated as a function of the air frictional velocity, wave age, and whitecap coverage. The model was validated by the dependences on winds and wave ages by field and laboratory measurements. The results supported the hypothesis that the large uncertainties in the traditional gas transfer velocities based on wind speed alone at moderate-to-high wind speeds can be ascribed to the neglect of the wind–wave effect, which is mainly attributed to the whitecap coverage as a function of the wind–sea Reynolds number.

Understanding how the ocean absorbs anthropogenic CO2 is critical for predicting climate change. We designed 'Sniffle', a new autonomous drifting buoy with a floating chamber, to measure gas transfer velocities and air–sea CO2 fluxes with high spatiotemporal resolution. Currently, insufficient 'in situ' data exist to verify gas transfer parameterizations at low wind speeds (

Two diel field campaigns under different weather patterns were carried out in the summer and autumn of 2013 to measure CO2 and CH4 fluxes and to probe the rates of gas exchange across the air–water interface in a subtropical eutrophic pond in China. Bubble emissions of CH4 accounted for 99.7 and 91.67% of the total CH4 emission measured at two sites in the summer; however, no bubble was observed in the autumn. The pond was supersaturated with CO2 and CH4 during the monitoring period, and the saturation ratios (i.e. observed concentration/equilibrium concentration) of CH4 were much higher than that of CO2. Although the concentration of dissolved CO2 in the surface water collected in the autumn was 1.24 times of that in the summer, the mean diffusive CO2 flux across the water–air interface measured in the summer is almost twice compared with that in the autumn. The mean concentration of dissolved CH4 in the surface water in the autumn was around half of that in the summer, but the mean diffusive CH4 flux in the summer is 4–5 times of that in the autumn. Our data showed that the variation in gas exchange rate was dominated by differences in weather patterns and primary production. Averaged k600-CO2 and k600-CH4 (the gas transfer velocity normalised to a Schmidt number of 600) were 0.65 and 0.55 cm/h in the autumn, and 2.83 and 1.64 cm/h in the summer, respectively. No statistically significant correlation was found between k600 and U10 (wind speed at 10 m height) in the summer at low wind speeds in clear weather. Diffusive gas fluxes increased during the nights, which resulted from the nighttime cooling effect of water surface and stronger turbulent mixing in the water column. The chemical enhancements for CO2 were estimated up to 1.94-fold in the hot and clear summer with low wind speeds, which might have been resulted from the increasing hydration reactions in water due to the high water temperature and active metabolism in planktonic algae. However, both the air and surface water temperatures decreased continually, and relatively lower temperature and overcast weather with occasionally light rain dominated the second campaign in the autumn. The concentration of dissolved oxygen in the surface water and U10 controlled gas transfer velocities of CO2 and CH4, respectively, in the cool autumn. When the surface water temperature was higher than the air temperature, higher CO2 flux was observed because the water body was unstable and overturned quickly, inducing quick CO2 emitted from plankton algae in surface water to the atmosphere.

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]

Two ideas regarding the structure of turbulence near a clear air‐water interface are used to derive a waterside gas transfer velocity kL for sparingly and slightly soluble gases. The first is that kL is proportional to the turnover velocity described by the vertical velocity structure function Dww(r), where r is separation distance between two points. The second is that the scalar exchange between the air‐water interface and the waterside turbulence can be suitably described by a length scale proportional to the Batchelor scale lB=ηSc−1/2, where Sc is the molecular Schmidt number and η is the Kolmogorov microscale defining the smallest scale of turbulent eddies impacted by fluid viscosity. Using an approximate solution to the von Kármán‐Howarth equation predicting Dww(r) in the inertial and viscous regimes, prior formulations for kL are recovered including (i) kL=2/15Sc−1/2vK, vK is the Kolmogorov velocity defined by the Reynolds number vKη/ν = 1 and ν is the kinematic viscosity of water; (ii) surface divergence formulations; (iii) kL∝Sc−1/2u∗, where u∗ is the waterside friction velocity; (iv) kL∝Sc−1/2gν/u∗ for Keulegan numbers exceeding a threshold needed for long‐wave generation, where the proportionality constant varies with wave age, g is the gravitational acceleration; and (v) kL=2/15Sc−1/2(νgβoqo)1/4 in free convection, where qo is the surface heat flux and βo is the thermal expansion of water. The work demonstrates that the aforementioned kL formulations can be recovered from a single structure function model derived for locally homogeneous and isotropic turbulence. Plain Language Summary: The problem considered here is a theoretical prediction of mass transfer across and air‐water interface. This interfacial transfer phenomenon is featured prominently in global carbon balances, methane, nitrous oxides, dimethyl sulfide, and other gases. It is used to assess the metabolic health of aquatic ecosystems and to determine evasion rates of volatile organic compounds from lakes, estuaries, reservoirs, and large water treatment plants. The novelty of the approach is to link a bulk quantity reflecting the efficiency of swirling motions near the air‐water interface to the sizes of eddies and their energetic content responsible for the aforementioned swirling motion. The proposed approach is shown to recover a number of equations describing gas transport across interfaces that summarize a large corpus of experiments and simulations. Key Points: Scaling laws empirically describing gas transfer velocity kL across marine and coastal systems are theoretically explainedNew theory predicts kL using Kolmogorov's universal structure function scaling laws for turbulence instead of surface renewal theoryWork shows how multiple mechanisms with different assumptions subject to eddy‐turnover time constraint lead to similar scaling laws for kL [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]

Carbon dioxide (CO2) fluxes between the ocean and atmosphere (FCO2) are commonly computed from differences between their partial pressures of CO2 (ΔpCO2) and the gas transfer velocity (k). Commonly used wind-based parameterizations for k imply a zero intercept, although in situ field data below 4 m s−1 are scarce. Considering a global average wind speed over the ocean of 6.6 m s−1, a nonzero intercept might have a significant impact on global FCO2. Here, we present a database of 245 in situ measurements of k obtained with the floating chamber technique (Sniffle), 190 of which have wind speeds lower than 4 m s−1. A quadratic parameterization with wind speed and a nonzero intercept resulted in the best fit for k. We further tested FCO2 calculated with a different parameterization with a complementary pCO2 observation-based product. Furthermore, we ran a simulation in a well-tested ocean model of intermediate complexity to test the implications of different gas transfer velocity parameterizations for the natural carbon cycle. The global ocean observation-based analysis suggests that ignoring a nonzero intercept results in an ocean-sink increase of 0.73 Gt C yr−1. This corresponds to a 28% higher uptake of CO2 compared with the flux calculated from a parameterization with a nonzero intercept. The differences in FCO2 were higher in the case of low wind conditions and large ΔpCO2 between the ocean and atmosphere. Such conditions occur frequently in the Tropics.

Boreal ecosystems contain a large fraction of the world's soil carbon and are warming rapidly, prompting efforts to understand the role of freshwaters in carbon export from these regions. We examined geomorphic controls on the magnitude of stream CO2 emissions and sources of stream dissolved inorganic carbon (DIC) across a heterogeneous river basin in SW Alaska. We found that watershed slope and precipitation interact to control gaseous carbon fluxes from streams, with the highest flux from low-gradient watersheds following rainstorms. Watershed slope influences C loading and stream CO2 fluxes by controlling carbon accumulation in watersheds and to a lesser extent by determining gas transfer velocity across the stream air-water interface. Low gas transfer velocity in flat streams offsets some of the effects of higher terrestrial C loading at those sites, resulting in lower than expected vertical CO2 fluxes. While the isotopic composition of stream DIC (Δ14C and δ13C) was highly variable across space and time, shifts toward contemporary, terrestrial sources after precipitation were most pronounced in flat watersheds. At base flow DIC was a mixture of modern and aged sources of biogenic and geologic origin (Δ14C, −198.3‰ to 27.9‰, n = 23). Aged (Δ14C-depleted) C sources contributed to food webs via a pathway from DIC to algae to invertebrate grazers. We observed coherent changes in stream carbon chemistry after large rainstorms despite considerable physical heterogeneity among watersheds. These patterns provide a way to extrapolate across boreal landscapes by constraining CO2 concentration and flux estimates by local geomorphic features. [ABSTRACT FROM AUTHOR]

The Sydney Harbour Estuary is a large drowned river valley adjacent to Sydney, a large urban metropolis on track to become a megacity; estimated to reach a population of 10 million by 2100. Monthly underway surveys of surface water p CO 2 were undertaken along the main channel and tributaries, from January to December 2013. p CO 2 showed substantial spatio-temporal variability in the narrow high residence time upper and mid sections of the estuary, with values reaching a maximum of 5650 μatm in the upper reaches and as low as 173 μatm in the mid estuary section, dominated by respiration and photosynthesis respectively. The large lower estuary displayed less variability in pCO 2 with values ranging from 343 to 544 μatm controlled mainly by tidal pumping and temperature. Air-water CO 2 emissions reached a maximum of 181 mmol C m −2 d −1 during spring in the eutrophic upper estuary. After a summer high rainfall event nutrient-stimulated biological pumping promoted a large uptake of CO 2 transitioning the Sydney Harbour Estuary into a CO 2 sink with a maximum uptake of rate of −10.6 mmol C m −2 d −1 in the mid-section of the estuary. Annually the Sydney Harbour Estuary was heterotrophic and a weak source of CO 2 with an air-water emission rate of 1.2–5 mmol C m −2 d −1 (0.4–1.8 mol C m −2 y −1 ) resulting in a total carbon emission of around 930 tonnes per annum. CO 2 emissions (weighted m 3 s −1 of discharge per km 2 of estuary surface area) from Sydney Harbour were an order of magnitude lower than other temperate large tectonic deltas, lagoons and engineered systems of China, India, Taiwan and Europe but were similar to other natural drowned river valley systems in the USA. Discharge per unit area appears to be a good predictor of CO 2 emissions from estuaries of a similar climate and geomorphic class. [ABSTRACT FROM AUTHOR]