CO2 Transfer Velocity Research Articles

Evaluation of wave-based parameterizations of air–sea CO2 gas transfer over global oceans

The parameterization of air–sea CO2 transfer velocity employed in the estimation of bulk fluxes over global ocean is typically established on wind speed but could suffer from the deviations induced by sea states. In this study, the effectiveness of wave-based formulations are substantiated by reproducing climatological air–sea CO2 flux and gas transfer velocity. Sea states play a significant role in facilitating CO2 transfer, particularly in mid to high latitude regions with high wind speeds. The variability in transfer velocity induced by sea states is estimated up to 19% at the wind speed of 15 m/s. The two wave-based formulations used in this study are combined using a critical value of the Reynolds number. The combined formulation further improves estimates of the CO2 gas transfer velocity.

On the short-term response of entrained air bubbles in the upper ocean: a case study in the north Adriatic Sea

Abstract. Air bubbles in the upper ocean are generated mainly by waves breaking at the air–sea interface. As such, after the waves break, entrained air bubbles evolve in the form of plumes in the turbulent flow, exchange gas with the surrounding water, and may eventually rise to the surface. To shed light on the short-term response of entrained bubbles in different stormy conditions and to assess the link between bubble plume penetration depth, mechanical and thermal forcings, and the air–sea transfer velocity of CO2, a field experiment was conducted from an oceanographic research tower in the north Adriatic Sea. Air bubble plumes were observed using high-resolution echosounder data from an upward-looking 1000 kHz sonar. The backscatter signal strength was sampled at a high resolution, 0.5 s in time and 2.5 cm along the vertical direction. Time series profiles of the bubble plume depth were established using a variable threshold procedure applied to the backscatter strength. The data show the occurrence of bubbles organized into vertical plume-like structures, drawn downwards by wave-generated turbulence and other near-surface circulations, and reaching the seabed at 17 m depth under strong forcing. We verify that bubble plumes adapt rapidly to wind and wave conditions and have depths that scale approximately linearly with wind speed. Scaling with the wind–wave Reynolds number is also proposed to account for the sea-state severity in the plume depth prediction. Results show a correlation between measured bubble depths and theoretical air–sea CO2 transfer velocity parametrized with wind-only and wind/wave formulations. Further, our measurements corroborate previous results suggesting that the sinking of newly formed cold-water masses helps bring bubbles to greater depths than those reached in stable conditions for the water column. The temperature difference between air and sea seems sufficient for describing this intensification at the leading order of magnitude. The results presented in this study are relevant for air–sea interaction studies and pave the way for progress in CO2 gas exchange formulations.

Dimensionless Parameterizations of Air-Sea <em>CO</em><sub>2</sub> Gas Transfer Velocity on Surface Waves

Accurate quantification of air-sea gas transfer velocity is critical for our understanding of air-sea CO2 gas fluxes, global carbon budget and climate responses. CO2 transfer velocity is predominantly subject to constraints of wave-related dynamic processes at the ocean surface layer but is typically parameterized with wind speed. This study proposes and compares two parameterizations which accommodate dimensionless wave terms. The validations are conducted using both laboratory and field measurements of CO2 transfer and wave statistics. A scaling of bubble-mediated gas transfer is implemented into the formula that is linked to wave breaking probability. The improved parameterizations are capable of collapsing combined laboratory and field data sets which comprise diversified conditions of wind, wave and wave breaking.

Oceanic eddy-induced modifications to air\u2013sea heat and CO2 fluxes in the Brazil-Malvinas Confluence

Sea surface temperature (SST) anomalies caused by a warm core eddy (WCE) in the Southwestern Atlantic Ocean (SWA) rendered a crucial influence on modifying the marine atmospheric boundary layer (MABL). During the first cruise to support the Antarctic Modeling and Observation System (ATMOS) project, a WCE that was shed from the Brazil Current was sampled. Apart from traditional meteorological measurements, we used the Eddy Covariance method to directly measure the ocean–atmosphere sensible heat, latent heat, momentum, and carbon dioxide (CO2) fluxes. The mechanisms of pressure adjustment and vertical mixing that can make the MABL unstable were both identified. The WCE also acted to increase the surface winds and heat fluxes from the ocean to the atmosphere. Oceanic regions at middle and high latitudes are expected to absorb atmospheric CO2, and are thereby considered as sinks, due to their cold waters. Instead, the presence of this WCE in midlatitudes, surrounded by predominantly cold waters, caused the ocean to locally act as a CO2 source. The contribution to the atmosphere was estimated as 0.3 ± 0.04 mmol m−2 day−1, averaged over the sampling period. The CO2 transfer velocity coefficient (K) was determined using a quadratic fit and showed an adequate representation of ocean–atmosphere fluxes. The ocean–atmosphere CO2, momentum, and heat fluxes were each closely correlated with the SST. The increase of SST inside the WCE clearly resulted in larger magnitudes of all of the ocean–atmosphere fluxes studied here. This study adds to our understanding of how oceanic mesoscale structures, such as this WCE, affect the overlying atmosphere.

Inferring Air‐Sea Carbon Dioxide Transfer Velocities From Sea Surface Scatterometer Measurements

AbstractC‐band microwave sea surface radar backscatter observations from the FINO‐2 tower in the western Baltic are analyzed with respect to their relevance for air‐sea CO2 transfer velocity parameterizations. The scatterometer measurements observed from a height of 25 m above the sea surface using a multifrequency scatterometer instrument of the University of Hamburg were obtained quasi‐simultaneously with eddy covariance CO2 flux measurements. Both data sets are merged here to derive a gas transfer velocity parameterization based on radar backscatter measurements. At the location of the FINO‐2 tower, the resulting time‐averaged gas transfer velocity amounts to 26.95 cm/hr. In combination with measurements available from the vicinity of the FINO‐2 platform, a time‐mean CO2 flux of 0.23 μmol·m−2·s−1 into the Baltic was estimated. Applied to monthly mean satellite‐based C‐band ASCAT scatterometer data, the newly derived gas transfer velocity parameterization provides estimates of seasonal and annual mean global maps of air‐sea transfer velocities. The new results agree in their general pattern with previous estimates using wind speed parameterization. However, the backscatter‐based transfer velocities appear smaller at higher latitudes. Globally averaged air‐sea CO2 fluxes would thereby be reduced by 20%. To what extent this is a robust result, or if it depends on the fact that the training data set did not represent conditions, has to be investigated in the future.

Winter CO2 emission and its production rate in cold temperate soils of northern Japan: 222Rn as a proxy for the validation of CO2 diffusivity

In the context of a changing seasonal snow-covered period and extent in northern high latitude regions, winter CO2 emissions are vulnerable to drastic climate change, prompting rigorous evaluation of regional carbon budgets. The significance of winter CO2 contributions to the atmosphere in these regions has likewise been recognized. In this study, concentrations of CO2 and 222Rn were observed in the snowpack and underlying cold temperate soils of northern Japan during two winter seasons. Winter CO2 emissions, estimated alternately by snow density, 222Rn tracer (under steady and non-steady states), and chamber, were 56 ± 2, 42 ± 6, 44 ± 15, and 34 ± 3 gCO2-C/m2/season, respectively, suggesting that contribution from average winter CO2 emission corresponds to 25% (range: 19–33%) of annual emission. Winter CO2 emission is highly regulated by soil temperature at 5 cm, depending on snow depth. Winter CO2 emission transport is not suppressed through the snowpack to the atmosphere; rather it is stimulated when seasonal snow depth is over the 40-cm threshold in the soils. The higher soil CO2 production of 0.11 ± 0.02 gCO2-C/m3/h estimated by 222Rn transfer velocity reflects vigorous microbial activity in soils underlying winter snowpack. Therefore, 222Rn is an important proxy for the estimation of winter CO2 emission and diffusivity in snowpack, and of CO2 production rate and transfer velocity in the seasonal, snow-covered.

Insights from year-long measurements of air–water CH<sub>4</sub> and CO<sub>2</sub> exchange in a coastal environment

Abstract. Air–water CH4 and CO2 fluxes were directly measured using the eddy covariance technique at the Penlee Point Atmospheric Observatory on the southwest coast of the United Kingdom from September 2015 to August 2016. The high-frequency, year-long measurements provide unprecedented detail on the variability of these greenhouse gas fluxes from seasonal to diurnal and to semi-diurnal (tidal) timescales. Depending on the wind sector, fluxes measured at this site are indicative of air–water exchange in coastal seas as well as in an outer estuary. For the open-water sector when winds were off the Atlantic Ocean, CH4 flux was almost always positive (annual mean of ∼0.05 mmol m−2 d−1) except in December and January, when CH4 flux was near zero. At times of high rainfall and river flow rate, CH4 emission from the estuarine-influenced Plymouth Sound sector was several times higher than emission from the open-water sector. The implied CH4 saturation (derived from the measured fluxes and a wind-speed-dependent gas transfer velocity parameterization) of over 1000 % in the Plymouth Sound is within range of in situ dissolved CH4 measurements near the mouth of the river Tamar. CO2 flux from the open-water sector was generally from sea to air in autumn and winter and from air to sea in late spring and summer, with an annual mean flux of near zero. A diurnal signal in CO2 flux and implied partial pressure of CO2 in water (pCO2) are clearly observed for the Plymouth Sound sector and also evident for the open-water sector during biologically productive periods. These observations suggest that coastal CO2 efflux may be underestimated if sampling strategies are limited to daytime only. Combining the flux data with seawater pCO2 measurements made in situ within the flux footprint allows us to estimate the CO2 transfer velocity. The gas transfer velocity and wind speed relationship at this coastal location agrees reasonably well with previous open-water parameterizations in the mean but demonstrates considerable variability. We discuss the influences of biological productivity, bottom-driven turbulence and rainfall on coastal air–water gas exchange.

Air-Sea CO2-Exchange in a Large Annular Wind-Wave Tank and the Effects of Surfactants

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 m s-1 to 22.8 m s-1), and we increased the water pCO2 concentration twice by more than 1000 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 µg L-1 to 1015 µg L-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 m s-1, surfactants in the SML can reduce the gas transfer velocity by 54%. Wind-wave tank experiments in combination with modelling 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.

Wave‐Related Reynolds Number Parameterizations of CO2 and DMS Transfer Velocities

AbstractPredicting future climate hinges on our understanding of and ability to quantify air‐sea gas transfer. The latter relies on parameterizations of the gas transfer velocity k, which represents physical mass transfer mechanisms and is usually parameterized as a nonlinear function of wind forcing. In an attempt to reduce uncertainties in k, this study explores empirical parameterizations that incorporate both wind speed and sea state dependence via wave‐wind and breaking Reynolds numbers, RH and RB. Analysis of concurrent eddy covariance gas transfer and measured wavefield statistics supplemented by wave model hindcasts shows for the first time that wave‐related Reynolds numbers collapse four open ocean data sets that have a wind speed dependence of CO2 transfer velocity ranging from lower than quadratic to cubic. Wave‐related Reynolds number and wind speed show comparable performance for parametrizing dimethyl sulfide (DMS) which, because of its higher solubility, is less affected by bubble‐mediated exchange associated with wave breaking.

Citizen science shows systematic changes in the temperature difference between air and inland waters with global warming

Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (Tw-Ta) as a proxy for sensible heat flux (QH). If QH is directed upward, corresponding to positive Tw-Ta, it can enhance CO2 and CH4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative Tw-Ta across small ponds, lakes, streams/rivers and the sea shore (i.e. downward QH), with Tw-Ta becoming increasingly negative with increasing Ta. Further examination of Tw-Ta using high-frequency temperature data from inland waters across the globe confirmed that Tw-Ta is linearly related to Ta. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative Tw-Ta with increasing annual mean Ta since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative Tw-Ta, thereby reducing CO2 and CH4 transfer velocities from inland waters into the atmosphere.

A quantitative evaluation of the factors influencing the air-sea carbon dioxide transfer velocity

The numerous factors influencing the air-sea carbon dioxide (CO2) transfer velocity have been discussed for many years, yet the contributions of various factors have undergone little quantitative estimation. To better understand the mechanism of air-sea transfer, the effects of different factors are discussed on the air-sea transfer velocity and the various parametric models describing the phenomenon are classified and compared. Then, based on GAS EX-98 and ASGAMAGE data, wind models are evaluated and the effects of some factors are discussed quantitatively, including bubbles, waves, wind and so on by considering their interaction through a piecewise average approach. It is found that the air-sea CO2 transfer velocity is not only the function of the wind speed, but is also affected by bubbles, wave parameters and other factors. Stepwise and linear regressions are used. When considering the wind speed, bubbles mediated and the significant wave height, the root mean square error is reduced from 34.53 cm/h to 16.96 cm/h. Discussing the various factors quantitatively can be useful in future assessments of a large spatial scale and long-term air-sea CO2 flux and global change.

Assessing the utility of satellite-based whitecap fraction to estimate sea spray production and CO2 transfer velocity

The utility of a satellite-based whitecap database for estimates of surface sea spray production and bubble-mediated gas transfer on a global scale is presented. Existing formulations of sea spray production and bubble-mediated CO2 transfer velocity involve whitecap fraction parametrization as a function of wind speed at 10 m reference height W(U10) based on photographic measurements of whitecaps. Microwave radiometric measurements of whitecaps from satellites provide whitecap fraction data over the world oceans for all seasons. Parametrizations W(U10) based on such radiometric data are thus applicable for a wide range of conditions and can account for influences secondary to the primary forcing factor, the wind speed. Radiometric satellite-based W(U10) relationship was used as input to: (i) the Coupled Ocean-Atmosphere Response Experiment Gas transfer (COAREG) algorithm to obtain CO2 transfer velocity and total CO2 flux; and (ii) the sea spray source function (SSSF) recommended by Andreas in 2002 to obtain fluxes of sea spray number and mass. The outputs of COAREG and SSSF obtained with satellite-based W(U10) are compared with respective outputs obtained with the nominal W(U10) relationship based on photographic data. Good comparisons of the gas and sea spray fluxes with direct measurements and previous estimates imply that the satellite- based whitecap database can be useful to obtain surface fluxes of particles and gases in regions and conditions difficult to access and sample in situ. Satellite and in situ estimates of surface sea spray production and bubble-mediated gas transfer thus complement each other: accurate in situ observations can constrain radiometric whitecap fraction and mass flux estimates, while satellite observations can provide global coverage of whitecap fraction and mass flux estimates.

Uncertainty in air–sea CO2 flux due to transfer velocity

From TOPEX/Poseidon data, the significant uncertainty in global air–sea carbon dioxide (CO2) flux in 2000 is calculated from 21 wind-speed-dependent and 5 sea-state-dependent CO2 transfer velocities and the sea-to-air partial pressure difference proposed by Takahashi, Sutherland, and Kozyr (2010). The sea-state-dependent parameterizations are calculated based on the significant wave height (SWH) and the radar backscatter coefficient measured using the Ku-band altimeter. Uncertainty in air‒sea CO2 flux is compared using 26 various transfer velocity formulas. The maximum differences in global monthly mean and 4° zonal mean values for gas transfer velocity among these formulas are 33.20 and 108.20 cm hour‒1, respectively. The corresponding differences for sea-to-air CO2 flux are 6.41 and 0.58 Pg C year‒1, respectively. Monthly mean global maps of gas transfer velocity and flux are also presented. The average value of the global mean, transfer velocity obtained using the 26 formula is 27.33 ± 9.75 cm hour‒1, and the averaged total global net air-to-sea CO2 flux is 2.77 ± 1.02 Pg C year‒1 after area weighting and Schmidt number correction. The sea-state-dependent parameterizations are near these values, providing a successful method to estimate the air‒sea CO2 transfer velocity and flux.

Global carbon dioxide emissions from inland waters

Carbon dioxide (CO2) transfer from inland waters to the atmosphere, known as CO2 evasion, is a component of the global carbon cycle. Global estimates of CO2 evasion have been hampered, however, by the lack of a framework for estimating the inland water surface area and gas transfer velocity and by the absence of a global CO2 database. Here we report regional variations in global inland water surface area, dissolved CO2 and gas transfer velocity. We obtain global CO2 evasion rates of 1.8(+0.25)(-0.25) petagrams of carbon (Pg C) per year from streams and rivers and 0.32(+0.52)(-0.26) Pg C yr(-1) from lakes and reservoirs, where the upper and lower limits are respectively the 5th and 95th confidence interval percentiles. The resulting global evasion rate of 2.1 Pg C yr(-1) is higher than previous estimates owing to a larger stream and river evasion rate. Our analysis predicts global hotspots in stream and river evasion, with about 70 per cent of the flux occurring over just 20 per cent of the land surface. The source of inland water CO2 is still not known with certainty and new studies are needed to research the mechanisms controlling CO2 evasion globally.

Global air-sea surface carbon-dioxide transfer velocity and flux estimated using ERS-2 data and a new parametric formula

Using data from the European remote sensing scatterometer (ERS-2) from July 1997 to August 1998, global distributions of the air-sea CO2 transfer velocity and flux are retrieved. A new model of the air-sea CO2 transfer velocity with surface wind speed and wave steepness is proposed. The wave steepness (δ) is retrieved using a neural network (NN) model from ERS-2 scatterometer data, while the wind speed is directly derived by the ERS-2 scatterometer. The new model agrees well with the formulations based on the wind speed and the variation in the wind speed dependent relationships presented in many previous studies can be explained by this proposed relation with variation in wave steepness effect. Seasonally global maps of gas transfer velocity and flux are shown on the basis of the new model and the seasonal variations of the transfer velocity and flux during the 1 a period. The global mean gas transfer velocity is 30 cm/h after area-weighting and Schmidt number correction and its accuracy remains calculation with in situ data. The highest transfer velocity occurs around 60°N and 60°S, while the lowest on the equator. The total air to sea CO2 flux (calculated by carbon) in that year is 1.77 Pg. The strongest source of CO2 is in the equatorial east Pacific Ocean, while the strongest sink is in the 68°N. Full exploration of the uncertainty of this estimate awaits further data. An effectual method is provided to calculate the effect of waves on the determination of air-sea CO2 transfer velocity and fluxes with ERS-2 scatterometer data.

Implementation of the Coupled Ocean-Atmosphere Response Experiment flux algorithm with CO2, dimethyl sulfide, and O3

[1] Updates for the Coupled Ocean-Atmosphere Response Experiment (COARE) physically based meteorological and gas transfer bulk flux algorithms are examined. The current versions are summarized and a generalization of the gas transfer codes to 79 gases is described. The current meteorological version COARE3.0 was compared with a collection of 26,700 covariance observations of drag and heat transfer coefficients (compiled from three independent research groups). The algorithm agreed on average to within 5% with observations for a wind speed range of 2 to 18 m s−1. Covariance observations of CO2 and dimethyl sulfide (DMS) gas transfer velocity k were normalized to Schmidt number 660 and compared to an ensemble of gas flux observations from six research groups and nine field programs. A reasonable fit of the mean k660 versus U10n values was obtained for both CO2 and DMS with a new version of the COARE gas transfer algorithm (designated COAREG3.1) using friction velocity associated with viscous (tangential) stress, u*ν, in the nonbubble term. In the wind speed range 5 to 16 m s−1, tracer-derived estimates of k660 are 10% to 20% lower than the CO2 covariance estimates presented here.

Application of ALOS/PALSAR to estimate carbon dioxide transfer velocities compared with satellite wind speed data 2007–2008

Gas exchange contributes to the mitigation of an anthropogenic greenhouse effect through absorption of excess atmospheric CO2 by the oceans. The gas transfer velocity requires computation in order to describe gas exchange and interaction. The absolute calibration of the relationship between air and sea CO2 transfer velocity and wind speed (U 10) has been under debate for a long time because the global averages of the CO2 exchange coefficients, deduced from many experimental data relationships, disagreed with each other. In this research, the CO2 transfer velocity () was derived from satellite wind speeds that were computed using three different parameterizations (k–U) and was then compared with the derived from Phased Array type L-band Synthetic Aperture Radar (PALSAR) (k P). To obtain the transfer velocity of CO2 between the atmosphere and ocean using PALSAR in the Bali Sea area close to the Indian Ocean, daily sea surface temperature, wind friction velocity (U*) from PALSAR images and oceanography data were collected and analysed. Additionally, values were computed from the QuikSCAT satellite wind speed and compared to three different k–U relationships suggested by different authors. The three algorithms of parameterization were used with QuikSCAT daily data and produced a value that was close to that of derived from PALSAR (k P).

Global average of air‐sea CO2 transfer velocity from QuikSCAT scatterometer wind speeds

The absolute calibration of the relationship between air‐sea CO2 transfer velocity, k, and wind speed, U, has been a topic of debate for some time, because k global average, 〈k〉, as deduced from Geochemical Ocean Sections Study oceanic 14C inventory has differed from that deduced from experimental k‐U relationships. Recently, new oceanic 14C inventories and inversions have lead to a lower 〈k〉. In addition, new measurements performed at sea in high–wind speed conditions have led to new k‐U relationship. Meanwhile, quality and sampling of satellite wind speeds has greatly improved. The QuikSCAT scatterometer has provided high‐quality wind speeds for more than 7 years. This allows us to estimate the global distributions of k computed using k‐U relationships and temperature‐dependent Schmidt numbers from 1999 to 2006. Given the difficulty of measuring in situ wind speed very accurately, we performed a sensitivity study of the 〈k〉 uncertainty which results from QuikSCAT U uncertainties. New QuikSCAT‐buoy U comparisons in the northern Atlantic Ocean and in the Southern Ocean confirm the excellent precision of QuikSCAT U (RMS difference of about 1 m s−1), but it is possible that QuikSCAT overestimates wind speeds by 5%, leading to a possible overestimation of k derived with quadratic relationships by 10%. The 〈k〉 values obtained with two recent experimental k‐U relationships are very close, between 15.9 and 17.9 cm h−1, and within the error bar of k average deduced from the new oceanic 14C inventory.

Influence of the Presence of Swell on Air-Sea CO2 Transfer Velocity

Field measurements at a sea observation tower were made to investigate the dependence of air-sea CO2 transfer velocity on swell conditions. Based on directional wave spectra, we classified swell conditions into three groups, i. e., Pure windsea, Swell-dominated sea and Cross swell. The present data show that the transfer velocity for Pure windsea increases monotonously with the wind speed, while the presence of swell contaminates the wind-speed dependence of the transfer velocity. Also, the transfer velocity for Cross swell seems to take larger values than the other conditions. Moreover, we examined how the transfer velocity nondimensionalized by the friction velocity depends on a nondimensional wave-energy dissipation rate. In such relations, there is no remarkable difference in three types of swell conditions. This means that the energy dissipation rate may be effective for the parameterization of the transfer velocity.

Effect of wind waves on air-sea gas exchange: proposal of an overall CO2 transfer velocity formula as a function of breaking-wave parameter

Effect of wind waves on air-sea gas exchange: proposal of an overall CO2 transfer velocity formula as a function of breaking-wave parameter