Gas Exchange Coefficient Research Articles

Temporal variability of atmospheric CO 2 of the Spanish Atlantic Coast

The variability of the molar fraction of atmospheric CO 2 ( xCO 2 a ) and wind speed and direction were investigated in a coastal embayment located in the west European coast, ría de Vigo, NW Spain, along daily and seasonal time scales. Observations in the ría showed that xCO 2 a on a short time scale presented a much wider variability than seawater molar fraction ( xCO 2 w ), in addition, a significant covariation between xCO 2 a and wind was found. A sluggish atmospheric renewal due to weak winds was associated with high values of xCO 2 a , whereas higher oceanic winds renovate the air column with more stable and constrained xCO 2 w values (from 350 to 370 ppm). The impact of anomalously high xCO 2 a on CO 2 air–sea fluxes is practically not significant, due to the kinetic control exerted by wind speed by means of the gas exchange coefficient. A seasonal cycle for the atmospheric molar fraction of CO 2 in the Southwest European Coast was obtained. Using this approach for xCO 2 a in calculating the air–sea CO 2 fluxes avoids under/overestimations of the fluxes on particular short periods of time, whilst using a mean xCO 2 a seasonal value for longer time scales has no significant effect on the final net magnitude of the air–sea flux.

Capillary wave gas exchange in the presence of surfactants

The effect of surfactants on gas exchange across an air/water interface populated with capillary waves, is con- sidered. Experiments were conducted on capillary waves having a wavelength of 2.87 mm in the presence of oleyl alcohol and stearic acid, as well as on surfaces which were surfactant-free. The presence of these surfactants decreased the gas exchange rate by at most a factor of two when the energy delivered to the tank was held constant. Thus, even in the presence of surfactants, pure capillary waves still caused significant gas exchange, indicating that partially damped capillary waves may play an important role in air/sea gas exchange. When the gas exchange coefficient was plotted as a function of mean square slope, the presence of surfactants was found to negligibly affect the gas exchange rate, with the possible exception of the high wave slope regime for stearic acid. This result suggests that it is principally the kinematics of wave motion which accounts for the enhancement of transport due to the capillary waves investigated here. Moreover, these results agree with those obtained from polychromatic, wind-generated waves, suggesting that, for non-breaking waves, knowledge of the statistics of the wave field may be all that is required to parameterize the gas exchange coefficient.

Assessing the seasonality of the oceanic sink for CO2 in the northern hemisphere

Seasonal CO2 fluxes are estimated from quarterly maps of ΔpCO2 (difference between the oceanic and atmospheric partial pressure of CO2) and associated error maps. ΔpCO2 maps were interpolated from pCO2 measurements in the North Atlantic and the North Pacific Oceans using an objective mapping technique. Negative values correspond to an uptake of CO2 by the ocean. The CO2 flux for the North Atlantic Ocean, between 10°N and 80°N, ranges from −0.69 GtC/yr, for the first quarter (January‐March), to −0.19 GtC/yr for the third quarter (July‐September) using the gas exchange coefficient of Tans et al. [1990], satellite wind speeds, and a correction for the skin effect. On annual average, the North Atlantic ocean (north of 10°N) is a sink of CO2 ranging from −0.23 ± 0.08 GtC/yr (gas exchange coefficient of Liss and Merlivat [1986] with Esbensen and Kushnir [1981] wind field) to −0.48 ±0.17 GtC/yr (gas exchange coefficient of Tans et al. with satellite wind field). The CO2 flux for the North Pacific, between 15°N and 65°N, ranges from −0.66 GtC/yr from April to June to zero from July to September. For the Atlantic, the errors are generally small, that is, less than 0.19 GtC/yr, but for the Pacific considerably larger uncertainties are generated due to the less extensive data coverage. The northern hemisphere ocean (north of 10°N) is a net sink of CO2 to the atmosphere which is stronger in spring (April‐June), due to the biological activity, with an estimate of −1.23 ± 0.40 GtC/yr averaged over this period. The annual mean northern hemisphere ocean flux is −0.86 ± 0.61 GtC/yr.

The rôle of capillary waves in oceanic air/water gas exchange

Recent experiments have demonstrated that millimeter-scale capillary waves can enhance the transport of CO2 by almost 2 orders of magnitude for moderate wave slopes. These results are used to create a model for the relative contribution of capillary waves to the gas exchange coefficient. The model input is wind speed u, and the output is Kf the fractional contribution of a specific range of capillary waves to the total gas exchange coefficient. Wind speed data, obtained via satellite, are used as the model input to obtain globally averaged values forKf. In spite of the enhancing effect which capillary waves provide in the laboratory, the maximum value of Kf predicted by the model is less than 10%, and global averages are less than 4%. The small values of Kf are primarily due to the small wave energies predicted by existing wave height spectra in the high wave number regime. The uncertainty in existing wave height spectra, and the importance of experimental validation of the high wave number regime is discussed. Some interesting aspects of capillary wave gas exchange are also expanded upon. Among these are the demonstration of a linear relationship between the capillary wave gas exchange coefficient and wavelength, and a peak in the contribution of capillary waves to gas exchange at a wavelength around λ= 3.6 mm.

Variability of ΔpCO2 in the subarctic North Pacific. A comparison of results from four expeditions

Time-space variability of surface seawater pCO2 is examined over the region (150°W-180°, 46°N-50°N) of the subarctic North Pacific where large meridional gradients of temperature and nutrient concentrations exist. The data were collected during four trans-Pacific expeditions in three different years (1985–1987), but within the same 30-day period of the year (August — September). Systematic measurement differences between the four data sets are estimated as < 10 μatm. The inter-expedition comparison suggests that surface seawater pCO2 in the study area is quite variable, with mean differences of up to 25 μatm and local differences up to 60 μatm. Spatial and interannual variability of surface seawater pCO2 were found to contribute significant uncertainty to estimates of the mean ΔpCO2 for the study area. Fluxes were calculated using ΔpCO2 values from the four expeditions combined with gas exchange coefficients calculated from four different wind fields giving a range of - 0.94 to + 4.1 mmol CO2 m-2 d-1. The range of fluxes from the study area is scaled to a larger area of the North Pacific to address how this variability can translate into uncertainties in basin-wide carbon air-sea exchange fluxes.

The effect of ventilation rate on proliferation and hyperhydricity of Dianthus caryophyllus L

Comparative studies of carnation micropropagation under four different ventilation rates showed that using gas-permeable filters, with gelled or liquid media and modifying the volume of culture medium, it was possible to establish a suitable hydric state to obtain good proliferation rates with gelled and liquid medium, as well as optimal acclimatization of microcuttings. The following parameters were measured: ventilation rate, gas exchange coefficients, relative water loss, increase of agar concentration, micropropagation rates, percentage of hyperhydricity, and acclimatization rates. Our results confirm that it is possible to avoid hyperhydric plants cultured in liquid medium with the use of ventilated culture vessels through the control of the water relations during the multiplication phase and, at the same time, keeping the micropropagation rate.

The total organic carbon export rate based on 13C and 12C of DIC budgets in the equatorial Pacific region

Carbon stable isotopic composition (δ 13C) of total dissolved inorganic carbon (DIC) is independent of DIC as a tracer for net organic carbon production rate in the surface ocean due to the Significantly different equilibration times for CO 2 and 13CO 2 during air-sea exchange. The rates of the organic carbon export and net physical supply of DIC, can therefore, be determined from the mixed layer mass balances for DIC and DIC 13 (DIC· 13C/ 12C), given an air-sea C0 2 flux estimated from wind speed and measured pCO 2. This method does not rely on specifying the absolute advetive and diffusive transports of DIC. The mass balance approach was applied to measurements of concentration and δ 13C of DIC made in the equatorial Pacific ocean in the Spring and Fall of 1991–1992 during WOCE P16C and US JGOFS EqPac cruises. The calculated organic carbon export rate out of mixed layer during Spring 1992 El Nin˜o was 3.2±1.4 mmol C m −2 d −1 (between 2°N to 2°S and 170°W to 110°W). No spatial trend for the total organic carbon export rate was determined despite the large variation in C02 degassing rate from ∼0.1 mmol C m −2 d −1 at 170°W to ∼4.0 mmol Cm −2 d −1 at 110°W over this region. Under the cold tongue conditions in the end of August 1992, a higher organic carbon export rate, about 11.5±6.3 mmol C m -2 d −1 was obtained on the equator at 140°W while the rate for the previous September (in 1991) was 4.9 ± 2.4 mmol C m −2 d −1. Most of the uncertainty in our estimated results from the uncertainty in gas exchange coefficient. The comparison between the estimated organic carbon export rates and the particulate organic carbon (POC) export rates derived from thorium budget and sediment drifting traps at the same stations (Buesseler et al., 1995; Murray et al., 1996) suggests that particulate organic carbon probably accounts for about 70% of the organci export. Since physical, rather than biological, processes primarily control the distribution of DIC, NO 3 O 2 and other nutrient-related properties in this region, it is very difficult to accurately obtain the net organic carbon export rate from advection rates and nutrient gradients, as demonstrated by a 3-D calculation.

Estimation of hydroxyl radical concentrations in the marine atmospheric boundary layer using a reactive atmospheric tracer

Hydroxyl radical (OH) concentrations in the atmospheric boundary layer over a number of remote ocean locations are calculated from the measured diurnal variation in atmospheric dimethylsulfide (DMS). By using averaged DMS data sets from extended periods, the calculation yields OH concentrations averaged over periods from several days to weeks. These average OH concentrations range from 7×105 to 2.9×106 molecules cm-3, corresponding to midday maxima of 3 to 12×106 molecules cm-3. The lowest values correspond to studies with the lowest light intensity (Antarctic summer and South Atlantic winter), and the highest values to regions with probable anthropogenic influence. In addition to the long term averages, daily average OH levels can be calculated for most days in a two week period from a cruise in the tropical eastern Pacific. These calculations are in good argeement with global average OH levels derived from other tracers, and are consistent with model OH calculations when allowance is made for variation in ambient ozone levels between the studies. Estimates of gas exchange made from the diurnal variation of DMS suggest that either the gas exchange coefficient of DMS or the boundary layer mixing depth may have been overestimated in past analyses.

The preindustrial atmospheric14CO2 latitudinal gradient as related to exchanges among atmospheric, oceanic, and terrestrial reservoirs

A three‐dimensional global tracer transport model (derived from a general circulation model) simulates geographic variations in atmospheric Δ14C in response to oceanic boundary conditions. Regional atmosphere‐ocean 14CO2 fluxes are controlled by regional wind‐dependent gas exchange coefficients (E), air‐sea pCO2 differences (ΔpCO2) and oceanic 14C/12C deficiencies. We find that various preindustrial oceanic scenarios reconstructed from reasonable sets of such air‐sea variables all produce model latitudinal gradients in atmospheric Δ14CO2 (the “NS Δ14C”) significantly greater than the measured preindustrial NS Δ14C of +4.4 ± 0.5‰ (45°N to 45°S), as estimated from pre‐twentieth century tree rings. The NS Δ14C is insensitive to regional ΔpCO2 but is strongly contingent on the chosen values for surface ocean Δ14C and E of southern high latitudes (&gt;50°S). The simultaneous seasonality in sea ice extent and high‐latitude wind speeds may in part explain the discrepancy between modeled and measured preindustrial NS Δ14C. Terrestrial 14C sinks with longer turnover times (such as peatlands) also potentially may help to reduce the NS Δ14C. With our best estimates for preindustrial surface ocean Δ14C, ΔpCO2, and E values for other regions, we find a “best fit” ocean scenario in which the preindustrial southern surface ocean Δ14C is −95 ± 10 ‰, its ΔpCO2 is 0 ± 20 ppmv, and its E is 0.088 ± 0.010 M m−2 yr−1 μatm−1 (about 20% reduced from the widely accepted value of 0.110). An independent estimate of +6.5 ± 0.5 kg yr−1 for the net preindustrial oceanic 14C uptake is an important constraint on global mean and Antarctic E.

Oceanic transport and storage of carbon emissions

Using a global carbon cycle model (GLOCO) that considers seven terrestrial biomes, surface and deep ocean layers based on the HILDA model and a single mixed atmosphere, we analyzed the response of atmospheric CO2 concentration and oceanic DIC and DOC depth profiles to additions of carbon to the atmosphere and ocean. The rate of transport of carbon to the deepest oceanic layers is rather insensitive to the atmosphereic-ocean surface gas exchange coefficient over a wide range, hence discrepancies between researchers on the precise global average value of this coefficient do not significantly affect predictions of atmospheric response to anthropogenic inputs. Upwelling velocity, on the other hand, amplifies oceanic response by increasing primary production in the upper ocean layers, resulting in a larger flux into DOC and sediments and increased carbon storage; experiments to reduce the uncertainty in this parameter would be valuable.

Thermal skin effect and the air‐sea flux of carbon dioxide: A seasonal high‐resolution estimate

Understanding the role the oceans play in sequestering anthropogenic CO2 is crucial to understanding global climate change. Correct parameterization of air‐sea flux of CO2 is an important challenge to modelers. Recently it has been demonstrated that the thin thermal layer at the surface of the ocean can lead to an underestimate of CO2 solubility (Robertson and Watson, 1992). We re‐evaluate the effect of the cool thermal skin and present a high‐resolution seasonal estimate of its effect on the air‐sea flux of CO2. We compare air‐sea flux estimates derived using both a mean wind field and a more realistic Rayleigh distribution of the wind field. Using the mean monthly wind stress and a linear relationship between wind speed and the gas exchange coefficient of CO2 (Tans et al., 1990), we estimate that excluding the southern ocean, the surface skin correction increases the air‐sea flux of carbon by 0.48 Gt yr‐1. This is 25% lower than the correction suggested by Robertson and Watson (1992) and the difference is attributed to the better temporal and spatial resolution of the present data set. When a more realistic representation of the temporally varying winds is used, the corrected carbon flux decreases to 0.36 Gt yr−1. Conservatively, adding a 10% contribution from the southern ocean, we estimate a mean global increase in CO2 flux due to the skin effect of 0.39 Gt C yr−1. This is 40% lower than the previous estimate of Robertson and Watson (1992). Finally, adopting the gas transfer parameterization of Liss and Merlivat (1984), we estimate a CO2 flux anomaly of only 0.17 Gt C yr−1 which is approximately 50% lower than the analogous estimate using the Tans et al. (1990) formulation and a full 75% lower than the estimate of Robertson and Watson (1992). These results suggest that both a proper representation of the wind speed/flux correlation and a realistic distribution of the wind field is essential in making large‐scale flux estimates. We also examine the seasonal variation of the thermal skin effect. The largest negative temperature gradients (‐0.75°C) are found during the northern hemisphere winter in the regions of the Kuroshio and the Gulf Stream Currents, whereas the central North Pacific has a small positive temperature gradient during the summer months.

Modelling atmosphere—ocean CO2transfer

Knowledge of the uptake of atmospheric carbon dioxide by the North Atlantic is important in understanding the global carbon budget. Obtaining an accurate value for the ocean sinks in the northern hemisphere temperate zone, in particular, would make it possible to be more quantitative about the enigmatic land sink in these latitudes. The global ocean sink is dominated by the North Atlantic, despite its small area in comparison with the North Pacific. Of the possible methods for calculating the uptake, potentially the most informative is that based on the gas exchange equation because information about the seasonal and spatial trends can be obtained. However, there still remain serious questions about which form of the gas exchange coefficient is appropriate to carbon dioxide. Here we summarize current knowledge of the gas exchange coefficient, including recent new evidence which supports lower gas exchange rates and which, therefore, generally supports the Liss-Merlivat prediction for less soluble gases. In view of the uncertainties still surrounding the direct calculation, we investigate methods for calculating the basin-wide uptake by exploiting what is known about the general circulation of the North Atlantic. The uptake can be estimated by dividing it into pre-industrial steady state and anthropogenic contributions. We make a new estimate of the pre-industrial flux based on the heat budget of the North Atlantic, and also consider two earlier calculations of the same quantity. Taking a best value, adding the better-known anthropogenic flux and making small corrections, we find a value of 0.7± 0.15 Gt C a-1for the uptake of the North Atlantic, north of 15° N, in the mid 1980s. Though larger than the gas exchange based estimates of Tanset al.(1990), this value is not enough to obviate the need which they deduced for a sizeable ‘land-based’ northern hemisphere sink for CO2.

Gas exchange in a contaminated estuary inferred from chlorofluorocarbons

A new method of quantifying mean rates of gas exchange for natural water systems using the distribution of pollutant chlorofluorocarbons (CFCs) is presented. Concentrations of dissolved CCl3F (F‐11), CCl2F2 (F‐12), and CC12FCC1F2 (F‐113) in the Hudson estuary are supersaturated with respect to the remote troposphere by as much as an order of magnitude. The most plausible source appears to be discharges from waste water treatment facilities. Loss of these compounds from the estuary water to the atmosphere occurs both upstream and downstream of the zone of input. Using a multi‐box model, gas exchange coefficients were calculated to be between 2 and 4 cm hr−1 based on the observed concentrations. This rate of gas exchange is similar to values determined in lakes and coastal bays and substantially lower than the mean value for the open ocean.

Assessment of the air‐sea exchange of CO2 in the south Pacific during austral autumn

Measurements of CO2 concentrations in the atmosphere and in the surface waters of the South Pacific Ocean were made by NOAA scientists between 1984 and 1989. These basin‐wide measurements were all taken during austral autumn and provide data for evaluation of the seasonal flux of CO2 from this region. The sensitivity of this flux to the uncertainty in the CO2 gas exchange coefficient was evaluated using four different wind data sets and two formulations for the wind dependence of gas transfer velocity. The integrated net flux of CO2 to the atmosphere during austral autumn (February to May) ranges from −0.03 (ocean influx) to +0.09 (ocean efflux) GT of carbon depending on the combination of wind field and wind‐dependent exchange coefficient used.

The changing patterns of ΔpCO2 between ocean and atmosphere

The average difference between the partial pressure of CO2 in ocean surface water and the overlying atmosphere (ΔpCO2 = pCO2,ocean) ‐ pCO2,atm) has been changing ever since the beginning of the anthropogenic era due to the dumping of CO2 as a waste gas into the atmosphere. However, the change in the difference has not been uniform over the surface of the oceans. This work assesses regional variations in the Earth scale patterns of sources (ΔpCO2&gt;0) and sinks (ΔpCO2&lt;0) during this anthropogenic transient. The regional correction, ΔpCO2*, is defined as the quantity that must be added to a region's preanthropogenic ΔpCO2 to give its present value. Using a five‐box ocean model with special surface regions for the prominent equatorial Pacific source and north Atlantic sink, we show that a larger magnitude of ΔpCO2* is necessary for these two regions than for the average ocean. Analytical results with a multiple one‐and‐one‐half‐box model, in which horizontal water exchanges are neglected, indicate that the ratio Of ΔpCO2* values for any two regions should vary inversely with the ratio of their gas exchange coefficients and nearly directly with the ratio of their deep‐to‐surface water piston velocities. The ΔpCO2* values for regions of the five‐box model vary by as much as a factor of six; predictions of the multiple one‐and‐one‐half‐box model are within a factor of two. Discrepancies from these predictions are believed to be due to horizontal and intermediate‐depth water exchanges, which tend to equalize the ΔpCO2* values. Our best estimate is that regions with large vertical water exchanges (equatorial and high‐latitude zones) and/or low gas exchange coefficients (equatorial zones) have ΔpCO2* magnitudes at least twice as large as the magnitude of ΔpCO2* for the average global ocean.

Fluxes of N2O at the sediment‐water and water‐atmosphere boundaries of a nitrogen‐rich river

The biogeochemistry of N2O was studied in a reach of a small river in Massachusetts which receives wastewater treatment plant effluent. Fluxes of N2O across the sediment‐water and water‐air interfaces were measured using a mass‐balance approach in which gas exchange coefficients were measured in situ. The observed atmospheric flux averaged 0.25 mg N2O m−2 h−1, and was among the highest reported fluxes for aquatic systems on a per‐area basis, although the significance of such rivers as global N2O sources may not be large. The largest fraction of the N2O released to the atmosphere was produced in the treatment plant per se. During the cold season, N2O was produced in the river sediment; during the warm season, the sediment was a sink for N2O. Throughout the year, water column production was minimal. During the season when N2O was consumed by the sediment, the benthic N2O flux could be predicted by a simple oxygen penetration depth model.

Field measurements of air‐sea CO2 exchange1

The direction and magnitude of air‐sea exchanges depend on the difference of gas partial pressure (ΔP) between phases and on a gas exchange coefficient (K). In this work, field air‐sea CO2 exchange and ΔPco2 have been measured simultaneously to study the in situ relations between parameters. Experiments have been carried out at three sites: the Bay of Calvi (Corsica), the Ligurian Sea, and the North Sea. Flux measurements were made with a direct chamber method to detect invasion or evasion of gas.Due to its bed of seagrass (P(Posidonia), the Bay of Calvi is the site of regular, daily, and yearly variations of surface CO2 content. Daily variations are often too small to overcome the variation of K and to find a correlation between hourly measured fluxes and daily changes of ΔP. The yearly variation of Pco2 in surface water (from 300 ppm in winter to 900 ppm in summer) has however permitted derivation of a linear relationship between mean fluxes and ΔP that fits well with fluxes measured at the two other sites under similar meteorological conditions and thus with nearly the same K. Calculated coefficients range between 10−5 and 4 × 10−5 m s−1 and so agree well with those determined with radiotracers or in wind tunnels.Measurements in the North Sea under different meteorological conditions show the influence of wind speed and sea state on exchanges measured by the in situ methodology used and allow the results to be compared with those obtained by others in wind tunnels.

Gas exchange on Mono Lake and Crowley Lake, California

Gas exchange coefficients have been determined for freshwater Crowley Lake and for saline Mono Lake through the use of a man‐made purposefully injected gas, sulfur hexafluoride (SF6). The concentration decreased from an initial value of 40×10−12 mol/L to 4×10−12 mol/L for Mono Lake and from 20×10−12 mol/L to 1×10−12 mol/L for Crowley Lake over a period of 6 weeks. Wind speed records from anemometers on the shore of each lake enabled us to determine the relationship between the gas exchange coefficient k and wind speed u. The average wind speed and average exchange coefficient for the experiment were identical for the two lakes (uav=2.9 m/s, kav=2.5 cm/h), despite a large difference in size and chemical composition.From laboratory experiments and theoretical calculations it is estimated that for wind speeds observed over Mono Lake from July until December 1984 the exchange of CO2 occurred 2–½ times faster than without chemical enhancement. This is a factor of 4 lower than needed to explain the high invasion rate of 14C produced by nuclear bomb tests.

Gas Exchange-Wind Speed Relation Measured with Sulfur Hexafluoride on a Lake

Gas-exchange processes control the uptake and release of various gases in natural systems such as oceans, rivers, and lakes. Not much is known about the effect of wind speed on gas exchange in such systems. In the experiment described here, sulfur hexafluoride was dissolved in lake water, and the rate of escape of the gas with wind speed (at wind speeds up to 6 meters per second) was determined over a 1-month period. A sharp change in the wind speed dependence of the gas-exchange coefficient was found at wind speeds of about 2.4 meters per second, in agreement with the results of wind-tunnel studies. However, the gas-exchange coefficients at wind speeds above 3 meters per second were smaller than those observed in wind tunnels and are in agreement with earlier lake and ocean results.

Gas exchange dependency on diffusion coefficient: Direct 222Rn and 3He comparisons in a small lake

A direct field comparison was conducted to determine the dependency of gas exchange coefficient (kx) on the diffusion coefficient (Dx). The study also sought to confirm the enhanced vertical exchange properties of limnocorrals and similar enclosures. Gas exchange coefficients for 222Rn and 3He were determined in a small northern Ontario lake, using a 226Ra and 3H spike to gain the necessary precision. The results indicate that the gas exchange coefficient is functionally dependent on the diffusion coefficient raised to the 1.22+ &gt; 12−35 power (kx = ƒ(Dx1.22)), clearly supporting the stagnant film model of gas exchange. Limnocorrals were found to have gas exchange rates up to 1.7 times higher than the whole lake in spite of the observation of more calm surface conditions in the corral than in the open lake.