Spatiotemporal patterns in methane flux and gas transfer velocity at low wind speeds: Implications for upscaling studies on small lakes

Abstract. Air/sea dimethylsulfide (DMS) fluxes and bulk air/sea gradients were measured over the Southern Ocean in February/March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (>15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s−1. Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A~flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data shows no obvious modification of the gas transfer-wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.

Download Full-text

Dimethylsulfide gas transfer coefficients from algal blooms in the Southern Ocean

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-1783-2015 ◽

2015 ◽

Vol 15 (4) ◽

pp. 1783-1794 ◽

Cited By ~ 31

Author(s):

T. G. Bell ◽

W. De Bruyn ◽

C. A. Marandino ◽

S. D. Miller ◽

C. S. Law ◽

...

Keyword(s):

Wind Speed ◽

Southern Ocean ◽

Eddy Covariance ◽

Algal Blooms ◽

Biologically Active ◽

Gas Transfer ◽

Transfer Coefficients ◽

Transfer Velocity ◽

Gas Transfer Velocity ◽

Wind Speeds

Abstract. Air–sea dimethylsulfide (DMS) fluxes and bulk air–sea gradients were measured over the Southern Ocean in February–March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (> 15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s−1. Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind-speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data show no obvious modification of the gas transfer–wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.

Download Full-text

Sniffle: a step forward to measure in situ CO2 fluxes with the floating chamber technique

Elem Sci Anth ◽

10.1525/elementa.275 ◽

2018 ◽

Vol 6 ◽

Cited By ~ 5

Author(s):

M. Ribas-Ribas ◽

L. F. Kilcher ◽

O. Wurl

Keyword(s):

Gas Transfer ◽

Tidal Basin ◽

Co2 Fluxes ◽

Spatiotemporal Resolution ◽

Transfer Velocity ◽

Gas Transfer Velocity ◽

Wind Speeds ◽

Drifting Buoy ◽

Floating Chamber

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 (<4 m s–1), which leads to underestimation of gas transfer velocities and, therefore, of air–sea CO2 fluxes. The Sniffle is equipped with a sensor to consecutively measure aqueous and atmospheric pCO2 and to monitor increases or decreases of CO2 inside the chamber. During autonomous operation, a complete cycle lasts 40 minutes, with a new cycle initiated after flushing the chamber. The Sniffle can be deployed for up to 15 hours at wind speeds up to 10 m s–1. Floating chambers often overestimate fluxes because they create additional turbulence at the water surface. We correct fluxes by measuring turbulence with two acoustic Doppler velocimeters, one positioned directly under the floating chamber and the other positioned sideways, to compare artificial disturbance caused by the chamber and natural turbulence. The first results of deployment in the North Sea during the summer of 2016 demonstrate that the new drifting buoy is a useful tool that can improve our understanding of gas transfer velocity with in situ measurements. At low and moderate wind speeds and different conditions, the results obtained indicate that the observed tidal basin was acting as a source of atmospheric CO2. Wind speed and turbulence alone could not fully explain the variance in gas transfer velocity. We suggest that other factors like surfactants, rain or tidal current will have an impact on gas transfer parameterizations.

Download Full-text

Impact of Nonzero Intercept Gas Transfer Velocity Parameterizations on Global and Regional Ocean–Atmosphere CO2 Fluxes

Geosciences ◽

10.3390/geosciences9050230 ◽

2019 ◽

Vol 9 (5) ◽

pp. 230

Author(s):

Mariana Ribas-Ribas ◽

Gianna Battaglia ◽

Matthew P. Humphreys ◽

Oliver Wurl

Keyword(s):

Wind Speed ◽

Ocean Model ◽

Global Ocean ◽

Gas Transfer ◽

Co2 Fluxes ◽

Average Wind Speed ◽

Transfer Velocity ◽

Gas Transfer Velocity ◽

Wind Speeds

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.

Download Full-text

Spatiotemporal variability of gas transfer velocity in a tropical high‐elevation stream using two independent methods

Ecosphere ◽

10.1002/ecs2.3647 ◽

2021 ◽

Vol 12 (7) ◽

Author(s):

Keridwen M. Whitmore ◽

Nehemiah Stewart ◽

Andrea C. Encalada ◽

Esteban Suárez ◽

Diego A. Riveros‐Iregui

Keyword(s):

High Elevation ◽

Spatiotemporal Variability ◽

Gas Transfer ◽

Transfer Velocity ◽

Gas Transfer Velocity

Download Full-text

Wind and fetch dependence of gas transfer velocity in an Arctic sea-ice lead determined from eddy covariance CO2 flux measurements

10.1002/essoar.10502449.1 ◽

2020 ◽

Author(s):

John Prytherch

Keyword(s):

Sea Ice ◽

Eddy Covariance ◽

Co2 Flux ◽

Arctic Sea Ice ◽

Gas Transfer ◽

Transfer Velocity ◽

Gas Transfer Velocity ◽

Arctic Sea ◽

Flux Measurements

Download Full-text

Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean

Biogeosciences ◽

10.5194/bg-9-957-2012 ◽

2012 ◽

Vol 9 (3) ◽

pp. 957-964 ◽

Cited By ~ 26

Author(s):

A. Kock ◽

J. Schafstall ◽

M. Dengler ◽

P. Brandt ◽

H. W. Bange

Keyword(s):

Nitrous Oxide ◽

Gas Exchange ◽

Mixed Layer ◽

North Atlantic Ocean ◽

Gas Transfer ◽

Potential Contribution ◽

Transfer Velocity ◽

N2o Production ◽

Gas Transfer Velocity ◽

Upwelled Water

Abstract. Sea-to-air and diapycnal fluxes of nitrous oxide (N2O) into the mixed layer were determined during three cruises to the upwelling region off Mauritania. Sea-to-air fluxes as well as diapycnal fluxes were elevated close to the shelf break, but elevated sea-to-air fluxes reached further offshore as a result of the offshore transport of upwelled water masses. To calculate a mixed layer budget for N2O we compared the regionally averaged sea-to-air and diapycnal fluxes and estimated the potential contribution of other processes, such as vertical advection and biological N2O production in the mixed layer. Using common parameterizations for the gas transfer velocity, the comparison of the average sea-to-air and diapycnal N2O fluxes indicated that the mean sea-to-air flux is about three to four times larger than the diapycnal flux. Neither vertical and horizontal advection nor biological production were found sufficient to close the mixed layer budget. Instead, the sea-to-air flux, calculated using a parameterization that takes into account the attenuating effect of surfactants on gas exchange, is in the same range as the diapycnal flux. From our observations we conclude that common parameterizations for the gas transfer velocity likely overestimate the air-sea gas exchange within highly productive upwelling zones.

Download Full-text

Surfactant control of gas transfer velocity along an offshore coastal transect: results from a laboratory gas exchange tank

Biogeosciences ◽

10.5194/bg-13-3981-2016 ◽

2016 ◽

Vol 13 (13) ◽

pp. 3981-3989 ◽

Cited By ~ 19

Author(s):

R. Pereira ◽

K. Schneider-Zapp ◽

R. C. Upstill-Goddard

Keyword(s):

Organic Matter ◽

Gas Exchange ◽

Trace Gas ◽

Gas Transfer ◽

Transfer Velocity ◽

Chl A ◽

Gas Transfer Velocity ◽

Biogeochemical Models ◽

Sea Surface Microlayer ◽

Laboratory Water

Abstract. Understanding the physical and biogeochemical controls of air–sea gas exchange is necessary for establishing biogeochemical models for predicting regional- and global-scale trace gas fluxes and feedbacks. To this end we report the results of experiments designed to constrain the effect of surfactants in the sea surface microlayer (SML) on the gas transfer velocity (kw; cm h−1), seasonally (2012–2013) along a 20 km coastal transect (North East UK). We measured total surfactant activity (SA), chromophoric dissolved organic matter (CDOM) and chlorophyll a (Chl a) in the SML and in sub-surface water (SSW) and we evaluated corresponding kw values using a custom-designed air–sea gas exchange tank. Temporal SA variability exceeded its spatial variability. Overall, SA varied 5-fold between all samples (0.08 to 0.38 mg L−1 T-X-100), being highest in the SML during summer. SML SA enrichment factors (EFs) relative to SSW were ∼  1.0 to 1.9, except for two values (0.75; 0.89: February 2013). The range in corresponding k660 (kw for CO2 in seawater at 20 °C) was 6.8 to 22.0 cm h−1. The film factor R660 (the ratio of k660 for seawater to k660 for “clean”, i.e. surfactant-free, laboratory water) was strongly correlated with SML SA (r ≥ 0.70, p ≤ 0.002, each n = 16). High SML SA typically corresponded to k660 suppressions ∼  14 to 51 % relative to clean laboratory water, highlighting strong spatiotemporal gradients in gas exchange due to varying surfactant in these coastal waters. Such variability should be taken account of when evaluating marine trace gas sources and sinks. Total CDOM absorbance (250 to 450 nm), the CDOM spectral slope ratio (SR = S275 − 295∕S350 − 400), the 250 : 365 nm CDOM absorption ratio (E2 : E3), and Chl a all indicated spatial and temporal signals in the quantity and composition of organic matter in the SML and SSW. This prompts us to hypothesise that spatiotemporal variation in R660 and its relationship with SA is a consequence of compositional differences in the surfactant fraction of the SML DOM pool that warrants further investigation.

Download Full-text

Effect of gas-transfer-velocity parameterization choice on CO<sub>2</sub> air–sea fluxes in the North Atlantic and European Arctic

Ocean Science Discussions ◽

10.5194/osd-12-2591-2015 ◽

2015 ◽

Vol 12 (6) ◽

pp. 2591-2616

Author(s):

I. Wróbel ◽

J. Piskozub

Keyword(s):

North Atlantic ◽

World Ocean ◽

Software Tool ◽

The Arctic ◽

Gas Transfer ◽

Transfer Velocity ◽

Gas Transfer Velocity ◽

The North ◽

European Arctic ◽

The North Atlantic

Abstract. The ocean sink is an important part of the anthropogenic CO2 budget. Because the terrestrial biosphere is usually treated as a residual, understanding the uncertainties the net flux into the ocean sink is crucial for understanding the global carbon cycle. One of the sources of uncertainty is the parameterization of CO2 gas transfer velocity. We used a recently developed software tool, FluxEngine, to calculate monthly net carbon air–sea flux for the extratropical North Atlantic, European Arctic as well as global values (or comparison) using several available parameterizations of gas transfer velocity of different dependence of wind speed, both quadratic and cubic. The aim of the study is to constrain the uncertainty caused by the choice of parameterization in the North Atlantic, a large sink of CO2 and a region with good measurement coverage, characterized by strong winds. We show that this uncertainty is smaller in the North Atlantic and in the Arctic than globally, within 5 % in the North Atlantic and 4 % in the European Arctic, comparing to 9 % for the World Ocean when restricted to functions with quadratic wind dependence and respectively 42, 40 and 67 % for all studied parameterizations. We propose an explanation of this smaller uncertainty due to the combination of higher than global average wind speeds in the North Atlantic and lack of seasonal changes in the flux direction in most of the region. We also compare the available pCO2 climatologies (Takahashi and SOCAT) pCO2 discrepancy in annual flux values of 8 % in the North Atlantic and 19 % in the European Arctic. The seasonal flux changes in the Arctic have inverse seasonal change in both climatologies, caused most probably by insufficient data coverage, especially in winter.

Download Full-text