Ribas-Ribas, Mariana and Helleis, Frank and Rahlff, Janina and Wurl, Oliver
(2018)
Air-sea CO2-exchange in a large annular wind-wave tank and the effects of surfactants.
Frontiers in Marine Science, 5.
p. 457.
ISSN 2296-7745
Abstract
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 from1.5ms−1 to 22.8ms−1), and we increased the water
pCO2 concentration twice by more than 950μatm 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 becausemeasurements 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.
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