Measurements of air–sea gas transfer velocities in the Baltic Sea

Abstract

Abstract. Heat transfer velocities measured during three different campaigns in the Baltic Sea using the active controlled flux technique (ACFT) with wind speeds ranging from 5.3 to 14.8 m s−1 are presented. Careful scaling of the heat transfer velocities to gas transfer velocities using Schmidt number exponents measured in a laboratory study allows us to compare the measured transfer velocities to existing gas transfer velocity parameterizations, which use wind speed as the controlling parameter. The measured data and other field data clearly show that some gas transfer velocities are much lower than those based on the empirical wind speed parameterizations. This indicates that the dependencies of the transfer velocity on the fetch, i. e., the history of the wind and the age of the wind-wave field, and the effects of surface-active material need to be taken into account.

Global Synthesis of Air-Sea CO2 Transfer Velocity Estimates From Ship-Based Eddy Covariance Measurements

“…They suggested that K 660 should be scaled with Sc -2/3 within this smooth regime; then once waves appear the Schmidt number exponent transitions to -1/2. Recent laboratory (Nagel et al, 2019) and field (Esters et al, 2017) observations imply that there is a smooth transition in n between -2/3 and -1/2. The intercepts of the K 660 -u * fits (over u * range of 0.1-0.5 m s -1 ) are negative for all cruises (Table 2), which implies different physical processes occurring at low wind speeds.…”

“…3 He and SF 6 differ from CO 2 from at least two perspectives: 1) they are much less soluble than CO 2 , and solubility is important in bubble-mediated gas exchange (Woolf, 1997;Asher and Wanninkhof, 1998); 2) they are inert, whereas the air-sea exchange of CO 2 is affected by chemical enhancement due to the carbonate kinetics (Wanninkhof, 1992). In addition, the derivation of K 660 from the 3 He/SF 6 measurements is highly sensitive to the Schmidt number exponent n, which is thought to deviate from -1/2 at low wind speeds (e.g., Esters et al, 2017;Nagel et al, 2019). It is arguably more robust to use K 660 directly derived from CO 2 measurements to estimate the global/regional CO 2 flux to avoid the aforementioned possible sources of uncertainty.…”

Air–sea gas exchange at wind speeds up to 85 m s−1

“…Figure 1 shows a schematic view of the resistances for bubble-mediated gas transfer R c = k −1 c in relation to the air- and water-sided resistances for transfer through the unbroken water surface. Many approaches have been made to quantify the bubblemediated gas transfer k c : gas transfer by single bubbles (Maiß, 1986;Patro et al, 2002), transfer in bubble clouds (Asher et al, 1996;Mischler, 2014) and breaking waves (Asher et al, 1995;Leifer and De Leeuw, 2002), as well as theoretical models based on bubble dynamics (Memery and Merlivat, 1985;Woolf and Thorpe, 1991). Bubble-mediated gas transfer also depends on bubble surface conditions.…”

“…Bubble-mediated gas transfer also depends on bubble surface conditions. It has been shown that surface active material reduces the gas transfer of single bubbles (Maiß, 1986;Patro et al, 2002), while it also decreases the bubbles' rise velocity (Alves et al, 2005). During the lifetime of a bubble, these surface conditions can change, as bubbles accumulate surface active material while moving through the water.…”

Air-sea gas exchange at hurricane wind speeds

“…The influence of fetch and thus waves on gas transfer has so far only been studied at wind speeds of less than 10 m s −1 and proved to be only significant at wind speeds lower than 6 m s −1 (Kunz and Jähne, 2018). However, recent measurements in the Baltic Sea using active thermography also showed large variations in the gas transfer velocity at wind speeds higher than 10 m s −1 , which is most likely related to the effect of surfactants (Nagel et al, 2019).…”