Quantifying the Mechanisms of Air-Sea Gas Exchange and Bridging Laboratory and Field by Imaging Measurements

对海气交换这一重要过程仍缺乏足够的认识。这主要是由于目前测量技术的局限性造成的。现场测量仅在4- 20米/秒的相当狭窄的风速范围内可用，并且仍然显示出相互矛盾的结果。对于较低的风速，不可能作出可靠的估计，因为并非所有技术（双示踪剂、涡度协方差和主动热成像法）都适合在如此低的风速下使用。到目前为止，现场测量只能对海气交换机制做出很小的贡献，而在风洞中进行实验室测量则可能更好。这里的问题是，这些设施的条件与公海的条件有很大的差异。普通的线性设施在风和波浪之间具有短的相互作用长度，因此仅产生远离与风平衡的风海的年轻风海（风隙）。即使是在海德堡Aeolotron这样的环形设施中，风场也与海洋不同：由于水深有限，波浪的传播速度不够快（波浪年龄差）。这里提出了一种全新的方法来克服所有这些限制，并在实验室中进行气体交换测量，该方法可以充分真实地模拟低风速和中等风速下的所有相关海洋条件。本文应用新的成像技术对海德堡气动加速器内非稳态条件下的气体传输速度进行了局部和瞬时的测量。以这种方式，当风速降低时，可以覆盖整个风场范围，包括衰减的风海。波龄差距可以通过在Aeolotron的大气中使用较重的气体（氩和氪）来克服。成像技术包括主动热成像测量跨水粘性边界层和一种新颖的光化学技术的质量边界层的图像和测量气体传递速度的热传递。此外，还将详细研究表面活性剂的影响，这对低风速非常重要。预期结果有两个方面：第一、海洋条件下海气交换机制的定量描述，因此，基于气体传输速度与控制它的参数的函数关系。这将包括可能的气体传输速度范围第二，一个简单的技术来测量气体传输在现场。该装置将仅由一个热成像仪组成，并从海面上的空间和时间的热模式推断气体传输速度和机制。因此，还可以核实实验室测量是否涵盖了所有相关的海洋条件。

The important process of air-sea gas exchange still lacks sufficient understanding. This is mainly caused by limitations in current measuring techniques. Field measurements are available only in a rather narrow range of wind speeds from 4--20 m/s and still show conflicting results. No reliable estimates are possible for lower wind speeds, because all techniques (dual-tracer, eddy covariance and active thermography) are not suitable to be used at such low wind speeds. Field measurements could only make minor contribution to the mechanisms of air-sea gas exchange so far.This is much better possible with lab measurements in wind-wave-tunnels. Here the problem is that the conditions in these facilities deviate significantly from those at the open ocean. The common linear facilities have a short interaction length between wind and waves and thus generate only young wind seas far away from a wind sea in equilibrium with the wind (fetch gap). Even in an annular facility with an infinite fetch as the Heidelberg Aeolotron, the wind field is different from the ocean: because of the limited water depth waves cannot travel fast enough (wave age gap).Here a radically new approach is proposed to overcome all these limitations and to perform gas exchange measurements in the laboratory which sufficently realistic simulate all relevant oceanic conditions at low and medium wind speeds. New imaging techniques are applied to measure the gas transfer velocity locally and instantaneously in the Heidelberg Aeolotron under non-stationary conditions. In this way the whole fetch range can be covered including decaying wind seas, when the wind speed is lowered. The wave age gap can be overcome by using heavier gases (argon and krypton) in the atmosphere of the Aeolotron. The imaging techniques include active thermography to measure the heat transfer across the aqueous viscous boundary layer and a novel opto-chemical technique to image the mass boundary layer and to measure the gas transfer velocity. Also the influence of surfactants, which is very important for lower wind speeds, will be studied in detail.The anticipated outcome is two-fold: Firstly, a quantitative description of the mechanisms of air-sea gas exchange for oceanic conditions and therefore a physically-based relation of the gas transfer velocity as a function of the parameters controlling it. This will include the range of gas transfer velocities that are possible in different wind speed regimes.Secondly, a simple technique to measure the gas transfer in the field. This device will consist of only a thermal imager and infers the gas transfer velocity and mechanism from the thermal patterns in space and time at the sea surface. Thus, it will also be possible to verify whether the laboratory measurements have covered all relevant oceanic conditions.