Turbulent Exchange: Aerosols, Bubbles And Gases

现在的共识是，随着大气中温室气体浓度的增加，全球气候正在发生变化。这些气体既有自然来源，也有人为来源和汇。对于二氧化碳来说，最大的汇是海洋，它吸收了燃烧化石燃料产生的30%到50%的二氧化碳。大气和海洋之间气体交换的方向取决于空气和水之间气体浓度的差异，以及一些改变交换速率的物理过程。这些过程中最重要的是接近地表的空气和水中的湍流混合。这随着风速的增加而增加，但这种关系因其他因素而变得复杂，例如海浪状态和海洋和大气近表层的热力学稳定性。在高风速时，波浪破碎会产生气泡，将空气混合到水柱中。气泡的存在增加了气体交换的速率，但这一过程的详细性质尚不完全清楚，对于描述气体传输速率的方程的确切形式也存在相当大的分歧。这在很大程度上是由于缺乏足够详细的测量结果。波浪破碎和气泡也与海雾气溶胶粒子的形成密切相关--这些作为云凝结核是重要的。气溶胶是由海面上的气泡破裂而产生的。气溶胶的形成速度通常表示为海面白云分数的函数，但生产速度的不确定度约为10倍。这表明，白盖分数本身并不能控制产量，但破浪产生的气泡的大小和数量等因素可能会随着波浪的大小或陡度等其他因素而变化。这个项目是英国对美国一艘研究邮轮的贡献，该邮轮旨在研究海浪破碎和气泡过程对海气交换的影响。我们将测量白帽比例、波浪状态、波浪破碎统计数据以及破碎波浪下的气泡特性。测量将通过配备波线的11米SPAR浮标以高空间分辨率测量局部波高，一个气泡相机测量表面附近的大气泡，以及两个声谐振器测量表面以下更深的小气泡。此外，还将部署一个单独的波浪浮标，对海浪频谱进行长期和独立的测量。在船上，我们将通过涡旋协方差技术直接测量气溶胶通量，以及热量、水蒸气、二氧化碳和动量的通量。我们来自NOAA和哈维大学的合作伙伴将测量几种不同气体的通量：二氧化碳、一氧化碳和二甲基硫。气体通量、白云和气泡属性的联合测量将允许根据现有的各种参数直接评估气泡对通量的影响。

There is now a consensus that global climate is changing in response to increasing atmospheric concentrations of greenhouse gases. These gases have natural as well as man-made sources and sinks. For carbon dioxide the largest sink is the ocean, which absorbs between 30% and 50% of the CO2 generated by the burning of fossil fuel. The direction of the exchange of gases between atmosphere and ocean depends on the difference in gas concentration between the air and water, and on a number of physical processes that modify the rate of the exchange. The most important of these processes is turbulent mixing in both the air and water close to the surface. This increases with wind speed, but the relationship is complicated by other factors such as the waves state and the thermodynamic stability of the near-surface layers of both ocean and atmosphere. At high wind speeds wave breaking generates bubbles, mixing air down into the water column. The presence of bubbles increases the rate of gas exchange, but the detailed nature of the process is not fully understood and there is considerable disagreement about the exact form of the equations that describe the rate of gas transfer. This is largely a result of a lack of sufficiently detailed measurements.Wave breaking and bubbles are also closely linked to the formation of sea-spray aerosol particles - these are important as cloud condensation nuclei. Aerosols are generated by the bursting of bubbles at the sea surface. The rate of aerosol formation is often expressed as a function of whitecap fractions on the sea surface, but there is an uncertainty of about a factor of 10 in the production rate. This suggests that whitecap fraction alone does not control the production rate, but that factors such as the size and number of bubbles produced by breaking waves may vary with other factors such as size or steepness of the wave. This project is a UK contribution to a US research cruise that aims to examine the impact of wave breaking and bubble processes on air-sea gas exchange. We will measure whitecap fraction, wave state, wave breaking statistics, and bubble properties beneath breaking waves. Measurements will be made from an 11-m spar buoy equipped with wave wires to measure the local wave height at high spatial resolution, a bubble camera to measure large bubbles near the surface, and 2 acoustical resonators to measure smaller bubbles deeper below the surface. A separate Waverider buoy will also be deployed to make longer term and independent measurements of the wave spectra. On the ship we will make direct measurements of aerosol fluxes via the eddy covariance technique, along with those of heat, water vapour, CO2, and momentum. Our partners from NOAA and the University of Hawi'i will measure fluxes of several different gases: CO2, CO, and DMS. The joint measurements of gas fluxes, and whitecap and bubble properties will allow the influence of bubbles on the flux to be evaluated directly against a variety of existing parameterizations.