Measurement of the abundance and optical significance of sub-micron sized particles in the ocean

30年来，可见光波段卫星图像一直是海洋科学的重要工具。通过它们，我们对海洋浮游植物的分布和季节变化有了大量的了解。已经有可能估计初级生产和海洋在从大气中吸收二氧化碳方面所起的作用。在陆架海域，可见光波段图像使我们能够绘制出从海床上搅拌起来的小型无机沉积物颗粒的浓度。在这两种应用中，粒子都是可见的，因为它们吸收和散射阳光。有一个重要的问题，目前还没有明确的答案，那就是究竟有多大的粒子主要负责散射卫星“看到”的光。人们通常认为，海洋中颗粒的大小分布遵循“Junge”分布，即随着颗粒大小的减小，颗粒的数量迅速增加。根据这个假设，并使用假设球形粒子的光学理论，已经表明，大多数光散射是由直径小于1微米的粒子进行的。如果这是真的，那就意味着卫星图像中看到的颗粒大多非常小，沉降速度慢，在海洋表面停留时间长。这将对那些解释这些图像并使用它们来验证海洋中粒子的数值模型的人产生重要影响。然而，从来没有人在海洋中直接观察到由荣格分布预测的这种小颗粒的数量。未受干扰的海水样本的照片显示，颗粒倾向于以“絮体”的形式聚集在一起。支持Junge分布的粒度分布测量使用了一种可能破坏絮凝体的破坏性技术，因此可能高估了小颗粒的数量。目前设计用于在不受干扰的情况下就地测量颗粒大小的仪器仅限于尺寸大于几微米的颗粒，因此大于遥感中重要的临界粒径。全息照相机可以对悬浮在水中的小颗粒进行聚焦成像。照相机拍下粒子的衍射图样，然后根据该图样用数学方法重建粒子。在小粒子的情况下，衍射图样比粒子本身大得多，因此全息技术确实可以重建非常小的粒子，比光的波长还小，这是任何其他方法都无法测量的。我们已经在实验室中演示了这项技术，并对0.5微米左右的颗粒进行了成像。通过进一步放大和改进光学，将有可能对0.2微米的粒子成像。在这个建议中，我们将把这项技术打包用于野外工作。通过使用不同的放大倍数和市售的原位粒度测量仪器，我们可以制作一套仪器来测量0.2微米到1毫米的未扰动粒度分布。这个包将首先在一个湍流池中使用，以“拍摄”絮凝过程。这一见解将用于构建粒径分布的新理论模型。实地工作将按季节周期在一个沿海地点进行，并在春季开花前后通过各种水类型在海上进行。我们还将改进粒子吸收和散射的测量。由于相机还可以测量粒子的形状，因此可以首次将观测到的光学特性与计算出的光学特性之间的差异与粒子形状进行比较。最后，我们将把完整的数据集放在一起，以确定在什么条件下，什么大小的颗粒主要负责在可见光波段卫星图像中看到的海洋信号。

Visible-band satellite images have been a valuable tool in marine science for 30 years. Through them we have learned a great deal about the distribution and seasonal variation of phytoplankton in the ocean. It has been possible to estimate primary production and the role the oceans play in taking up carbon dioxide from the atmosphere. In shelf seas, visible band images have enabled us to map out the concentration of small inorganic sediment particles stirred up from the sea bed. In both of these applications the particles are visible because they absorb and scatter sunlight. An important question, to which the answer is not at all clear at present, is exactly how large the particles are that are mainly responsible for scattering the light that is 'seen' by the satellites. It is often assumed that the size distribution of particles in the ocean follows a 'Junge' distribution, in which the number of particles increases rapidly as the size of the particles decrease. With this assumption, and using an optical theory which assumes spherical particles, it has been shown that most of the light scattering is performed by particles smaller than 1 micron in diameter. If this were true, it would mean that the particles seen in satellite imagery are mostly very small with slow settling speeds and long residence times in the surface of the ocean. This would have important implications for those who interpret these images and who use them to verify numerical models of particles in the ocean. However, no-one has ever directly observed in the sea the numbers of such small particles predicted by the Junge distribution. Photographs of undisturbed samples of seawater show that particles tend to gather together in 'flocs'. The measurements of particle size distribution which support the Junge distribution use a disruptive technique which potentially breaks up flocs and hence possibly over-estimates the number of small particles. Current instruments designed to measure the size of particles in situ and without disturbance are limited to particles greater than a few microns in size and hence greater than the critical particle size thought to be important in remote sensing. Holgraphic cameras enable focused images of small particles suspended in water to be made. The camera images the diffraction pattern of the particle and the particle is then reconstructed mathematically from this pattern. In the case of small particles, the diffraction pattern is much larger than the particle itself and so the holographic technique can reconstruct very small particles indeed, smaller than the wavelength of light, which cannot be measured in any other way. We have demonstrated this technique in the laboratory and imaged particles down to about 0.5 micron. With further magnification and improved optics it will be possible to image particles down to 0.2 micron. In this proposal we will package this technology for field work. By using different magnifications and commercially available in situ particle sizing instruments, we can make a package of instruments for measuring the undisturbed particle size distributions from 0.2 micron to 1 mm. This package will first be used in a turbulence tank to 'film' the flocculation process. The insight this gives will be used to construct new theoretical models of the particle size distribution. Field work will be carried out at one coastal site over a seasonal cycle and at sea through a variety of water types before and after the spring bloom. We will also make improved measurements of absorption and scattering by particles. Because the camera also measures the shape of the particles, differences between observed and calculated optical properties can be compared, for the first time, to particle shape. Finally, we will put together the complete data set to determine what size particles, under what conditions, are primarily responsible for the signals seen in visible band satellite images of the oceans.