New particles in the atmosphere: two non-classical examples

小颗粒在大气中扮演着几个非常重要的角色。它们提供了可以发生奇异化学反应的表面，它们充当凝结核(“种子”)，水在这些凝结核上凝结形成冰粒和云滴。粒子还可以将可见光和紫外线反射回太空(使地球降温)，并吸收传出的红外线辐射(导致“温室效应”)。由于热力学的原因(减少的熵)，实际上很难在大气中从气体成分中形成新的粒子。这一过程被称为均相成核：几个分子凝聚在一起形成一个稳定的团簇(大小约为1纳米)，然后成为进一步生长的基石。这项提议将审查发生在大气层非常不同区域的两个均质成核的例子：在海洋边界层中由海洋中生物产生的碘物种形成的氧化碘粒子；以及在大气层中部由流星体烧蚀的金属和硅氧化物形成的陨烟粒子。对于这两个系统，我们希望跟踪粒子从单个分子到包含约100万个分子(直径约50纳米)的粒子的演变，以了解是什么控制了粒子的生长速度和形状。初步工作表明，这种颗粒是类分形体(蓬松的)，具有较大的表面积，这往往有利于化学反应。例如，陨石烟雾颗粒可能会影响控制平流层臭氧的某些化学物质。我们还将研究这些粒子作为冰凝结核的性质。最近在南极洲海岸上空观察到高浓度的碘氧化物，可能是地表附近冰核的来源。流星烟雾粒子最有可能是夜光云的核心。这些冰云在盛夏期间在高纬度约83公里处形成，在19世纪末首次被观测到。这引发了人们的猜测，即它们是中层大气气候变化的早期指标。最后，我们将讨论陨石烟雾粒子如何从大约80公里处落到地球表面的问题。探测到这些粒子的方法有两种，一种是用飞行在70公里以上的火箭载仪器捕获它们(在这个项目中，我们打算分析其中一些捕获到的粒子)，另一种是通过在格陵兰岛和南极洲的冰芯中探测到宇宙中的铱和铂。我们将使用气象局的大气环流模型来研究现今条件下和冰川盛期期间的运输路径，以解释数十万年来的冰芯记录。

Small particles play several very important roles in the atmosphere. They provide surfaces on which exotic chemical reactions can take place, and they act as the condensation nuclei ('seeds') on which water condenses to form ice particles and cloud droplets. Particles may also reflect visible and UV sunlight back to space (cooling the earth), and absorb outgoing infra-red radiation (contributing to the 'greenhouse' effect). For thermodynamic reasons (decreasing entropy), it is actually quite difficult to form new particles in the atmosphere from gaseous constituents. This process is known as homogeneous nucleation: a few molecules condense together to form a stable cluster (about 1 nm in size), which then becomes the building block for further growth. This proposal will examine two examples of homogeneous nucleation that occur in very different regions of the atmosphere: iodine oxide particles, which form in the marine boundary layer from iodine species that are produced biogenically in the ocean; and meteoric smoke particles, which form in the middle atmosphere from the metals and silicon oxides that ablate from meteoroids. For both these systems, we want to follow the evolution of the particles from single molecules to particles containing about 1 million molecules (diameter about 50 nm), in order to understand what controls the rate of growth and the shapes of the particles. Preliminary work shows that the particles are fractal-like ('fluffy'), with large surface areas which often facilitate chemical reactions. For example, meteoric smoke particles may influence some of the chemistry controlling ozone in the stratosphere. We will also examine the properties of these particles as ice condensation nuclei. Iodine oxides have recently been observed at high concentrations over coastal Antarctica, and could be a source of ice nuclei near the surface. Meteoric smoke particles are most likely the nuclei for noctilucent clouds. These ice clouds, which form around 83 km at high latitudes during mid-summer, were first observed at the end of the 19th Century. This has led to speculation that they are an early indicator of climate change in the middle atmosphere. Finally, we will address the question of how meteoric smoke particles descend from around 80 km to the earth's surface. The particles have been detected both by capturing them with a rocket-borne instrument flying above 70 km (in this project we propose to analyze some of these captured particles), and by detecting cosmic iridium and platinum in ice cores from Greenland and Antarctica. We will use the Met Office's general circulation model to study the transport pathways during present-day conditions and during a glacial maximum, in order to interpret the ice core record over several hundred thousand years.