New particles in the atmosphere: two non-classical examples

小颗粒物在大气中起着几个非常重要的作用。它们提供了可以发生奇异化学反应的表面，并且它们充当凝结核（“种子”），水在其上凝结形成冰粒和云滴。粒子也可以将可见光和紫外线反射回太空（冷却地球），并吸收向外的红外线辐射（有助于“温室”效应）。由于热力学原因（熵减少），实际上很难在大气中由气态成分形成新的粒子。这个过程被称为均匀成核：一些分子凝聚在一起形成一个稳定的簇（大小约为1 nm），然后成为进一步生长的构建块。本提案将审查在大气层非常不同的区域发生的均质成核的两个例子：氧化碘粒子，它在海洋边界层中由海洋中生物产生的碘物质形成;流星烟雾粒子，它在中层大气中由流星体烧蚀的金属和氧化硅形成。对于这两个系统，我们希望跟踪粒子从单个分子到含有约100万个分子（直径约50 nm）的粒子的演变，以了解是什么控制了粒子的生长速率和形状。初步工作表明，这些颗粒是分形状的（“蓬松的”），具有大的表面积，通常有利于化学反应。例如，流星烟雾颗粒可能会影响平流层中控制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。最近在南极洲沿海观测到高浓度的碘氧化物，可能是表面附近冰核的来源。流星烟雾粒子最有可能是陨石云的核。这些冰云在仲夏形成于高纬度地区约83公里处，最早是在世纪末被观测到的。这导致人们猜测它们是中层大气气候变化的早期指标。最后，我们将讨论流星烟雾粒子如何从大约80公里处下降到地球表面的问题。这些粒子的探测方法有两种：一种是用飞行在70公里以上的火箭携带的仪器捕获它们（在本项目中，我们建议分析一些捕获的粒子），另一种是在格陵兰岛和南极洲的冰芯中探测宇宙铱和铂。我们将使用气象局的大气环流模型来研究在当今条件下和冰川最大期的运输途径，以解释几十万年来的冰芯记录。

Summary 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.