Microscale dynamics and light scattering characteristics of ice crystals in contrails

在2019冠状病毒病大流行之前，全球航空业预计将以每年3%至4%的速度增长。由于全球旅行限制和政府的财政支持与环境条件挂钩，这被称为“绿色重启”，该行业受到的冲击比大多数行业都大。2019年，全球航空业的二氧化碳排放量占人为辐射强迫总量的3%。然而，包括氮氧化物（NOx）、硫酸盐和颗粒物（PM）在内的航空排放也有助于气候强迫。飞机发动机排放的PM促进了尾迹的形成，尾迹是你在天空中看到的飞机后面形成的可见的线状云。飞机尾迹是由冰晶组成的，当排气羽流中的水蒸气凝结在烟灰颗粒（直径约100纳米）上形成液态水滴，然后随着烟灰冷却而冻结。飞机尾迹会影响地球的辐射平衡，并产生比航空二氧化碳排放更显著的变暖效应，然而，减少飞机尾迹影响的监管框架受到重大科学不确定性的阻碍。尽管存在这些不确定性，但解决飞机尾迹问题提供了一个显著而迅速地减少航空环境足迹的机会。尾迹对气候的影响仍然存在很大的不确定性，部分原因是缺乏对冰成核的微物理学及其对光学性质的影响的物理理解。冰核的生长和形状取决于PM表面性质的动力学：(i) PM倾向于疏水，减少冰核，但阳光和臭氧氧化PM表面成为亲水，有助于冰的形成；（ii）其他大气物质（碳氢化合物、硫氧化物、氮氧化物）可能在水积聚之前凝结在PM上并改变其表面性质；（iii） PM孔隙度可能很重要，因为水进入孔隙形成冰，然后帮助冰生长；（iv）尾迹冰粒可以进一步吸收大气中的可用水蒸气，从而减少天然卷云的发生、覆盖面积和光学特性，从而抵消尾迹的净增温效应。上述行为可能显著改变冰晶的光学特性，这必须包括在模式中，以减少轨迹气候影响估计的不确定性。很少有研究通过实验研究飞机发动机烟尘颗粒冰核的微物理性质，这些烟尘颗粒的直径通常小于100纳米。目的和目的实验研究在飞机发动机排气羽流冷却条件下，烟灰颗粒上形成的冰晶的形状、大小和光散射特性的时间演变。学生将在不同温度、不同气体成分和不同停留时间的环境中，使用不同成分和孔隙度的悬浮PM进行对照实验。悬浮粒子将被不同的光谱光特性照射，以量化其光学特性。这些发现将被纳入评估飞机尾迹对气候影响的模型中。通过将现有的高分辨率尾迹模型与一般大气环流模型（包括气溶胶与云的相互作用）耦合，将对自然卷云光学特性所产生的尾迹的间接强迫进行量化。该项目的贡献在于：(i)首次纳入尾迹和自然卷云之间的相互作用和反馈；（ii）更准确地量化尾迹对地球表面温度的影响，包括精确的光学特性、大气过程和反馈。这些建模工具将用于评估减少尾迹变暖效应的策略的潜力。

ContextUp until the COVID-19 pandemic, global aviation was forecasted to grow at a rate of 3 to 4% per annum. The industry has suffered more than most due to global travel restrictions and government financial support is being tied to environmental conditions, which is being called the 'green restart.' In 2019, CO2 emissions from the global aviation industry represented 3% of the total anthropogenic radiative forcing. However, aviation emissions including nitrogen oxide (NOx), sulphates and particulate matter (PM) also contribute to climate forcing. PM emitted by aircraft engines facilitate the formation of contrails, which are visible line shaped clouds that form behind aircraft you see in the sky. Contrails are made up of ice crystals and are formed when water vapour in the exhaust plume condenses onto soot particles (approximately 100 nm in diameter) to form liquid water droplets which then freeze as the plume cools. Contrails affect the earth's radiation balance and have a warming effect that is more significant than that of aviation's CO2 emissions, yet regulatory frameworks for reducing contrail impacts are held back by significant scientific uncertainties. Notwithstanding these uncertainties, tackling contrails presents an opportunity to significantly and rapidly reduce aviation's environmental footprint. Significant uncertainties in the climate effect of contrails remain, in part due to a lack of physical understanding of the microphysics of ice nucleation and its consequences on optical properties. The growth and shape of ice nucleates depends on the dynamics of PM surface properties: (i) PM tends to be hydrophobic, reducing ice nucleation, but sunlight and ozone oxidise PM surface that becomes hydrophilic, assisting ice formation; (ii) other atmospheric substances (hydrocarbons, SOx, NOx) may condense on PM and modify surface properties before water accumulates; (iii) PM porosity may be important, since water enters in pores and forms ice, which then assists ice growth; (iv) contrail ice particles could further absorb available water vapour in the atmosphere, thereby reducing the occurrence, coverage area and optical properties of natural cirrus, which could offset the net warming effect of contrails. The above behaviour may modify significantly the optical properties of ice crystals, which must be included in models to reduce uncertainties of estimates of contrail climate impact. Few studies have experimentally investigated the microphysics of ice nucleation on soot particles from aircraft engines, which are typically less than 100 nm in diameter.Aims and objectivesThe aim is to experimentally investigate the temporal evolution of the shape, size and light scattering properties of ice crystals that form on soot particles in conditions representative of cooling aircraft engine exhaust plumes. The student will perform controlled experiments by using suspended PM of variable composition and porosity in an ambient of different temperatures and gas compositions with variable residence times. The suspended particles will be illuminated by different spectral light characteristics to quantify the optical characteristics. The findings will be incorporated in models that evaluate the climate impact of aircraft contrails. The indirect forcing of contrails due to the effect on the optical properties of natural cirrus will be quantified by coupling an existing high-resolution contrail model with a general atmospheric circulation model, including aerosol-cloud interactions. This contributions of the project are that it will: (i) incorporate the interactions and feedbacks between contrails and natural cirrus for the first time; and (ii) more accurately quantify the impact of contrails on Earth's surface temperature with the inclusion of accurate optical properties, atmospheric processes and feedbacks. These modelling tools will be used to evaluate the potential for strategies to reduce the warming effect of contrails.