Microscale dynamics and light scattering characteristics of ice crystals in contrails

背景在COVID-19疫情爆发前，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，以及政府的财政支持与环境状况挂钩，该行业遭受的损失比大多数行业都要大，这被称为“绿色重启”。“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，这是一种在天空中看到的飞机后面形成的可见线状云。凝结尾迹由冰晶组成，当排气羽流中的水蒸气凝结在烟灰颗粒（直径约100 nm）上形成液态水滴时就会形成，然后随着羽流冷却而冻结。凝结尾迹影响地球的辐射平衡，并具有比航空二氧化碳排放更显著的变暖效应，但减少凝结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是一个显著和迅速减少航空环境足迹的机会。凝结尾迹对气候的影响仍然存在很大的不确定性，部分原因是对冰成核的微观物理学及其对光学特性的影响缺乏物理认识。冰核的生长和形状取决于颗粒物表面性质的动力学：（i）颗粒物倾向于疏水，减少冰核，但阳光和臭氧氧化颗粒物表面，成为亲水，有助于冰的形成;（ii）其他大气物质（碳氢化合物、SOx、NOx）可能凝结在PM上，并在水积聚之前改变表面性质;（iii）PM孔隙度可能很重要，因为水进入孔隙并形成冰，然后帮助冰生长;（iv）凝结冰粒子可进一步吸收大气中的水蒸气，从而减少天然卷云的出现、覆盖范围和光学性质，可以抵消凝结尾迹的净变暖效应。上述行为可能会大大改变冰晶的光学特性，必须将其纳入模型，以减少轨迹气候影响估计的不确定性。很少有研究实验研究了飞机发动机，这是典型的小于100 nm的直径，Aims和objectivesThe的目的是实验研究的时间演变的形状，大小和光散射特性的冰晶上形成的碳烟颗粒，在代表冷却飞机发动机排气羽流的条件下的碳烟颗粒上的冰成核的微观物理。学生将通过使用悬浮颗粒的可变组成和孔隙度在不同的温度和气体成分的环境中与可变的停留时间进行控制实验。悬浮颗粒将被不同的光谱光特性照射以量化光学特性。这些发现将被纳入评估飞机尾迹对气候影响的模型中。由于对自然卷云光学特性的影响，将通过将现有的高分辨率尾迹模型与包括气溶胶-云相互作用在内的一般大气环流模型相结合，对尾迹的间接强迫进行量化。该项目的贡献是：㈠首次纳入了尾迹和自然卷云之间的相互作用和反馈; ㈡更准确地量化了尾迹对地球表面温度的影响，包括准确的光学特性、大气过程和反馈。这些建模工具将用于评估减少凝结尾迹升温效应的战略潜力。

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.