Microscale dynamics and light scattering characteristics of ice crystals in contrails

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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.