Hydrophilic properties of soot particles exposed to OH radicals: A possible new mechanism involved in the contrail formation

Abstract

This work investigates the role of soot particles in the early steps of formation of condensation trails (contrails). Contrails are thin linear clouds that form behind cruising aircrafts and can evolve into persistent clouds, and therefore can contribute to the radiative forcing of the atmosphere. Mitigating their contribution is considered an efficient way for decreasing the radiative forcing. Condensation freezing is proposed as a major pathway leading to the heterogeneous nucleation of ice particles in the aircraft exhausts. Soot particles, freshly produced in the flames and not significantly oxidized in the post-flame, are hydrophobic. Therefore, many theoretical and experimental approaches that investigate contrails formation introduce prior activation by surface adsorption of sulfur compounds issued from kerosene combustion. Even though the role of sulfur content of the fuel is unclear, and may only weakly impacts contrails formation, many simulations still rely on this assumption. In this work, to elucidate the role of sulfur compounds on the activation of soot chemically aged with OH radicals, a comparative study is performed with soot sampled from a turbulent jet flame burning a sulfur-containing fuel (kerosene) and a sulfur-free fuel (diesel). Soot aging experiments are performed in the atmospheric simulation chamber CESAM, with controlled generation of OH radicals. Soot activation (ratio between the number of nucleated droplets and seeding particles) is measured in water supersaturation conditions. OH exposure significantly enhances soot activation regardless of the sulfur presence in the fuel. The possibility for soot oxidation by OH radicals formed in aeronautical engines to be sufficient to promote soot activation is explored. Large eddy simulation in the high pressure turbine is used to model the fluid particle and OH trajectories and their chemical evolution. Residence time in the turbine is found sufficient to activate soot particles, opening a new possible route to explain ice particles formation.