Study of the impact of altered flight trajectories on soot-cirrus: a EC-REACT4C study

The emission, dispersion and transport of aviation black carbon particles in the atmosphere may trigger additional cirrus clouds (‘soot-cirrus’) or change the background distribution of upper tropospheric ice particles. These effects and the sensitivity of aerosol and ice accumulation to changes in flight altitude have been studied with the University of L’Aquila climate-chemistry model (ULAQ-CCM), using emission inventories from the collaborative European project ECREACT4C. Formation of background upper tropospheric ice particles is included by means of homogeneous and heterogeneous freezing of super-cooled aerosols: this scheme considers the basic physical processes that eventually determine the number of ice crystals Ni forming during an adiabatic ascent of air, including the link of Ni on temperature and updraft speed. Background ice particle formation is dominated by the homogeneous freezing process, producing the largest population in the tropical upper troposphere. Ice changes from aviation are dominated by the heterogeneous freezing mechanism, with the ice particle number density increasing with increasing BC particles from aircraft emissions. RF changes produced by flight vertical displacement are negative/positive for upward/downward displacement. A 2000 ft upward shift of aircraft routes, in fact, brings more emissions into the stratosphere, where lower amounts of condensable water vapour are present and less amounts are transported from below due to a rapid decrease of sub-grid updraft velocities above the tropopause.