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We investigate the evolution of contrails during the vortex phase using numerical simulations. Emphasis is placed on microphysical properties and on the vertical distribution of ice mass and number concentration at the end of the vortex phase. Instead of using a 3D model which would be preferable but computationally too costly, we use a 2D model equipped with a special tool for controlling vortex decay. We conduct a great number of sensitivity studies for one aircraft type. It turns out that atmospheric parameters, namely supersaturation, temperature, stability and turbulence level have the biggest impact on the number of ice crystals and on the ice mass that survives until vortex breakup and that therefore makes up the persistent contrail in supersaturated air. The initial ice crystal number density and its distribution in the vortex, are of minor importance.