  We study the effect of an imposed vertical magnetic field on the turbulent
mass diffusion properties of magnetorotational turbulence in protoplanetary
discs. It is well-known that the effective viscosity generated by the
turbulence depends strongly on the magnitude of such an external field. In this
letter we show that the turbulent diffusion of the flow also grows, but that
the diffusion coefficient does not rise with increasing vertical field as fast
as the viscosity does. The vertical Schmidt number, i.e. the ratio between
viscosity and vertical diffusion, can be close to 20 for high field magnitudes,
whereas the radial Schmidt number is increased from below unity to around 3.5.
Our results may have consequences for the interpretation of observations of
dust in protoplanetary discs and for chemical evolution modelling of these
discs.
