  Quantification of the radial transport of gaseous species and solid particles
is important to many applications in protoplanetary disk evolution. An
especially important example is determining the location of the water snow
lines in a disk, which requires computing the rates of outward radial diffusion
of water vapor and the inward radial drift of icy particles; however, the
application is generalized to evaporation fronts of all volatiles. We review
the relevant formulas using a uniform formalism. This uniform treatment is
necessary because the literature currently contains at least six mutually
exclusive treatments of radial diffusion of gas, only one of which is correct.
We derive the radial diffusion equations from first principles, using Fick's
law. For completeness, we also present the equations for radial transport of
particles. These equations may be applied to studies of diffusion of gases and
particles in protoplanetary and other accretion disks.
