  The observed slow rotation and abundance peculiarities of certain blue
horizontal branch (BHB) stars suggests that atomic diffusion can be important
in their stellar atmospheres and can lead to vertical abundance stratification
of chemical species in the atmosphere. To verify this hypothesis, we have
undertaken an abundance stratification analysis in the atmospheres of six BHB
stars, based on recently acquired McDonald-CE spectra. Our numerical
simulations show that the iron abundance is vertically stratified in the
atmospheres of two stars in M15: B267 and B279. One star WF2-2541 in M13 also
appears to have vertically stratified iron abundance, while for WF4-3085 the
signatures of iron stratification are less convincing. In all cases the iron
abundances increase towards the lower atmosphere. The other two stars in our
sample, WF4-3485 and B84, do not show any significant variation of iron with
atmospheric depth. Our results support the idea that atomic diffusion dominates
other hydrodynamic processes in the atmospheres of BHB stars.
