  We investigate the role of perpendicular diffusion in shaping energetic ion
spectrum in corotating interaction regions (CIRs), focusing on its
mass-to-charge ($A/Q$) dependence. We simulate a synthetic CIR using the
EUropean Heliospheric FORcasting Information Asset (EUHFORIA) and model the
subsequent ion acceleration and transport by solving the focused transport
equation incorporating both parallel and perpendicular diffusion. Our results
reveal distinct differences in ion spectra between scenarios with and without
perpendicular diffusion. In the absence of perpendicular diffusion, ion spectra
near CIRs show a strong $(A/Q)^{\epsilon}$ dependence with $\epsilon$ depending
on the turbulence spectral index, agreeing with theoretical predictions. In
contrast, the incorporation of perpendicular diffusion, characterized by a weak
$A/Q$ dependence, leading to similar spectra for different ion species. This
qualitatively agrees with observations of energetic particles in CIRs.
