Axial thermal gradients play a key role in monolithic diesel particulate filters. For complete particulate regeneration, the ignition location must be near the leading edge of the diesel particulate filter. A new reduced model in terms of the capacitance-weighted and mixing cup average temperature is formulated here using the Liapunov-Schmidt reduction technique to describe the ignition behavior and account for the axial variation. The major advantage of the reduced model is that one can neglect the details of the fine scale dynamics and focus on the large scale dynamics. This new, reduced model has the ability, to accurately predict two important ignition characteristics: ignition time and ignition length. Based on the reduced model results, it is shown that the thermal dispersion [a coupling of transverse diffusion (short-scale) and axial convection (long-scale)] plays a dominant role on ignition. (c) 2007 American Institute of Chemical Engineers.