An after-treatment system, comprising: a particulate filter configured to remove contaminant material from a flow contained in a filter housing forming a radio frequency resonant cavity; and a filter load monitoring system configured to determine the amount of contaminant material accumulated in the particulate filter, the load monitoring system comprising: at least one probe configured to transmit radio frequency signals through the resonant cavity and particulate filter; at least one probe configured to receive radio frequency signals transmitted through the resonant cavity and particulate filter; and a control unit configured to: determine the amount of material accumulated in the particulate filter; and initiate an action based on the amount of material accumulated in the particulate filter characterized in that: the at least one probe which is configured to transmit radio frequency signals through the resonant cavity and particulate filter is further configured to transmit radio frequency signals over a frequency range suitable for generating multiple resonant modes; the radio frequency signals received by the at least one probe define resonance curves; and the control unit is further configured to determine, by sampling the resonance curves and comparing changes in the response of the radio frequency signals for the multiple resonant modes, a spatial distribution of the material in the particulate filter. a particulate filter configured to remove contaminant material from a flow contained in a filter housing forming a radio frequency resonant cavity; and a filter load monitoring system configured to determine the amount of contaminant material accumulated in the particulate filter, the load monitoring system comprising: at least one probe configured to transmit radio frequency signals through the resonant cavity and particulate filter; at least one probe configured to receive radio frequency signals transmitted through the resonant cavity and particulate filter; and at least one probe configured to transmit radio frequency signals through the resonant cavity and particulate filter; at least one probe configured to receive radio frequency signals transmitted through the resonant cavity and particulate filter; a control unit configured to: determine the amount of material accumulated in the particulate filter; and initiate an action based on the amount of material accumulated in the particulate filter characterized in that: the at least one probe which is configured to transmit radio frequency signals through the resonant cavity and particulate filter is further configured to transmit radio frequency signals over a frequency range suitable for generating multiple resonant modes; the radio frequency signals received by the at least one probe define resonance curves; and the control unit is further configured to determine, by sampling the resonance curves and comparing changes in the response of the radio frequency signals for the multiple resonant modes, a spatial distribution of the material in the particulate filter. the at least one probe which is configured to transmit radio frequency signals through the resonant cavity and particulate filter is further configured to transmit radio frequency signals over a frequency range suitable for generating multiple resonant modes; the radio frequency signals received by the at least one probe define resonance curves; and the control unit is further configured to determine, by sampling the resonance curves and comparing changes in the response of the radio frequency signals for the multiple resonant modes, a spatial distribution of the material in the particulate filter.