A mechanical apparatus including a combustion engine, a conduit for transporting an exhaust stream away from the engine, a first catalytic NOx storage and conversion region disposed along the conduit, and a second catalytic NOx storage and conversion region disposed along the conduit at a location downstream from the first catalytic NOx storage and conversion region is disclosed. Each of the first catalytic NOx storage and conversion region and the second catalytic NOx storage and conversion region includes at least one catalytic metal configured to convert NOx to NO2 when the engine is supplied a lean air/fuel mixture, and at least one NOx adsorbing compound configured to adsorb NO2 for storage when the engine is supplied a lean air/fuel mixture, and wherein the one of the first and the second catalytic NOx storage and conversion regions has a lower concentration of catalytic metal than the other of the first and second catalytic NOx storage and conversion regions.