Under on-road driving conditions, the engine load and speed and the cooling effect of ambient air may affect the functioning of exhaust aftertreatment devices. In this paper, we studied the effects of these parameters on the functioning of the combination of a Diesel Oxidation Catalyst and a Particle Oxidation Catalyst (DOC+POC). In the engine tests, the engine load and speed were observed to affect the nonvolatile particle reduction efficiency curve of the DOC+POC; while the nonvolatile core particle (Dp ≺ 15 nm) reduction was high (97-99%) in all the engine test modes, the reduction of soot varied from 57% at low load to 70% at high load. Because the change in engine load and speed affected both the exhaust temperature and flow velocity, the effects of these parameters were measured separately in an aerosol laboratory. Results indicated that compared to the exhaust temperature, the exhaust flow velocity had a more significant role from the viewpoint of the nonvolatile particle reduction efficiency of the DOC+POC system. At three engine test modes, the external cooling did not affect the particle reduction of the DOC+POC. Instead, in one case (low engine load, low engine speed) the external cooling affected the particle reduction. However, this observation cannot be explained by thermophoretic forces.