Most importantly, these particles were found to acquire high positive charges at exhaust temperatures exceeding approximately 300 \({}^{\circ}\)C at the inlet of the SCR system. While the exact charge distribution is not precisely quantified, it was shown that even the lowest particles detected (\(\sim\)5-10 nm) can carry as high as two positive charges on average. The proper operation of any diffusion-charger-based instrumentation requires that the charge state of the particles is precisely controlled by the corona charger. The corona charger efficiency is an essential part of the instrument kernel function and accordingly the data inversion. The thermally treated automotive exhaust aerosol is predominantly composed of fractal soot [17] that is bipolarly charged with an overall neutral charge [18]. It is well established that fractal particles are more efficiently charged than compact spherical/cubic particles [19, 20, 21, 22, 23]. Accordingly, the regulation requires that the calibration of DC systems is performed with neutralized soot-like fractal particles. However, if particles entering the DC are pre-charged, the actual charging efficiency may be different than established during calibration [24,25] and this will affect the data inversion process. The actual effect will depend on the charge level and size of the particles as well as the design characteristics of the corona. The charging efficiency of commercial corona chargers is typically less than one for neutral particles smaller than 30 nm [6]. Therefore, when highly charged particles of the same polarity in this size range (like those produced in the SCR) pass through them, their charge level can only increase and will thus be much higher than if they were neutral. In this study, two technical solutions for the conditioning of pre-charged particles were developed:  
* The use of an electrofilter (hereinafter EF) upstream of a corona charger to remove all charged particles below the lowest detection size required by the regulation before entering the sensor. The concept is similar to the use of an elevated ion trap to adjust the cut-off size of DC-based instruments [26]. However, by applying the electric field upstream of the corona, the EF only tackles the charged fraction of the sampled aerosol, thus selectively targeting the particles formed in the SCR. * The use of a negative and a positive corona charger in tandem to condition (neutralize) the sample aerosol by exposing it to a bipolar ion-environment (hereinafter referred to as TC = Tandem Negative-Positive Corona). Similar approaches have been previously successfully established in other commercial instruments [27]. The design and parametrization of both solutions were designed for application on the DC-based detector from Naneos (Windisch, Switzerland) employed in the PN-PEMS from AVL GmbH (Graz, Austria). Their performance in conditioning charged aerosol was first characterized by monodisperse graphite aerosol of different charge levels. Subsequently they were introduced in AVL PN-PEMS systems and tested alongside the original AVL PN-PEMS system, a commercial PN-PEMS on-board system (OBS) from Horiba (Kyoto, Japan) utilizing a CPC, and a stationary laboratory grade reference PN system from AVL (Graz, Austria) in exhaust measurements of two HD engines equipped with SCR systems. The results of the present study proved that the previously reported overestimations of DC-based PN-PEMS instrumentation originates from the formation of positively charged nanoparticles in the SCR of heavy-duty diesel engines. The study also illustrates that the two technical solutions evaluated can effectively suppress the interference of these positively charged nanoparticles in RDE measurements with DC-based PN-PEMS systems.