## 4 Discussion  
Several studies have reported that urea injection in HD engine SCR systems leads to non-volatile particle formation [14; 16; 43]. These particles exhibit a mode that peaks below 20 nm and accordingly they are only partially detected by the regulated PN methodology which requires a 50% counting efficiency at 23 nm. It was also recently found that these particles can acquire a large positive charge at elevated exhaust temperatures that can increase the average exhaust aerosol charge to as high as +3 depending on the background (bipolarly charged) soot concentrations [14]. Figure 7: (**a**) Real-time non-volatile PN traces at steady cruise driving of the HD vehicles at 1000 rpm and 55% load as determined with the reference (REF) and PEMS (LD, LD + TC, OBS) instrumentation (colored lines). The increase in the SC inlet temperature (black dashed curve) was accompanied with an increase in the average particle charge (grey dashed line), which was accompanied by a similar rise in the recordings with the default LD PEMS system (dotted blue curve). (**b**) Number weighted size distribution over the same test. Numbers indicate the SMPS scan sequence, with the corresponding total non-volatile PN rates shown in panel a as yellow dots. HD = Heavy-duty; SMPS = Scanning mobility particle size. The presence of positive charges on particles in the nucleation mode range (<30 nm) can pose challenges for corona-charger-based sensors. They typically employ positive coronas that are more stable in their control and produce less ozone [41]. At typical corona settings, the average charge acquired on initially uncharged particles is less than +1 at sizes below 50 nm. When positively charged particles at this size range enter the positive ion environment of the corona, their charge level can only increase (although in practice the change in the charge level will be minor owing to the repulsive electric field of these positively charged nanoparticles). Accordingly, in the presence of such particles, the actual charge level of the aerosol exiting the corona will be higher than anticipated for neutral particles, as will be the measured signal and therefore the reported concentrations [24]. Such effects explain the overestimations observed in experimental evaluations of DC-based PEMS instrumentation with SCR equipped HD engines [12]. Since a large fraction of the formed particles lies below the 23 nm detection threshold of the regulation, one straightforward solution would be to remove all charged particles below this size via an electrofilter. A more advanced solution is to expose the pre-charged aerosol in a bipolar ion environment produced by a combination of a negative and a positive corona charger. Both technical solutions were realized and incorporated in the corona charger section of AVL's PN-PEMS system (LD). Dedicated exhaust measurements with an SCR-equipped HD engine and an SCR equipped vehicle confirmed that the overestimation observed with the original PEMS (LD) was associated with the formation of positively charged nanoparticles in the SCR. Both technical solutions were found to efficiently address interferences from such particles, leading to deviations of less than \(\pm\)30% from the reference technique at levels above 1 \(\times\) 10\({}^{11}\) #/kWh. Over the same tests, the default (LD) system showed much higher differences which increased with decreasing concentrations, reaching up to 120% at emission levels of 1 \(\times\) 10\({}^{11}\) #/kWh. The dependence of the overestimation on the cycle emission levels reflects the relative contribution of neutral soot particles on the tailpipe aerosol. Cold start operation and the fill state of the diesel particulate filter (DPF) will have a strong effect on the concentration of engine exhaust particles reaching the tailpipe [36]. On the other hand, the concentration of particles forming on the SCR was found to be relatively stable while charge builds up only at elevated exhaust temperatures (above \(\sim\)300 \({}^{\circ}\)C) [14].