## 3 Results  
### Characterization of Technical Solutions  
#### 3.1.1 Unipolar Corona of Light-Duty (LD) System  
The experimentally determined charging efficiencies for the unipolar corona of the AVL PN-PEMS sensor are shown with dots in Figure 3a. The charging efficiency for neutralized graphite particlesexhibits a power law dependence with an exponent of 1.34 at both settings tested. In the default configuration, it ranged from 0.25 at 23 nm to 1.8 at 100 nm. The operation at the elevated N\({}_{i}t\) settings resulted in approximately 35% higher levels, spanning from 0.3 to 2.4 over the same size range. The measured slope is similar to what has been reported for other commercial unipolar corona chargers tested with soot aggregate particles (i.e., 1.41 [40] and 1.36 [21]). The absolute values were within the range values of other commercial unipolar coronas tested with soot particles (0.23 to 1.72 [21] and 0.53 to 4.23 [40] from 23 to 100 nm).  
Calculations of the charge probabilities with the Fuchs model yield less steep efficiency curves with slopes of 1.15-1.19 over the size range of the experimental data (23-200 nm) (Figure 3b). This discrepancy is in line with the well-known differences in charging mechanism between spherical particles assumed in Fuchs theory and aggregate graphite particles [20]. The experimental data at the lowest size ranges, where morphology effects are not expected to be significant, suggest \(N_{i}t\) products of 1.5 \(\times\) 10\({}^{12}\) m\({}^{-3}\)s for the default configuration and 2.5 \(\times\) 10\({}^{12}\) m\({}^{-3}\)s for the elevated corona settings employed in the campaign.  
The Fuchs model was also employed to evaluate the performance of such a unipolar corona charger when challenged with particles carrying +1 and +2 charges (dashed lines in Figure 3a). When sampled particles are already positively charged, their charge level can only increase when entering the positive ion environment of the corona. Their charge level asymptotically approaches that of initially neutralized particles and flattens out at their original charge below a certain size. Advanced DC sensors utilize some mechanism to apply a weight on the charging efficiency and thus reduce the contribution of large sizes in the signal [8; 9]. Therefore, the effect of such a distortion in the charging efficiency caused by charged nanoparticles will be further amplified in advanced DC configurations. Increasing the \(N_{i}t\) product would be beneficial as it would bring closer the efficiency curves for neutralized and  
Figure 3: (a) Experimentally determined charging efficiencies (dots) for the corona charger of the light-duty (LD) portable emissions measurement system (PEMS) for neutralized graphite particles at the default and elevated \(N_{i}t\) settings. Lines show the calculated charging efficiencies from Fuchs theory for N\({}_{i}t\) products of 1.5 \(\times\) 10\({}^{12}\) m\({}^{-3}\)s (green lines) and 2.5 \(\times\) 10\({}^{12}\) m\({}^{-3}\)s (orange lines), and for initially neutralized, singly and doubly charged particles (b) Experimentally determined charging efficiencies (dots) for the TC, for neutralized, singly and doubly charged particles. Lines show the calculated charging efficiencies from Fuchs theory for \(N_{i}t\) products of 2 \(\times\) 10\({}^{12}\) m\({}^{-3}\)s and 3 \(\times\) 10\({}^{12}\) m\({}^{-3}\)s on both stages, and for initially uncharged, singly and doubly charged particles.  
charged particles. However, there are practical limitations in increasing the ion-concentrations and thus alternative more efficient approaches are more suitable.