## 3 Results
### Exhaust Aerosol Measurements  
#### 3.2.1 Cycle-Average Emissions  
Figure 5 provides a summary of the relative performance of all four PN-PEMS systems (OBS, LD, LD + EF, LD + TC) in quantifying the cycle-average emission rates. The dashed line corresponds to the maximum allowed deviation of PN-PEMS instrumentation from reference PN measurements in the LD regulation (UNECE 2017/1154) [7] (50% or 1 \(\times\) 10\({}^{11}\) #/kWh, whichever is greater). This curve should only serve as a guideline since (a) it applies for measurements conducted at different sampling locations and (b) it is derived from the passenger car application and thus does not consider the higher and more dynamic range of exhaust temperatures and flows at the tailpipe of their HD counterparts.  
The results obtained with the DC-based instrument that did not incorporate a charge conditioning stage (LD) exhibited a much higher variability, yielding on average higher emission rates than all other PEMS instruments. The difference from the reference PN was 31% \(\pm\) 17% at emissions above 2 \(\times\) 10\({}^{11}\) #/kWh and 65% \(\pm\) 38% at lower emission levels. Similarly to what was observed in previous campaigns [12; 13], the overestimation was not systematic and in 5 out of the 22 HDV tests the instrument agreed within \(\pm\)20% with both the reference PN and the remaining PEMS solutions. When a clear overestimation was observed, the effect was found to increase with decreasing emission levels, from approximately 30% at 5 \(\times\) 10\({}^{11}\) #/kWh to as high as 110% at -1 \(\times\) 10\({}^{11}\) #/kWh.  
The cycle average results for both technical solutions (LD + EF and LD + TC) were found to agree with the reference PN technique within \(\pm\)20% at levels down to 2 \(\times\) 10\({}^{11}\) #/kWh. More specifically the difference was \(-\)8% \(\pm\) 7% (average \(\pm\) one standard deviation) for the LD + EF and \(-\)5% \(\pm\) 6% for the LD + TC. At levels below 2 \(\times\) 10\({}^{11}\) #/kWh, the deviations increased to 17% \(\pm\) 26% and 16% \(\pm\) 25% for the LD + EF and LD + TC, respectively. The CPC-based PEMS system (OBS) exhibited similar  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**d (nm)** & **23** & **30** & **41** & **50** & **70** & **100** & **200** \\ \hline +1 & \(-\)32\% & 14\% & 27\% & 24\% & 17\% & 13\% & 6\% \\ +2 & \(-\)100\% & \(-\)99\% & 19\% & 41\% & 41\% & 30\% & 13\% \\ \hline \end{tabular}
\end{table}
Table 2: Percentage difference in the charging efficiencies between singly (+1) or doubly charged (+2) particles and neutralized particles by introducing the electrofilter (EF) upstream of a unipolar corona operating at N\({}_{i}\)t product of 2.5 \(\times\) 10\({}^{12}\) m\({}^{-3}\)s for various diameters (d).  
Figure 5: Percentage difference in the cycle-average emission results between the PN-PEMS systems and the reference PN for all conducted tests, as a function of the reference PN emission rates. Each point is a test cycle. Dotted lines show acceptable differences for systems sampling from the same position.  
performance with the deviation from the reference technique being 13% \(\pm\) 7% and 31% \(\pm\) 17% at levels above and below 2 \(\times\) 10\({}^{11}\)\(\#\)/kWh, respectively. The similar performance of the LD + EF and LD + TC solution to the OBS suggests that the observed overestimation with the LD is indeed associated with the presence of charged particles.