## 3 Results and Discussion
### PEMS vs. Laboratory Instruments  
The following figures summarize the differences between the PEMS and the laboratory reference instruments. To put the results into perspective, two curves are also given: 6 \(\times\) 10\({}^{11}\) #/kWh or 63% (whichever is larger) and 1 \(\times\) 10\({}^{11}\) #/kWh or 40% (whichever is larger). The first curve corresponds to the Euro VI limit and on-road margin that takes into account the PEMS measurement uncertainties. The second curve is a reasonable uncertainty for the Euro 7 proposed \"hot\" limit of 2 \(\times\) 10\({}^{11}\) #/kWh. Furthermore, an uncertainty of 40% was taken into account for the proposed limits in the Euro 7 assessment of SPN limits for heavy-duty vehicles [10]. These values are also close to the PEMS uncertainty based on theoretical estimations [30].  
Figure 8 plots the deviations of Horiba's 23 nm and 10 nm OBS from the reference instruments. The differences were well within the \"tighter\" curve, but with a few exceptions for both the 10 nm and 23 nm PEMS. These had to do with pressure sensitivity, as discussed in Figure 6. The differences were not related to any ambient temperature effect. For the D1 vehicle, the differences between the PEMS and reference instrument were at the same levels, irrespective of the ambient temperature. The same applied to the CNG vehicle. The CNG points with negative differences could be cases where the reference instrument was overestimating due to condensate spikes, as discussed in Figure 7.  
Figure 9 plots the deviations of AVL's 23 nm and 10 nm MOVE from the reference instruments. The differences were also within the \"tighter\" curve, with the exception of the hot tests of D2 when the electrofilter was not active. As discussed in Figure 5, this had to do with the charged urea-originating particles that resulted in an overestimation of the instrument's signal. For the 23 nm PEMS, which had no electrofilter, the impact of charged urea particles was still negligible, as the majority of these particles are typically smaller than 23 nm. The \(>\)23 nm levels were \(<\)10\({}^{11}\) #/kWh, which is \(<\)17% of the 6 \(\times\) 10\({}^{11}\) #/kWh laboratory limit. Nevertheless, the heavy-duty 23 nm PEMS has an electrofilter or pre-charger installed. The difference during active regeneration (around 30%, see Appendix A) was also \(<\)40%, even without the electrofilter activated, because these particles are mainly soot penetrating the DPF. Due to the low filter efficiency [77], many particles go through the DPF [17], and the concentration of any urea particles formed downstream of the DPF is relatively low. The ambient temperature had no effect on the PEMS. With the CNG vehicle, the negative differences could be condensate spikes of the reference instrument  
Figure 8: Difference of Horiba’s OBS PEMS from laboratory reference instruments. Results for 10 nm instruments, unless otherwise specified. CNG = compressed natural gas; HVO = hydrotreated vegetable oil; SPN = solid particle number.  
(see Figure 7), while the \(>\)100% differences at the 10\({}^{9}\) #/kWh range are due to the low concentrations, close to the background levels of the specific PEMS.