## 3 Results and Discussion
### Real-Time Graph Examples  
For a better understanding of the final results, some examples are given in this section. Figure 4 plots real-time emissions during the cold start part of a WHVC at \(-7\)\({}^{\circ}\)C for vehicle D2. Two tests on two different days are plotted. In the first one (Figure 4a), the AVL MOVE PEMS was without the electrofilter activated, while in the second, it was with the electrofilter activated (Figure 4b). The small difference at the absolute levels of the two tests has to do probably with the DPF fill state. The two reference instruments at the tailpipe were almost indistinguishable, indicating that the mean size of the particles was >23 nm and thus soot in nature. The PEMS was measuring slightly lower: on average, 4% and 1% for the two tests. Thus, the electrofilter had no obvious impact on the absolute levels during the cold start, where relatively large particles are expected. Figure 5 plots the real-time emissions during the hot motorway part of a WHVC at \(-7\)\({}^{\circ}\)C. As previously, two tests from the same vehicle on two different days are plotted. In the first one, the AVL MOVE PEMS was without the electrofilter activated (Figure 5a), while in the second, it was with the electrofilter activated (Figure 5b). The absolute levels of the two tests were quite similar. The two reference instruments at the tailpipe had a big difference, indicating that the mean size of the particles was <23 nm, most likely from urea. The PEMS was measuring much higher (+110% at an emission level of 1 \(\times\) 10\({}^{11}\) #/kWh) than the 10 nm reference instrument at the first test, but almost identical at the second with the electrofilter activated. This indicates that the particles were indeed highly charged urea particles [62], captured efficiently by the electrofilter [46]. Figure 6 plots the real-time emissions of a WHVC at 23 \({}^{\circ}\)C, where 23 nm and 10 nm PEMS OBS from Horiba are compared with reference instruments at the tailpipe. At cold start (Figure 6a), the reference instruments were close to each other, and thus the mean particle size was >23 nm. Both PEMS had higher spikes than the reference instruments, which indicates either faster response time or a small pressure dependency. Pressure fluctuations may be caused by the dynamic pressure generated by exhaust flow in addition to the back (static) pressure caused by the connection to the CVS. For the specific cold start phase (900 s), the overestimation of PEMS was 53% (10 nm) and 66% (23 nm). The (under)pressure recorded inside the optical device of the PEMS fluctuated between 91 and 103 kPa in the tests in Figure 6 and other cycles with vehicle D3, while in the tests with  
Figure 4: Real-time solid particle number (SPN) emissions during a cold start of WHVC at an ambient temperature of \(-7\)\({}^{\circ}\)C: (**a**) PEMS without the electrofilter activated and (**b**) PEMS with the electrofilter activated. The PEMS was the MOVE from AVL. PEMS = portable emissions measurement system; TP = tailpipe; WHVC = worldwide harmonized heavy-duty vehicles cycle. vehicle D1, it fluctuated between 90 and 95 kPa. It is not clear what might have resulted in higher backpressure in the specific setup. The main difference between the two vehicles was that there were many instruments sampling close to each other. However, it is unlikely that the low sampling flow rates of the instruments might have affected each other. For this diesel vehicle, the 45\({}^{\circ}\) probe was placed facing the flow, so the instrument was more sensitive to such pressure fluctuations. With the CNG vehicle, the probe was placed the other way around to be protected by big droplets of condensate. The real-time signals with vehicle D1 are not plotted because the reference instruments were at the CVS; any difference might be partly misleading due to diffusion and smoothening taking place in the connection tube to the dilution tunnel.