## 3 Results and Discussion
### Real-Time Graph Examples
Figure 5: Real-time solid particle number (SPN) emissions during the hot motorway part of a 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. Figure 6: Real-time solid particle number (SPN) emissions during a worldwide harmonized heavy-duty vehicles cycle (WHVC) at an ambient temperature of 23 \({}^{\circ}\)C: (**a**) cold start and (**b**) last motorway part. The PEMS was the OBS from Horiba. PEMS = portable emissions measurement system; TP = tailpipe reference system. Figure 6b plots the motorway part of the cycle. The differences at accelerations were still evident, while at steady conditions, the absolute levels of the 10 nm instruments were in agreement. The 23 nm PEMS was measuring higher than the 23 nm laboratory reference. For the motorway part of this cycle, the difference was 7% for the 10 nm PEMS and 46% for the 23 nm PEMS. This difference indicates a small difference in the cut-off size of the CPCs inside the PEMS and the reference instrument [58]. Differences in the 23 nm CPCs' calibrations are not uncommon and have been reported by other researchers as well [74]. However, for 10 nm instruments, the calibration material is not so important, and the differences become smaller [55]. Figure 7 plots the first 1200 s of a RWC with the CNG vehicle. Figure 7a plots the 23 nm Horiba OBS and the 23 nm reference at the tailpipe. Figure 7b plots the AVL MOVE 10 nm and the 10 nm reference at the tailpipe. The relative humidity, as measured inside the PEMS instruments with their internal sensors, is also given. The emissions, due to the particulate filter of the CNG vehicle, were very low, as expected [75], close to the background (detection levels) of the instruments. For PEMS, the maximum zero level allowed in the regulation is 5000 #/cm\({}^{3}\). There was a spike at the beginning of the test with the ignition on and the first idling seconds. Another spike was at around 300 s, where the speed increased to 30 km/h, along with an increase in the relative humidity levels (>30%) inside the instruments. At the beginning of the cycle, the water content of the exhaust gas was condensing due to the low ambient temperature. As the temperature increased, condensation stopped. It is not clear whether this spike at around 300 s was due to true particle emissions, as not all instruments measured it, and in some cases, there was a difference in timing. Based on the real-time signals, we cannot conclude that there was a dependency of the instrument's signals on the humidity. However, condensates inside the instruments can create spikes [76]. Water condensates can trap many particles, which are released when the droplet evaporates in the heated section of the instrument. For the specific test, the cold start emissions were 2 \(\times\) 10\({}^{10}\) #/kWh according to PEMS 23 nm and double according to the reference instruments. Figure 7: Real-time solid particle number (SPN) emissions during a real-world cycle (RWC) at an ambient temperature of \(-7\)\({}^{\circ}\)C with the compressed natural gas (CNG) vehicle: (**a**) Horiba OBS 23 nm and (**b**) AVL MOVE 10 nm. PEMS = portable emissions measurement system; RH = relative humidity.