## 2 Materials and Methods  
The assessment of the 10 nm instruments was done at the vehicle emissions laboratory (no. 7) of the Joint Research Centre (JRC) in Ispra, Italy. Four heavy-duty vehicles were tested with various cycles at ambient temperatures of \(-7\)\({}^{\circ}\)C, \(22\)\({}^{\circ}\)C, and/or \(35\)\({}^{\circ}\)C (Table 1). The diesel-fueled vehicles are abbreviated as D1-D3, while the CNG-fueled vehicle is abbreviated as CNG. The diesel Fuel B7 was the market fuel with a maximum 7% content of fatty acid methyl ester (FAME), but the HVO fuel was purchased. The CNG was a market quality of unknown composition. The commercial natural gas of the area typically contains 85-96% methane; heavier hydrocarbons, such as ethane, propane, and butane; and inert diluents, such as N\({}_{2}\) and CO\({}_{2}\). The tests were parts of various projects, and thus, the instruments were operated by the technical staff of the laboratory. As they were not dedicated tests for the 10 nm instruments, they can be considered representative of the current situation and not optimized cases for the PEMS. The test cycles included the worldwide harmonized heavy-duty vehicles cycle (WHVC), in-service conformity like cycles (Regulation (EU) 582/2011 and 2019/1939), and verification testing procedure (VTP) cycles (Commission Regulation (EU) 2017/2400); the last two are abbreviated as real-world cycles (RWC). As the study focused on comparing instruments, the integrated emissions for the period of interest were calculated. For all cycles starting with the engine at ambient temperature, the first 900 s were evaluated separately and are abbreviated as \"cold\" engine start. The remaining of the cycle (duration 5500-11,000 s after the 900 s, depending on the cycle and vehicle) was then evaluated \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Abbreviation** & **D1** & **D2** & **D3** & **CNG** \\ \hline \hline \multicolumn{5}{c}{**Vehicles**} \\ Emission standard & Euro VI-E & Euro VI-E & Euro VI-E & Euro VI-E \\ Fuel & HVO & HVO & B7 & CNG \\ Category & N3 & N3 & N3 & N3 \\ Engine displacement (l) & 13 & 13 & 13 & 13 \\ Aftertreatment for SPN & DPF & DPF & DPF & PF \\ Payload & 60\% & 55\% & 65\% & 60\% \\ \hline \hline \multicolumn{5}{c}{**Test conditions**} \\ Ambient temperature (\({}^{\circ}\)C) & \(-7\), \(22\), \(35\) & \(-7\) & \(22\) & \(-7\), \(22\), \(35\) \\ Test cycles & RWC & RWC, & RWC, & \\  & & WHVC & WHVC & RWC \\ \hline \multicolumn{5}{c}{**Reference laboratory instruments**} \\ APC/ET 23 nm at CVS & Y & Y & Y & - \\ APC/ET 10 nm at CVS & Y * & - & Y & - \\ APC/CS 23 nm at TP & - & Y & Y & Y \\ APC/CS 10 nm at TP & - & Y & Y & Y \\ \hline \multicolumn{5}{c}{**Portable emissions measurement systems (PEMS)**} \\ OBS 23 nm & Y & - & Y & Y \\ OBS 10 nm & Y & - & Y & - \\ MOVE 23 nm & - & Y & - & - \\ MOVE 10 nm & - & Y & - & Y \\ \hline \hline \end{tabular}  
* An additional CS was installed for some tests with vehicle D1. CNG = compressed natural gas; CS = catalytic stripper; CVS = constant volume sampler (dilution tunnel); D = diesel vehicle; DPF = diesel particulate filter; ET = evaporation tube; HVO = hydrotreated vegetable oil; PF = particulate filter; RWC = real-world cycle; SPN = solid particle number; TP = tailpipe; WHVC = worldwide heavy-duty vehicles cycle.  
\end{table} 
Table 1: Overview of vehicles and instrumentation. separately and considered \"hot\" part. Similarly, all tests in which the engine coolant was not at ambient temperature (usually >70 \({}^{\circ}\)C) at the engine starting (WHVC hot and some RWC) are considered \"hot\" cycles, and the integration was done from the beginning of the cycle. Thus, the analysis did not follow the Euro VI step E methodology with moving average windows [52], considering cold start after the coolant temperature has reached 30 \({}^{\circ}\)C, hot emissions after the coolant temperature has reached 70 \({}^{\circ}\)C, and weighing cold and hot start emissions at 14% and 86%, respectively. Figure 1 gives an overview of the setup and the instruments that were used.