## 2 Materials and Methods
The particle number instruments were reference laboratory-grade instruments and portable emission measurement systems (PEMS). As reference instruments, two advanced particle counters (APCs) from AVL (Graz, Austria) were used, both fulfilling the SPN requirements of the heavy-duty regulation (UNECE Regulation 49). One was connected to the dilution tunnel with a constant volume sampler (CVS) and one at the tailpipe. Both could measure both >23 and >10 nm particles. In current regulations (UNECE Regulation 49), the reference instrument is connected to the dilution tunnel (or proportional partial flow system). Recently, a technical regulation gave the possibility to connect directly at the tailpipe with equivalent results [53]. The instrument at the dilution tunnel consisted of a hot diluter (150 \({}^{\circ}\)C), an evaporation tube (350 \({}^{\circ}\)C), a secondary diluter at ambient temperature, and two butanol-based condensation particle counters (CPCs) with 50% detection efficiencies at approximately 23 nm and 10 nm, respectively. CPCs optically count particles after growing them to micrometer size in a super-saturated environment of the working fluid (butanol in this case). For the tests with the D1 vehicle, a catalytic stripper (350 \({}^{\circ}\)C) from Catalytic Instruments (Rosenheim, Germany) was added to minimize volatile artifacts. Based on the manufacturer's calibration certificate, due to additional diffusion and thermophoretic losses in the catalytic stripper, a 1.4 correction factor was applied for these tests. The correction gives good results when the particle size distribution has a geometric mean diameter larger than 30 nm. As it will be shown in the Results section, this was the case for the D1 vehicle. The instrument at the tailpipe was similar to the one at the dilution tunnel, with the main difference that there was a catalytic stripper (350 \({}^{\circ}\)C) inside the device in place of the evaporation tube [54; 55; 56; 57]. Additionally, a 1 m heat line at 150 \({}^{\circ}\)C was used to connect the instrument to the tailpipe. Both instruments were not always available (see Table 1). Nevertheless, the two instruments at the two different locations were compared for some tests with vehicles D2 and D3. For the tests at low ambient temperatures (\(-7\) \({}^{\circ}\)C), the instruments in the test cell were covered with heated blankets. For the high ambient temperature tests (35 \({}^{\circ}\)C) with the CNG vehicle, there was no cooling unit, resulting in high temperatures at the CPCs, and the tests were not considered in the analysis. The PEMS were two OBS from Horiba (Kyoto, Japan), measuring >23 nm and >10 nm, respectively, and two MOVE from AVL (Graz, Austria), measuring >23 nm and >10 nm,  
Figure 1: Experimental setup. Not all instruments were available for all tests. OBS and MOVE are CPC- and DC-based portable emissions measurement systems (PEMS), respectively (see Table 1). APC = advanced particle counter; CPC = condensation particle counter; CS = catalytic stripper; DC = diffusion charge; CVS = constant volume sampler; ET = evaporation tube. respectively. The OBS consisted of a diluter with recirculating air at ambient temperature, a catalytic stripper (350 \({}^{\circ}\)C), a secondary dilution bridge diluter, and an isopropanol CPC: one instrument was equipped with a 23 nm CPC and the other was equipped with a 10 nm CPC [58,59]. The MOVE consisted of a two stage hot diluter (>150 \({}^{\circ}\)C and 60 \({}^{\circ}\)C, respectively) with dilution ratios around 2:1 to 3:1 each and a catalytic stripper (300 \({}^{\circ}\)C) in between. A pulsed-mode diffusion charger (DC)-based instrument was used to count particles [60,61]. Particles were charged in a corona charger, and their (modulated) current was measured with an electrometer. The particle number concentration was calculated by the measured current using a calibration factor. The 10 nm MOVE was equipped with an electrofilter (EF). Tests were conducted with both the EF activated and not.