## 2 Materials and Methods
### Setup and Measurement Protocol
#### 2.1.2 Exhaust Measurements  
Figure 2 shows the layout of the instrumentation in the exhaust aerosol measurements. The measurements took place in three laboratories at approximately 23 \({}^{\circ}\)C: an engine dynamometer and two chassis dynamometers. In the first one, a bi-fuel HD engine (5.2 L, 154 kW, 706 Nm) was used, which was retrofitted with a diesel particulate filter (DPF) and an SCR in addition to the pre-existing diesel oxidation catalyst. The engine operated on different mixtures of diesel and compressed natural gas. In the two chassis dynamometers, the same diesel HD vehicle (13 L, 335 kW, 2200 Nm) was tested. The vehicle was equipped with diesel oxidation catalyst, DPF, SCR and ammonia slip catalyst. All tests were conducted with market fuel diesel (EN-590) with sulphur content <10 ppm. The natural gas was provided by gas cylinders of G20 (100% methane) or G25 (86% methane) quality. The lubricant was 5W-30. Urea dosing was controlled by the engine control unit. The measurement instruments were located within 1.5 m (engine) to 4 m (vehicle) from the after-treatment devices and sampling from the tailpipe. The reference PN instruments (all from AVL) [32] in all laboratories were UNECE 49 regulation compliant systems (called REF from now on). They dilute the sample at least 10 times with hot air at 150 \({}^{\circ}\)C and then remove the semi-volatile particles in an evaporation tube at 350 \({}^{\circ}\)C. A final dilution with room temperature filtered air cools down the sample bringing it to the appropriate concentration levels for the CPC. In addition to the default reference CPC having a 50% counting efficiency at 23 nm (REF@23), a second reference one having a corresponding 50% efficiency at 10 nm (REF@10), was employed sampling in parallel. The two technical solutions (EF and TC) were installed in two PN-PEMS compliant with the light-duty requirements (2017/1154) (called LD from now on). Note that there are no PN-PEMS technical requirements for HD applications yet. The original DC-based PN-PEMS (AVL, Graz, Austria) consists of a primary diluter using hot air at 150 \({}^{\circ}\)C (dilution 2:1), followed by an evaporation tube in series with a catalytic stripper (both set at 300 \({}^{\circ}\)C) and a secondary dilution 3:1 at 60 \({}^{\circ}\)C. The diluted sample is transferred to the particle detector (Naneos, Switzerland) [33] with a 1.3 m heated line at 60 \({}^{\circ}\)C. In the detector, particles are first charged in a positive unipolar diffusion charger and subsequently passed through a specially designed electrostatic precipitator to which a pulsating electric field is applied. The resulting modulation in the charge state of the sampled aerosol is subsequently  
Figure 2: The layout of the employed instrumentation in the heavy-duty (HD) engine and vehicle testing. Samples were extracted from tailpipe using a dedicated sampling probe for each instrument. A CPC with a 10 nm cut-off size (REF@10) was connected in the reference particle number instrument sampling in parallel to the default 23 nm CPC (REF@23). In selected tests, an electrometer and an SMPS were connected in the LD + EF system to measure the total charge and size distribution of the emitted particles during steady-state operation. LD = light-duty; TC = tandem negative positive corona charger; EF = electrofilter; SMPS = scanning mobility particle size; OBS = on-board system; REF = reference PN; CPC = condensation particle counter. detected in a Faraday cage electrometer [33]. The corona of the charger was set to operate at elevated ion concentrations in the EF configuration. For some tests, the particle detector was replaced with a scanning mobility particle sizer (SMPS) and an electrometer for particle charge estimation. The same PEMS units (LD, LD + EF, LD + TC) were used at all three laboratories. A CPC-based PN-PEMS from Horiba (Kyoto, Japan) (OBS from now on) [34] was additionally used. The first diluter (10:1) of the OBS is located directly at the sample probe at the tailpipe.