## 2 Materials and Methods
### Particles Instrumentation  
Various instruments were used to measure particles from the tailpipe (TP) and the dilution tunnel with constant volume sampling (CVS). One advanced particle counter (APC) from AVL (Graz, Austria) [28] with 23 nm and 10 nm condensation particle counters (CPCs) was connected directly at the dilution tunnel and a second one at the tailpipe with a 0.5 m heated stainless steel line at 150 \({}^{\circ}\)C. Both APCs had a catalytic stripper (CS) operating with wall temperature set at 350 \({}^{\circ}\)C [29]. The difference between the two APCs was that the one at the dilution tunnel had condensation particle counters (CPCs) from TSI (Shoreview, MN, USA), while the one at the tailpipe from AVL. The counting efficiencies at 23 nm and 10 nm were approximately 50% and 65%, respectively, for both TSI and AVL counters. These efficiencies are defined in the respective 23 and 10 nm solid particle methodologies in the global technical regulation (GTR 15) for light duty vehicles [6,15]. The solid particle number (SPN) emissions from these instruments will be abbreviated as: SPNx,y, where x is 23 nm or 10 nm and y is the dilution tunnel (CVS) or tailpipe (TP). The particle number concentration reduction factor (PCRF), which is the dilution with the average losses at 30 nm, 50 nm, and 100 nm, was 1000 (primary dilution 100) for both instruments. This value has been found to minimize any interference from volatile particles [30].  
For the measurement of total particle number (TPN) concentration (i.e., solid and volatile particles), two systems were used: one at the dilution tunnel and the other at the tailpipe. The one at the dilution tunnel consisted of a porous diluter (PD) with a cold (i.e., not heated) dilution of approximately 10:1. The particle number concentration and size distribution were measured with an engine exhaust particle size (EEPS) from TSI. The covered size range was 5.6 nm to 560 nm. The second system at the tailpipe, based on the DownToTen project [31], consisted of 0.8 m heated stainless-steel line at 150 \({}^{\circ}\)C, a porous diluter (PD) with dilution 10:1, an ejector diluter (ED) with dilution 10:1, and a bifurcated flow diluter (BD) with dilution 20:1. The filtered air of the PD and ED was not heated. The BD dilutes without the need of external air supply by splitting the flow: One part is filtered and used internally as dilution air. The reasons for having a third diluter and not a higher dilution at the first PD were twofold: Firstly, a primary dilution ratio of around 10-25:1 has been found to maximize the nucleation mode formation [19,27,32]. Secondly, as we did not control the real-time dilution ratio by gas measurements (only with pressure measurements), the lower dilution ratio had lower uncertainty compared to a higher dilution. The system was characterized at the laboratory for particle losses and were found to be around 30% more in the 10 nm to 23 nm range and 50% more in the sub-10 nm range than the losses in the \(>\)23 nm range [13,33]. The particle number concentration was determined with two CPCs from TSI: one with 65% counting efficiency at 10 nm and the other 50% counting efficiency at 5 nm. The total particle number (TPN) emissions from these instruments will be abbreviated as: TPNx,y, where x is 4 nm, 6 nm (approximately the lower size detected by the EEPS), or 10 nm and y is a dilution tunnel (CVS) or tailpipe (TP).