## 2 Materials and Methods  
Figure 1 presents the experimental setup. The tests were conducted at the vehicles' emissions laboratory (VELA 1) of the Joint Research Centre (JRC) of the European Commission. The facility is used for light-duty and L-category vehicles (such as mopeds and motorcycles) testing. Before this campaign a Euro 4 motorcycle was tested (6 tests) and before that motorcycle, a gasoline vehicle (6 tests). The tests of this campaign were conducted with a Euro 4 moped with a 50 cm\({}^{3}\) 4-stroke engine with three-way catalyst. For the specific moped high differences between tailpipe and dilution tunnel were found in another study [11]. Between the tests of this study and the previous study the moped was driven \(>\)500 km in urban roads. Steady state test at the maximum speed of the moped (45 km/h) were run. The tailpipe of the moped was connected to a 6 m stainless steel transfer tube. The first 4 m  
Figure 1: Experimental setup with two PMP systems at position 1 (PMP #1) and 2 or 3 (PMP #2). Exhaust gas temperatures at the different positions are also given. Square inset plots the open transfer tube configuration. Oval inset plots the cold dilution TP configuration with a mixing diluter. CPC = Condensation particle counter; CVS = Constant volume sampling; ET = Evaporation tube; PMP = Particle Measurement Programme; PND = Diluter; TP = Tailpipe. were flexible and heated (heater set to 80 \({}^{\circ}\)C) and the last 2 m were fixed and insulated. The moped and the first 4 m of the transfer line were inside a climatic chamber kept at 23 \({}^{\circ}\)C. The whole exhaust gas was then diluted in a dilution tunnel with constant volume sampling (CVS). The last 2 m of the transfer line and the CVS were in another climatic room kept at 20 \({}^{\circ}\)C.  
Samples were taken at three locations: (1) close to the tailpipe of the moped; (2) at the end of the transfer tube; and (3) at the dilution tunnel (CVS). The temperatures were also measured at these 3 locations, but they are available in real time only for position (1). Two identical and freshly calibrated measurement systems were used: one (PMP #1) was measuring always from the tailpipe (position 1). The second (PMP #2) was measuring from the CVS (position 3) and for a few tests from the end of the transfer tube (position 2). The systems fulfilled the light-duty vehicles regulations and the PMP specifications [4]. The PMP system of this study (AVL particle counter APC 489, Graz, Austria) [12] consisted of a hot diluter at 150 \({}^{\circ}\)C, an evaporation tube at 350 \({}^{\circ}\)C, a cold diluter with filtered ambient air at 20 \({}^{\circ}\)C, and full-flow butanol-based condensation particle counters (CPCs) with 50% counting efficiency at 23 nm (model 3790 from TSI, MN, Shoreview, MS, USA), 10 nm (model 3772 from TSI, USA) and 4 nm (model 3752 from TSI, USA). The thermal pre-treatment of the PMP systems evaporates volatile and semivolatile particles in order to allow only nonvolatile (solid) particles to be counted [4]. The CPCs optically detect particles that grow in the measurement range of the optics by condensation of butanol [13]. The PMP #1 was connected to the tailpipe with a stainless steel 0.5 m tube heated at 120 \({}^{\circ}\)C. This was the default configuration and was repeated in the middle and the end of the measurement campaign (tests 1, 5, 7) (Table 1). For test 2, the 6 m transfer tube was left open at the side normally connected to the moped (Figure 1, left inset). The underpressure from the CVS sucked some ambient air also from the opening. The dilution was estimated to be around 2:1. This configuration (open transfer tube) is allowed in the EU motorcycles/mopeds regulation. Here the target was to see whether the nonvolatile particles are formed due to the cold dilution. For test 3, the 0.5 m heated tube of PMP #1 was switched off and a simple mixer diluter with filtered ambient air was added upstream (dilution 6:1). Target was to see whether cold dilution forms the nonvolatile particles.