## 1 Introduction  
In the European Union (EU) the solid (or nonvolatile) particle number (SPN) emissions are regulated since 2011 for compression ignition (diesel) light-duty vehicles, since 2013 for heavy-duty diesel engines, since 2014 for positive ignition heavy-duty engines and gasoline direct injection vehicles, and since 2017 for non-road engines (19-560 kW), inland waterway vessels (>300 kW), and rail traction engines [1,2]. For light-duty vehicles the tests are complemented by on-road real driving emissions (RDE) tests since 2017 (Regulation (EU) 2017/1151).  
While the SPN sampling on the road is done directly from the tailpipe of the vehicle, the sampling in the laboratory is done after dilution of the whole exhaust in a dilution tunnel with constant volume sampling (CVS). The laboratory particle number systems consist of a hot diluter (\(\geq\)150 \({}^{\circ}\)C), an evaporation tube (350 \({}^{\circ}\)C), and a particle detector, typically a condensation particle counter (CPC) with 50% counting efficiency at 23 nm, specifications based on the Particle Measurement Program (PMP) [3,4]. For the on-road tests, the portable emissions measurement systems (PEMS) have simpler specifications: they must contain an evaporation tube with wall temperature >300 \({}^{\circ}\)C. The comparison of the PEMS and the PMP systems in the laboratory, the so-called validation test, gives differences up to 50% [1]. Approximately 30% difference is due to calibration uncertainties, mainly due to the fact that the PEMS technical requirements are not so strict (e.g., there is a 30% margin for the counting efficiencies). The rest 20% is due to the different sampling locations (tailpipe vs. CVS). There are not many studies that have examined the particle dynamics between the two locations [5; 6; 7]. It was found that agglomeration (coagulation), diffusion, and thermophoresis are the main processes that result in decreased number concentration at the dilution tunnel. The differences for modern vehicles with emission levels \(<\)1 \(\times\) 10\({}^{12}\) particles/km are \(<\)20% [6], but they were higher (40%) for older diesel vehicles without particulate filters [5]. The opposite, i.e., higher concentration levels at the CVS than at the tailpipe, are rarely seen [8]. When seen, they are typically attributed to the volatile artefact, i.e., formation of volatile particles in the measurement system downstream of the evaporation tube due to re-nucleation of semi-volatile species [9; 10]. Recently, it was mentioned that nonvolatile particles can also be formed in the tube to the dilution tunnel [11]. However there was no clear explanation of where and how they are formed. It was speculated that either they are formed at the transfer tube due to pyrolysis or that they are low volatility compounds that due to kinetic restrictions do not evaporate in the evaporation tube of the measurement system.  
The objective of this paper is to investigate when (where) and how these nonvolatile particles appear. The term \"appearance\" instead of formation is used, because these particles may already exist at the tailpipe of the vehicles, but below the measurement range of the instruments. Particle number systems with different lower size detection limits (4 nm, 10 nm, 23 nm) at the tailpipe, the transfer tube, and the dilution tunnel are compared under cold and hot dilution conditions measuring the exhaust gas of a moped.