## 5 Conclusions  
In order to investigate the differences reported between tailpipe and dilution tunnel particle number emissions, two identical systems were used to measure the exhaust gas of a moped under different sampling conditions. The systems comprised an evaporation tube at 350 \({}^{\circ}\)C in order to remove volatile and semi-volatile particles and condensation particle counters (CPCs) with lower detection limits at 4 nm, 10 nm, and 23 nm to measure nonvolatile particles. One system was measuring directly from the tailpipe under hot conditions, while the other was measured from the dilution tunnel or from the end of the transfer tube from tailpipe to dilution tunnel. For one test, a cold dilution was used upstream of the system connected to the tailpipe. For another test, the transfer tube was left open at the side connected to the moped. A final test was done with the system connected to the dilution tunnel with its heaters off.  
The results showed that, while the 23 nm CPCs were measuring similar nonvolatile particle concentrations, the 10 nm and 4 nm CPCs started to deviate after 5 min of testing, with the CPCs at CVS measuring one order of magnitude higher. The concentrations at the CVS were at similar levels when the heaters were off and on, indicating that the particles pre-existed (i.e., they were emitted by the moped). The test with the system sampling from the end of the transfer tube showed that the high concentration was already detectable before the mixing at the dilution tunnel, indicating that the particles grew in the transfer tube and not in the dilution tunnel. Separate tests with cold dilution at the tailpipe confirmed that it is not the cold dilution that forms or grows the particles. A test with the transfer tube open at the moped side did not show high concentrations at the dilution tunnel, indicating that the pre-existing particles grew due to the desorbed material from the transfer tube at high exhaust gas temperatures.  
The term nonvolatile artefact can be used to describe the appearance (growth) of these particles at the sub-23 nm CPCs at the dilution tunnel. However, the term should be used carefully, because in this study, the particles were not formed in the transfer tube, but pre-existed (i.e., were emitted by the moped) at sizes below the detection limit of the instruments; however they were grown due to the desorbed material from the transfer tube.  
Future work for more detailed characterization and understanding of these particles needs sampling systems with low particle losses and particle counters that can measure from 1 nm. In addition to mopeds emissions, similar studies should be conducted to motorcycles, compressed natural gas (CNG) vehicles, and vehicles with particle filters during regenerations.  
This research received no external funding.  
The author would like to thank [PERSON], [PERSON], [PERSON] and [PERSON] for their helpful comments at an earlier draft version.  
The author declares no conflict of interest.  
The opinions expressed in this manuscript are those of the author and should in no way be considered to represent an official opinion of the European Commission. Mention of trade names or commercial products does not constitute endorsement or recommendation by the author or the European Commission.