## 4 Discussion  
This study confirmed the findings of the previous study [11], that higher concentration of sub-23 nm nonvolatile particles may be measured at the dilution tunnel (\(2.6\times 10^{12}\) p/km) than at the tailpipe (\(6.4\times 10^{10}\) p/km). It was shown for the first time that the higher concentrations were also detected at the end of the transfer tube to the dilution tunnel. The cold dilution that takes place at the dilution tunnel did not change their concentration. Also the concentration of total particles (including volatiles) was similar indicating that no additional homogeneous nucleation took place, thus all volatile material condensed (or heterogeneously nucleated) on pre-existing particles. Cold dilution at the tailpipe and subsequent thermal pre-treatment or direct hot dilution had similar low concentrations of sub-23 nm particles confirming that it is not the cold dilution forming or growing the particles. Dilution of the whole exhaust and subsequent measurement at the dilution tunnel resulted in similar concentrations with direct hot measurements from the tailpipe. The dilution of the whole exhaust gas decreased the exhaust gas temperatures and consequently desorption of condensable species from the transfer tube that enhance the growth of particles. Extending these findings, it can be assumed that during dilution and cooling of the exhaust gas in the atmosphere, these particles would not form or they would remain \(<4\) nm. Combining the previous findings, a likely scenario is that the moped emitted nanoclusters (particles in the 1-3 nm size range). Traffic related nanocluster-sized (1-3 nm) particles containing low volatility compounds have been reported recently [16; 17; 18; 19; 20]. Mopeds can emit hydrocarbons or polycyclic aromatic hydrocarbons (PAHs) [21; 22; 23], some of them with condensation/evaporation temperature (boiling point) around 350 \({}^{\circ}\)C (e.g., icosane, PAHs such as phenanthrene, anthracene). These nanoclusters may grow in the transfer tube to sizes detectable by the instrumentation due to heterogeneous nucleation and condensation of volatile material either emitted from the moped or  
Figure 5: Solid particle number (SPN) emission rates for particles \(>4\) nm at the tailpipe (TP), the end of the transfer tube heated at 80 \({}^{\circ}\)C or 150 \({}^{\circ}\)C, and the dilution tunnel (CVS). The temperature at position 2 (end of transfer tube) is also plotted. Tests 6a and 6b. desorbed from the transfer tube due to the high exhaust gas temperatures. The contribution of the desorbed material was confirmed by the open transfer tube test. Even though the dilution was only 2:1, no growth of particles took place. This means that high exhaust gas temperatures are needed to desorb semi-volatiles that will condense on the pre-existing particles during their residence time in the transfer tube and the gradual drop of the temperature. The reason that the particles are were not evaporated in the evaporation tube at 350 \({}^{\circ}\)C is due to their low volatility and kinetic restrictions (finite residence time in the evaporation tube) of the particle number measurement system [24]. This can partly explain why the nonvolatile particles were seen with the mopeds (residence time 15 s) [11], a motorcycle (residence time 3.5 s) [25], and less with another one (residence time 2.8) [25]. Pyrolysis as a mechanism of formation can be excluded, because increasing the temperature of the transfer tube from 80 \({}^{\circ}\)C to 150 \({}^{\circ}\)C did not increase or accelerate the formation or growth of these particles. Their formation in the particle number systems was excluded in a previous study and was confirmed in this one too. In the previous study with the same moped [11] a counter with 50% counting efficiency of 2.5 nm still could not detect these particles at the tailpipe.