**5. Conclusions**

This study summarized the issues that have been raised in the literature regarding solid particle number (SPN) measurements of L-category vehicles (e.g., mopeds and motorcycles), namely: Lower particle size that should be counted, possibility of artefacts, use of long lines from the vehicle to the dilution tunnel in combination with low exhaust flow rates, extraction of a sample from the tailpipe for analysis, exhaust flow rate determination, time alignment, and cold start. In this study we quantified the uncertainties in emission measurements of a moped, with emphasis on SPN and CO2 and concluded on the setup with the highest accuracy for regulatory purposes.

The dilution tunnel measurements needed a 24–31% correction for CO2 and 19–73% for SPN to account for exhaust extracted upstream of the dilution tunnel from instruments sampling from the tailpipe (in our case 23–28% of the mean total exhaust flow) (bleed o ff). Furthermore, the particles losses in the transfer tube from the moped to the dilution tunnel decreased the SPN emissions by an additional 30% (dilution tunnel vs. tailpipe). The exhaust flow was determined via the di fference between total and dilution air flow, the CO2 tracer method, and the carbon balance. In this study, we estimated about 5% of uncertainty associated to the average exhaust flow, which propagates to the final emission results. The wrong estimation of the exhaust flow during the first minute of the cold start, due to the "dead" volume in the transfer tube to the dilution tunnel, resulted in a >50% error in SPN. A time misalignment of ±1 second gave di fferences of the SPN emissions on the order of ±5%.

Based on the results of this study, dilution tunnel SPN measurements in open configuration without additional instruments sampling from the tailpipe would give the most accurate results for regulatory purposes in terms of particle emissions. Tailpipe measurements for regulatory purposes need further investigations when specific exhaust flow meters will be available.

The specific moped had SPN emissions of 5 × 10<sup>11</sup> p/km: Lower than, but close to the passenger cars limit when considering particles larger than 23 nm. The emissions were double when smaller particles were included and exceeded the limit indicating that for this category a lower cut-o ff size is more appropriate. With high dilution ratios (around 1000:1) and the tailpipe in open configuration, and consequently, lower exhaust temperatures, the possibility of particle losses due to agglomeration or formation of particles due to high temperatures can be minimized.

**Author Contributions:** Conceptualization, B.G.; formal analysis, B.G.; writing—original draft preparation, B.G.; writing, review and editing, A.A.Z., T.L., M.C., F.F., and Y.D.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to acknowledge D. Lesueur, P. Le Lijour, and M. Sculati for the experimental support.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Disclaimer:** The opinions expressed in this manuscript are those of the authors and should in no way be considered to represent an o fficial opinion of the European Commission. Mention of trade names or commercial products does not constitute endorsement or recommendation by the authors or the European Commission.
