## 4 Discussion  
The pollutant with the lowest variance is CO\({}_{2}\). The reproducibility values can slightly vary depending on the laboratories that are considered. As an example, Table 2 gives the bags CO\({}_{2}\) emissions of different combinations of laboratories and using (or not) of PEMS. The column of the \"13 labs\" gives the results of the official inter-laboratory exercise with all 13 participating laboratories, where no PEMS were used. The \"eight labs (no PEMS)\" column refers to the official results without any PEMS connected of the eight laboratories that repeated later the tests using the Golden PEMS. The \"8 labs (with PEMS)\" column refers to the eight laboratories results when they used the Golden PEMS. The \"five labs\" column refers to the official results without any PEMS connected of the 5 laboratories that repeated later the tests using their own PEMS in parallel to the Golden PEMS. Regarding the mean value, there were up to 1.5 g/km of CO\({}_{2}\) difference (149.1 vs. 150.6 g/km). The mean values of the 8 laboratories when using PEMS or when not using PEMS had 1 g/km difference. However, this difference is not due to the addition of PEMS: extracting a flow from the exhaust gas with the PEMS would reduce the final bag result if no correction of the automated systems for the extracted flow would be applied (but an increase was seen). In addition, this extracted flow is very small: around 3 L/min from each PEMS, for exhaust flow rates of 100 L/min (idle) to >2000 L/min (high speed). This means that the difference of 1 g/km is due to the variability of the different set of measurements. The reproducibility values vary from 2.1% to 2.9%, and they are in all cases lower than the reproducibility of the Golden PEMS (3.9%) or the Lab PEMS (5.5%), even when considering an equal number of laboratories (eight for Golden, five for Lab PEMS). The (laboratory) reproducibility values of this study and others publicly available from inter-laboratory correlation exercises with gasoline cars are given in Table 3. The results are in \begin{table}
\begin{tabular}{c c c c c} \hline  & **13 Labs** & **8 Labs** & **8 Labs** & **5 Labs** \\  & **(No PEMS)** & **(No PEMS)** & **(with PEMS)** & **(No PEMS)** \\ \hline Laboratories & 13 & 8 & 8 & 5 \\ Mean value [g/km] & 149.2 & 149.6 & 150.6 & 149.1 \\ Reproducibility & 2.3\% & 2.1\% & 2.9\% & 2.1\% \\ Repeatability & 1.7\% & 0.8\% & 1.6\% & 1.1\% \\ \hline \end{tabular}
\end{table} 
Table 2: Bag CO\({}_{2}\) [g/km] mean emissions, repeatability and reproducibility of the inter-laboratory exercise for different combinations of laboratories. Figure 2: Mean differences of PEMS (Golden PEMS: one instrument, Lab PEMS: five instruments) from bag (tunnel) results for the World harmonized Light-duty Test Cycle (WLTC). Dashed lines show relative permissible tolerances. Solid lines show absolute permissible tolerances [16; 17]. Error bars show one standard deviation. good agreement and, according to the studies of Table 3, the CO2 reproducibility is around 2-3%, CO is typically 15-25%, NOx and PN between 20% and 40%. The PEMS reproducibility values are on the same order. Table 4 examines whether the PEMS absolute levels are also in good agreement with the regulated bags methodology. Indeed in most cases, the mean differences to the bags are within the scatter of the measurements (not statistically significant differences). The results confirm that there is no bias from PEMS with the exception of PN (see below). The differences of one (Golden) PEMS to the laboratory results were smaller than from many (laboratory) PEMS. The previously mentioned results (Appendix A) showed that the PEMS reproducibility is at the same levels (for CO and NOx) or only slightly higher (for CO2 and PN) compared to the laboratory results. A detailed analysis showed that for PN there is a bias, while for CO2 the scatter is higher (Table 4). Although, not in the scope of this paper, the following points should be considered:  
* The installation of the flowmeters is critical.