## 4 Discussion
Sudden curves, sharp bends or diameter changes can result in high errors. Additionally, uncertainties at idle and low flow rates can result in over-or underestimation of the CO2 (e.g., Reference [14]). * The exhaust flow measurement uncertainty at idle can influence the CO2 results (for the other pollutants the effect is small because cars typically have low emissions at idle and low loads/speeds) [25; 26]. The zero calibration should be conducted with the dilution tunnel operating in order to minimize any influence of the under-pressure created by the constant volume sampler (CVS). Investigations with \"open\" CVS (i.e., the flowmeter is connected to the tubing via a cone that permits ambient air also to enter) would minimize pressure and pulsations from the CVS and should be further investigated in order to evaluate the true performance of the flowmeter on the road. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Emission Level** & \(\mathbf{CO_{2}}\) & \(\mathbf{CO}\) & \(\mathbf{NO_{x}}\) & \(\mathbf{PN}\) & **Comments** \\ \hline Euro 5 & +7.1\% (3.6\%) & - & +1.5\% (15\%) & +6\% (20\%) & 1 Golden PEMS [14] \\ Euro 6 & \(-1.0\%\) (4.0\%) & \(-0.7\) (14\%) & +2.0\% (9\%) & +36\% (32\%) & 1 Golden PEMS \\  & +0.9\% (3.4\%) & +1.5\% (13\%) & +11.3\% (18\%) & +41\% (33\%) & 5 Lab PEMS \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mean differences of PEMS from bags from inter-laboratory exercises with gasoline vehicles. In brackets the relative standard error (Coefficient of Variation), i.e., the standard deviation of the differences divided to the mean difference. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Emission Level** & \(\mathbf{CO_{2}}\) & \(\mathbf{CO}\) & \(\mathbf{NO_{x}}\) & \(\mathbf{PN}\) & **Comments** \\ \hline Euro 3 [1] & 1.8 & 7 & 24 & 30 & 4 labs, NEDC [11] \\ Euro 4 [1; 2] & 2.0 & 5 & 18 & 44 & 4 labs, NEDC [12] \\ Euro 3 & 5.0 & 40 & 40 & - & 9 labs, many cycles [24] \\ Euro 6 & 1.6 & 15 & 29 & 9 & 11 EU labs, WLTC [15] \\ Euro 6 eq. & 1.3 & 12 & 24 & - & 16 Asian labs, WLTC [15] \\ Euro 5 [3] & 2.1 & - & 37 & 22 & 7 labs, WLTC [14] \\  & 2.4 & - & 36 & _20_ & _7 labs, 1 Golden PEMS_ \\ Euro 6 & 2.9 & 22 & 23 & 29 & 8 labs, WLTC, this study \\  & 3.9 & 25 & 25 & _24_ & _7 labs, 1 Golden PEMS_ \\  & _5.5_ & _20_ & _31_ & _39_ & _5 labs, 5 Lab PEMS_ \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Reproducibility levels (%) from inter-laboratory exercises with gasoline vehicles. In italics values based on PEMS. * The difference of the PN results can be high: partly due to PEMS uncertainties, but mainly due to lower PN concentrations reaching the dilution tunnel (diffusion losses, thermophoretic losses, coagulation etc.) [22]. As a final check, the consistency of the PEMS was evaluated by comparing the laboratory PEMS measuring simultaneously with the Golden PEMS, using their own flow meters, for both the WLTC and the RDE cycles. The results are plotted in Figure 3. The mean differences are small and similar between the two cycles. The good agreement of the Golden and Lab PEMS for PN, even though they consisted of different counting technologies (CPCs and DCs) indicates that the higher emissions at the tailpipe are true and not a calibration issue. Small differences between PN systems at the same location was expected for CPC based systems, but higher differences for DC-based systems have been seen [27], especially at lower emission levels. Some attention needs to be given to the 5 g/km mean difference of the CO\({}_{2}\) results. This difference can be partly explained by the general slight overestimation of Lab PEMS and a slight underestimation of the Golden PEMS (see Figure 2). A closer look at the data revealed that a big part of this difference came from one laboratory. Excluding this laboratory from the analysis, reduced the difference to <1 g/km with scatter (one standard deviation) of 3 g/km. Comparing the mean exhaust flow rates calculated by the PEMS revealed that the reason for the higher results of that laboratory was due to the exhaust flow meter.