## 4 Conclusions  
The PEMS from two manufacturers were compared with laboratory grade analyzers connected to the tailpipe, the dilution tunnel, and/or the bags, as prescribed by the regulation. Two vehicles, one spark ignition (SI, gasoline) and one compression ignition (CI, diesel), were used to evaluate the performance of the PEMS during the WLTC and a lab simulated RDE cycle. The CO\({}_{2}\) concentrations from the PEMS and tailpipe gas analyzers were within 2.5%. However, the PEMS that measured the 'wet' concentrations (heated NDIR) showed higher concentrations (+4%) at the cold start of the SI vehicle. The NO\({}_{\rm x}\) concentration from the PEMS analyzers were within 5% for the mean levels of 20 ppm or higher, but the differences increased to 15% at 7 ppm and 30% at 1 ppm. Similar behavior was seen for both techniques (CLD and NDUV). The exhaust flow measurements had an uncertainty of around \(\pm\)4 kg/h, that is, \(\pm\)10% for exhaust mass rates equal or lower than 40 kg/h, but around 5% at higher flow rates. The distance specific CO\({}_{2}\) emissions (in g/km) of the PEMS were typically lower (but within 10%) than the bag (diluted) results, while the laboratory tailpipe emissions were higher (but within 10%). The differences were mainly attributed to the exhaust flow measurement uncertainties. The distance specific NO\({}_{\rm x}\) emissions of PEMS were within 10% of the bag (diluted) results in most cases. The results of this study underline that current technology PEMS provide measures with good accuracy, particularly the gas analyzers, which present results close to the levels of laboratory grade equipment. However, the biggest contribution of the differences between the PEMS and bags (diluted) measurements comes from the exhaust flow measurements. At the moment, there is no simple way to check the flow meters of PEMS, and the validation test on the chassis dynamometer remains the most valid check. Systems without exhaust flow meters that estimate the exhaust flow (however, they are not considered PEMS) will probably have a higher uncertainty. Moreover, for technologies with  
Figure 10: NO\({}_{\rm x}\) emission differences of PEMS and tailpipe analyzers from bags (WLTC) or dilution tunnel (lab-RDE). **Left panel:** SI vehicle. **Right panel:** CI vehicle. The dashed lines show permissible tolerance according to the regulation [4]. low NO\({}_{\rm x}\) levels, PEMS should improve the current relative high uncertainty at low concentrations, near their zero levels. These findings are important, because they confirm that air quality studies and emission inventories based on PEMS measurements have a low measurement uncertainty (values as reported above) for vehicles with emission levels of approximately 100 mg/km and higher. The introduction of the real driving emissions test procedure will contribute to the reduction of urban air pollution, because new vehicles will emit lower emissions; however, the PEMS relative measurement uncertainty will be higher. The results of this study can help in a better estimation of the PEMS measurement uncertainty during the annual reviews, as follows: (1) the uncertainty of NO\({}_{\rm x}\) at lower levels was quantified and (2) the flow meter uncertainty was almost constant (and not relative). What should be further investigated is whether there is any drift of the analyzers on the road as well as any influence of the environmental conditions (temperature and pressure) that would further increase the differences and uncertainty of the measurements. In addition, investigation on how acceleration and power can influence the PEMS measurement could be an add-on to the current research. Conceptualization, B.G. ; data curation, R.A.V. ; writing (original draft preparation), R.A.V. and B.G. ; writing (review and editing), G.D. and L.S. The work of [PERSON] was covered by the National Council of Scientific and Technological Development (CNPq-Brazil) for his Doctoral financial support (202097/2014-5).