## 1 Introduction  
Air quality control at an urban scale is one of the biggest challenges for many countries and cities worldwide. One of the main sectors contributing to air quality issues is the transport sector; in 2016, it was responsible for about 23% of the carbon dioxide (CO\({}_{2}\)) emissions in Europe [1], and light duty vehicles contributed around 44% of the transport sector emissions [2]. The road transport contributed to 39% of the NO\({}_{x}\) emissions in Europe in 2015 [3]. The European Union (EU) has set emission limits for certification tests, where all new vehicles must comply with, not only during type approval, but also during normal use [4]. The European passenger cars (M1) and vans (N1) pollutant emission standards, also called 'Euro' standards, are currently in stage six (Euro 6), and define the acceptable limits for the exhaust emissions of pollutants for new vehicles sold in the EU member states, namely CO, HC, and NO\({}_{x}\), as well as the particulate matter and particle number. To verify the compliance with the targets, specific vehicle certification procedures are used, which include performing driving cycles (pre-defined speed profiles) on a chassis dynamometer under controlled laboratory conditions. Since 2017, in EU, the new Worldwide harmonized Light vehicle Test Procedure (WLTP) has been implemented and the new test cycle is the Worldwide harmonized Light vehicle Test Cycle (WLTC) [4]. Even though the WLTC is based on the driving patterns of many countries, there are still concerns that will not close the gap (mainly of CO\({}_{2}\)) between the real driving emissions and the type approval [5]. In addition, to reduce the differences found between the laboratory and on-road pollutant emissions and to limit the use of illegal strategies, the EU incorporated a real driving emissions (RDE) test procedure for the Euro 6d standard from 2017 [4]. EC Regulation (EU) 2017-1151 [4] establishes the rules for RDE testing, stating that the vehicle must be equipped with a portable emissions measurement system (PEMS), which is, in general, a compact equipment composed of portable analyzers, an exhaust mass flow meter (EFM), a weather station, and a Global Positioning System (GPS). All of this equipment must be integrated and use an acquisition frequency of 1 Hz. The regulation also establishes the trip requirements, such as the maximum and minimum duration, covered distance, ranges of speed, and the ambient boundary conditions, such as the maximum and minimum altitude and temperature [6]. The trip must cover a wide range of real world conditions, having defined shares of urban, suburban (also called rural), and highway (also called motorway) operation. PEMS are a useful tool for emission inventories because they provide emissions under a wide range of operating conditions, including those that would otherwise be difficult to replicate in the laboratory (e.g., large road gradients, strong accelerations, and variations in altitude). PEMS measurements have increased in the last years and will further increase, not only because they are a part of the type approval and in-service monitoring EU regulation, but because they are robust and reliable tools for the market surveillance of vehicles on the roads [7,8] and a support for vehicle manufacturers in the development of new vehicles. Member states will be able to take measures (including ordering vehicle recalls and revoking type-approval certificates) against non-compliant vehicles sold in their national markets. The new regulation for the type-approval and market surveillance of motor vehicles was voted for in 19 April 2018. and will replace Directive 2007/46/EC in September 2020. As summarized previously, PEMS have many sub-systems synchronized to a main platform. Thus, uncertainty associated to the measurements can be propagated through the data used for emissions calculations. According to a study by the Joint Research Centre (JRC) of the EU [9] the main sources of uncertainty are the exhaust flow meter (EFM), the gas analyzers, and their drift.