## 1 Introduction  
Vehicle emissions performance has been controlled since the 70's in laboratories [1]. In current regulations worldwide, light-duty vehicles follow prescribed test cycles on chassis dynamometers and the whole exhaust gas is diluted in a dilution tunnel. A sample of the diluted exhaust gas is collected in bags and analyzed at the end of the test. However, for heavy-duty applications, only the engines with their after-treatment devices are type approved.  
The first on-road testing started in the 90's using Portable Emissions Measurement Systems (PEMS) [2]. PEMS were introduced in regulations for in-use compliance (in United States (US) since 2007) or in-service conformity (in the European Union (EU) since 2014) of heavy-duty vehicles without the need of removing the engine from the vehicle for testing it in the laboratory. Recently, PEMS testing was introduced for type-approval of light-duty vehicles (in EU since 2017) and will be necessary for in-service conformity with the introduction of the 4 th RDE (Real-Driving Emissions) package. The measurement uncertainty of PEMS is taken into account in the limits that vehicles have to meet during the on-road testing: In the US with the measurement allowance program [3], in the EU with the measurement uncertainty framework study [4].  
Testing with PEMS for market surveillance purposes will begin in 2020 in the EU [5]. For such testing, the instruments will have to fulfil the requirements of an under development standard from the European Committee for Standardization (CEN) (Comite Europeen de Normalisation), which will assess the PEMS performance in the laboratory under various boundary conditions (e.g., ambient temperature and pressure, vibrations) [6]. Additionally, the testing laboratories, which are not designated technical services, will have to be accredited according to the ISO (International Organization for Standardization) 17025 [7] and 17020 [8]. ISO 17025 accredited laboratories typically participate in inter-laboratory exercises (or proficiency testing or round robins as they are sometimes called), described in the ISO 17043 [9], to determine whether they are within the repeatability and reproducibility limits of a reference method. The inter-laboratory exercises are also used to assign a certified value to a reference material. They also aim at evaluating the performance of a method through the repeatability and reproducibility criteria. For example, inter-laboratory exercises (round robins) were conducted when the particle number method (e.g., [10; 11; 12; 13]), the particle number PEMS procedure [14], or the new World Harmonized Light-duty Test Procedure (WLTP) [15] were to be introduced in the regulations. Such exercises are also common practice in the automotive industries to assess their uncertainty, however, the publicly available data is limited.  
On-road PEMS testing has become a necessary source for emission inventories because they provide the \"true\" vehicle emissions under a wide range of operating conditions. Laboratory testing has limitations to simulate some real-life cases, for example, large road gradients, strong accelerations, and variations in altitude. In some cases, it was found that laboratory and on-road emissions had differences by a factor of ten. Today PEMS are robust and reliable tools for on the roads; however, the studies addressing their measurement uncertainty are limited.  
The main objective of this study is to present the repeatability and reproducibility of the PEMS measurement method and compare these values with the respective values of the regulated laboratory method. The results of this study are a first step in defining (internal) acceptable limits for the participating laboratories. The possibility to run inter-laboratory exercises only with PEMS, without chassis dynamometers testing, will also be discussed.