## 1 Introduction  
Particulate matter (PM) is a mixture of particles and droplets in the air, consisting of a variety of components such as organic compounds, metals, acids, soil, and dust. Traffic, industrial activities, domestic fuel burning, but also natural dust and salt are the main contributors in urban PM. The concentrations of PM continue to exceed the European Union (EU) limit values in large parts of Europe [1]. The contribution of road traffic to PM concentrations is important: around 14-25% to the PM concentrations in Europe [2,3], but higher in Asia [2,4]. The share of heavy-duty vehicles' emissions to the respective regional emissions inventories is significant and disproportionate to their activity. In 2008, heavy-duty vehicles, which represented \(<\)5% of the vehicle population in California, China and Brazil, contributed 44-57% of the road-traffic PM emissions [5]. The type approval of a heavy-duty engine with the aftertreatment system is conducted in an engine dynamometer where the engine revolutions and torque are varied following a prescribed test cycle. In Europe, since Euro VI (2013), for on-road engines the cycle is the WHTC (World Harmonised Transient Cycle). The test is conducted twice: with engine starting with coolant and oil temperature at ambient conditions (20-30 \({}^{\circ}\)C) and the engine starting warmed up (the results are weighted 14% and 86% respectively). Additionally, a stationary cycle is tested: the WHSC (World Harmonised Stationary Cycle). Other cycles are used in other regions of the world: for instance the FTP (Federal Test Procedure) in the United States of America (USA). For off-road engines the relevant cycle is the NRTC (Non-Road Transient Cycle). The exhaust is diluted in a dilution tunnel with constant volume sampling (CVS), where samples of PM mass, solid particles number (SPN) and gases are taken. PM mass can also be measured with a proportional (to the exhaust flow) partial flow dilution (PD) system since November 2006 (Commission Directive 2005/78/EC) for transient cycles [6]. The proportionality is necessary for the filter method in order to mimic the full dilution tunnel (CVS), where the dilution changes inversely with the exhaust flow. However, for real time signals and instruments this is not necessary, because the emissions can be calculated in a second-by-second basis. Gaseous components can be measured directly from the tailpipe since 2006 for transient cycles. SPN measurements could also be made directly from the tailpipe, but at the moment this is not allowed. However, in 2021 SPN measurements will be conducted with PEMS (portable emissions measurement systems) for type approval and in-service conformity of vehicles, which have higher measurement uncertainty from the laboratory grade equipment and procedures [7]. The samples are taken from the tailpipe of the heavy-duty vehicles with fixed dilution. Thus, the main question that is raised is whether direct tailpipe measurements could be allowed for the type approval of engines in the engine dynamometers in the future regulations. The measurement uncertainty should remain at the same levels as with the full dilution tunnel or the proportional sampling. There are a few comparisons of the CVS or PD systems with the tailpipe systems for heavy-duty engines for SPN [8, 9, 10]. Nevertheless, a thorough investigation is missing. The SPN limit for heavy-duty engines (6\(\times\)10\({}^{11}\) p/kWh) (\"p\" will be used for particles from now on) was introduced in 2013 (Euro VI) for compression ignition (diesel) engines (Commission Regulation (EU) 582/2011) and in 2014 for positive ignition engines (Commission Regulation (EU) 133/2014). The methodology is based on the recommendations of the PMP (particle measurement programme) group which define sampling from the CVS or the PD system with hot (\(>\)150 \({}^{\circ}\)C) dilution of the sample, thermal pre-treatment in an evaporation tube at 350 \({}^{\circ}\)C and counting with a CPC (condensation particle counter) which has a 50% detection efficiency at 23 nm [11].