**1. Introduction**

Air pollution continues to have significant impacts on the economy and the health of the European population, particularly in urban areas. Europe's most serious pollutants in terms of adverse health effects are particulate matter (PM), nitrogen dioxide (NO2), and ground-level ozone (O3) [1]. PM is damaging to ecosystems and cultural sites, responsible for reduced visibility, and an important global risk factor for human health. Automotive exhaust PM, due to its important contribution to urban PM, has been subject to progressively more stringent regulations [2].

PM mass emissions are determined gravimetrically by collecting diluted exhaust gas on a filter. In addition to the PM method, the non-volatile (solid) particle number (SPN) emissions are measured by a specified method [3] in the European Union (EU). The SPN sampling is conducted via Constant Volume Sampling (CVS) of the whole exhaust in a dilution tunnel where the exhaust is diluted. The laboratory particle number systems consist of three parts: First, the exhaust gases pass through a hot diluter (≥150 ◦C), followed by an evaporation tube (350 ◦C), and lastly by a condensation particle counter (CPC) with 50% counting efficiency at a particle diameter of 23 nm. The methodology and specifications follow the recommendations of the Particle Measurement Program (PMP) [4]. The accuracy of the methodology was estimated to be around 15%. Inter-laboratory correlation exercises, which include the uncertainty of the source (i.e., the car or engine) found a reproducibility of approximately 35% [4,5].

Since 2011, SPN emissions are regulated in the EU for compression ignition (diesel) light-duty vehicles; the limit value is 6 × 10<sup>11</sup> particles per km (p/km from now on). Since 2013, on-road compression ignition (diesel) heavy duty-engines are included in the regulations, and since 2014, positive ignition heavy duty-engines with limits of 6 × 10<sup>11</sup> p/kWh. A limit of 6 × 10<sup>11</sup> p/km was applied to gasoline direct injection vehicles in 2014 [6]. However, for up to three years a particle number emission limit of 6 × 10<sup>12</sup> p/km could be applied upon request of the manufacturer. Some non-road engines (power range 19–560 kW), inland waterway vessels (>300 kW), and rail traction engines are regulated for SPN since 2017 with a limit of 1 × 10<sup>12</sup> p/kWh. Since 2017, emission testing of light-duty vehicles includes on-road real driving emissions (RDE). Recently, the European Commission decided to extend the lowest detection size of the SPN methodology to 10 nm, to cover cases that have a high fraction of particles in the 10 nm to 23 nm size range (e.g., some vehicles with spark ignition engines) [2].

The only vehicle class so far not covered by SPN limits is the L-category (two- or three-wheel vehicles and quadri-cycles, such as quads and minicars). Nevertheless, moped and motorcycles account about 10% of the passengers mobility fleet [7,8] and contribute significantly to air pollution [9]. Recent EU regulations aim to reduce the share of total road-transport emissions from L-category vehicles as compared to other road vehicle categories with a focus on PM and ozone precursors, such as nitrogen oxides and hydrocarbons. Among other provisions a new test cycle was introduced, the gaseous compounds emission limits were tightened, and a PM mass limit was introduced. In addition, feasibility and cost-effectiveness studies on durability, on-board diagnostics (OBD), in-service conformity, off-cycle emissions, and particle number emissions were required (Regulation (EU) 168/2013). A first investigation on the feasibility and necessity of introducing a SPN limit for L-category vehicles was conducted in 2015 [10]. That research showed that L-category vehicles can have high SPN emissions with a high percentage of them not counted by the current lower limit of 23 nm. However, it was argued that decreasing the lower detectable size could result in volatile artefacts (namely, the formation of volatile particles in the measurement system via nucleation of semi-volatile species—these re-nucleated particles form downstream of the evaporation tube and may be measured by the particle-measuring instrument) and more research was therefore considered necessary. An ad-hoc environmental study concluded that introducing specific SPN limits for L-category vehicles would first require better understanding of the emissions performance of such vehicles, as new emission control technologies at Euro 5 step would become available (from 2020 on) [11]. A detailed analysis of the data concluded that L-category vehicles are a significant contributor to vehicular particulate emissions, and their relative contribution will increase if no measures will be taken [12]. Consequently, a specific particle number limit should be considered for L-category vehicles. A recent study with the latest technology mopeds and motorcycles (all fulfilling the Euro 4 emission standards) reported that, although the PM mass emissions were <1.5 mg/km for all vehicles tested, two motorcycles and the moped were close to the SPN limit for passenger cars and four motorcycles exceeded the limit by a factor of up to four [13]. Even though the repeatability was good (about 10% deviation from the mean), steady state tests with the moped showed big differences between the tailpipe and the dilution tunnel sampling points for sub-23 nm particles. Detailed studies showed that the di fferences originated mainly from the growth of pre-existing particles with sizes even below 4 nm to the measuring size range of the instruments; however, particles grew due to condensation of desorbed material from the walls of the transfer tube [14,15].

The previous studies sugges<sup>t</sup> that the current legislative SPN methodology cannot be directly extended to L-category vehicles. On the one hand, the mean particle size is close to or lower than the lower size of the current methodology (23 nm) [10], on the other hand sub-23 nm artefacts are more probable with this category of vehicles than with light-duty or heavy-duty category vehicles [16]. Some of the existing facilities are used for both motorcycles and light-duty vehicles and consequently might not be optimized for mopeds. The low exhaust flow rate may lead to high residence times in the transfer lines and consequently high particle losses. Extraction of a sample from the tailpipe for analysis (e.g., real time gas analyzers) (sometimes the term bleed o ff is used) at the moment is not taken into account in any automation system for SPN emissions (neither in the light-duty regulation). Although this e ffect might not be important for light-duty vehicles, for mopeds it may result in a significant error. The recently introduced RDE regulations require measurements from the tailpipe. The equivalency of the tailpipe and dilution tunnel locations has been investigated for heavy-duty engines [17] and for light duty vehicles [18], but not for L-category vehicles. The few studies that did some preliminary investigations found big di fferences both in size distributions and number concentrations [19]. However, there are no studies that have quantified these di fferences. Thus, there is a need to summarize the open issues for L-category vehicles, especially for particle number measurements, and quantify the measurement uncertainties.

In the International vocabulary of metrology (VIM) [20], Type A evaluation of uncertainty is defined as the method of evaluation of uncertainty by the statistical analysis of series of observations. Repetitions of a test and statistical analysis are classified as Type A uncertainty. Type B evaluation of uncertainty is defined as the method of evaluation of uncertainty by means other than the statistical analysis of series of observations, for instance, calibration reports and guides. This approach was recently followed to estimate the uncertainty of PEMS (portable emissions measurement systems) [21]. In this study we followed a Type B analysis where we tried to collect all parameters that can contribute or influence the final result and quantified them based on theoretical or experimental data. We started from the equations that are used to calculate emissions both at the tailpipe and the dilution tunnel and then discussed possible errors and inaccuracies for each parameter. Even though we did two to three repetitions of each test condition, and thus Type A uncertainty was available, we did not cover the calculation of the combined standard uncertainty and expanded uncertainty, as foreseen in the Guide to the Expression of Uncertainty in Measurement (GUM) [22]. The objective was to identify and quantify the sources of uncertainty/errors that repetitions cannot identify (e.g., tests can be repeatable but erroneous). The focus was on SPN emissions. Gas analyzers and SPN instruments with various cut-o ff sizes were used to cover current and future regulations. A moped in a facility for motorcycles and passenger cars was chosen as the worst case scenario due to its small weight and engine capacity and the necessity for long transfer lines. Finally, recommendations for appropriate setups and procedures were given.

#### **2. Materials and Methods**

## *2.1. Experimental Setup*

The experimental measurements were performed at the Vehicle Emissions Laboratory (VELA 1) of the European Commission's Joint Research Centre. The facility is an emission test cell for L-category vehicles (such as mopeds and motorcycles) and light-duty vehicles (such as small passenger cars). Herein, we consider emission-test measurements performed with a Euro 4 moped with a 50 cm<sup>3</sup> 4-stroke engine with a three-way catalyst (>1000 km at the odometer). As shown in Figure 1, which presents schematically the experimental setup, the tailpipe of the moped was connected to a 6 m

stainless steel transfer tube. The first section of the transfer tube, the first 4 m, was flexible and heated (heater set to 80 ◦C), while the second section, the last 2 m, was fixed, insulated, and kept in a climatic room at 20–24 ◦C. The moped and the first 4 m of the transfer line were inside a different climatic chamber kept at 21–25 ◦C. The whole exhaust gas was diluted in the dilution tunnel with constant volume sampling (CVS) set to 3.5 m<sup>3</sup>/min. With this flow rate, the under-pressure at the tailpipe of the moped is small, and similarly its influence on emissions [23]. The dilution tunnel and the CVS were in the climatic room kept at 20–24 ◦C.

**Figure 1.** Experimental setup. Gases and SPN were sampled with separate probes at the tailpipe or the dilution tunnel. CPC = Condensation Particle Counter; ET = Evaporation Tube; HC = Hydrocarbons; PND = Particle Number Diluter; SPN = Solid Particle Number.

For gaseous pollutants, two identical AMA i60 (AVL, Graz, Austria) benches were used to measure dry CO and CO2 with non-dispersive infrared detectors, NOx with chemiluminescence detectors, and total hydrocarbons (HC) with flame ionization detectors. One system was connected to the tailpipe and the other to the full dilution tunnel with CVS.

The particle measurement systems we used in this study were AVL particle counters (APC 489, Graz, Austria) [24], which consisted of the following parts: A hot diluter at 150 ◦C, an evaporation tube at 350 ◦C, a cold diluter with filtered ambient air at 20 ◦C, and three Condensation Particle Counters (CPCs). As shown in Figure 1, the CPCs had different 50% counting efficiencies: At 23 nm (model 3790 from TSI, USA), at 10 nm (model 3772 from TSI, USA), and at 4 nm (model 3752 from TSI, USA) [25,26]. Two such SPN measurement systems were used. They were identical and freshly calibrated. One (SPN #1) measured always at the tailpipe and the other (SPN #2) at the dilution tunnel (CVS). The SPN #1 was connected to the tailpipe with a stainless steel 0.5 m heated tube at 120 ◦C.

The transfer tube that conducts the exhaust flow from the moped tailpipe to the dilution tunnel was usually connected to the moped. For some tests, however, the connection was left open on the side normally connected to the moped (Figure 1, left inset). The tailpipe instruments were sampling at the moped tailpipe, before the opening. The CVS under-pressure sucked ambient air also from the opening in addition to that from the external line for main dilution. The dilution factor of this first dilution is not known; based on some steady state tests it is expected to be around 2:1 at the maximum speed and higher at lower speeds [14,19]. This configuration (open transfer tube) is allowed in the EU motorcycles/mopeds regulation, and it has been used previously [23]. We note, however, that the closed configuration is more common [7,27–32].

The test cycle was the Worldwide harmonized Motorcycle Test Cycle (WMTC) class 1 consisting of a cold engine start phase followed by the same phase in hot engine conditions for a total of 7.6 km and 45 km/h maximum speed [33].
