10 nm. This proposal has triggered discussions about situations where a large number of sub-23 nm particles appear. For example, engines fueled with compressed natural gas (CNG) are considered soot-free due to the low particulate matter mass they produce. However, many studies have shown that they are not SPN-free [12; 13; 14; 15], and most importantly, the particle concentration below 23 nm can be high [16; 17; 18; 19]. Diesel vehicles have the issue of sub-23 nm particles produced during urea injection [17]. Urea dosing is used to reduce NO\({}_{\text{x}}\) emissions, but in some cases, \"solid\" particles appear downstream of the selective catalytic reduction for the NO\({}_{\text{x}}\) system [20]. Studies have found higher emissions of about +1 \(\times\) 10\({}^{11}\) #/kWh by lowering the size from 23 nm to 10 nm [21; 22; 23]. These particles are believed to be ammonium salts [24; 25] or urea decomposition products (cyanuric acid and ammelide) [21; 26; 27; 28]. A recent study also found that different urea solutions can result in different increases in the SPN concentration for the same engine, and it was attributed to their different metallic element contents [29]. A different topic, but also of importance for the lower proposed limits, is the measurement uncertainty of SPN PEMS [30]. While a margin of 0.63 (i.e., uncertainty 63%) is applicable for Euro VI testing, the Euro 7 proposal limits do not have any additional margin, but an approximately 40% uncertainty has been considered. The SPN instruments consist of a diluter, a heated section at \(\geq\)300 \({}^{\circ}\)C, and a particle detector [8]. The combined systems must have a detection efficiency of around 40% at 23 nm. At approximately 50-100 nm, the efficiency has to be 100% (\(\pm\)30%), and at 200 nm, it is \(\leq\)200%. For 10 nm measurements, the heated section has to include a catalytic stripper to oxidize volatiles, and the efficiency of 50% is at 15 nm. At larger sizes (50-100 nm), the efficiency is 100% (\(\pm\)15%). Two technologies are used as particle detectors: condensation particle counters (CPCs) (e.g., [31]) or diffusion chargers (DCs) (e.g., [32]). CPCs, in general, are considered more accurate because they count each particle after it has grown to the micrometer range in a supersaturated vapor region. DCs charge the particles in a corona charger and then measure the charge of the particles (or the escaping charge) with an electrometer. The charging efficiency depends on the size of the particles [33], so there is some additional uncertainty when converting current to particle number concentration. There are also some concerns regarding the lower detection limit of DC-based PEMS for low-emitting vehicles. The assessment of PEMS for heavy-duty applications is limited [34], in particular for the 10 nm systems. The reason is that the 23 nm SPN PEMS testing has been required for a few years (2021), and the 10 nm SPN PEMS are not regulated yet. Thus, until recently, researchers have been using different techniques to measure on-board the vehicle not only for heavy-duty vehicles (e.g., [35; 36; 37; 38]) but also for light-duty ones (e.g., [39; 40]). While research studies with SPN PEMS exist (e.g., [41; 42]), the assessment of PEMS is limited. One of the few studies with heavy-duty engines and with three PEMS manufacturers found differences to the 10 nm reference instruments of 50% [22]. Studies with light-duty vehicles have found differences of 50% for four 10 nm instrument manufacturers and a wide range of engine technologies [43]. But higher differences have also been reported in the past. For example, 23 nm diffusion charger-based systems had differences of 70-100% [44; 45]. One of the reasons was the sensitivity of diffusion chargers to existing charges on the particles [46]. Pre-chargers or electrofilters were found to be efficient in neutralizing or removing these particles [46]. This technology has not been tested since then. There are also some other boundary conditions that need further testing.