## 1 Introduction  
Diesel vehicles used to contribute significantly to the air pollution due to their high particulates (soot) and NO\({}_{\mathrm{x}}\) emissions [1]. The typical particle number size distribution consisted of the accumulation (soot) mode with a mean size around 50-90 nm [2, 3]. A smaller solid \"core\" mode was also detected in some engine operating points [4, 5]. The picture has changed completely in the last years because Diesel particulate filters (DPFs) and selective catalytic reduction (SCR) for NO\({}_{\mathrm{x}}\) systems have brought the soot and NO\({}_{\mathrm{x}}\) emission levels close to the ambient air levels. This is the outcome of the development of the more stringent emission levels over the years and the introduction of the real driving emissions (RDE) regulation [6]. Cold start and low ambient temperatures emissions can be still an issue for NO\({}_{\mathrm{x}}\), but not so much for particulate emissions [7, 8]. Nevertheless, emissions during active regenerations is still a topic of concern [9, 10]. The increase in the gaseous pollutants during active regenerations is taken into account in the regulationsby the so-called k\({}_{\rm i}\) factor, which considers the emissions during the regeneration and the distance between regenerations. For particulates, there is no such factor defined, and most importantly, while the increase for gaseous pollutants is of the order of some percentage units (<5%), for particulates, it can be orders of magnitude [11; 12]. Nevertheless, many studies have shown that the weighted emissions of solid particles larger than 23 nm remain below the limit of 6 \(\times\) 10\({}^{11}\)\(\#\)/km [12; 13]. However, this is not always the case [14] and will be even more challenging with the lower emission limits and lower cut-off size of 10 nm as planned for Euro 7 [6; 14; 15]. Even though they are not regulated, volatile particles, weighted or not, are exceeding the limit for solid particles [9; 10; 13; 16; 17; 18]. Note that the term \"volatile\" particles is used to characterize particles that form a nucleation mode upon cooling and dilution in the atmosphere or the sampling system, but are in the vapor phase at the tailpipe conditions (high exhaust gas temperature) [19]. Thus, the \"volatiles\" include nitrates, sulfates, and intermediate and semi-volatility compounds, while \"solids\" are soot and metals [19; 20]. There are health and environmental concerns for volatile particles. The water-soluble organic carbon generates reactive oxygen species [21; 22], and polycyclic aromatic hydrocarbons are toxic and carcinogenic [23]. Furthermore, the volatiles contribute to the formation of secondary aerosol, which is also associated with adverse health effect [24]. As volatile particles might contribute to adverse health effects and their number concentrations is measured in ambient air monitoring stations, their thorough characterization is necessary. This is not a simple task, however [19]. Many studies have shown that the sampling conditions (dilution ratio, dilution air temperature, etc.) have a significant impact on the measured volatile particle number concentration [25; 26]. Furthermore, there are no standardized protocol or studies comparing different measurement systems [19]. To make things worse, outgassing of the lines can also impact the results, making the pre-history and pre-conditioning of the vehicle and the facility important [27].  
The objective of this study is to determine the emissions of two diesel vehicles focusing on the regeneration events. Different sampling locations and instruments will give an insight on challenges and provide recommendations for future measurements especially for volatile particles.