Experimental Assessment of an Electrofilter and a Tandem Positive-Negative Corona Charger for the Measurement of Charged Nanoparticles formed in Selective Catalytic Reduction Systems  
[PERSON]  
1 Institute for Internal Combustion Engines and Thermodynamics, Graz University of Technology, 8010 Graz, Austria; [EMAIL_ADDRESS]  
AVL GmbH; 8020 Graz, Austria; [EMAIL_ADDRESS]  
[PERSON]  
2 AVL GmbH; 8020 Graz, Austria; [EMAIL_ADDRESS]  
[PERSON]  
3 Naneos Particle Solutions GmbH, 5210 Windisch, Switzerland; [EMAIL_ADDRESS]  
[PERSON]  
4 European Commission, Joint Research Centre, 21027 Ispra, Italy  
4 Correspondence: [EMAIL_ADDRESS]; Tel. : +39-0332-78-5312  
and reduced visibility and is recognized as an important global risk factor for disease [2]. Vehicle PM emissions have been addressed with very low PM mass limits in the United States of America (USA) and the introduction of the non-volatile particle number (PN) method in the European Union (EU). This method counts non-volatile (sometimes called solid) particles, defined as those surviving a thermal pre-treatment stage at 350 \({}^{\circ}\)C, with a diameter greater than 23 nm [3]. The limit value is 6 \(\times\) 10\({}^{11}\) particles per km (#/km) for passenger cars or per kWh (#/kWh) for heavy duty (HD) engines. At the same time, there is an on-going discussion about reducing the lower detection size to 10 nm in order to cover engines that have a high fraction of sub-23 nm particles (e.g., some spark ignition vehicles) [4]. Recently, a PN limit was introduced for in-service conformity testing of light-duty vehicles (2017) in the EU and will be introduced for heavy-duty vehicles in the next regulatory amendment Euro Vle (2021). Owing to the challenges imposed by onboard measurements, including power consumption, weight and harsh operating conditions, the specifications for onboard sensors are relaxed. While the reference laboratory procedure does not explicitly specify a sensor principle, the specifications necessitate the use of a full flow condensation particle counter (CPC). A CPC optically counts the particles after their growth in a condensation section. The regulation requires that the CPCs have a detection efficiency of 50% \(\pm\) 12% at 23 nm and above 90% at 41 nm [3,5]. PN-PEMS systems can use any type of CPCs or diffusion charger (DC) based detectors to count particles. The key element of a DC-based detector is a unipolar charger that electrically charges the particles. The charging efficiency follows a power-law dependence on particle diameter with an exponent that is typically around 1.1-1.6 [6]. The EU regulation 2017/1154 [7] requires a much tighter size dependence, thus necessitating the use of advanced DC-based sensor concepts to reduce the size influence [8,9]. Measurements with light duty exhaust [10] suggested equivalent performance between PN-PEMS instrumentation utilizing such advanced DC sensors and CPCs. Differences of either PN-PEMS system was less than \(\sim\)\(\pm\)35% from laboratory-grade PN instrumentation when all sampling from tailpipe [11]. Subsequent measurements with HD vehicles and engines equipped with selective catalytic reduction (SCR) systems, however, revealed quite high differences (>50%) between advanced DC and CPC based PN PEMS systems [12,13]. Follow-up investigations [14] indicated the particles forming following urea injection in the SCR as a possible reason for the observed differences. The exact chemistry and formation mechanism of these particles is not well understood. It has been suggested that these are ammonium sulfates and bisulfates [15,16] or byproducts of the ammonium reduction process (i.e., ammelides, ammelines) [14]. These produced particles were found to be non-volatile (in the sense that they survive thermodilution at 350 \({}^{\circ}\)C) exhibiting a size distribution that peaks below 20 nm, i.e., below the regulatory limit of 23 nm.