Airborne particulate matter (PM) is one of the most relevant pollutants to human health, with both short- and long-term exposure linked to increased mortality ^[1][2][4,5]. A review that summarised European data from 2000 to 2012 showed that transport was a major contributor to urban air pollution, with 27% of PM originating from road transport in the cities ^[3][9] (the percentage was 20% for overall Europe). The stricter emission standards (the so-called Euro standards), along with fuel improvements and traffic management, decreased both primary and secondary aerosol contributions from vehicles ^[4][10]. In 2018 the road transport contribution to primary PM (overall Europe) was 11% ^[5][11].

Controlling the PM mass does not necessarily address the problem of ultrafine particles concentration in the air. The sources dominating the particle number emissions are different to those dominating the mass emissions ^[6][7][19,20]. The particle number concentration is highly affected by road transport and the mass by aged and transported aerosol ^[6][19]. In 2010, the major global particle number source was road transport (40%) ^[8][21]. In Europe, the percentage was 60%, ranging from 32% to 97% ^[9][22]. Another study with data from 2013 to 2016 showed that road transport was the main contributor in all cities examined, with a contribution up to 94% (annual average) ^[10][23]. The recent WHO (World Health Organization) Guidelines recommended the measurement of ambient ultrafine particles at monitoring stations and suggested that low particle number counts should be considered less than 1000 #/cm³ (as a 24-h mean), while typical levels observed in urban background areas typically exceed 10,000 #/cm^3 [11][24]. The non-volatile part is typically less than one-third ^[12][25].

The methodology to assess the emissions of a vehicle is traditionally based on the weight increase in a filter sampling diluted exhaust over a test ^[13][26]. The vehicle is fixed on a chassis dynamometer, and the driver follows a pre-defined speed profile (test cycle). The exhaust is diluted in a dilution tunnel, where various analysers determine the pollutants. A part of the diluted exhaust gas is extracted and passes through a filter. The quantity collected during the test determines the PM mass emissions. With the introduction of particulate filters at the exhaust aftertreatment of diesel vehicles early 2000, the collected mass on the filter reached values close to the background levels. For this reason, the PMP (Particle Measurement Programme) group was tasked by the UNECE (Unite Nations Economic Commission for Europe) with developing a complementary method to PM mass with better sensitivity at low emission levels. The JRC (Joint Research Centre) of the European Commission actively participated in the PMP, providing data and drafting the relevant regulations since almost the beginning. The candidate method that was finally selected was based on counting solid particles with diameter > 23 nm ^[14][27]. The choice of not measuring smaller particles, which are very sensitive to the sampling conditions, was based on technical reasons (better accuracy, repeatability, acceptable cost) rather than on health reasons. Additionally, the limits were selected on the basis of what was achievable with the best available technology (wall flow particulate filters) and not on health-related considerations. The reason for this choice was based on the fact that, while a clear relationship between adverse effects and exposure to PM had been already demonstrated, there was a lack of robust epidemiological studies on the health effects of ultrafine particles levels ^[15][28]. The solid particle number (SPN) methodology has been included in the regulation of many regions worldwide.

2. Current SPN Regulations

2.1. Light-Duty Vehicles

In the EU (European Union), a SPN limit of 6×10¹¹ #/km applied for the first time in 2011 (Euro 5b) for diesel vehicles (Regulation (EC) 692/2008). Regulation (EU) 459/2012 introduced the same limit for GDIs in 2014 (Euro 6), with the possibility of a 10 times higher limit for the first 3 years, upon request of the vehicle manufacturer. Regulation (EU) 2017/1151 (Euro 6c) repealed Regulation (EC) 692/2008 and introduced the worldwide harmonized light vehicles test procedure (WLTP). Real-Driving Emissions (RDE) testing with portable emissions measurement systems (PEMS) was introduced in 2017 (Euro 6d-Temp) initially only for type approval (Regulations (EU) 2016/427, 2016/646) with a conformity factor (CF) of 1.5 for particle number (Regulation (EU) 2017/1154) and later for in-service conformity (ISC) (Regulation (EU) 2018/1832). The CF for on-road SPN with PEMS remained the same for Euro 6d that entered into force in 2020. All RDE regulations were consolidated in Regulation (EU) 2017/1151. Regulation (EU) 2018/858 replaced Directive 2007/46/EC and introduced a new EU type-approval framework (from September 2020), with an effective market surveillance system to control the conformity of vehicles already in circulation (from September 2019).

2.2. Heavy-Duty Vehicles

In the EU, SPN limits were initially applied to compression ignition (CI) (diesel) engines in 2013 (Euro VI) (Regulation (EU) 582/2011), and in 2014 to positive ignition (PI) engines (Regulation (EU) 133/2014). The limit is 6×10¹¹ #/kWh for the combined (14% cold, 86% hot) worldwide harmonized transient cycle (WHTC), and 8×10¹¹ #/kWh for the worldwide harmonized stationary cycle (WHSC) (only diesel engines). The methodology (Regulation (EU) 64/2012) refers to UN Regulation 49.

Regulation (EU) 582/2011 introduced on-road PEMS testing for ISC (instead of removing the engine from a vehicle and testing it on the engine dynamometer) for the gaseous pollutants. Regulation (EU) 2019/1939 (Euro VI step E) added the cold start in the evaluation of the emissions and SPN with a CF=1.63 with PEMS from 2021 for CI engines and 2023 for PI engines.

2.3. Non-Road Mobile Machinery

In the EU, Regulation (EU) 2016/1628 repealed Directive 97/68/EC in 2016 and introduced SPN limits to non-road engines (19–560 kW), inland waterway vessels (> 300 kW), and rail traction engines in 2019 (Stage V). The procedures and test cycles (non-road transient cycle – NRTC and non-road stationary cycle – NRSC) are described in Regulation (EU) 2017/654 and 2017/655 for on-road monitoring. At the moment, there is no particle number limit for on-road testing with PEMS.

2.4. Worldwide

Worldwide, many countries in Asia have introduced SPN limits in the laboratory (6×10¹¹ #/km for light-duty vehicles, 6×10¹¹ #/kWh for heavy-duty engines) in the last years (e.g. Singapore, Korea, China, India). On-road testing will be required from 2023 in India (CF to be defined) and China (CF=2.0-2.1). Korea has limits only for diesel vehicles, India includes also gasoline vehicles with direct injection engine, while China all technologies (with China 6).

3. Future SPN Regulations

3.1. Type Approval and In-Service Conformity

The improved SPN methodologies (e.g. possibility of having a catalytic stripper at 23 nm systems, or measuring particles from 10 nm) have been included in the latest amendments to the global technical regulation GTR 15 (light-duty). There is also a Consolidate Resolution ready for heavy-duty regulations which, in addition to the improved SPN methodologies, expands the SPN methodology for sampling directly from the tailpipe.

The PEMS methodology counting from 10 nm will be included in the future in the GTR RDE light-duty regulation. It is expected to have the same requirements for heavy-duty testing, but at the moment, there is no GTR or UN Regulation foreseen for on-road heavy-duty vehicles testing. There is no official announcement regarding whether other countries will introduce the 10 nm methodologies (laboratory or on-road) in their regulations.

Since 2018, the Commission has started the development of a future-proof light-duty and heavy-duty emissions regulation, i.e., the Euro 7/VI. During the process of consultation, various organisations have commented on some aspects of the current EU regulation and/or had suggestions for the future regulations ^[16][17][18][66,67,68]. It has been suggested to  (i) lower the 23 nm to 10 nm and (ii) include all technologies in the limits without exceptions (iii) include all emission events and sources (e.g. regeneration, urea particles, crankcase ventilation emissions) (iv) cover all normal conditions of use (cold start, low temperatures, use of auxiliaries).

Currently there is no SPN limit for L-category vehicles (i.e. mopeds and motorcycles). Studies for L-category vehicles showed that measuring only >23 nm misses a large part of the emitted particles ^[19][85]. On the other hand, due to high exhaust gas temperatures, measurements of particles >10 nm were subject to artefacts ^[20][86]. Recent studies showed that this can be avoided with the appropriate system and sampling configuration ^[21][22][87,88]. It is important to assess the emission levels of the recently introduced Euro 5 L-category vehicles compared to the future Euro 7 light-duty vehicles ^[23][89].

3.2. Periodical Technical Inspection

The previous discussion focused on methodologies and instrumentation for type-approval, in-service conformity (in-use compliance), and market surveillance purposes, which fall under the responsibility of the OEMs (original equipment manufacturers) and the respective authorities. This paragraph discusses the periodic inspection of vehicles under the responsibility of each car owner. Traditionally, this test measured the exhaust opacity during unloaded acceleration from idle to a high engine speed ^[13][26]. Opacity is not sensitive enough for DPF (diesel particulate filter)-equipped vehicles, and other methods have been investigated over the years. The better sensitivity of diffusion chargers (DCs) and condensation particle counters (CPCs) led to the introduction of simplified SPN methodologies for the vehicles’ inspection ^[24][90].

The first to introduce the particle number in inspection was Switzerland for construction machinery. For light-duty and heavy-duty vehicles, the Netherlands and Belgium will also introduce a simplified SPN methodology in July 2022, and Germany in January 2023. The methodology prescribes a measurement at idle with a sensor that detects solid particles >23 nm. The technical specifications of the instruments were based on the technical specifications of the instruments for construction machinery in Switzerland and the on-board systems of the European real-driving emissions (RDE) regulation (i.e., PEMS). The work was conducted by the informal new periodic technical inspection (NPTI) technical working group consisting of Swiss, German, and Dutch governmental organisations, the VERT (Verification of Emission Reduction Technologies) association, metrological institutes, scientists, and equipment manufacturers. The JRC of the European Commission is also working on the topic to prepare common recommendations for particle number instrumentation and procedures for PTI purposes. Details about the simplified methodology and the specifications can be found elsewhere ^[25][91].

3.3. Brake Emissions

Due to the low exhaust emissions from modern vehicles, the contribution of traffic-related non-exhaust emissions to air pollution has become relatively more important and is likely to increase ^[26][27][92,93]. In June 2021, GRPE mandated the PMP group to develop a GTR on brake emissions from light-duty vehicles. The draft is expected to be ready in June 2022, with most probable adoption in January 2023 at GRPE. The sampling methodology and the instruments’ technical specifications are already under development. In addition to mass, a particle number limit of total particles (i.e., including volatiles) >10 nm is expected.