## 1 Introduction  
Soot is one of the main risk factors causing millions of deaths, ranked 6 th highest by the global burden of disease (GBD) analysis in 2018 from 2016 data [1]. On the same note, the World Health Organisation (WHO) in a report from 2016 [2] approximated the number of deaths that can be linked to particulate pollution close to 4.2 million yearly, with more than 90% of people exposed to dangerous quantities of PM2.5 (particulate matter <2.5 microns) worldwide. This is because soot is a well-known cause of diseases of the lungs and heart [3; 4] with the smallest, PM0.1, being the most harmful, as they can penetrate deeper and result in various conditions in multiple organs [3; 5; 6; 7]. In environmental terms, black carbon (which also includes soot) is one of the main global warming agents, third compared to CO\({}_{2}\) and CH\({}_{4}\)[8; 9]. This is due to its light absorption characteristic; it can heat its surroundings with a warming impact of about 450-1500 times higher than CO\({}_{2}\) per unit mass [10]. However, due to its short life in the atmosphere, black carbon emissions reduction can have more rapid benefits, slowing down climate change at a more significant rate [9; 10]. From simulation work, it was shown that eliminating fossil fuel organic matter and black carbon in the atmosphere can bring net warming down by 20-45% within only 3-5 years, while the same effect with CO\({}_{2}\) would be achieved by reducing it by a third and would take about 50-200 years [9; 11]. Transport alone is responsible for 26% of black carbon [10] emissions as soot is emitted from internal combustion engine vehicles (ICEVs) as a combustion by-product. Although the shift to electric vehicles is now imminent with ICEV sales banned or restricted by countries from 2025 onwards [12], these will continue to exist on the road without limitation in the used market. Another at least 25 years are expected with ICEVs around [13]. For this reason, together with soot's short atmospheric life, research to reduce soot emissions and understand the particles being emitted is greatly significant to progress in the climate change fight. Footnote 1: The authors are grateful to the anonymous referee for their comments. Footnote 2: The authors are grateful to the anonymous referee for their comments. Footnote 3: The authors are grateful to the anonymous referee for their comments. Footnote 4: The authors are grateful to the anonymous referee for their comments  
using new gasoline particulate filters (GPFs), which are the same concept as the diesel ones but with different design requirements. Current regulation, Euro 6, restricts only >23 nm particles not to be more than 6 \(\times\) 10\({}^{11}\)/km [27; 28]. This threshold is due to knowledge from past diesel studies and, above all, due to past measurement limitations, which could measure with a maximum detection efficiency of 50% at 23 nm and 90% at 41 nm (d50 = 23 nm and d90 = 41 nm) [29]. This limit is defined by the particle measurement protocol (PMP) and is known as SPN23 (solid particle number), based on the accuracy of particle number counters (PNCs) as reference measuring devices. Over the last years, as more evidence is showing more presence of SPN below 23 nm also for past technologies [28; 30; 31], new regulations are expected to get tighter with Euro 7. The Consortium for Ultra-low Vehicle Emissions (CLOVE), which is made up of emissions experts across Europe, has proposed for Euro 7 a limit of 1 \(\times\) 10\({}^{11}\)/km for particles >10 nm [13]. In Table 1, CLOVE propositions for Euro 7 are summarised. Eventually, the European Commission on 10 November 2022 proposed the limits [32] highlighted in the last row of the table, which may become effective from 2025. [PERSON] et al. [33] carried out a study comparing SPN10 to SPN23 emissions, which showed that with a 6 \(\times\) 10\({}^{11}\)/km limit, SPN10 would still accept most of the vehicles but with 1 \(\times\) 10\({}^{11}\)/km might become too stringent, affecting a large quantity of existing vehicles.