## 2 General Context
### Policy Context
For PM\({}_{10}\), there are two limit values set to 40 and 50 \(\upmu\)g m\({}^{-3}\)--annual and daily bases. PAH concentrationsare also regulated [28]. In contrast, no limit values are set for BC or OC, but there is an obligation to measure these substances in rural areas to ensure that adequate information is available on PM\({}_{2.5}\) concentration and composition in the background. This information is essential to judge the enhanced levels in more polluted areas (such as urban background, industry related locations and traffic-related locations), assess the possible contribution from long-range transport of air pollutants, support source apportionment analysis and for use in modelling in urban areas to further understand specific pollutants such as particulate matter. So far, no limit values have been set for PN concentrations in ambient air. Road traffic remains a major emitter of primary carbonaceous species [29] although, at a global scale, primary PM anthropogenic emissions are mainly driven by residential combustion [30] and wildfire (Table 1). In the EU, the so-called \"Euro\" regulations [37; 38; 39] are the legal framework to fix emission limit values (ELVs) for all types of vehicles. There are no specific standards on carbonaceous PM emissions in Euro regulations. However, carbonaceous species are a major constituent of PM particles emitted at vehicles exhaust (see Section 2.2). As summarised in Table 2, the last Euro 6 regulation includes ELVs for PM in mg km\({}^{-1}\) and also for particle numbers (PN) in # km\({}^{-1}\) for both passenger cars and light commercial vehicles [17]. BC emissions are among the core regulatory policy issues in all engine manufacturers, and they have not yet been adequately addressed by policy makers [40]. The way emissions are determined depends on the driving cycle used for emission measurements, as described in [41; 42] for NOx emissions several years before the \"Dieselgate\" scandal [43]. For diesel vehicles, there was a clear divergence between real emissions and homologation limits values for NOx emissions. Europe has taken a leading role in introducing particle number (PN) limits, effectively forcing the introduction of filters on diesel power train exhausts [44]. These provisions have been mirrored in the Chinese standards (Figure 1). The U.S. Tier 3 (US-LEV-III) standard, which will be phased in by 2025, sets a PM limit of approximately 1.9 mg km\({}^{-1}\) over the federal test procedure (FTP) cycle (Figure 1). Although no PN limit exists in the United States, California's LEV III standards set a PM limit of approximately 0.6 mg km\({}^{-1}\) over the same cycle, which is deemed comparable in stringency to Europe and China's PN limits. This limit will be fully phased in by 2028. Regarding heavy-duty vehicles (HDV), for instance the Chinese Ministry of Ecology and Environment (MEE) released in July 2018 the final rule for the China VI emission standard for HDV, hereafter referred to as China VI. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **PM\({}_{10}\)** & **PM\({}_{2.5}\)** & **PM\({}_{1}\)** & **BC** & **OC** & **OM** \\ \hline _Agriculture_ & 6555 & 3848 & 2883 & 337 & 1313 & 2364 \\ _Residential combustion_ & 23,078 & 21,857 & 20,742 & 4163 & 8852 & 15,329 \\ _Industrial processes_ & 12,162 & 8340 & 4135 & 462 & 633 & 823 \\ _Large-scale combustion_ & 11,561 & 6420 & 3812 & 136 & 164 & 248 \\ _Oil and gas, mining_ & 1706 & 571 & 412 & 226 & 93 & 120 \\ _Transport—road_ & 3339 & 2925 & 2524 & 1349 & 1116 & 1451 \\ _Transport—non-road_ & 861 & 823 & 795 & 363 & 217 & 283 \\ _Waste_ & 1388 & 1272 & 876 & 97 & 751 & 977 \\ _International shipping_ & 1856 & 1758 & 1612 & 120 & 398 & 517 \\ _International aviation (a)_ & 30 & 30 & 28 & 10 & 10 & 13 \\ _Global anthropogenic_ & 62,537 & 47,843 & 37,819 & 7264 & 13,548 & 22,125 \\ _Forest and savannah fires (b)_ & 48,207 & 33,014 & 33,014 & 2268 & 19,489 & 31,363 \\ _Global total_ & 110,744 & 80,858 & 70,834 & 9532 & 33,037 & 53,489 \\ \hline \hline \end{tabular}
\end{table}