## 4 Mitigation Measures for Road Traffic PM Emissions and Its Impact on Carbonaceous Emissions
### Non-Exhaust Emissions  
Direct emissions from brakes, tyres and road surface wear are the major sources of non-exhaust emission (NEE). While the exhaust emissions decrease continuously due to improved exhaust after-treatment and filter technologies, the NEE share is increasing because of rising vehicle mileage. According to the recent results of the European Environment Agency (EEA), a significant fraction of 33.9% and 26.7% of the road transport-related PM is assigned to automobile tyre and brake wear for PM\({}_{10}\) and PM\({}_{2.5}\), respectively. Further studies consider brake wear separately and report contributions of 21% to the traffic-related PM\({}_{10}\) in urban areas [217]. Brake wear was also reported to give 16%-55% by mass to total non-exhaust traffic-related PM\({}_{10}\) emissions in urban environments [218]. A complete review of NEE is provided in [PERSON] et al. [219]. The dedicated EEA guidebook chapter [220] covers emissions of particulate matter (PM) including BC which are due to road vehicle tyre and brake wear (NFR code 1.A.3.b.vi), and road surface wear (NFR code 1.A.3.b.vii), as depicted in Table 6. The focus is on primary particles--in other words, those particles emitted directly as a result of the wear of surfaces--and not those resulting from the resuspension of previously deposited material. Resuspension of dust road depends of the site climatology, traffic flow and meteorological parameters. As part of resuspended dust can be anthropogenic and biogenic in origin and remain difficult to disentangle from direct emissions of brakes, tyre and road wear, we must be cautious to avoid double counting. Consensus exists in the scientific literature that non-exhaust emissions are becoming the dominant source of PM from road traffic and that PM [219], and particularly PM\({}_{2.5}\). Despite these demonstrated negative externalities, non-exhaust emissions have been only tangentially addressed by public policies to date. Given the magnitude of the aggregate social costs they entail, and the fact that the transition to electric vehicles will not lead to significant reductions in non-exhaust emissions, policy makers should invest resources in determining how to optimally reduce them via targeted policy instruments [221]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Category** & **Euro Standard** & **PM\({}_{2.5}\) EF (mg km\({}^{-1}\))** & **BC/PM\({}_{2.5}\) (\%)** & **OM/BC (\%)** & **Uncertainties (\%)** \\ \hline \multirow{8}{*}{L-Categories **\({}^{**}\)**} & Conventional 2 stroke & 176 & 10 & 900 & 50 \\ \cline{2-5}  & Euro 1 2 stroke & 45 & 20 & 400 & 50 \\ \cline{2-5}  & Euro 2 2 stroke & 26 & 20 & 400 & 50 \\ \cline{2-5}  & Conventional 4 stroke & 14–176 & 15 & 560 & 50 \\ \cline{2-5}  & Euro 1 4 stroke & 14–40 & 25 & 300 & 50 \\ \cline{2-5}  & Euro 2 4 stroke & 3.5–7 & 25 & 300 & 50 \\ \cline{2-5}  & Euro 3 4 stroke & 0.96–4 & 25 & 250 & 50 \\ \hline \end{tabular}  
* Notes: The values originate from available data in the literature and engineering estimates of the effects of specific technologies (catalysts, DPEs, etc.) on emissions. The estimates are also based on the assumption that low-sulphur fuels (<50 ppm t. S) are used. Hence, the contribution of sulphate to PM emissions is generally low. In cases where advanced after-treatment is used (such as catalysed DPEs), then EC and OM does not add up to 100%. The remaining fraction is assumed to be ash, nitrates, sulphates, water and ammonium salts. ** L-categories are _Mopads and Motorbikes_.  
\end{table} 
Table 5: _Cont._According to [PERSON] et al. [222] (and references therein), tyre wear contribution to PM\({}_{10}\) was reported to be up to approx. 11 mass %. The highest absolute concentration of tyre wear in ambient air is reported to be 3.4 \(\mu\)g m\({}^{-3}\) (without mass contribution from road wear). The BC fraction is on average equal to 15% of TSP for tyre and brake wear in other recent studies [223; 224].