## 4 Mitigation Measures for Road Traffic PM Emissions and Its Impact on Carbonaceous Emissions
### Impact of After-Treatment Systems on Particulate Matter Emissions
Besides catalyst type, space velocity of exhaust gas, and emission form are the other parameters affecting the efficiency. With the after-treatment emission control systems, it is possible to reduce the damage of the pollutant emissions on air pollution, to meet emission standards and requirements, and to prevent the harmful effects of pollutant emissions on environment and human health [236]. A recent study [237] was performed focussing on (i) ultrafine particles, BC, BTEX, PAHs, carbonyl compounds, and NO\({}_{2}\) emissions from Euro 4 and Euro 5 diesel and gasoline passenger cars, (ii) the effect of driving conditions (e.g., cold start, urban, rural andmotorway conditions), and (iii) the impact of additive and catalysed DPF devices on vehicle emissions. The results showed that compared to hot-start cycles, cold-start urban cycles increased all pollutant emissions by 2 fold. The sole exception was NO\({}_{2}\), which was reduced 1.3-6 fold. Particulate and BC emissions from the gasoline engines were significantly higher than those from diesel engines equipped with DPF. Moreover, the catalysed DPF emitted approximately 3-10-fold more carbonyl compounds and particles than additive DPF, respectively, during urban driving cycles, while the additive DPF vehicles emitted 2- and 5-fold more BTEX and carbonyl compounds during motorway driving cycles. Regarding particle number distribution, the motorway driving cycle induced the emission of particles smaller in diameter (mode at 15 nm) than the urban cold-start cycle (mode at 80-100 nm). The results showed a clear positive correlation between particle, BC, and BTEX emissions, and a negative correlation between particles and NO\({}_{2}\). Additionally, there is a clear correlation between BC emissions and PN or PM\({}_{10}\) emissions. The study showed a significant decrease in aldehyde emissions compared to other studies such as [238,239]. The aldehyde emissions from the gasoline vehicles decreased from 12 mg km\({}^{-1}\) for the Euro 1 vehicles to 1 mg km\({}^{-1}\) for the Euro 4 vehicles. The aldehyde emissions from the diesel vehicles decreased from 30 mg km\({}^{-1}\) for the pre-Euro vehicles to 0.5 mg km\({}^{-1}\) for the Euro 4 vehicles. A significant decrease in PAH emissions is observed compared to [239,240]. PAH diesel emissions decreased from 17 \(\upmu\)g km\({}^{-1}\) for pre-Euro vehicles to 1 \(\upmu\)g km\({}^{-1}\) for Euro 3 vehicles and remained below 0.2 \(\upmu\)g km\({}^{-1}\) for Euro 5 vehicles. PAH gasoline emissions were quite low (below 8 mg km\({}^{-1}\)) and close to the detection limit. To investigate the effects of after-treatment systems on emission of pollutants, a recent study [241] characterised the chemical composition of particles emitted from three diesel and four gasoline Euro 5 light-duty vehicles on a chassis dynamometer facility. BC was the dominant emitted species with emission factors (EFs) varying from 0.2 to 7.1 mg km\({}^{-1}\) for gasoline cars and 0.003 to 0.08 mg km\({}^{-1}\) for diesel cars. For gasoline cars, the OM EFs varied from 5 to 103 \(\upmu\)g km\({}^{-1}\) for direct-injection (GDI) vehicles, and from 1 to 8 \(\upmu\)g km\({}^{-1}\) for port fuel injection (PFI) vehicles, while for the diesel cars it ranged between 0.15 and 65 \(\upmu\)g km\({}^{-1}\). PAH emissions from the GDI technology were 4-fold higher compared to the vehicles equipped with a PFI system during the cold start cycle, while the nitro-PAHs fraction was 25-fold much more appreciable in the GDI emissions. For two of the three diesel vehicles, the PAH emissions were close to the detection limit, but for one, which presented an after-treatment device failure, the average PAH EF was 2.04 \(\upmu\)g km\({}^{-1}\). Emissions of nanoparticles (below 30 nm), mainly composed by ammonium bisulphate, were measured during the passive regeneration of the catalysed diesel particulate filter (CDPF) vehicle.