## 3 Results and Discussion
### Emissions from diesel-powered Vehicles under Different Emission Regulations  
NOx emissions from diesel-powered engines mostly occur in the form of NO, a portion of which gets further oxidized to form NO\({}_{2}\) within DOCs prior to emission from the tailpipe [19]. Figure 5 depicts emission trends concerning diesel vehicles during the NIER drive-cycle performed in accordance with different emission regulations imposed over time. As can be observed, with the exception of a few measurements, the proportion of NO\({}_{2}\) within NOx emissions increases with an increase in vehicle speed. However, the magnitudes of these increases are relatively small. In EURO-3-compliant vehicles, NO\({}_{2}\) accounts for only 1-3% of the total NOx emissions. As regard EURO-6- and EURO-5-compliant vehicles, the corresponding proportions of NO\({}_{2}\) within NOx emissions lie in the range of 12-16% and 21-28%, respectively. This trend is similar to that reported in extant studies [3; 20; 21], thereby illustrating the conversion of NO to NO\({}_{2}\) in the presence of a catalyst and high-temperature conditions within after-treatment components.  
Figure 4: Results obtained from chassis dynamometer tests performed on gasoline vehicles incorporating different combustion technologies and operating at different speeds—(**a**) Trends concerning CO\({}_{2}\)-emission rate; (**b**) Trends concerning emission rates of CO, NOx, non-methane hydrocarbons (NMHC), CH\({}_{4}\), NH\({}_{3}\), and SO\({}_{2}\)  
EURO-6-compliant emission regulations on diesel vehicles are currently in effect in Korea. EURO 6 features stricter regulations on NOx and PM emissions compared to EURO [1]. Emissions from diesel vehicles manufactured in accordance with a given regulation standard are controlled via the use of improved combustion technology and after-treatment devices. Figure 6b depicts proportions of regulated, unregulated, and GHG gases within the non-CO\({}_{2}\) emissions from diesel-powered vehicles. CO\({}_{2}\) emissions from EURO-6-compliant vehicles can be observed to be lower as a result of mixed displacement. NOx, CO, and NMHC are dominant gas species emitted by EURO-3-compliant vehicles without DPFs. NOx constitutes 89% of the total non-CO\({}_{2}\) emissions from EURO-5- and EURO-6-compliant vehicles. Although emitted concentrations of gas species other than CO\({}_{2}\) have, over the years, been reduced owing to strengthened regulations, the proportions of NOx emitted from diesel-powered vehicles has remained relatively high. Increased concentration of NOx gases in the atmosphere may result in the production of hazardous O\({}_{3}\) gas in combination with VOCs (Volatile Organic Compounds) [22; 23]. Furthermore, NOx gases serve as precursor species that react with PMs and other gas species to produce secondary aerosols. Therefore, diesel vehicles that emit large amounts of NOx gases must be regulated to counter the adverse effects on human health as well as atmospheric pollution.  
Figure 5: Observed trends in the variation of NO\({}_{2}\)/NOx ratio and standard deviation concerning emissions from diesel vehicles as per different emission-regulation standards for different diesel-vehicle speeds.  
Figure 6: Results obtained from chassis-dynamometer tests performed on diesel vehicles complying with different emission-regulation standards and operating at different speeds. (**a**) Trends concerning CO\({}_{2}\)-emission rates; (**b**) Trends concerning emission rates of CO, NOx, NMHC, CH\({}_{4}\), NH\({}_{3}\), and SO\({}_{2}\) gases.