## 2 Methods
### Gaseous-Component Analysis  
During chassis-dynamometer tests, exhaust gases from test-vehicle tailpipes were directly sampled using a constant-volume sampler (CVS) tunnel (CVS flow rate: 20 m\({}^{3}\) min\({}^{-1}\)). Gases collected within the CVS tunnel were diluted by the factor of \(\sim\)10-95 using purified dilution air treated by activated carbon and passed through a high-efficiency particle (HEPA) filter. Subsequently, diluted gases within the CVS tunnel were drawn into Tedlar bags, and gas samples were analyzed to determine average concentrations of target pollutant gas species through use of a flame ionization detection analyzer (HORIBA; FIA-720) for total hydrocarbons (THCs), chemiluminescence detection analyzer (HORIBA; CLA-720) for NOx, non-dispersive infrared analyzer (HORIBA; AIA-721A and AIA-722) for CO and CO\({}_{2}\), and gas chromatography flame ionization detection analyzer (HORIBA; GFA-720) for CH\({}_{4}\). NMHC concentration within exhaust gases sampled from diesel-powered vehicles was defined as THC minus CH\({}_{4}\), whereas NMOG content within gases sampled from gasoline- and LPG-powered vehicles was defined as 1.04 times the NMHC concentration [1]. After removal of hydrocarbons from gas samples collected from the CVS tunnel, SO\({}_{2}\) concentration within the remaining gas was measured using a pulsed fluorescence detection analyzer (Thermo Scientific; 43C). NH\({}_{3}\), NO, and NO  
Figure 1: Speed profiles concerning test cycles considered in this study.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Cycle** & **Total Duration (s)** & **Distance (km)** & **Average Speed (km h\({}^{-1}\))** & **Maximum Speed (km h\({}^{-1}\))** \\ \hline NIER \({}^{-}\)01 & 873 & 1.1 & 4.7 & 33.4 \\ NIER-05 & 864 & 4.2 & 17.3 & 68.1 \\ NIER-09 & 926 & 8.8 & 34.1 & 70.9 \\ NIER-12 & 840 & 15.3 & 65.4 & 83.3 \\ \hline \hline \end{tabular} \({}^{a}\) NIER: National Institute of Environmental Research.  
\end{table}
Table 3: Specifications of test cycles considered in this study.  
concentrations were monitored via quantum cascade laser spectroscopy (HORIBA; MEXA-1400 QL-NC). NH\({}_{3}\) was directly extracted from the tailpipe using heated tubing (at 115 \({}^{\circ}\)C). This is because gaseous NH\({}_{3}\) demonstrates strong adsorption on the walls of the dilution tunnel, thereby inducing errors in measurement of its actual concentration. Measurements concerning NO and NO\({}_{2}\) concentrations within exhaust gas samples were also similarly performed to determine the ratio of NO\({}_{2}\) to NOx within undiluted vehicle exhaust. Emission rate from CVS tunnel was quantified using Equation (1):  
\[\text{Emission rate, g km${}^{-1}$ from CVS tunnel}\ =\ \frac{\text{V}_{\text{mix}} \times\rho_{\text{i}}\times\text{C}_{\text{i}(\text{corrected})}}{\text{d}} \tag{1}\]  
where V\({}_{\text{mix}}\) is total volume (m\({}^{3}\)) in the CVS tunnel during durations of running cycle, \(\rho_{\text{i}}\) is the density of gases (kg m\({}^{-3}\)), and C\({}_{\text{i}(\text{corrected})}\) is the concentration of exhaust gases which is corrected for the CVS tunnel background. d is the total driving distance (km) of each cycle. The direct emission rate was calculated using the following Equation (2):  
\[\text{Emission rate, g km${}^{-1}$ from direct measurement}\ =\ \frac{\text{C}_{\text{i}(\text{corrected})} \times\rho_{\text{i}}\times\text{flow}_{\text{(cvs tunnel)}}}{\text{Speed}_{ \text{(avg)}}\times\text{DF}} \tag{2}\]  
where C\({}_{\text{i}(\text{corrected})}\) is the concentration which is corrected by the instrument blank periodically, flow(cvs tunnel) is the CVS flow rate (m\({}^{3}\) min\({}^{-1}\)) during cycles. Speed(avg) is the average speed (km h\({}^{-1}\)) of each cycle, and DF is the dilution factor reported from the measurement of the CVS tunnel system.