## 3 Results and Discussion  
### Characteristics of Gaseous Emissions from Vehicles Powered by Different Fuel Types  
Results obtained from chassis dynamometer tests performed as described in the previous section are depicted in Figure 2. A non-DPF-equipped diesel vehicle manufactured prior to the adoption of strengthened pollutant-emission regulations (and hence, considered old) was observed to emit higher concentrations of gaseous pollutants compared to modern vehicles. Except for NOx emissions from gasoline- and LPG-powered vehicles, the aggregate pollutant emission was observed to decline corresponding to an increase in average vehicle speed. The observed high emission volume during the NIER-01 drive cycle indicates that the functional behavior of after-treatment systems could not be realized until attainment of an appropriate working temperature to facilitate sufficient purification of gas emissions during low-speed operation [11; 12]. Investigation of NH\({}_{3}\) emission from vehicles is important from the viewpoint of estimating ambient air pollution in urban areas because gaseous NH\({}_{3}\) can act as a precursor, thereby facilitating SIA production via chemical reactions with NOx and SOx [13]. Figure 2f depicts the observed trends concerning NH\({}_{3}\) emission during chassis-dynamometer tests performed at different average speeds. In gasoline- and LPG-powered vehicles, NH\({}_{3}\) emission declines with an increase in average vehicle speed. The corresponding relationship can, therefore, be expressed in terms of an emission factor for calculating the total emission of on-road vehicles. Diesel vehicles, in general, emit low amounts of NH\({}_{3}\). That said, diesel vehicles equipped with a urea-based selective catalytic reduction (SCR) system tend to encounter a problem referred to as \"ammonia slip\" emission [14]. Emissions from such vehicles, however, were not investigated in this study. At present, in Korea, NH\({}_{3}\) emissions from large and extra-large passenger cars and trucks are regulated to values not exceeding 10 ppm using the engine driving test [1]. Although results obtained from chassis-dynamometer tests would, in general, differ from those obtained under actual on-road conditions, findings of this study, nonetheless, demonstrate that NH\({}_{3}\) emissions are possible from small- and medium-sized vehicles. Further investigations must be undertaken to determine which vehicles must be subject to additional regulations to prevent the formation of secondary pollutants due to NH\({}_{3}\) emission. Emission of sulfur oxides (SO\({}_{2}\), SO\({}_{3}\), and H\({}_{2}\)SO\({}_{4}\)) from vehicles is, in general, detrimental to the durability of catalyst devices. Additionally, sulfur oxides react with atmospheric NH\({}_{3}\) to produce ammonium sulfate ((NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\))--a secondary pollutant [15]. Most sulfur oxides are produced by the reaction of sulfur present within fuel with oxygen during combustion of fuel-air mixtures within automotive engines. Additionally, a recent study has revealed the conversion of SO\({}_{2}\) to SO\({}_{3}\) within high-temperature after-treatment devices [16]. In Korea, regulations concerning chemical composition of fuels mandate less than 10 ppm of sulfur within gasoline and diesel; the corresponding regulation for LPG mandates less than 40 ppm of sulfur. However, studies concerning actual sulfur-oxide emissions are relatively rare. Figure 2g depicts trends concerning SO\({}_{2}\) emissions when using different fuel types. As observed, SO\({}_{2}\)-emission rates vary in the range of 0.0001-0.0005 g km\({}^{-1}\) for gasoline vehicles, 0.0001-0.0009 g km\({}^{-1}\) for LPG vehicles, 0.0019-0.0096 g km\({}^{-1}\) for diesel vehicles, and 0.0041-0.03 g km\({}^{-1}\) for diesel vehicles without DPF.