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The study of PM chemical composition from two types (GDI and PFI) were conducted at Transportation Pollution Research Center (TPRC) of the National Institute of Environmental Research (NIER), which is a certifying institute for vehicle exhaust emissions in Korea. All tests were performed in accordance with the World Forum for Harmonization of Vehicle Regulations, an international standard method used by the United Nations Economic Commission for Europe (UNECE).

The High Resolution Time of Flight Aerosol Mass Spectrometer (Hereafter, HR-ToF-AMS) contains a high-resolution time-of-flight mass spectrometer that can characterize the elemental composition of the organic carbon content of the PM₁. The specific chemical breakdown of the exhaust products provides more insight on the health and environmental effects due to the combustion of different fuels. The exhaust sample passed through diffusion drier and a critical orifice, restricting the flow to 0.1 LPM, before entering the HR-ToF-AMS. Diffusion dryers were used to control sample humidity (<40% RH) (Fig. 1), reducing uncertainties due to bounce-related changes in collection efficiency and reduced particle transmission through the aerodynamic lens. The sample air enters the HR-ToF-AMS through a critical orifice (100 μm pin hole) into an aerodynamic lens, creating a narrow particle beam. The particles are accelerated in the supersonic expansion of gas molecules into vacuum at the end of the aerodynamic lens. Particle packets are selected by chopper for separation as a function of size in the PToF region. Non-refractory particles are vaporized, and the fragments are then ionized and sent through the ToFMS region. The ion flight time to the detector corresponds to a specific mass-to-charge ratio, m/z. The operation of the Aerodyne HR-ToF-AMS has been described in detail elsewhere (DeCarlo et al., 2006; Drewnick et al., 2005; Jimenez et al., 2003; Jayne et al., 2000).

Fig. 1. 
Schematic diagram of experiment setup using HR-ToF-AMS.

The HR-ToF-AMS was calibrated for nitrate ionization efficiency (IE) through introduction of 350 nm ammonium nitrate particles (typical value IE_NO3 ~1.53e-07 ions/molecule). Relative ionization efficiencies (RIEs) for other aerosol types were taken from published values (note that RIE of NH₄ of 4.5 was measured during nitrate ionization efficiency). The timeline of Composition-Dependent Collection Efficiency (CDCE) was calculated based on HR-ToF-AMS chemical composition and experiments under low humidity conditions and was used for the AMS quantitative analysis (Middlebrook, 2011). The measured CO₂ concentration of gas-phase from dilution tunnel in HORIBA was used to correct CO₂ concentration every HR-ToF-AMS data point.

Primary data processing will be started with integrating the results of all calibrations, logged events, and background data (air beam, etc.) with raw data to produce the initial unit mass resolution (UMR) data; m/z calibrations, baseline signal, and single ion signal areas are also verified. Primary data processing uses the SQUIRREL program (v1.56) in Igor Pro 6 (WaveMetrics, Lake Oswego, OR), and products include species timelines, average diurnal variations, and the spectra that comprise the PMF input. The secondary data processing addresses error in high-resolution peak shape and assigns fragments to species families using the IGOR-based PIKA program (v1.15). High-resolution mass spectrometry allows discernment between fragments that share the same UMR bin as described earlier.

We tested direct emission from two types of gasoline vehicles (PFI and GDI) using HR-ToF-AMS. The HR-ToF-AMS was operated under V-mode only. In this study, 20 sec time resolution was used for determining particle composition, including concentrations of nitrate, sulfate, ammonium, and organic matter for the different speeds of GDI and PFI vehicles (10, 3, 80, and 110 km/h). Fig. 1 showed the schematic diagram of experiment setup using HR-ToF-AMS.

Fig. 2 showed the emission rates of organic and inorganic compounds for two types of vehicles. The emission rates were calculated from eq. (1).

Fig. 2. 
Mean emission rates for organics and inorganics components (nitrate, sulfate and ammonium) from PFI and GDI vehicles.

Where, C and Q denoted mass concentrations of chemical species (μg/m³) for PM measured by HR-ToF-AMS and flow rate (20 m³/min) of dilution tunnel (HORIBA), respectively. S and DF indicated the speed of vehicles (km/h) and dilution factor (unitless quantity). Dilution factor (DF) was obtained from dilution tunnel in HORIBA to calculate the total concentations of organic and inorganic aerosol emitted from vehicles.

The overall mean of emission rate during for GDI was slightly higher than that of PFI as 10.1 μg/km and 9.1 μg/km, respectively. In General, GDI engine emits more PM than PFI engine because of stratified-charge operation (Zhao et al., 1999). Liang et al. (2013) reported that the PFI vehicle’s PM mass and total particle number declined about 76% and 77% from those from the GDI vehicle. However, this study did not show significant decrease in PM emission rate. The portion of organic species for PFI and GDI were similar as ~94% and ~93%, respectively. Though portion of inorganic aerosol was too low compared to organic aerosol, the emission rate of inorganics were different between PFI and GDI. Sulfate and ammonium for GDI were relatively higher than that for PFI, but nitrate did not show any significant difference between PFI and GDI. It is indicated that the GDI engine emitted more ammonium sulfate than PFI, even the emission rate was too small. It should be noted that emission rate of nitrate was higher than sulfate and ammonium in both engine types, because on-road vehicles (including gasoline and diesel engines) mainly emitted NO_x that can be converted to particulate nitrate.

Fig. 3 showed the variation of emission rate for organics and inorganics depending on vehicle speeds. It is interesting that the emission rates of both PFI and GDI were significantly decreased with increasing the speed. The emission rate drastically decreased at low speed of vehicle, especially 10 to 35 km/h, whereas the emission rate for organic and inorganics were slightly decreased as vehicle speed was increased. The equations of power law fitting between emission rate and speed can be helpful to estimate PM emission for the different speeds between PFI and GDI vehicles.

Fig. 3. 
Variation of emission rates for organics and inorganics components depends on speed from PFI and GDI vehicles. The dashed lines indicated the power law fitting between emission rate and speed.

The aerosol oxidation state is important for potential increase of organic mass and of hygroscopic property. HR-ToF-AMS can separate quantification of different ions with the same nominal mass with precise characterization of the elemental composition of each ion (e.g. C_xH_y⁺, C_xH_yO₁⁺, and C_xH_yO_z⁺). This more intensive information can help for enhancing of understanding about organic compounds in the atmosphere. The mean concentrations of organics were 9.7 μg/m³ and 7.9 μg/m³ for PFI and GDI, respectively. The portion of the oxidized fragment families (C_xH_yO₁⁺, and C_xH_yO_z⁺) accounts for ~94% in PFI and ~78% in GDI, while ~6 % and ~22% come from the C_xH_y⁺ family in PFI and GDI, respectively (Fig. 4). The relatively high fractions of C_xH_y in organic aerosol (OA) was emitted from GDI comparing it from PFI. It means that the organic species from GDI are potentially increased with oxidation of PM in atmosphere resulting in the increase of mass concentration.

Fig. 4. 
Mass fraction of oxidation state for organic components from (a) PFI and (b) GDI.

To identify the reasonable factors, the organic components in the triangle plot are transformed into the Van Krevelen diagram that can indicate bulk changes in oxygenation in ambient datasets and may be used to explore reaction mechanisms in isolated air masses (Fig. 5). Ng et al. (2011) reported that Van Krevelen space which divided ΔH : C/ΔO : C slope suggests that ambient oxidized organic components aging results in net changes in chemical composition that are equivalent to the addition of both acid and alcohol/peroxide functional groups without fragmentation (i.e. C-C bond breakage), and/or the addition of acid groups with fragmentation.

Fig. 5. 
Van Krevelen-triangle diagram for the organic components from organics with HR-AMS data for (a) PFI and (b) GDI. Each symbol indicated emitted organic components at specific speed.

On the Van Krevelen-triangle diagram, the data position in the apex of the triangle space (higher O : C and lower H : C) indicates highly oxidized particles consistent with other ambient datasets (Ng et al., 2011). Hydrocarbon-like organic components locates 1.5-2 for H : C and 0-0.4 for O : C. Semi-volatile oxidized organic components occupy 1.3-1.6 for H : C and 0.1-0.6 for O : C, while less-volatile oxidized organic components are more oxidized with lower H : C than semi-volatile oxidized organic components as 1.2-1.5 (Aiken et al., 2008; Aiken et al., 2007). In van Krevelen-triangle diagram, distribution of organic components from PFI and GDI showed distinct pattern. Organic components from PFI were mainly distributed in high O : C and low H : C, however, that from GDI were spread over the van Krevelen-triangle diagram, reflecting organic component from PFI was more oxidized that than from GDI as described in section 3.2. It should be noted that the organic components from PFI at less than 80 km/h were mainly more oxidized organics, and the less oxidized organics were emitted at high speed, 110 km/h. For GDI, organic components at low speed (10 km/h) were composed of more oxidized organics, however, the less oxidized organic were gradually increased with increasing the speed. The organic mass emitted from PFI vehicles are hardly increased in ambient due to fully oxidized organic components at the most of speeds, while organic mass emitted from GDI can be potentially increased in ambient by further oxidation process of relatively less oxidized organic aerosol at the most of speeds (Fig. 5).