## 3 Results and Discussion
### Long-Term Trend of LDV and HDDV Emission Factors  
In a previous section, we compared results from our tunnel measurement with results from [PERSON] et al. [14] and concluded that on-road gasoline and diesel vehicle emission factors significantly dropped in the decade between studies. However, this conclusion is based on only two studies in the same city and may not represent the situation over the entire United States. Also, there is significant uncertainty in the diesel emission factors, because we used the regression method in that comparison. Therefore, to further confirm the decreasing trend of on-road vehicle emission factors, we summarizedemission factors reported by previous tunnel studies [12; 14; 26; 27; 28; 29; 30; 31; 32; 33] in the United States since the 1990s and compared them with the change of federal vehicle emission standards. The results are presented in Figure 6. All data presented in Figure 6 and the cited previous studies are summarized in Tables S1 and S2. As shown in Figure 6, over the past three decades, measured LDV and HDDV emission factors decreased as the emission standards became stricter. It indicates that stricter emission standards have been effective at reducing on-road vehicle emissions in the U.S. For example, CO and NOx emission factors for LDV have dropped by 70-80% since 2002. Total PM emissions from LDV have also decreased. This decrease is driven by reductions in OC emissions, whereas EC emissions have been essentially flat. Decreasing LDV emissions of OC and stable EC measured in tunnels echo similar trends observed in dynamometer tests [13; 17]. The long-term trend of gas-phase pollutants and PM emitted by HDDV measured in the tunnel are presented in Figure 6c,d, respectively. Tunnel measurements demonstrate a rapid decrease in HDDV NOx emissions associated with 2010 reductions in emissions limits; tunnel studies conducted before 2010 typically measured HDDV NOx emissions \(>\)40 g NO\({}_{2}\) kg-fuel\({}^{-1}\), whereas 2010 data from [PERSON] et al. [32] show lower emissions (\(\sim\)30 g NO\({}_{2}\) kg-fuel\({}^{-1}\)), and our data indicate the lowest emissions. Presumably, fleet penetration of SCR for NOx control, which we did not measure directly, increased from 2010 to 2014. Like NOx in 2010, the HDDV PM emission standard experienced a step change in 2007. Our measurements in 2014 show a significant decrease (\(\sim\)30% decrease) in both EC and OC relative to tunnel studies in the mid-2000s, prior to the introduction of DPFs into the on-road fleet. The introduction of  
Figure 6: Emission factors of (**a**) gaseous pollutants emitted by LDV, (**b**) particulate pollutants emitted by LDV, (**c**) gaseous pollutants emitted by HDDV, and (**d**) particulate pollutants emitted by HDDV measured in tunnels in the United States since the 1990s. The emission factors of LDV and HDDV simulated by EPA MOtor Vehicle Emission Simulator (MOVES) are also presented in the figure as dashed lines. The data and cited previous studies of emission factors are summarized in Table S1 (LDV) and Table S2 (HDDV). For CO emission factors in 2014, we used the average winter emission factors from this study. For emission factors measured in 1992 and reported by [PERSON] et al., only the uphill measurements conducted in the Fort McHenry Tunnel are shown. diesel after-treatments has also contributed to reductions in HDDV CO emissions in 2010 and 2014 relative to the 1990s. Although both LDV and HDDV emission factors measured in tunnels decreased in the past three decades, they did not decrease at the same rate. Thus, the relative significance of HDDV to total on-road vehicle emissions also changed over time. In Table 2 we summarize the ratio of HDDV-to-LDV emission factors measured in previous tunnel studies and in this study in the United States. The HDDV-to-LDV ratio of NOx emission factors reflects changes in vehicle regulations.