## 3 Results and Discussion
### Emission Factors of LDV and HDDV  
The hourly averaged emission factors shown in Figure 2 are mixed emission factors of gasoline and diesel vehicles. They cannot be directly compared with other tunnel studies, since different tunnels may have different traffic volumes and vehicle composition (% _fuel_D). The strong correlations between the emission factors and the %_fuel_D, especially for NOx, OC, and EC, offer a chance to apportion LDV and HDDV emissions. Following previous studies [14; 43; 44], we extended the linear regression shown in Figure 3 to the %_fuel_D range of 0 to 1 to extrapolate the LDV emission factor (% _fuel_D = 0) and the HDDV emission factor (% _fuel_D = 1). The results of linear regressions are presented in Figure 5, and the uncertainty ranges of linear regressions are presented in Figure S4. Figure 5: The (**a**) NOx, (**b**) OC, and (**c**) EC emission factors as a function of %_fuel_D over the %_fuel_D range of 0–1. The linear regression lines of spring and winter measurements are shown as red and green lines, respectively. Black dashed lines are linear regression results of [PERSON] et al [14]. Symbols with error bars represent mean values and standard deviations measured in recent dynamometer studies by [PERSON] et al [13]. An advantage of using the linear regression to determine LDV and HDDV emission rates is that it allows us to compare them directly to measurements by [PERSON] et al. [14], which were conducted in a different tunnel (Squirrel Hill tunnel) on the same highway (I-376) in Pittsburgh \(\sim\)10 years prior to our measurements. Emission factors from [PERSON] et al. [14] are shown as black dashed lines in Figure 5. These paired studies show that over the course of \(\sim\)10 years, emissions of NOx, OC, and EC from LDV and HDDV decreased by 50-70%, with the exception of EC emissions from LDV, which only decreased by about 8%. Our results indicate that the emission regulation policies and the advancement of vehicle and emission control technologies were effective at reducing on-road vehicle emissions. Figure 5 and Table 1 also compare our tunnel-based emission factors to those of a recent dynamometer study by [PERSON] et al. [13]. [PERSON] et al. [13] classified all tested LDV into three groups based on the vehicle model year: pre-LEV (Low Emission Vehicle; before 1994), LEV-I (1994-2003), and LEV-II (2004 and later). In order to compare our results with those of with [PERSON] et al. [13], we calculated the fractions of pre-LEV, LEV-I, and LEV-II LDV based on the vehicle inspection records of Allegheny County in 2010 and then calculated a weighted-average LDV emission factor of dynamometer measurements. The average LDV NOx emission factors measured on a dynamometer were about 35% higher than winter tunnel measurements. OC measured by [PERSON] et al. [13] was lower than tunnel measurements for either season. Dynamometer emission factors (EFs) for EC fell within the range of our tunnel measurements. Differences in NOx and OC emissions may reflect the impact of transient drive cycles on the dynamometer versus high-speed cruise conditions in the tunnel, and OC emissions may be further impacted by differences in gas-particle partitioning in the tunnel versus the constant volume sampler used with the dynamometer [40,45,46]. Major changes were instituted in 2007 (for PM) and 2010 (for NOx) for regulating HDDV emissions. [PERSON] et al. [13] tested HDDV meeting both pre-2007 (no after treatment) and post-2010 (equipped with diesel particulate filter (DPF) and selective catalytic reduction (SCR)) standards. The average dynamometer emission factors of HDDV with and without after-treatment devices are presented in Figure 5 to compare with the tunnel measurements. Tunnel-measured HDDV emissions of NOx, OC, and EC were all similar to dynamometer measurements of untreated (pre-2007) HDDV. Tunnel estimates of HDDV emissions may be biased high because of extrapolation and uncertainty in %_fuel_D, as described below, and by the uphill grade in the tunnel.