## 3 Results and Discussion
### Seasonal Variation of Emission Factors  
As discussed above, the hourly averaged emission factors shown in Figure 2 are mixed emission factors of gasoline and diesel vehicles and were greatly affected by % \(fuel_{\rm D}\). In order to remove the  
Figure 2: Diurnal patterns of the hourly averaged (**a**) NO\({}_{x}\), (**b**) OC, (**c**) EC, and (**d**) CO emission factors measured in the tunnel on weekdays in spring (right axis) and the heavy-duty diesel vehicle (HDDV) fuel fraction (left axis). Bars represent the standard deviation of the measurement. OC and EC data are not available at 04:00 and 05:00 because the instrument ran a blank sample each day in that period. \(R^{2}\) values show the correlation between hourly averaged % \(fuel_{\rm D}\) and emission factor. dependence of emissions factors on % _fuel_D and directly show the potential seasonal trend, in Figure 3, we performed a linear regression of emission factors as a function of % _fuel_D. The linear regression was performed separately for winter and spring measurements. It was performed for NOx, OC, and EC in the % _fuel_D range of 0%-40%. The data points used for the linear regression were hourly averaged diurnal emission factors and hourly averaged % _fuel_D. The uncertainty of the linear regression was calculated as the simultaneous functional bounds of the linear fit at the 95% confidence level. As shown in Figure 3, we observed a seasonal variation in emissions of NOx and EC. NOx and EC emissions were both elevated in winter for the entire % _fuel_D range. The NOx emission factor we measured in winter was, on average, 1.7 times larger than in spring at all % _fuel_D, which is larger than the seasonal changes measured by [PERSON] et al. (50%) [36] and [PERSON] et al. (13%) [22]. Regardless of absolute differences, our results contribute to the growing body of evidence that on-road vehicle NOx emissions are higher in winter. We measured EC emission factors in winter that were 1.4 times higher than in spring, whereas [PERSON] et al. [36] and [PERSON] et al. [22] measured slightly lower wintertime emissions. Uncertainty ranges of the NOx and EC emission factors we measured in winter and spring did not overlap, except for EC emission factor at % _fuel_D = 0 (i.e., our estimated EC emissions for gasoline vehicles). OC emissions, on the other hand, were not systematically different between winter and spring. The spring and winter measurements were conducted over left and right lanes, respectively. Since diesel trucks are expected to primarily travel in the right lane, there is a possibility that the higher  
Figure 3: The (**a**) NOx, (**b**) OC, and (**c**) EC emission factors as a function of % _fuel_D over the % _fuel_D range of 0–0.4. Circles represent measurement data, and the color represents the average ambient temperature during the measurement. The linear regression lines of spring and winter measurements are shown as red and green lines, respectively. Red and green shadings are the simultaneous functional bounds of the linear fit at the 95% confidence level for spring and winter measurements, respectively. emission factors measured during winter are simply a result of the traffic distribution between lanes. Therefore, we further verified that the seasonal variation of emission factors was not an artifact of sampling over different lanes. We collected data during weekdays in a single week in spring 2014 and compared the diurnal pattern of NOx emission factors measured over left and right lanes in Figure S3. Overall, there was good agreement. From 5:00 to 20:00, emission factors over the two lanes were identical within experimental uncertainty. The %_fuel_D ranged from 3% to 13% over this time. NOx emissions measured over the right lane were 30-40% higher between 20:00 and 3:00, when %_fuel_D was between 7% and 30%.