## 3 Results and Discussion  
### Pollultant Concentrations and Emission Factors  
The background-corrected time series of gaseous and particulate pollutants measured in the tunnel in a typical week (3 February to 9 Feb, 2014) are presented in Figure S2. All measured species showed strong diurnal patterns, and these diurnal trends are good indicators of the traffic conditions. CO\({}_{2}\) and CO concentrations showed two peaks during weekdays, one during the morning rush hour (around 08:00), and the other one in the afternoon rush hour (15:00-17:00). During weekends, these species presented only one peak during the afternoon. These trends are consistent with the overall traffic patterns, driven by the number of gasoline vehicles.  
The time series of NO\({}_{\rm x}\), OC, and EC concentrations showed a different temporal pattern than CO and CO\({}_{2}\). On weekdays, NO\({}_{\rm x}\), OC, and EC had only one peak around midday. This seemed to be associated with diesel truck activity, since the peak volume of HDDV in the tunnel was also around noon. Weekend concentrations of NO\({}_{\rm x}\) were generally much lower than on weekdays, consistent with reduced HDDV traffic. One exception occurred on Saturday at ~01:00-02:00, when a spike in NOx and CO was observed. This spike may be indicative of high diesel truck volumes during that specific time.  
The hourly averaged diurnal patterns of NO\({}_{\rm x}\), OC, EC, and CO emission factors measured on weekdays in the spring campaign are presented in Figure 2. The diurnal trends of emission factors measured in the winter campaign were similar. Unlike the NO\({}_{\rm x}\), OC, and EC concentrations, which followed the trend of the HDDV volume and peaked around midday, the NO\({}_{\rm x}\), OC, and EC emission factors closely followed the trend of the %_fuel\({}_{\rm D}\)_ and had the highest value during overnight hours (0:00-4:00), dropping by about 65% in daytime. This is because HDDV emit much higher NO\({}_{\rm x}\), OC, and EC than light-duty gasoline vehicles (LDV), and the fraction of HDDV in the tunnel was much higher during overnight hours than in daytime.  
The diurnal trends of NO\({}_{\rm x}\), OC, and EC emission factors measured in this study are similar to those measured by [PERSON] et al. [14] at a nearby Pittsburgh tunnel in 2002. They separated the measured emission factors into three time periods--the early morning (high truck, 0:00-06:00), morning rush hour (low speed, 07:00-09:00), and midday (high speed, 10:00-16:30)--and found that the NO\({}_{\rm x}\), OC, and EC emission factors were much higher in the early morning than in other time periods and the emission factors in the rush hour were slightly lower than at midday, since the %_fuel\({}_{\rm D}\)_ was lower during rush hour.  
Unlike NO\({}_{\rm x}\), OC, and EC, the diurnal pattern of the CO emission factors measured in the tunnel does not follow the trend of %_fuel\({}_{\rm D}\)_. The hourly averaged CO emission factors were relatively constant during the entire day, suggesting that CO emissions trends are driven by LDV. From midnight to 3 am, the CO emission factors were slightly higher and more variable. It may be because during these hours, the background CO ratio was also higher (Figure 1) and the calculated CO emission factors were more subject to the influence of the background air.