## 3 Result and Discussion
### Particle Size Distribution Using SMPS  
Figure 4 illustrates the seasonal variations in the particle number concentration. In the range of distribution of particle size from 14.1 nm to 532.8 nm, the number concentration in seasons in the inside of the tunnel appeared as in the following order: Spring (1.29 \(\times\) 10\({}^{11}\) N/m\({}^{3}\)), Summer (1.18 \(\times\) 10\({}^{11}\) N/m\({}^{3}\)), Fall (1.15 \(\times\) 10\({}^{11}\) N/m\({}^{3}\)), and Winter (0.42 \(\times\) 10\({}^{11}\) N/m\({}^{3}\)). Whereas in the outside of the tunnel, it appeared as in the following order: Summer (6.50 \(\times\) 10\({}^{10}\) N/m\({}^{3}\)), Spring (4.92 \(\times\) 10\({}^{10}\) N/m\({}^{3}\)), Fall (4.06 \(\times\) 10\({}^{10}\) N/m\({}^{3}\)), Winter (2.01 \(\times\) 10\({}^{10}\) N/m\({}^{3}\)). The seasonal number concentration in the inside of the tunnel appeared higher by more than twice as much as that of the outside of the tunnel. The inside of the tunnel is a semi-closed environment free from effects of external sources thereby the level of dilution of particles, discharged from vehicles, is relatively low, and is dependent upon the wind speed therein, number of traveling vehicles, and types of each vehicle. Thus, the difference in the number concentrations of particle size distribution, between the inside and outside of the tunnel, was employed for the analysis of the number concentration generated by the discharge from respective vehicles. The number concentration in the winter time, in the inside and outside of the tunnel, appeared approximately 34.9 \(\pm\) 3% and 40.4 \(\pm\) 10% lower, respectively, than those of spring, summer, and fall. It was concluded that it would be associated with final condensation diameter, particle number concentration, and the environment of discharge such as solar radiation, temperature, and humidity etc., [29; 30]. Particle number concentration is related with the particle formation, which is dependent on the condensation nuclei; it is also dependent upon temperature and humidity in the atmospheric environment [31]. Figure 3: Traffic volume related to types of vehicles in each season [27]. The particle size distributions of each season, obtained during the period of the present study, are presented in Figure 5. The mode of the particle number concentration in the inside and outside of the tunnel appeared in the range 61.5-68.5 nm in summer, fall, and spring, whereas it appeared with smaller value in winter distributing in the range 21.7-25.9 nm. This was attributed to the temperature and humidity in the atmosphere affecting the final condensation diameter. It was announced in results of the previously conducted studies that the growth of particulate matters is significantly associated with (1) temperature, (2) humidity, and (3) reaction time [32; 33; 34; 35]. To analyze the daily variation in the diurnal pattern of the particle number concentration, the hourly average values thereof in a day were calculated and are presented in Figure 6. The values increased from 4 to 6 AM and remained constant until 9 PM. Thereafter, the values decreased. The number and speed of vehicles per hour were compared with the corresponding particle number concentration. The traffic volume of vehicles passing through the tunnel increased from 5 to 7 AM and then the traffic volume was kept constant to 0 AM; the variation corresponds to the variation in the particle number concentration observed each hour. The major cause of variation in the particle number concentration was attributed to the discharge of exhausts from vehicles passing through the tunnel. Figure 4: Evolution of seasonal particle size distribution in the inside- and outside of the tunnel estimated by using SMPS. Figure S2 represents the pattern of diurnal variation of the mode of number concentration of inside and outside of the tunnel employed for the analysis of effects of temperature on the final condensation diameter. The inside of the tunnel exhibited relatively constant mode of particle diameters (68.5 nm).