## 3 Results and Discussion
### Operational Stage  
Table 2 displays the average pollution concentrations (gases and particulate matter) recorded on 30 June 2021, during the tunnel's operational phase, at Spots 1 (south portal), 2 (Chainage 1 + 000), 3 (Chainage 4 + 000), and 4 (Chainage 8 + 000). Included among them are NO, CO, H\({}_{2}\)S, CO\({}_{2}\), O\({}_{2}\), O\({}_{3}\), NH\({}_{3}\), NO\({}_{2}\), PM\({}_{10}\), PM\({}_{2.5}\), and PM\({}_{1}\). Several gases were not detected, including NO, H\({}_{2}\)S, O\({}_{3}\), and NH\({}_{3}\). CO was measured to be 1.94 ppm and 5.80 ppm at Spots 1 and 3, respectively. Whereas CO\({}_{2}\) was measured at Spots 1 to 4 to be 459 ppm, 369 ppm, 525 ppm, and 484 ppm, respectively. All four locations had appropriate O\({}_{2}\) levels (>19.5 ppm), ranging from 22.0 ppm to 24.0 ppm. NO\({}_{2}\) concentrations were measured to be 0.002 ppm at all four locations. The measured amounts of gases were well below the Occupational Safety and Health Administration's alarming thresholds (i.e., 35 ppm for CO, 5000 ppm for CO\({}_{2}\), and 0.2 ppm for NO\({}_{2}\)). In addition to mechanical ventilation, natural ventilation was available after tunnel breakthrough, and hence the concentration of gases was within the acceptable limits. Spot 1 at Chainage 1 + 000 near the south portal showed a high concentration of PM\({}_{10}\) (0.512 ppm), whereas Spot 3 at Chainage 8 + 000 near the north portal showed a high concentration of PM\({}_{2.5}\) (0.208 ppm). The high concentrations of PM\({}_{10}\) and PM\({}_{2.5}\) in the vicinity of the portals are caused by the suspension of road dust. Due to the piston effect from both sides, PM\({}_{1}\), with a size of 1 \(\upmu\)m, can travel with the vehicle and was concentrated in the middle of the tunnel. At Spots 2, 3, and 4, the PM\({}_{10}\) levels surpassed the maximum limit (0.054 ppm) (0.515 ppm, 0.298 ppm, and 0.371 ppm, respectively). Similarly, PM\({}_{2.5}\) levels were above the maximum limit (0.06 ppm) for Spots 2, 3, and 4 (0.076 ppm, 0.054 ppm, and 0.208 ppm, respectively). In contrast, PM\({}_{1}\) levels were 0.104 ppm at Spot 1, 0.094 ppm at Spot 2, 0.342 ppm at Spot 3, and 0.061 ppm at Spot 4. However, the maximum allowable PM\({}_{1}\) concentration is not established in any standard. Studies have ascribed the source of particulate matter to road traffic [42; 43; 44; 45; 46; 47]: PM\({}_{10}\) and PM\({}_{2.5}\) is ascribed to non-exhaust emissions including wear and tear (brakes, tires, and clutches), re-suspension of road surface dust, and abrasion [47; 48], while PM\({}_{1}\) is ascribed to vehicle exhaust [49; 50]. Figure 3 depicts the morphological and elemental analyses of the dust samples collected at Spots 1 to 5. In general, angular and subangular particles dominated the micro-morphologies of the dust samples. A few spherical particles were also seen at Spot 1. The dust samples collected at Spots 1 and 5 showed more angular particles and a broad range of particle sizes (the south and north portals, respectively). Overall, the particle sizes were less than 200 \(\upmu\)m. Attached to the surfaces of larger particles were smaller particles with a size of 1 \(\upmu\)m. The EDS examination revealed that the dust particles were mostly composed of O, C, Si, Fe, Al, Ca, Mg, Zn, Pb, and Pd. The chemical composition of particles inside the tunnel was identical to that of mineral dust at all five spots, caused by exhaust and non-exhaust vehicle emissions [17; 51; 52]. Figure 4 demonstrates that the composition of road dust at all five spots includes the major elements Si \(>\) Ca \(>\) Na \(>\) Mg \(>\) Fe \(>\) K and the trace elements Zn \(>\) Pb \(>\) Ni \(>\) Ag \(>\) Co \(>\) Cd. The chemical composition relates to the mineralogy and traffic of airborne dust (brake and tire wear). These results are comparable with those of [PERSON] [44] and [PERSON] et al.