## 5 Conclusions  
Several conclusions can be drawn from this study:  
1. The pollutant emission of the upper vent under a vertical discharge pattern can be considered as the free buoyant jet diffusion under the impact of the momentum of ambient wind. The maximum concentration of pollutants \(C_{\rm max}\) gradually decreased with the increase in the horizontal distance \(r\) from the upper vent under varied ambient wind velocities \(U\). The environmental impact radius \(r_{\rm criti}\) of the pollutant increased with the increase of \(U\) when \(U\leq 3\) m/s, and was free from the impact of \(U\) after \(U>3\) m/s.
2. The emission under the horizontal discharge pattern can be regarded as the semi-confined buoyant jet diffusion under the impact of the momentum of ambient wind. \(C_{\rm max}\) first remained unchanged with the rising of the horizontal distance \(r\) from the upper vent under various \(U\), and then gradually decreased. The attenuation rate of \(C_{\rm max}\) was nearly independent of the impact of \(U\) in the range approaching the upper vent. However, when it was far from the upper vent, the greater the value of \(U\), the faster the attenuation of \(C_{\rm max}\), and the smaller the environmental impact radius \(r_{\rm criti}\).
3. When the discharge capacity was constant, the environmental impact radius of the upper vent under both vertical and horizontal discharge patterns decreased with the increase of the discharge velocity \(V_{0}\). When there was ambient wind, \(V_{0}\) was more sensitive to the impact of \(r_{\rm criti}\) for the vertical outlet, whereas \(V_{0}\) was more sensitive to the impact of \(r_{\rm criti}\) for the horizontal outlet when there was no ambient wind. Moreover, the environmental impact radius, \(r_{\rm criti}\), of the vertical outlet was smaller than that of the horizontal outlet for the range of parameters considered.
4. It is worth noting that the upper vent featuring \"emission in high concentration and low wind velocity\" can improve more significantly the environmental air quality inside a longitudinal ventilation tunnel when the total air volume and discharge capacity are constant. In this regard, the annual environment wind velocity of the tunnel and the impact of the upper vent on the environment inside and outside the tunnel should be considered before designing top vents for tunnels.  
Conceptualization and Writing, X.Z.; Funding acquisition, X.Z. and K.W.; Supervision, K.Z.; Validation, X.Z. and K.Z.; Writing--Original Draft Preparation, X.Z.; Writing--Review and Editing, K.W. and K.Z. All authors have read and agreed to the published version of the manuscript. This research was funded by the Natural Science Basic Research Program of Shaanxi (Program No.2020)Q-779), Shaanxi Provincial Education Department (Program No. 20 JK0840), Ningbo Transportation Science and Technology Plan Project (202113) and the Natural Science Foundation of Zhejiang province (No. LY19E080028). We choose to exclude this statement because this study did not involve humans or animals.  
**Informed Consent Statement:** We choose to exclude this statement because this study did not involve humans.  
**Data Availability Statement:** We choose to exclude this statement because this study did not report any data.  
**Conflicts of Interest:** The authors declare no conflict of interest.