## 3 Results and Discussion
### Field Survey Results and Analysis
#### 3.2.1 Physical test results and analysis
The thermal environment of the station hall was the worst, maintaining the highest temperature nearly the entire day. Changes in the flow rate had little influence on the thermal environment of the tubular space; the relative position was the decisive factor for the temperature there. Figure 10: Box diagram of the physical test results from the platform layer. b: CO\({}_{2}\) concentration  
In the diurnal variation curve for CO\({}_{2}\) concentration, there was almost no consistent fluctuation pattern for the same type of tubular space. From the overall curvilinear relation in Figure 12, it can be seen that there were two main trends. The first was the stable value for the whole day; the range of change was not large, as illustrated by test points tp8, tp13, and tp14. The second was the dramatic changes during two time periods, 8:00-9:00 a.m. and 4:30-7:30 p.m., which indicated that the concentration of CO\({}_{2}\) was significantly affected by indoor people flow, and the elevated concentration occurred during morning and evening peak hours. In addition, tp3, tp4, tp5, tp6, tp7, tp9, tp10, and tp12 exposed a lack of air adjustment capacity when a large number of people gathered together. Figure 11: Comparison curves of hourly temperature data for each point during the monitoring day. (**a**) Middle tubular space; (**b**) Tube entrance (connection to building); (**c**) Tube entrance (connection to subway); (**d**) Station hall. c: PM2.5/PM10  
As shown in Figure 13, the PM2.5 and PM10 data for each test point varied with time. This study took the middle of the tubular space to be the object of analysis. There were change rules over time for four test points: tp1, tp4, tp8, and tp11. The survey found that three points--tp1, tp4, and tp8--had a sudden increase in PM concentration at 11:00 a.m. and 6:00 p.m., almost two times that of other times. Tp11 was located in the tubular space near the #14 subway line, and was affected by the piston wind from the subway platform. The variations in PM concentration throughout the day were large, and the regularity presented was closely related to the subway's operation time. Figure 12: Comparison curves of hourly CO\({}_{2}\) concentration data for each point during the monitoring day. (**a**) Middle tubular space; (**b**) Tube entrance (connection to building); (**c**) Tube entrance (connection to subway); (**d**) Station hall. d: Wind speed  
The test point height was 1.5 m (tp1, as shown in Figure 14a). One operating cycle for a train (arrival, stay, and departure) was around three minutes. Due to the piston effect caused by the train's operation, the tunnel wind reached remarkable levels. The wind speed when the train was arriving lasted for 15 s, with an average wind speed of 1.2 m/s. The stage during which the train stayed at the station lasted for 40-50 s (at the test point it was 48 s), with an average wind speed of 0.84 m/s. The train's departure stage lasted longer, about 120 s, with a maximum instantaneous wind speed of up to 3.6 m/s and an average wind speed of 1.79 m/s. During each three-minute cycle, the maximum wind speed was 3.6 m/s during the departure stage, while the minimum wind speed was 0 m/s during the period when the train stayed at the station (as shown Figure 14b). The wind direction was opposite during the arrival and departure stages. These two wind directions were off set when the train stayed at the station, presenting a brief state of calm. According to the coupling experiment in the Beijing subway conducted by [PERSON] and others, the maximum wind speed near the train could reach 7.5 m/s at the test point height of 2 m [56] (tp2, as shown in Figure 14a).