# Study on the Impact of Ventilation Methods on Droplet Nuclei Transmission in Subway Carriages
## 3 Results
### Particle Reach Probability  
Figure 10 presents a quantitative comparison of the PRP under six scenarios with different ventilation methods. For the purpose of statistical data analysis, particle sizes were divided into 20 size intervals. However, for brevity, only three key intervals are listed. In the PRP legend, category D1 denotes aerosol particles (diameter < 5 \(\upmu\)m), D2 encompasses the intermediate size range of 30-50 \(\upmu\)m, while D3 denotes the maximum size in the cough diameter distribution of this study (diameter 100 \(\upmu\)m). The overall trend shown in Figure 10 indicates that the PRP decreases as the diameter decreases. This suggests that particles with greater mass are more likely to settle closer to the coughing source, while lighter particles tend to remain suspended in the air. Cases (a)-(c) correspond to the SFRC system, where it can be observed that particle sizes within the aerosol range exhibit a strong tendency to remain airborne even at a distance of 4 m from the infected individual. Figure 9: Comparison of the maximum reach of cough particles and the range of pollutants in different ventilation systems, along with a comparison of the weighted average pollution range and maximum reach indicators. Although the situations of the two ventilation systems differ slightly, this trend is consistently observed across the six examples. Another extreme is that, at different air supply speeds, the largest particles in diameter distribution for both ventilation methods settle at distances of less than 2 m from the ground. Regarding Figure 10, in the MV system (d)-(f), neither intermediate nor smaller diameter particles exhibit a tendency to settle at close distances, and the flow distances are significantly greater than those in the SFRC system. The overall trend of PRP variations for different particle sizes remains unchanged; however, within the social safety distance of 2-3 m, both intermediate and small diameter particles show a higher probability of reaching their destination, with no significant differences in the sedimentation distances of the final particles in the distribution. Figure 10: The probability of particle arrival in both ventilation systems. (**a**) SFRC: V = 0.2 m/s; (**b**) SFRC: V = 0.3 m/s; (**c**) SFRC: V = 0.4 m/s; (**d**) MV: V = 0.8 m/s; (**e**) MV: V = 1.2 m/s; (**f**) MV: V = 1.6 m/s. D1, D2, and D3 refer to small (\(<\)5 \(\mu\)m), intermediate (30–50 \(\mu\)m), and large (100 \(\mu\)m) particle sizes, respectively. By comparing Figures 10c and 10f, several conclusions can be drawn. Figure 10c,f illustrates the effect of different ventilation methods on PRP under the same high air supply speed. For example, in the SFRC system, the sedimentation distance for D2 particles is nearly 200% shorter than that in the MV system. Additionally, at lower air supply speeds, the PRP of medium-sized aerosol particles increases significantly. This clearly demonstrates that improving ventilation under a more efficient system reduces the risk of airborne transmission. Characteristics of Air Supply Velocity on the Deposition, Suspension, and Expulsion of Cough Droplets from Patient  
To objectively evaluate the dispersion of droplet nuclei within a carriage, Figure 11 shows the proportion of suspended, deposited, and escaped droplet nuclei as a percentage of the total release at four different time points: 5 s, 20 s, 60 s, and 120 s. The results indicate that due to the patient's coughing and the continuous breathing of other passengers, the number of suspended, deposited, and escaped droplet nuclei inside the subway car increases over time. At all four time points, the number of suspended droplets in the SFRC system is significantly lower than in the MV system. At the early stages of droplet dispersion, the escaped and deposited droplet nuclei behave oppositely in comparison to the MV system. For example, at t = 20 s, the suspended, escaped, and deposited droplet fractions in Case 1 were 38%, 25%, and 37%, respectively, while in Case 4, they were 66%, 0%, and 34%.