Table 1.

Key parameters defining the two ventilation scenarios.

	Scenario 1	Scenario 2	Notes
Computational domain	Dimensions as shown in Fig. 2 but only includes the seating area with approximate length of 15.5 m and carriage height of 2.3 m.		Different carriages have heights between 2.2 m and 2.4 m (Wang et al., 2014).
Inlet air flow rate (m3/h) for the simulated passenger seat area	5500	4000	Ventilation rates of 6200 m3/h and 4400 m3/h are for the entire CHR1 and CHR2 saloons respectively. Specified rates are for passenger seating area.
Inlet	1.2 m wide running along the ceiling above the centre of the saloon, with a temperature of 18 °C and velocity of 0.08333 m/s.	0.05 m high on each side wall just under the luggage racks with a temperature of 18 °C and velocity of 0.7276 m/s.	In CHR2 saloons there are also inlet vents (gaspers) located above the windows, which are adjustable by passengers (http://www.360doc.com/content/16/0903/12/, 2020). However, these are not modelled.
Outlet	0.15 m high vents at the bottom of each side wall running along the length of the saloon with a pressure of 101325 Pa.	At the bottom of each seat base with dimensions equal to that of the seat base with a pressure of 101325 Pa.	In CHR2 saloons the seat bottom vent opening has dimensions 0.48 × 0.49 m2 (Wang et al., 2014).
Mass ratio of recirculated air to dumped air	0.66	0.59	Derived from the known ventilation and fresh air rates.
Filtration efficiency	20%		G-trains assumed to have EU grade 3 and better filters.
Seats	Represented as obstacles. The seat base is square with side length of 0.5 m with top face 0.45 m above the floor. Top of seat back is 1.45 m above floor.		See Fig. 5.
Passengers	Represented as a simple set of solid obstacles with dimensions (depth × height × width); Head: 0.2 m × 0.2 m × 0.16 m, Body trunk: 0.2 m × 0.7 m × 0.4 m, Lap: 0.35 m × 0.1 m × 0.4 m, and Each leg: 0.1 m × 0.45 m × 0.12 m. A heat release rate of 50 W/m2 is applied to the full exposed surface of all passengers.		See Fig. 5.
Index locations	Seat A, B, C, D and F in Row 6.
Quanta source	(1) A 0.04 m wide and 0.05 m high inlet with 0.35 m3/h flow rate (0.049 m/s, 30 °C) representing pulmonary ventilation rate located at the mouth of index patient (2) The scalar value at the inlet is set to 1.0 (3) Quanta generation rate is 14 quanta/h per index patient.		Quanta generation rate assumed for a source at rest (Buonanno et al., 2020, Wang et al., 2021, Dai and Zhao, 2020). Also see Supplementary Material Section S2.
Mesh (cells)	(1) Scenario 1: 2,245,320 (378 × 60 × 99). (2) Scenario 2: 2,170,476 (378 × 58 × 99).
Timestep size (s)	0.5
Target volume for quanta concentration calculation	For individual passengers, the simulated quanta concentration is the average value in a volume 0.16 m wide, 0.20 m deep and 0.2 m high, adjacent to the passenger nose.
Face covering	(1) 40% of population wear face masks (2) Face mask efficiency a (50%) for index patient and b (30%) for susceptible.		(1) Assumed value according to video footage (Youtube video clip, 2020), see also Supplementary Material Section S7.1. (2) See (Davies et al., 2013). For reference surgical mask efficiency is reported to be between 35% and 75% (Rossettie et al., 2020).