= 1. In an assessment of the indoor air quality of an enclosed office space, an inert, colorless, odorless and non-flammable tracer gas sulfur hexafluoride, SF6 (m.w. 146.07), was used to estimate the ventilation efficiency of the air-conditioning system. It was assumed that once released into the office indoor atmosphere, SF6 would not be removed by any physical and chemical processes other than being ventilated out of the office space via the air-conditioning system. The total volume of the office space was 1.65 x 104 m. Immediately after the release of the tracer gas, the averaged mixing ratio of SF6 in the office space was measured to be 250 ppbv. After 8 hours, the averaged mixing ratio of SF6 in the office space was decreased to 40.2 ppby. The ambient temperature of the office space was 20 C and the indoor atmospheric pressure was 1.013 105 N m. (a) From the above data, estimate the removal rate constant of SF6 by the air- conditioning system of office space using the One-box Model given below. (8 marks) (b) Estimate the mixing ratio of SF6 in the office space 24 hours after the release of the tracer gas. (4 marks) (c) What assumptions have you made in your estimation in (a) using the One-box model? Briefly discuss whether those assumptions are valid in the present situation