An air purification apparatus (100) is disclosed that comprises a flow channel (110) extending between an inlet (111) and an outlet (113); a pollutant removal structure (120) and an air displacement apparatus (130) in the flow channel; a branched sensor channel (140) including: a first branch (142) extending between an ambient air port (141) and a further outlet (143) in the flow channel between the inlet and the air displacement apparatus; and a second branch (144) extending between the ambient air port and a further inlet (145) in the flow channel between the air displacement apparatus and the outlet, the first branch and second branch sharing a branch section; at least one sensor (150) in the shared branch section; a valve arrangement (161, 163) in the branched sensor channel adapted to exclusively disconnect the first branch from the flow channel in a first configuration and exclusively disconnect the second branch from the flow channel in a second configuration; and a controller (170) adapted to control the valve arrangement.