Apparatus (1) for generating acknowledged flow (Q), comprising a first passage (2) with ends (3,4) for acknowledged flow (Q) inlet and outlet, a discharge electrode (5) for generating airborne unipolar ions (8) positioned inside the first passage (2), a counter electrode (6) adapted to attract said airborne ions (8), thereby being adapted to cause a net flow (7) of airborne ions (8) and thereby generating an airflow (Q) in the direction of the net flow of airborne ions (8), sensing element (12, 13), the output of which is a function of the concentration of the airborne electric charge (8, 11), means (17) for switching or modulating a parameter which affects the output of the sensing element (12,13) and means for determining the volumetric flow (Q) on the basis of the time response which switching or modulation creates to the sensing element (12,13) output. 11. Use of apparatus (1) as described in the previous claims for determining ultrafine particle concentration. Process for generating acknowledged flow.