A measuring system (10) and method measure a concentration of components of a gas mixture of gas/aerosol. A reaction support (14) has a flow channel (42) that forms a reaction chamber (46) with an optically detectable reactant (48) that reacts with at least one component or with a reaction product of the component. The flow channel (42) is at least partially filled with particles (100, 102, 104, 110) which have a pre-flow starting position and to which a gas flow is applied through the flow channel (42) in a flow position. The particles (100, 102, 104, 110) are designed (configured) in such a manner that the particles (100, 102, 104, 110) in the starting position and the particles (100, 102, 104, 110) in the flow position can be optically distinguished. The invention also relates to an optical flow sensor (109) for determining a flow of a fluid.