A measuring device (10) and a measurement method measure a concentration of gaseous/aerosol components of a gas mixture. A reaction carrier (14) has a flow channel (42) defining a reaction chamber (46) having a optically detectable reaction material (48), that reacts with a gas mixture component or with a reaction product. The measuring device (12) includes a gas-conveying assembly (2) with a gas-conveying apparatus (28) conveying the gas mixture and a detection assembly (3), which has a lighting apparatus (37) for lighting the reaction chamber (46), an optical sensor (38) for sensing the optically detectable reaction, and an evaluating unit (4) evaluating sensor data and determining a concentration of the component of the gas mixture. The detection assembly (3) senses a speed of a reaction front (6) propagating in the flow direction in the reaction chamber (46) and determines a preliminary concentration from the speed of the reaction front (6).