A sensor system is provided for measuring particle concentration and mass concentration in an aerosol. An optical sensor (33) is used for measuring a particle concentration and a mechanical sensor (32) is used for measuring a mass of collected particles. A particle concentration in the aerosol is monitored using the optical sensor (33), until detection of a particle generating event. Upon detection of a particle generating event, a mass measurement using the mechanical sensor (32) is performed and the mass measurement is used to calibrate the optical sensor (33). This approach enables the lifetime of the mechanical sensor to be extended, because it is only used when events are detected. The optical sensor, which typically is less accurate for mass sensing, is calibrated by the mechanical sensor.