A mobile aerosol spectrometer (MAS) was used for in situ characterization of environmental aerosols in the size range between 10 nm and 25 μm. The device combines a differential mobility analyser with a laser and a white light aerosol spectrometer. Classification errors were estimated theoretically, considering the influence of the particles' refractive index and shape. Experimental intercomparison with a cascade impactor showed lower errors than predicted. A mechanism of error compensation is proposed to explain this discrepancy. In a 1-month field study in Frankfurt city the MAS has shown the capability of resolving fast changes in particle concentration and detecting periods of particle production. | A mobile aerosol spectrometer (MAS) was used for in situ characterization of environmental aerosols in the size range between 10 nm and 25 μm. The device combines a differential mobility analyzer with a laser and a white light aerosol spectrometer. Classification errors were estimated theoretically, considering the influence of the particles' refractive index and shape. Experimental intercomparison with a cascade impactor showed lower errors than predicted. A mechanism of error compensation is proposed to explain this discrepancy. In a 1-month field study in Frankfurt city the MAS has shown the capability of resolving fast changes in particle concentration and detecting periods of particle production.