A mobile aerosol spectrometer for automatic in situ evaluation of particle size and number concentration in environmental aerosols is described. The instrument is able to evaluate size distributions of ambient particles in the range 0.01-25 μm, and to detect alterations in these distributions with a time resolution of a few minutes, capabilities not at present offered by any single commercially available instrument. The satisfactory performance of the spectrometer was demonstrated in field trials at a rural site (Heligoland Island). Environmental aerosols at this site are usually characterized by a particle number concentration of about 5000 cm-3. During the passage of a warm air front, however, a 6-fold increase in this concentration was detected, resulting from enhanced particle production in the nucleation mode below 0.05 μm. | A mobile aerosol spectrometer for automatic in situ evaluation of particle size and number concentration in environmental aerosols is described. The instrument is able to evaluate size distributions of ambient particles in the range 0.01-25 μm, and to detect alterations in these distributions with a time resolution of a few minutes, capabilities not at present offered by any single commercially available instrument. The satisfactory performance of the spectrometer was demonstrated in field trials at a rural site (Heligoland Island). Environmental aerosols at this site are usually characterized by a particle number concentration of about 5000 cm-3. During the passage of a warm air front, however, a 6-fold increase in this concentration was detected, resulting from enhanced particle production in the nucleation mode below 0.05 μm.