This paper describes the development of an instrument for online detection of certain types of organic aerosol particles in the atmosphere using the laser-induced fluorescence (LIF) technique. Aerosol particles, sampled from the atmosphere to an optical chamber, is first detected by scattering of a 635-nm continuous wave (CW) laser beam and then excited by a 266-nm pulsed laser, to induce fluorescence emission. The fluorescence in the 300-600 nm wavelength range is spectrally dispersed by a grating spectrometer and then detected by a 32-anode photomultiplier tube (PMT). The performance of the instrument was tested using laboratory-generated particles with known fluorescence properties. We found that pure tryptophan particle as small as 0.3 mu m was detectable with fluorescence. Preliminary results from ambient measurements and cluster analysis are presented and the interpretation of the classification is discussed. (C) 2012 Elsevier Ltd. All rights reserved.