DETECTION LIMITS FOR SULFATES AND NITRATES IN AEROSOL-PARTICLES BY RAMAN-SPECTROSCOPY

Detection limits by Raman spectroscopy are reported for sulfates and n itrates as trace constituents in aerosol particles. The aerosol partic les under investigation are either single suspended solid particles wi th a diameter of 15 mum or present in a monodisperse particle stream g enerated by a vibrating orifice with a droplet diameter of 45 mum. The visible lines of a continuous wave (cw) Argon ion laser are used as t he excitation source. Using the state-of-the art charge coupled device (CCD) detector, high sensitivity is achieved even for spontaneous Ram an scattering. For suspended solid particles containing sodium nitrate as the trace constituent, the detection limit (S/N = 1) is about 4 x 10(-15) moles or 3 x 10(9) molecules. For solution droplets, the detec tion limit is 2.5 millimolar for both sulfates and nitrates ions in aq ueous solutions. A comparison of the sensitivities between spontaneous and stimulated Raman scattering (SRS) shows that the sensitivity of t he spontaneous Raman scattering is at least 20-40 times higher than th at of SRS.