Chemical Characterization of Flowing Polydisperse Aerosols by Raman Spectroscopy

We have applied Raman spectroscopy to the in-situ measurement of chemical composition of polydisperse flowing aerosols. Monodisperse and polydisperse aerosols in the size range 0.3 to 1.8 m, composed of diethylsebacate (DES) and ammonium sulfate, were generated. The particles were irradiated with 514.5 nm laser light and Raman spectra were collected. The Raman intensities of DES at 2935 cm^-1 and ammonium sulfate at 981 cm^-1, normalized by the nitrogen carrier gas Raman intensity at 2313 cm^-1, were approximately proportional to the aerosol mass loading over the particle size range studied. Calculations based on previous theoretical studies support this observation. The mass loading ranged from 0.17 to 12.8 g/m³ for DES and 20 to 138 mg/m³ for ammonium sulfate. The method was applied to mixing aerosol streams containing DES and ammonium sulfate in a turbulent jet. The Raman system, with a sensitive volume of 0.02 mm³, was used to measure radial and axial concentration profiles in the mixing region. The results compared well with turbulent mixing theory. The primary limitation for application of the method is the low signal to noise ratio.