Raman Scattering by Particles

There are few methods suitable for on-line chemical analysis of aerosol particles. Raman spectroscopy offers the possibility of identifying the chemical species in aerosol particles because the spectrum is specific to the molecular. structure of the material, especially to the vibrational and rotational modes of the molecules. Raman spectra have been obtained for individual micron-sized particles placed on surfaces, levitated optically or by an eiectrodynamic balance, or by monodisperse aerosols suspended in a flowing gas. A few measurements have also been made for chemically mixed and poly disperse aerosols. The Raman spectrum of a spherical particle differs from that of the bulk material because of morphology-dependent resonances that re.su It when the Raman scattered photons undergo Mie scattering in the particle. Methods have been developed for calculating the modified spectra (McNulty el al., 1980). 

Both measurements and calculations based on Raman theory indicate that the scattering intensity is approximately proportional to the particle volume (or mass) over certain 

1 For a given mass of particles with the optical properties of carbon spheres, determine the panicle. size producing tnaximum extinction fori ss 0.436/im, Assume itionod is perse particles. 

2 De te rmi ne the part ic le concemrai i on ( g/m ) n ecessary to scatter an amou moflightequal to that of air at 20 C and I atm. Assume a panicle refractive index of 1.5 and a wavelength of 0.5 /Jin. Do the calculation for 0. i-, 0.5-, and l /.im particles of unit density. Compare your result with the average conccntnitton in the Los Angeles atmosphere, about l(X) g/m.  

3 The California visibility standard requires that the visibility be greater than 10 miles on days when the relative humidity is less than 709fc. Consider a day when the visibility controlling 

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