Aerosol Optical Extinction Measurements Using a Pulsed Laser

A recently developed portable instrument, the pulsed cavity ring-down transmissometer/nephelometer tandem (CRDT/N) allows for the in situ measurement of optical properties of aerosols. An Nd:YAG laser excited CRDT (λ = 532, 1064 nm) is shown to be capable of sensitively measuring the aerosol extinction coefficient (bext) at the two wavelengths simultaneously. Adding a nephelometer yields a measurement of scattering coefficient (bscat) of the same aerosol. From scattering and extinction measurements, aerosol absorption coefficients (babs) can be calculated from bext = bscat + babs. Furthermore, intensive properties single scattering albedo (ω0 = bscat/bext) can also be determined. The pulsed cavity-ring down transmissometer/nephelometer (CRDT/N) is validated through a series of laboratory experiments. First agreement between extinction measurements from the cavity ring-down and scattering measurements from the nephelometer is demonstrated for purely scattering particles. Then agreement between measured extinction and Mie theory is demonstrated when using sizeselected purely scattering particles. Overall, measurements agree with Mie theory to within 7%, although observed differences could be due to problems with size selection. The limits of detection and quantitation for this instrument are shown to be bext = 4.0 Mm and 13.4 Mm, respectively.