Infrared fingerprint-region aerosol spectroscopy

Infrared (IR) spectroscopy of aerosols is important for climate science, astronomy and industrial safety, but the spectral characteristics of light scattered from an aerosol are complicated by the dual contributions of the bulk absorption and by particle-size-dependent Mie scattering. At shorter wavelengths, spectra correspond well to the transmission profile of the bulk material, as our earlier work using wavelengths of 3.2–3.55 μm showed [I]; however, at longer wavelengths Mie-scattering strongly modulates the underlying chemical absorption signature. Spectroscopy in the “fingerprint region” (ca. 6.5–20 μm) is routinely used to identify unknown vapor and condensed phase chemicals with high confidence, but extending this to aerosolized chemicals requires scattering effects to be taken into account. Here we show how this can be done, presenting a combination of novel theoretical calculations and broadband laser measurements for aerosolized diethyl phthalate (DEP).