Visible spectroscopy of aerosol particles collected on filters: iron-oxide minerals

Diffuse reflectance spectrometry was used to investigate the reflectance characteristics of aerosol particles captured on bulk filters from Bermuda, Barbados, and Izaña. First derivatives of the spectra were examined for signals from two iron-oxide minerals, hematite and goethite, at 555 and 435 nm, respectively, and the spectra and peaks were evaluated relative to the iron concentrations on the filters. The percent reflectance in the yellow, orange, and especially red bands increased with increasing iron concentrations while the normalized reflectances in the violet, blue, and green bands decreased. Pronounced peaks in the first derivative values of the reflectance spectra indicate the presence of hematite and goethite in the brown samples whereas the gray samples lack significant peaks for hematite and goethite. The first derivative values at these characteristic wavelengths were strongly related to the iron concentrations in the samples, implicating iron-oxide minerals as important influences on the aerosols’ reflectance spectra. These results indicate that diffuse reflectance spectroscopy has the potential of providing a rapid non-destructive method for identifying two iron-oxide minerals at concentrations typical of those on atmospheric filters.