Optical Properties of Humic Material Standards: Solution Phase and Aerosol Measurements

Aerosols generated from aqueous samples of readily obtainable humic material standards are often used as proxies for organic particulates found in the atmosphere in various investigations, such as consideration of radiative forcing effects. Here, we present results for the retrieved complex index of refraction, m = n + ik, at a wavelength of 403 nm for aerosols prepared from six humic material standards using a calibrated cavity ring-down spectrometer: a humic acid sodium salt, Pahokee peat humic and fulvic acids, Elliott soil humic and fulvic acids, and Suwannee river fulvic acid. In addition, we have conducted UV–vis spectrometric studies to measure the mass absorption coefficients, molar absorptivities, and absorption Ångstrom exponents of bulk aqueous solutions of the humic materials. We find clear differences between the humic acid (HA) and fulvic acid (FA) samples with the HA having larger values for the imaginary part of the refractive index, k. The mean value for the HA samples is k = 0.170 while the mean is k = 0.037 for the FA materials. We have examined correlations between the retrieved refractive index and humic material characteristics obtained from spectroscopic and elemental analysis, including aromatic content and the oxygen-to-carbon atomic ratio, where the molar absorption coefficient yields the strongest correlation. Finally, we compare the humic material optical properties to those of authentic and laboratory generated organic carbon samples in order to assess the usefulness of these humic standards as proxies for light absorbing aerosol.