Humic-like substances have been identified in atmospheric aerosols that resemble their cousins, humic and fulvic acids, that are known to play important roles in groundwater chemistry. Past work on groundwater humic and fulvic acids made use of diffuse reflectance spectroscopy to characterize key functional groups, particularly carboxylate and hydroxy species (1). Similar complex organics, i.e. HULIS in aerosols and precipitation samples have been found to have similar chemical functionalities and properties, although they are typically smaller in molecular weight. Likely HULIS formation processes will be discussed along with a brief comparison of the spectral properties of these materials to aqueous humic and fulvic acids. HULIS materials have strong absorption bands in both UV-visible and IR and are therefore of interest with regard to radiative balance and climate forcing as absorbing aerosols. HULIS can be formed by secondary photochemical reactions leading to water soluble and species that can undergo organic condensation reactions that can lead to the formation of these “polymers” in acidic aerosols. Similar HULIS can also be produced directly by primary combustion processes involving biomass burning. Other atmospheric oxidation and aging of HULIS can also occur by reaction with ozone, OH and NO3 radicals, as well as reaction with soluble oxidants such as H2O2 , organic peracids, and organic peroxides. The potential importance of HULIS in relation to climate and fine aerosol impacts will be emphasized as these carbonaceous aerosols contribute as strongly absorbing aerosol substances that are additional to black carbon. This work is supported by the Office of Science (BER) U.S. Department of Energy under Grant No. DE-FG02-07ER6429, as part of the DOE Atmospheric Science Program.

1. N.A. Marley, J.S. Gaffney, and K.A. Orlandini, “Characterization of Aquatic Humic and Fulvic Materials by Cylindrical Internal Reflectance Infrared Spectroscopy,” Chapter 7 in Humic/Fulvic Acids and Organic Colloidal Materials in the Environment, ACS Symposium Series 651, American Chemical Society, Washington, D.C., pp. 96-107, 1996.