Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 5 2005 10.5194/acpd-5-9801-2005 http://www.atmos-chem-phys-discuss.net/5/9801/2005/ http://www.atmos-chem-phys-discuss.net/5/9801/2005/acpd-5-9801-2005.html http://www.atmos-chem-phys-discuss.net/5/9801/2005/acpd-5-9801-2005.pdf 9801 9860 2005-10-11 Atmospheric HULIS: how humic-like are they? A comprehensive and critical review E. R. Graber Y. Rudich Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, Bet Dagan, 50250, Israel Department of Environmental Sciences and Energy Research, The Weizmann Institute of Science, Rehovot, 76100, Israel A class of organic molecules extracted from atmospheric aerosol particles and isolated from fog and cloud water has been termed humic-like substances (HULIS) due to certain resemblance to terrestrial and aquatic humic and fulvic acids. In light of the interest that this class of atmospheric compounds currently attracts, we comprehensively review HULIS properties, as well as laboratory and field investigations concerning their formation and characterization in atmospheric samples. While sharing some basic features, accumulating evidence suggests that atmospheric HULIS differ substantially from terrestrial and aquatic humic substances. Major differences between HULIS and humic substances, including smaller average molecular weight, lower aromatic moiety content and higher aliphatic moiety and polysaccharide contents, as well as others, are highlighted. Several alternatives are proposed that may explain such differences: (1) the possibility that mono- and di-carboxylic acids and mineral acids abundant in the atmosphere prevent the formation of large humic ''supramolecular associations''; (2) that large humic macromolecules are destroyed in the atmosphere by UV radiation, O₃, and OH-radicals; (3) that ''HULIS'' actually consists of a complex, unresolved mixture of relatively small molecules rather than macromolecular entities; and (4) that HULIS formed via abiotic and short-lived oxidative reaction pathways differs substantially from humic substances formed over long time periods via biologically-mediated reactions. It is concluded that a significant effort towards adopting standard extraction and characterization methods is required to develop a better and more meaningful comparison between different HULIS samples. The essential differences as denoted throughout, point to the possibility that HULIS may not be nearly as humic-like as hitherto believed.