Atmospheric HULIS: How humic-like are they? A comprehensive and critical review

[摘要] A class of organic molecules extracted from atmospheric aerosol particlesand isolated from fog and cloud water has been termed HUmic-LIke Substances(HULIS) due to a certain resemblance to terrestrial and aquatic humic andfulvic acids. In light of the interest that this class of atmosphericcompounds currently attracts, we comprehensively review HULIS properties, aswell as laboratory and field investigations concerning their formation andcharacterization in atmospheric samples. While sharing some importantfeatures such as polyacidic nature, accumulating evidence suggests thatatmospheric HULIS differ substantially from terrestrial and aquatic humicsubstances. Major differences between HULIS and humic substances, includingsmaller average molecular weight, lower aromatic moiety content, greatersurface activity, better droplet activation ability, as well as others, arehighlighted. Several alternatives are proposed that may explain suchdifferences: (1) the possibility that mono- and di-carboxylic acids andmineral acids abundant in the atmosphere prevent the formation of largehumic "supramolecular associations"; (2) that large humic macromoleculesare destroyed in the atmosphere by UV radiation, O₃, and OH^- radicals; (3) that "HULIS" actually consists of a complex, unresolvedmixture of relatively small molecules rather than macromolecular entities;and (4) that HULIS formed via abiotic and short-lived oxidative reactionpathways differ substantially from humic substances formed over long timeperiods via biologically-mediated reactions. It should also be recalled thatthe vast majority of studies of HULIS relate to the water soluble fraction,which would include only the fulvic acid fraction of humic substances, andexclude the humic acid (base-soluble) and humin (insoluble) fractions ofhumic substances. A significant effort towards adopting standard extractionand characterization methods is required to develop a better and meaningfulcomparison between different HULIS samples.

[学科分类] 大气科学