Natural and Unnatural Organic Matter in the Atmosphere: Recent Perspectives on the High Molecular Weight Fraction of Organic Aerosol

This chapter provides an overview of research related to aerosol-derived HUmic-Like Substances (HULIS) and the low volatility/high molecular weight organic aerosol fraction, from the comprehensive 2006 review by Graber and Rudich of the similarities and differences between terrestrial/aquatic humic substances and HULIS, to new insights on the sources and formation, atmospheric distribution and climatic, ecological, and epidemiological influences of HULIS from more recent observations. HULIS are primarily formed through secondary oxidation of gaseous biogenic and anthropogenic precursors followed by oligomerization or polymerization reactions and atmospheric aging but can also be directly emitted e.g., from biomass burning smoke or airborne soil. These diverse processes and sources cause HULIS' physicochemical and optical characteristics to evolve over time and vary spatially, making conclusions about its climate forcing effects uncertain. Recent evidence shows that HULLS is present world-wide and in both fine and coarse aerosol size fractions and can impact water optical qualities and nutrient cycling in some ecosystems, form complexes with metals or organic pollutants, and generate reactive oxygen species, which can harm exposed cells. A brief review of mass spectrometric techniques that have been applied to study HULIS or have been recently developed but not yet extensively utilized is included, as are possible future research directions.