5.2 Formation and Transformation of Humic-like Substances (hulis) in Atmospheric Aerosols: Relation to Climate Forcing

Humic-like substances (HULIS) have been identified in the water soluble component of atmospheric aerosols, comprising as much as 50% of the water soluble aerosol species at both urban and rural sites (Cappiello, et al., 2003; Limbeck, et al., 1996). These substances have been found to resemble the aquatic humic and fulvic acids, which are known to play important roles in groundwater chemistry. These aquatic humic and fulvic acids are complex organic polymers that have strong uv-visible absorption profiles attenuating the penetration of UVB radiation in the water column (Gaffney et al., 1996; Yacobi et al., 2003). They also have strong IR absorptions that include intense carboxylate and ether bands between 800-1700 cm (Gaffney et al., 1996; Marley et al., 1996; Kim et al., 1990). These materials are also known to complex metals and radionuclides very strongly leading to transport over much longer distances than previously estimated (Marley et al, 1993; Penrose et al., 1990). The HULIS are similar complex organic polymers found in atmospheric aerosols and precipitation samples that have similar chemical functionalities and properties as the aquatic humic materials, although they are typically smaller in molecular weight (Graber and Rudich, 2006). The HULIS also have strong absorption bands in both UV-visible and IR spectral regions and are therefore of interest with regard to radiative balance and climate forcing (Andreae and Gelencser, 2006; Hoffer et al., 2006). The principal absorbing aerosol species has long been thought to be black carbon (BC) derived from incomplete combustion processes. The HULIS are an additional carbonaceous aerosol species that has the potential of contributing to the absorbing aerosols in addition to BC. The HULIS can be formed by secondary photochemical reactions leading to water soluble species that can subsequently undergo organic condensation reactions resulting in the formation of these polymeric materials in acidic aerosols (Limbeck, et al., 1996; Holmes and Petrucci, 2006). HULIS can also be produced directly by primary combustion processes involving biomass burning (Hoffer et al., 2006). In addition, atmospheric oxidation and aging of the HULIS can also occur by reaction with ozone,