Photolability of pyrogenic dissolved organic matter from a thermal series of laboratory-prepared chars

While pyrogenic dissolved organic matter (pyDOM) is known to be photolabile, the rates and components of pyDOM that are lost via photochemical degradation, and how these vary with pyrogenic source, are poorly understood. Thus, pyDOM was leached from an oak thermal series and a grass chars (250-650 °C) and photoirradiated in a solar simulator. About 10-20% of oak char leachate organic C was mineralized over five days, with greater proportions lost from leachates of higher temperature parent chars. Ultraviolet and fluorescence spectroscopy suggested that mainly aromatic components (e.g., fulvic-, humic-, aromatic-like) were lost. Quantification of benzenepolycarboxylic acids (BPCAs), molecular markers indicated that 75-94% of condensed aromatic C was lost during the first five days of photoincubation, with preferential loss of larger aromatic clusters. Using a 2-component exponential decay model, this most photolabile fraction was calculated to have experimental half-lives of about 1 day. It represented 16 to 23% of the dissolved C, was primarily condensed aromatics, and was likely lost through primary photoreactions. A non-condensed component was lost at half-lives of about 1-2 d, likely through radical-driven propagation reactions. Using the same model, about 43% of pyrogenic C was predicted to be photomineralized over the course of 1 year. These results highlight the contrasting reactivity of condensed and non-condensed portions of pyDOM, and both should be considered when evaluating the potential of pyDOM to alter aquatic ecology and the environmental mobility of priority pollutants.