Photochemistry after fire: Structural transformations of pyrogenic dissolved organic matter elucidated by advanced analytical techniques

Abstract Pyrogenic organic matter (pyOM) is the solid residue left after pyrolysis of organic matter, a process occurring in nature during forest fires. Upon rain events, pyrogenic dissolved organic matter (pyDOM) leaches into the rain water and is transferred to the aquatic environment where it has been identified as an important component of the global carbon cycle. Overall, pyDOM consists of highly condensed aromatic compounds (ConAC) and poorly characterized aliphatic molecules. While the ConAC are thought to be persistent in the environment, a large ConAC accumulation that would match the calculated fluxes of pyDOM formation and export has not been observed. Photo-degradation has been identified as a major pyDOM sink, and it can potentially account for the discrepancy in the global mass balance of pyDOM. Photochemistry of pyDOM has been studied to a certain extent, but neither the structural transformation pathways of pyDOM nor how they relate to pyrolysis temperature and parent feedstocks are well understood. In response to this need, we have examined the products after photo-irradiation of pyDOM derived from a temperature series (250, 400, 525, 650 °C) of oak biochars made under controlled laboratory conditions, which can be considered model pyrogenic substances. Ultrahigh resolution mass spectrometry (FT-ICR-MS), one-dimensional 1H NMR, and two-dimensional 1H-1H total correlation spectroscopy (TOCSY) NMR were used to evaluate the compositional changes that occurred with photo-irradiation. In agreement with previous studies, pyDOM aryl groups were the most photo-labile, and alkyl and O-alkyl groups were photo-produced. A new finding is that polysubstituted olefinic moieties of pyDOM were also photo-degraded. Using TOCSY we identified several small molecular weight compounds whose abundances changed non-linearly across the combustion continuum which suggests that different radical processes occur in the different char leachates. Using mass spectrometric data we observed that pyDOM from oak chars made at lower temperatures (250 and 400 °C), as well as pyDOM from a grass char prepared at 650 °C, photo-transformed to a more hydrogenated and oxygenated composition. In contrast, pyDOM from higher temperature char leachates (525 and 650 °C) transformed predominantly into smaller aliphatic compounds. To explain these differences, we propose a photo-transformation mechanism for ConAC which includes oxygenation of ConAC, ring-opening reactions, and sequential decarboxylation yielding small aliphatic N-rich molecules (marine-like composition). These structures could be further polymerized by reactive oxygen species to form straight-chain alkanes if not consumed by microorganisms. Results from these controlled experiments reveal that the extent of photo-transformation appears to be a function of production temperature and parent feedstock. This study enhances our understanding of ConAC and pyDOM, their terrestrial-to-marine transfers, geochemical stability, and eventual fate in sunlit environments. Here we also show the value of structural characterization of pyDOM, and reveal the many unknowns left to be answered for this complex and analytically challenging type of organic matter.