Effects of Environmental Aging on Wildfire Particulate and Dissolved Pyrogenic Organic Matter Characteristics

An extensive body of research demonstrates that the molecular composition of pyrogenic organic matter (PyOM, also known as "black carbon") and dissolved PyOM (PyDOM) influences its properties and fate in the environment. The properties of PyOM and PyDOM are also well-known to be governed by their production conditions (e.g., combustion temperature). However, numerous physical and oxidative aging processes in the environment influence PyOM's chemical properties with unclear impacts on its cycling and fate. Here, the solid and aqueous leachate chemical characteristics of wildfire-derived chars aged 1, 4, and 30 years from the Great Dismal Swamp, USA, were examined to explore the effects of environmental aging. Solid-state C-13 NMR spectroscopic and elemental analyses show the most aged char to have the highest H/C and O/C ratios and lowest aromatic contents, yet it also had the highest benzene polycarboxylic acid (BPCA) marker compound yield and degree of aromatic condensation. In PyDOM leachates, increased char aging correlated with increased dissolved organic carbon and BPCA yields as well as with lower H/C and O/C ratios derived from molecular formulas obtained by ultrahigh-resolution mass spectrometry. These results are inconsistent with established combustion continuum molecular-level relationships, suggesting that aging processes govern long-term PyOM and PyDOM molecular compositions. We suggest that hydrological and oxidative aging processes of PyOM selectively remove an oxygenated, less aromatic fraction yielding lower H/C and O/C ratios in residual PyOM. Subsequently, only the most highly condensed aromatic compounds are available for oxidation and transport as PyDOM. Thus, while a combustion continuum describes the initial PyOM and PyDOM molecular characteristics, an aging continuum may better describe those characteristics on annual to decadal time scales, and PyOM losses as PyDOM are likely greater than those estimated from laboratory or fresh chars.