Fluorescence and Absorbance Indices for Dissolved Organic Matter from Wildfire Ash and Burned Watersheds

Wildfiresgenerate significant amounts of ash and burned soilsthat can leach altered dissolved organic matter (DOM) to watersheds.In this work, we analyzed the absorbance and fluorescence spectrumof DOM leached from 40 ash and soil samples collected from two coniferforest burn scars. DOM fluorescence quantum yield at 350 nm (& phi;(f350)) was elevated in all ash and burned soil leachates. WildfireDOM fluorescence was also consistently shifted toward the ultraviolet(UV) region relative to unburned materials, which limited the utilityof several pre-defined optical indices, such as the humification index(HIX) and SUVA(254), for wildfire DOM differentiation. Incontrast, E-2:E-3 and & phi;(f350) resolveddifferences between ash, burned mineral soil, and unburned soil DOMacross an assortment of physical settings. Mixtures of a freshwaterDOM reference material and wildfire ash leachate confirmed that specificfluorescence peaks A and C, & phi;(f350), and E-2:E-3 were well correlated (R (2) > 0.92) to mass increases of ash-derived DOM. These findingswerecompared to 31 surface water samples collected from burned tributarieswithin the Cameron Peak Fire burnscar. Surface waters draining fromthe burn scar were also elevated in & phi;(f350), E-2:E-3, SUVA(254), FI, and UV-shifted fluorescencepeaks A and C at intensities comparable to ash mixing experiments. Wildfire ash and burned soil can leachaltered dissolvedorganic matter (DOM). This study investigated optical tracers forwildfire-generated DOM.