Singlet Oxygen Quantum Yields of Pyrogenic Dissolved Organic Matter from Lab-Prepared and Wildfire Chars

Wildfires or prescribed fires release pyrogenic dissolved organic matter (pyDOM) into the environment, which can photochemically produce singlet oxygen (¹O₂) in sun-lit surface waters. ¹O₂ quantum yields (Φ_Δ) are well-studied for non-pyrogenic DOM, but little is understood about the ¹O₂ generation from pyDOM, especially the Φ_Δ values from real wildfire samples and their wavelength dependence. In this study, time-resolved ¹O₂ phosphorescence was used to determine the wavelength-dependent Φ_Δ values for pyDOM generated from wildfire char and a series of lab-prepared chars produced by combusting oak and pine wood. Wildfire and most lab-prepared pyDOM generally had similar Φ_Δ values (2.1–2.7%) at 365 nm compared to the reference Suwannee River Natural Organic Matter (SRNOM) isolate (2.4%). Interestingly, pyDOM from the highest combustion temperature char was found to possess extremely low Φ_Δ values compared to SRNOM and other pyDOM at all excitation wavelengths. In addition, it was revealed that the predicted steady-state concentration of ¹O₂ from pyDOM was similar to that from SRNOM, indicating that the addition of pyDOM from wood chars may not strongly impact surface water photochemistry.