In wildfire-prone areas, dissolved wildfire ashes can significantly impact watershed recovery, the drinking water of downstream communities, and long-term aquatic biogeochemistry. Furthermore, the fundamental photochemical properties of wildfire leachates are not well understood. This study measured fluorescence quantum yield, singlet oxygen yield, and photodegradation rates of a range of wildfire material leachates. Over forty wildfire ashes, mineralized soils, and unimpacted soils from a range of forest settings were collected from two Colorado wildfire scars. Ash, mineralized or burned soils, and unburned reference soils were returned to the lab, dried, and sieved. Dissolved organic matter (DOM) was extracted with dilute CaCl2. Fluorescence quantum yield at 350 nm (Φf-350) of wildfire DOM exhibited a range of values across burn scar conditions. Grey ash and duff collected from soil surfaces 2-4 weeks after the fire exhibited the greatest Φf350 (4.0 ±0.5%). This was significantly different than the Φf350 of mineralized or burned soils (2.5±0.8%) and unburned soil leachates (1.3± 0.4%) Singlet oxygen (1O2) is a reactive oxygen species (ROS) and aquatic oxidant produced by DOM, but little is known about the role of wildfire DOM in this ROS production.1O2 yields (Φ1O2) were determined via irradiation for a subset of ash and mineralized soil leachates. Like Φf-350, Φ1O2 of ash leachates was greatest (7.6 ± 3.4%), followed by mineralized soil leachates (4.6±0.7%), and unburned soil leachates (3.9±1). Natural wildfire ash Φ1O2 was notably greater than previous reports of Φ1O2 of laboratory-generated dissolved black carbon (4.2±1.5). Positive relationships between increasing E2:E3, Φf350, and Φ1O2 suggest that fresh, grey ash leachates with greater molar absorptivity than other wildfire materials may play an important role in environmental ROS production in the environment that is not well documented. Interestingly, photobleaching experiments comparing ash DOM to unimpacted soil DOM revealed ash leachates lost total fluorescence and Φf-350 at greater rates than control soils. This suggests that the aromatic features of ashes can absorb more solar light and may degrade faster in the environment. Future work is needed to understand the implications of ash photochemistry in additional environmental contexts and during water treatment.

Singlet oxygen, fluorescence quantum yield, and photodegradation of wildfire ashes in solution

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