Using fluorescence spectroscopy to detect photochemical changes of dissolved wildfire byproducts
Nearly 80% of the United States’ freshwater originates in forested landscapes at risk of Å·ÃÀ¿Ú±¬ÊÓƵ (United States Geological Survey (USGS), 2018), which influence both the terrestrial landscape and hydrologic regime by introducing a heterogeneous spectrum of thermally altered carbon compounds, known as pyrogenic carbon (PyC) (Bird et al., 2015). Given the projected increase in both wildfire frequency and intensity, understanding the coupling of hydrologic transport and chemical fractionation that Å·ÃÀ¿Ú±¬ÊÓƵ impose on water sources is critical (Myers-Pigg et al., 2017). Research has begun to show that PyC can be quite mobile and reactive with turnover times of decades or years in soils rather than previously assumed millennia timescales, emphasizing the importance of dissolved PyC (DPyC) translocation from soils to rivers (Bird et al., 2015; Dittmar et al., 2012). While riverine PyC transport has been identified as a key component of the global PyC cycle, the extent to which photodegradation contributes to both short-term and long-term DPyC chemical fraction has yet to be resolved. We investigate the role of photodegradation as a major driver altering aquatic PyC physical and chemical properties. Artificial PyC was made by burning organic matter (leaves and soil) at various temperatures to isolate distinct portions of the PyC spectrum. Each temperature range of the PyC spectrum was separately leached, filtered, and the dissolved fraction was placed outside and exposed to natural sunlight for various exposure times ranging from zero to 28 days. This photodegradation experiment took place in Boulder, Å·ÃÀ¿Ú±¬ÊÓƵ during the summer months to maximize daily sun exposure. Photochemistry was confirmed by monitoring the photochemical formation of hydrogen peroxide via fluorescence spectroscopy. The dissolved organic matter was primarily characterized using excitation-emission matrix (EEM) fluorescence spectroscopy. Our results confirm that 1) DPyC is susceptible to photodegradation, and 2) there are distinct fluorescence signatures and trends that can differentiate PyC from natural organic matter, underscoring that burn severity matters.