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Tire tread particles are microplastics (< 5 mm) and leach organic chemicals into aquatic environments. It is important to understand the behavior of tire wear compounds in sunlight-exposed waters in terms of their persistence, removal, and transformation. Therefore, we conducted photolysis experiments with leachates from laboratory-generated tire tread particles (TTP) over 72 h in a solar simulator to evaluate the behavior of leached compounds and fluorescent components over time. Compared to initial leachates, simulated sunlight exposure resulted in ~12 % decrease in dissolved organic carbon, 11 % reduction in the total fluorescence of leachates, and ~30 % removal of the 213 chromatographic features detected by nontargeted analysis (NTA) using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry. A decrease in total chemical abundance determined by NTA was observed, with normalized peak areas decreasing by 36.4% in the 72 h photoirradiated samples and by 13.6% in the dark samples. Fifty-three compounds were tentatively identifiable based on mass spectral matching and among them, 12 compounds were confirmed with authentic standards. Among the 53 compounds, 19 compounds were photo-labile, 27 were photo-resistant, and 7 were photo-transformation products. NTA also identified compounds previously unreported as tire-related compounds. Parallel factor analysis (PARAFAC) modeling of three-dimensional excitation-emission-matrix (EEM) data identified five fluorescent components. PARAFAC component C4 (excitation/emission peak at 285/445 nm) was found to be a fluorescent analog for 6PPD. Rapid double exponential decay kinetics were observed for the 6PPD-like component during photoirradiation. Similarly, the peak fluorescence of commercially available 6PPD exposed to simulated sunlight was reduced by >90 % in the first 0.5 h of photoirradiation. 6PPD photodegradation resulted in the production of a fluorescent transformation product resembling PARAFAC Component C2 (with emission at 360 nm). These results prove that EEM fluorescence analyses can serve as a rapid method for kinetics analysis of 6PPD, and may be combined with NTA compound tentative identification to track the behavior of other TTP-derived compounds in experimental studies. 

Wildfires can pose environmental challenges in urban watersheds by altering the physical and chemical properties of soil. Further, invasive plant species in urban riparian systems may exacerbate changes in geomorphological and soil processes after fires. This research focuses on the 2018 Del Cerro fire, which burned upland and riparian areas surrounding Alvarado Creek, a tributary to the San Diego River in California. The study site has dense and highly flammable non-native vegetation cover (primarily Arundo donax) localized in the stream banks and has primarily native vegetation on the hillslopes. We estimated the post-fire organic matter and particle distributions for six time points during water years 2019 and 2020 at two soil depths, 0–15 cm and 15–30 cm, in upland and riparian areas. We observed some of the largest decreases in organic matter and particle-size distribution after the first post-fire rainfall event and a general return to initial conditions over time. Seasonal soil patterns were related to rainfall and variability in vegetation distribution. The riparian soils had higher variability in organic matter content and particle-size distributions, which was attributed to the presence of Arundo donax. The particle-size distributions were different between upland and riparian soils, where the riparian soils were more poorly graded. Overall, the greatest change occurred in the medium sands, while the fine sands appeared to be impacted the longest, which is a result of decreased vegetation that stabilized the soils. This research provides a better understanding of upland and riparian soil processes in an urban and Mediterranean system that was disturbed by non-native vegetation and fire. 

In barren alpine catchments of the Colorado Rocky Mountains, microorganisms are typically carbon (C)-limited, and C-limitation can influence critical heterotrophic processes, such as denitrification. In these remote locations, organic matter deposited during dust intrusion events and other forms of aerosol deposition may be an important C source for heterotrophs; however, little is known regarding the biodegradability of atmospherically deposited organic matter. This study evaluated the extent to which organic matter in Holocene dust and other types of atmospheric deposition in the Colorado Rocky Mountains could support metabolic activity and be biodegraded by alpine bacteria. Microplate bioassays revealed that all atmospheric deposition samples were able to activate microbial metabolism. Decreases in dissolved organic carbon (DOC) concentrations over time in biodegradability incubations reflect the presence of two pools of dissolved organic matter (DOM), a rapidly decaying pool with rate constants in the range of 0.0130–0.039 d^–1and a slowly decaying pool with rate constants in the range of 0.0008–0.009 d^–1. Changes in the fluorescence excitation-emission matrix of solutions evaluated over time indicated a transformation of organic matter by bacteria resulting in a more humic-like fluorescence signature. Fluorescence spectroscopic analyses, therefore, suggest that the degradation of non-fluorescent DOM in glutamate and dust-derived C sources by bacteria results in the production of fluorescent DOM.