Photodissolution of charcoal and fire-impacted soil as a potential source of dissolved black carbon in aquatic environments

This study investigates the effect of photodissolution on the production of dissolved black carbon (DBC) from particulate charcoal and a fire-impacted soil. A soil sample and a char sample were collected within the burn vicinity of the 2012 Cache La Poudre River wildfire and irradiated in deionized water with artificial sunlight. Photoexposure of the suspended char and soil significantly enhanced production of DBC after 7 days continuous exposure to the simulated sunlight. The increase was coupled with an increase in the DBC polycondensed character. In agreement with this, characterization using Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) showed an increase in the number of BC molecular formulae detected and in their average molecular weight, suggesting that increasing photoexposure is required for dissolution of larger, more polycondenced DBC compounds. An increase in molecular signatures with lower H/C ratio and higher O/C ratio after 7 days photoexposure suggested increasing functionality of newly produced DBC with irradiation time, and therefore photooxidation as a potential mechanism for the photodissolution of BC. The photoproduced DBC was also strongly coupled with the photoproduced bulk dissolved organic carbon (DOC). The results suggest that photodissolution may be a significant and previously unrecognized mechanism of DBC translocation to aquatic systems.