Investigating Atmospheric Inputs of Dissolved Black Carbon to the Santa Barbara Channel During the Thomas Fire (California, USA)

The Thomas Fire ignited on December 4, 2017 and burned for over one month. As the Thomas Fire burned, Santa Ana winds carried a thick plume of smoke and ash over the Santa Barbara Channel. We sought to determine whether the deposition of Thomas Fire ash to the Santa Barbara Channel had a measurable effect on the concentration and stable carbon isotopic composition (delta C-13) of dissolved black carbon (DBC) in coastal waters. DBC is the condensed aromatic fraction of thermally altered organic carbon quantified using the benzenepolycarboxylic acid (BPCA) method. DBC delta C-13 signatures were determined via BPCA-specific stable carbon isotopic analysis. Surface water DBC concentrations beneath the smoke plume were up to 13% higher than other sampling stations. Via controlled leaching experiments, we found that Thomas Fire ash released a considerable amount of DBC in seawater (1.4 g-DBC per kg of ash organic carbon), which was further enhanced by photodissolution. By combining in situ and experimental data, we constructed an isotopic mixing model to estimate inputs of ash-derived DBC to marine surface waters. Although we were able to detect slight elevations in DBC concentrations beneath the smoke plume, the ash-derived contributions were too small to meaningfully shift the delta C-13 signature, which resulted in an observed mismatch between modeled and measured DBC delta C-13 values. Few studies have investigated the immediate impacts of wildfire on coastal biogeochemistry. Therefore, our work provides an important foundation for understanding atmospheric contributions of fire-derived DBC to coastal margins.

Automated summary

The Thomas Fire in December 2017 led to increased dissolved black carbon (DBC) concentrations in coastal waters, released by ash and further enhanced by photodissolution. However, the ash-derived contributions were not significant enough to shift the delta C-13 signature, leading to a mismatch between modeled and measured values. This study provides insights into atmospheric contributions of fire-derived DBC to coastal margins.