Linking Combustion-Derived Magnetite and Black Carbon: Insights from Magnetic Characterization of PM_2.5 in Downwind East Asia

Combustion-derived magnetite has recently attracted attention for its health risks and potential impact on atmospheric heating/cooling. This study provides new observational insights into the relationship between black carbon (BC) and magnetite at a remote site in East Asia, Japan, focusing on combustion sources, seasonal trends, and potential overestimation of BC by the light-absorbing magnetite. Magnetic measurements of PM_2.5 samples, complemented by detailed chemical analyses, reveal similar temporal variations between BC and magnetite while demonstrating that the relative abundance of magnetite to BC varies by combustion source, driving seasonal trends. Magnetite abundance during combustion episodes was found to follow the order: coal > oil > biomass, with mass concentrations roughly estimated via magnetization to be 9-10%, 5-6%, and <2% of BC, respectively. Furthermore, magnetite was estimated to contribute up to 5% of the BC absorption coefficient, suggesting the considerable overestimation of BC depending on the source. Although regionality and source mixing should be further verified, these findings show that magnetic measurements of archived samples can offer valuable contributions to reconstructing long-term combustion trends or overestimates in conventional observations of BC.