Coastal Eutrophic Ecosystem as an Overlooked Pool of Atmospheric Formic Acid: Disentangling Biogenic and Abiotic Contributions

Formic acid (HCOOH), a key driver of secondary organic aerosol and cloud condensation nuclei formation, is systematically underestimated in atmospheric models due to inadequate source characterization. To address this gap, an integrated field campaign was conducted at a South China Sea (SCS) coastal site during the summer, focusing on gaseous HCOOH (HCOOH_g). Measurements revealed that the average concentrations of HCOOH_greached as high as 6.7 μg/m³and 4.0 μg/m³under land and marine breeze, respectively. On marine breeze days, HCOOH_gexhibited significant correlations with chlorophyll-a (Chl-a) and particulate organic carbon (POC) in seawater, implicating dual production pathways from both biogenic activities of phytoplankton and abiotic processes of dissolved organic matter degradation. Quantification of marine-sourced HCOOH_grevealed that these processes collectively accounted for around 16% of HCOOH_g, with abiotic contributions double the biogenic contributions. Based on the established parametrization linking HCOOH_gto marine tracers (POC and Chl-a), the spatial distribution of marine-sourced HCOOH_gwas extrapolated over the SCS, identifying pronounced hotspots in eutrophic coastal regions. This work reveals the coastal eutrophic ecosystem as a pivotal yet unaccounted HCOOH_gsource, emphasizing the necessity of coupling the interactions between marine biogeochemical processes and atmospheric chemical components into air quality and climate modeling.