Incomplete understanding of secondary organic aerosol (SOA) sources creates significant uncertainty in air quality management and climate change assessment. Aqueous-phase chemical reactions are a major SOA source, but the impact of anthropogenic emissions on aqueous SOA (aqSOA) is poorly understood. This study uses compound-specific dual-carbon isotopic fingerprints (δ¹³C and Δ¹⁴C) of dominant aqSOA molecules (e.g., oxalic acid) to trace precursor sources and formation mechanisms. Significant stable carbon isotope fractionation in aqSOA molecules indicates extensive aqueous-phase processing. Contrary to the assumption that aqSOA is primarily biogenic, radiocarbon analysis reveals that fossil precursors contribute over half of the aqSOA molecules. This large fraction of fossil-derived aqSOA significantly impacts total water-soluble organic aerosol load, affecting air quality and anthropogenic radiative forcing projections. The findings highlight the importance of fossil emissions in aqSOA formation and their climate and air quality implications.
