Organic aerosol (OA) is a dominant component of atmospheric fine particulate matter, with secondary organic aerosols (SOAs) comprising the majority. SOAs, formed from the oxidation of precursor gases, consist largely of oxygenated and hygroscopic compounds, harming respiratory health and significantly impacting—though uncertainly—atmospheric radiative forcing. Current global models often underpredict SOA magnitude, distribution, and dynamics, indicating gaps in our understanding of their sources and formation processes. A growing body of research suggests that aqueous-phase chemical reactions in cloud droplets and wet aerosols are a crucial, previously underestimated SOA formation pathway. Aqueous SOA (aqSOA) formation often involves a mixture of anthropogenic and biogenic emissions, and the relative contribution of each is a significant research area for both air quality and climate modeling. However, the complexities of aqueous chemical processes and anthropogenic-biogenic interactions make definitive conclusions difficult to reach. Model simulations often suggest that aqSOA primarily derives from biogenic precursors like isoprene oxidation products. Fossil precursors, considered less polar and hydrophilic, are thought less likely to partition into the aqueous phase and subsequently form aqSOA. However, recent East Asian studies suggest substantial fossil precursor contributions to aqSOA formation. For instance, rapid aqueous-phase oxidation of polycyclic aromatic hydrocarbons from fossil fuel combustion has been observed in Beijing's winter haze, potentially explaining observed SOA increases. Radiocarbon-based estimates attribute approximately 50% of water-soluble OA to fossil sources in the East Asian outflow, contrasting with lower percentages (<30%) in Europe, the US, and South Asia. A quantitative estimation of fossil contributions to aqSOA formation remains challenging due to a lack of reliable tools for tracing sources and formation mechanisms in ambient aerosols.

Previous research has largely focused on the formation of aqSOA from biogenic and biomass precursors, particularly isoprene. Studies have shown the importance of aqueous-phase reactions in secondary organic aerosol formation, but the contribution of anthropogenic precursors, especially those from fossil fuels, has been less explored. Earlier work suggested that fossil fuel-derived SOA is less water-soluble and therefore less likely to contribute significantly to aqSOA. In contrast, studies in East Asia have pointed to substantial contributions of fossil fuel precursors to water-soluble organic aerosols. This paper aims to quantify these contributions using a novel approach.

This study leverages the recent development of compound-specific dual-carbon isotope fingerprinting (δ¹³C-Δ¹⁴C) of aqSOA molecules to quantify and characterize aqSOA sources and atmospheric chemical processes. The Δ¹⁴C content directly constrains the origin of aqSOA tracers, while δ¹³C fingerprints differentiate atmospheric processes and reactions. The technique was applied to purified oxalic acid and other abundant organic acids (e.g., glyoxylic acid). Oxalic acid, representing the highest oxidation state of OA, serves as a proxy for aqueous processing. Organic acids with high O/C ratios (around 1–2) are abundant SOA components and key end-products of aqueous-phase photochemical oxidation. Aerosol samples were collected year-round from a regional receptor site (Heshan Atmospheric Environmental Monitoring Superstation) in South China, encompassing both continental outflow and South China Sea (SCS) air masses. The dual-carbon isotopic compositions, chemical indicators, and meteorological parameters were analyzed to elucidate the relative contributions of fossil and biomass aqSOA precursors and their evolution processes. The analysis was extended to other regions of China to confirm the widespread contribution of aqueous-phase fossil precursor transformation to organic aerosols. The Heshan site experiences an East Asian monsoon climate, with oceanic monsoon conditions during summer (June-August 2017) and continental outflow conditions during winter (September 2017-March 2018) Samples were categorized into "coastal background" and "continental outflow" groups based on air mass transport regimes. Chemical analysis included measurements of gas-phase pollutants, PM2.5 components, organic matter (OM), anthropogenic water-soluble inorganic constituents (WSICanth), and aerosol liquid water (ALW). Molecular compositions of diacids and related aqueous processing tracers were determined. The δ¹³C and Δ¹⁴C isotopic compositions of oxalic acid and other compounds were measured using GC-IRMS and Accelerator Mass Spectrometry (AMS). Backward air mass trajectories were used to identify source regions. Statistical correlations between various parameters were evaluated to understand the relationships between different factors.

The study's key findings include: 1. Significant differences in aerosol composition were observed between coastal background and continental outflow air masses, with concentrations of pollutants and PM2.5 components being considerably higher during continental outflow. 2. Oxalic acid δ¹³C values were significantly lower in continental outflow samples (-24.6 ± 2.7‰) than in coastal background samples (-19.9 ± 2.3‰), suggesting extensive aqueous-phase processing. 3. The strong correlation between oxalic acid concentration and ALW (r²=0.78, P<0.001) further supports the dominance of aqueous-phase formation of oxalic acid. 4. Radiocarbon analysis revealed that fossil sources contributed substantially to oxalic acid, with 55±10% in continental outflow and 33±9% in coastal background samples. 5. Fossil contributions were also significant for other aqSOA precursors, indicating a dominant role of fossil fuel sources in aqSOA formation. 6. The proportion of anthropogenic oxalic acid (fossil and biomass burning) was 78% in continental outflow samples and 39% in coastal background samples. 7. Analysis of samples from five major Chinese cities confirmed the significant contribution of fossil precursors to aqSOA formation, particularly in winter. 8. Aqueous-phase processing of fossil precursors is a significant, yet often overlooked, pathway for SOA formation, especially in regions with high anthropogenic emissions and humid conditions. 9. The positive feedback loop among ALW, inorganic particles, and aqSOA compounds was demonstrated, suggesting that controlling anthropogenic emissions is crucial for mitigating organic particulate pollution. 10. The study challenges the prevailing paradigm that aqSOA are largely biogenic, demonstrating significant anthropogenic, fossil-fuel-derived contributions.

This study's findings directly address the limited understanding of aqSOA sources by demonstrating the significant and previously underestimated contribution of fossil-fuel emissions. The results challenge the existing paradigm, largely based on model simulations, that aqSOA are predominantly biogenic. The large contribution of fossil-derived carbon to aqSOA has substantial implications for aerosol climate forcing and regional air quality. Aqueous-phase processing of fossil precursors can amplify the impact of anthropogenic emissions on water-soluble OA, changing aerosol chemical properties and hygroscopicity. The findings highlight the need for a comprehensive understanding of anthropogenic emissions in aqSOA formation to improve accuracy in climate and air quality projections. The observed positive feedback loop among ALW, inorganic particles, and aqSOA underscores the need for broad controls on various anthropogenic emissions to effectively reduce organic particulate pollution.

This research provides compelling evidence, using compound-specific dual carbon isotope analysis, for the substantial contribution of fossil fuel emissions to aqueous secondary organic aerosol (aqSOA) formation in China. The results challenge the prevailing assumption of aqSOA's primarily biogenic origin and highlight the importance of considering aqueous-phase processing of fossil precursors in climate and air quality models. Future research should focus on extending these analyses to other aqSOA constituents and regions to further refine our understanding of the chemical mechanisms involved and the global implications of this phenomenon.

The study primarily focuses on oxalic acid as a proxy for aqSOA, and while it is a major component, it may not fully represent the diversity of aqSOA compounds. The isotopic fractionation during atmospheric processing might not be fully quantified, and the relative contributions of different emission sources to the isotopic variations in oxalic acid were not fully constrained. Furthermore, while the study highlights aqueous-phase processing, it notes that biomass burning and gas-phase photochemical aging also contribute to oxalic acid formation, and differentiating these pathways remains challenging in field measurements.